EP2247471A2

EP2247471A2 - Deformation collision sensor and method for testing the function thereof

Info

Publication number: EP2247471A2
Application number: EP09706095A
Authority: EP
Inventors: Manfred Sondergeld
Original assignee: Gebrueder Schmidt Fabrik fuer Feinmechanik GmbH and Co KG
Current assignee: Gebrueder Schmidt Fabrik fuer Feinmechanik GmbH and Co KG
Priority date: 2008-01-29
Filing date: 2009-01-17
Publication date: 2010-11-10
Also published as: WO2009095165A3; DE102008008082B4; DE102008008082A1; WO2009095165A2

Abstract

A deformation collision sensor (10) comprises a deformable hollow space (12), with at least one acoustic sensor element (22) connected to the hollow space (12), with an evaluation unit (26) connected to the sensor element (22), and with a thermoacoustic actuator (24) connected to the hollow space (12) for coupling preferably standing sound waves (25) into the hollow space (12). The evaluation unit (26) is designed for evaluating signals (23) that are created in the sensor element (22) by the sound waves (25). The acoustic actuator (24) preferably emits sound waves (25) in the frequency range between 5 and 500 Hz.

Description

Deformation collision sensor and method for checking its function

The invention relates to a collision sensor with a deformable cavity, with at least one acoustic sensor element connected to the cavity, with an evaluation unit connected to the sensor element, and with an acoustic actuator connected to the cavity for coupling preferably standing sound waves in the cavity, wherein the evaluation unit is designed to evaluate signals generated by the sound waves in the sensor element.

The invention further relates to a plurality of methods for verifying the function of a collision sensor having a deformable cavity.

A collision sensor of the type mentioned above is known from DE 10 2004 034 877 A1 and WO 2006/008298 A1. From DE 10 2004 003 199 Al collision systems and methods of the type mentioned above are known.

It is known to equip motor vehicles with so-called collision sensors, which detect when the motor vehicle collides with an obstacle. The obstacle may be another moving vehicle, a moving object, a space-locked obstacle or a pedestrian. Depending on the type of collision, different devices are known, in particular for a frontal collision, a side impact, a rear impact, a rollover and a personal impact.

Collision sensors are also used in industrial safety technology, where high protection and safety requirements may even have to be met in compliance with standards in order to safely and reliably protect man and machine in dangerous situations of industrial use. These are machine and personnel protection devices of all kinds, especially for machine safety in assembly, handling and robotics, as well as for personal safety and accident prevention in access control, access control, and attendance monitoring. Particularly in work areas with potential danger, there are high demands on safety devices for personal safety.

The permanent monitoring functions required for this purpose are realized by sensors which are sensitive to the environment and deliver a control signal directly and immediately after the occurrence of a dangerous situation to a downstream safety device. In addition to non-contact sensors of different types, contacting sensory security elements are used in particular in the presence of pinch edges, such as, for example, tactile switches, contact strips, contact buffers and contact mats. Such, to be monitored crushing edges can be found, for example, on doors, gates, protective gates, road and area boundaries of traffic areas, etc. The monitoring is eg. For sliding gates, rolling gates, lifts, theater stages, machine doors, automatic doors, for example. In public transport and on transport vehicles , Forklift trucks, driverless transport systems, high bay warehouses, hangar doors, and passenger boarding bridges, these lists being merely exemplary and not exhaustive.

Depending on the design, the known sensory switching elements utilize different physical effects and are defined by their limit values for the action of force, the duration of action, the switching path and the overtravel path for the eradication of the impact energy.

The contact strips and contact buffers used in this case are often formed as hollow chamber profiles of a rubber-like or otherwise elastic material. In addition, a sensor tube is integrated in the contact buffers, which has longitudinally opposite, electrically conductive and mutually insulated contact surfaces, which are electrically shorted at a locally impressed from the collision force by partially squeezing the sensor tube and generate a signal in this way.

In a type of collision sensor of interest in the context of the present invention, as used, for example, for passenger protection in vehicles, a gas-filled cavity is arranged in the vehicle or at the pinch edge and is deformed in the event of a collision event. In order to detect this deformation, it is known to increase the pressure in the cavity (DE 1 944 289 A, DE 43 22 488 A1, DE 195 04 353 A1), the air flow emanating from the deformed cavity (DE 102 44 730 A1, DE 102 44 732 A1) or the temperature rise (DE 100 57 258 C1, DE 101 03 047 C1) and trigger a signal when certain limit values are exceeded. The signal in turn activates a safety system, in a vehicle such as an airbag, a belt tensioner, a roll bar, a pedestrian protection device and the like. More. In another type of collision sensors, a sound event occurring during the collision is detected and evaluated.

From EP 0 305 654 Bl a device for triggering a safety device is known. In this device, structure-borne sound sensors, such as microphones, mounted on a motor vehicle. The structure-borne sound sensors generate an electrical structure-borne sound signal in the event of a collision of the motor vehicle. This signal is fed to a spectrum analyzer which generates an acoustic power spectrum from the structure-borne sound signal. The output of the spectrum analyzer controls a tripping unit for an occupant safety device.

From US Pat. No. 4,842,301 A it is known to record the structure-borne sound signals occurring in the event of a vehicle collision by means of piezoelectric sensors. In this case, a frequency band between 100 kHz and 1 MHz is evaluated by a corresponding bandpass filter is used.

US 4 346 914 A discloses a collision sensor device in which an acoustic waveguide is looped along the vehicle support structure and welded thereto at certain points. At the free ends of the waveguide are piezoelectric sensors, which also operate in the range between 100 kHz and 1 MHz. In the event of a collision of the vehicle, an acoustic wave in the waveguide is excited via the welding points and accordingly a corresponding signal is generated in the sensors. A deformation of the waveguide is not provided in this device.

The above known devices that evaluate a sound signal have the disadvantage that structure-borne noise signals are detected that were not caused by a collision of the vehicle, such as a heavy slamming a vehicle door. This can lead to false triggering of the occupant safety device. EP 0 445 907 A2 describes a collision sensor device for motor vehicles. In this device, an acoustic waveguide is also arranged in the vehicle, but acoustically isolated from its support structure. In case of collision, the waveguide is deformed. The deformation excites an acoustic wave in the waveguide and this wave is in turn detected by sensors. The sensor signals are supplied to a processor, which generates a trigger signal for an occupant safety device from the sensor signals if certain limit values have been exceeded. The acoustic decoupling of the waveguide from the vehicle structure ensures that only collision events are detected in which a deformation of the waveguide occurs.

DE 100 34 524 A1 discloses a method for detecting an accidental deformation of a component of a motor vehicle. In this case, a component of the motor vehicle is repeatedly excited with a defined frequency pulse and analyzes the structure-borne noise spectrum resulting from the excitation.

In DE 102 59 527 Al an impact sensor is described in which a deformation of a fastening screw of a bumper is detected by means of ultrasound. For this purpose, the screw is provided with an ultrasonic transmitter and an ultrasonic receiver. The ultrasonic signal propagates from the transmitter in the longitudinal direction of the screw, is reflected at the end and runs back to the receiver. In the case of a change in length caused by a collision, the transit time of the ultrasound signal changes, which can thus be evaluated as a criterion for the occurrence of a collision.

The above-mentioned DE 10 2004 034 877 A1 and WO 2006/008298 A1 disclose an impact sensor in which a piezoelectric crystal sound transmitter couples ultrasonic signals with a frequency of more than 10 kHz into a deformable cylindrical cavity of 1.50 m in length such that in the cavity creates a standing wave. The sound transmitter is mounted on one end face and a sound receiver on the opposite end side. In a collision of the vehicle, the cavity is deformed and thus changes from the sound receiver received signal. To verify whether a collision has actually taken place, the sound transmitter can again emit a test pulse and the signal received by the sound receiver is then evaluated.

This known collision sensor has the disadvantage that a standing wave in the cavity is difficult to maintain, if at a length of the cavity of 1.50 m, the wavelength at 10 kHz is only 3.3 cm, ie 45 periods of the sound signal in the waveguide stand. Then, for the coupling of the sound signal of the sound transmitter as well as for the decoupling of the sound signal at the sound receiver important position of the nodes or Schwingungsbäuche changes even with the least change in the resonant frequency of the cavity, especially due to temperature change, so strong that in terms of quality For example, between 100 and 1000, a signal drop occurs in the cavity that could be misinterpreted as a collision. For the known sensor, a frequency stabilization by wobbling the sound frequency at the sound transmitter is therefore already proposed in the publication itself, which is however complicated and prone to failure. The use of sound signals in the ultrasonic range also has the disadvantage that it can lead to interference with the sound events that occur as a so-called "material cry" in a collision.

The above-described collision sensors are all limited to detecting a collision event. They thus require a collision sensor, which is in perfect condition immediately before the collision.

In the known collision sensors, however, there is the problem that in long-term use, for example, the motor vehicle, the cavity is deformed by other causes or form holes or cracks in the cavity. This changes the acoustic characteristics of the sensor and a reliable triggering of the occupant safety device is no longer guaranteed. This can lead to life-threatening situations in the event of a serious collision. Particularly critical is a crack formation in the cavity wall, which has a leakage of the compressed air result and falsifies the pressure increase to be detected or the air flow to be detected or even prevented. Likewise, a permanent narrowing of the cavity cross-section caused by external force or a complete local clamping of a cavity area can cause the air displacement to be detected beyond the bottleneck not to be forwarded to the sensor element which detects the pressure increase or the air flow. This is the case when the deformation caused by the collision takes place beyond the bottleneck in a volume portion of the cavity facing away from the sensor element.

From the aforementioned DE 10 2004 003 199Al a device for impact detection is known. The device has a waveguide into which a microwave signal of 2.4-2.5 or 5.72-5.875 GHz is coupled by means of a transmitter. The device measures the transit time of the microwave signal between the transmitter and a receiver located at the opposite or the same end of the waveguide. If the waveguide is not deformed, results in a predetermined duration. If the waveguide is deformed by an impact, the transit time changes, and this transit time change is detected as an impact. The known device also allows a functional test by comparing the measured transit time before or during operation of the device with stored data.

The known device thus has the disadvantage that a complex microwave apparatus is needed. Further, the transit time measurement is a relatively inaccurate measurement method because the travel time of a wave in an elongated waveguide is significantly more affected by changes in length than in the cross-sectional shape.

The collision sensors described so far also have the inherent disadvantage that the sound characteristic is firmly impressed by the mechanical Inrienaufbau and must be adapted to the particular application by re-design of the hollow chamber profile always new. That leads to an undesirable because costly variant variety. Canted installations of the collision sensors are also only possible to a limited extent and in the case of very large radii of curvature, so that a frictional adaptation of the collision sensors to the objects to be monitored often succeeds only by embossing a plurality of collision sensors and consequently leads to increased assembly costs.

Furthermore, because of the simple switching function, false alarms can not be distinguished from actual hazardous situations. Permanent monitoring of the safety contact for functionality is also not possible with the normally open contact.

The invention is therefore the object of developing a collision sensor of the type mentioned in such a way that the above-mentioned disadvantages are avoided. According to one aspect of the invention, the sensor according to the invention should be inexpensive to manufacture and easy to assemble, preferably an immediate detection of the described incidents, in particular a deformation (clamping or constriction) or a leak (hole or crack) of the cavity should be possible. According to a further aspect of the invention, this detection is to be performed independently of which physical principle (air pressure, air flow, temperature, sound, etc.) the collision-induced deformation of the cavity is detected.

Finally, it should be ensured that there is no disturbing interaction of the inventive review of the function and the actual function of the collision sensor.

In a collision sensor of the type mentioned, this object is achieved in that the acoustic actuator generates the sound waves using the thermoacoustic effect and preferably emits sound waves in the frequency range between 5 and 500 Hz. The present invention thus utilizes the so-called thermoacoustic or thermo-pneumatic effect for generating sound in a sensory cavity resonator. In this case, the sound is, for example, coupled via a driven with a pulsed or alternating electric current heating element directly into the cavity and the heating element immediately surrounding air due to the inhomogeneous and transient temperature distribution in the vicinity of the heating element by means of induced pressure fluctuations in the air to form sound waves excited ,

The thermoacoustic effect as such has been known for a long time, the first investigations on a heating flame date back to the year 1777; See review article by M. Altenbokum: "The phenomenon of thermoacoustics" in KI Refrigeration Air Conditioning Technology, May 2007, pages 24 to 26.

The first sound transmission with the help of the thermoacoustic effect succeeded in 1880 by means of a thin heating wire, to which a large-area membrane was coupled; See M. Preece, "On Some Thermal Effects of Electric Currents," Proceedings of the Royal Society of London, May 27, 1880.

In the context of the present invention, a sounding membrane is not required.

Since the signal generation is carried out analogously according to the invention, the switching point can be set electronically in the downstream evaluation unit and thus u.U. be easily and individually adapted to different operating conditions by means of preselection functions.

The collision sensor can be integrated into existing contact buffers - even with shared use of the existing cavity - with little effort, so that therefore the compulsion to variant variety is significantly reduced and often completely eliminated. The new collision sensor is thus inexpensive to manufacture and easy to assemble.

If required, the new sensor also supplies a so-called life-zero signal and can be used to record the temporal impact curve, as it not only provides a pure switching function. It thus makes it possible to detect and discriminate against harmless or undesired events, so that false tripping is avoided.

If, for example, the cavity is bent or cranked when used in a contact strip, this leaves the impressed sound signal largely unimpaired.

Furthermore, the complete collision sensor system consisting of sensor element, actuator, cavity and evaluation unit can be completely monitored for functionality by self-test, which significantly increases operational reliability and reliability.

In a method of the type mentioned above, the object underlying the invention is therefore achieved in that a preferably standing sound wave is coupled into the cavity via a thermoacoustically acting actuator, and that the propagating in the cavity sound event is detected and compared with a predetermined setpoint , wherein preferably the sound wave with a frequency in the range between 5 and 500 Hz coupled into the cavity and / or the sound wave is coupled at predetermined operating conditions of an object to be monitored, in particular a motor vehicle for a predetermined period of time in the cavity.

The object underlying the invention is completely solved in this way.

In fact, the invention also makes it possible to have a collision sensor in or on an object to be monitored, in particular in a motor vehicle, with respect to its object To monitor operability, in particular to automatically monitor the entire measuring chain, consisting of cavity and sensor element, so that a fault of the type described above is displayed immediately and can be rectified immediately.

So that it does not come to undesirable interactions with the actual collision detection, the verification of the functionality is on the one hand only for a short time, and preferably only made when a collision detection is not required.

The use of a very low frequency has the advantage that the coupling and decoupling of the sound wave into and out of the cavity is not critical with respect to fluctuations of the ambient temperature.

In a particularly preferred embodiment of the invention, the sensor element is further designed to receive a collision signal which has been generated by a collision event acting on the collision sensor in the cavity.

This measure has the advantage that when a sound signal is evaluated for detecting the collision event, the sensor element and the evaluation unit are used not only in the manner described for monitoring the function of the collision sensor. Rather, the sensor element then also serves for the actual function, namely the detection of a collision event. This double function of the sensor element and the evaluation unit reduces the required installation space, for example, in the motor vehicle as well as the production costs.

Alternatively, however, especially in the event that other physical effects than the sound for the detection of a collision event to be evaluated, another sensor element to be connected to the cavity, which is designed to receive a collision signal, which acts by a force acting on the collision sensor Collision event was generated in the cavity. This measure has the advantage that one is free in the selection of the physical effect.

Preferably, the cavity is filled with a fluid, in particular with air, and the sensor element and / or the further sensor element detects a physical parameter from the group: fluid pressure, fluid flow, fluid temperature.

Particularly preferred is the use of a thermal flow sensor element, because this has an optimal sensitivity for the present application.

In particularly preferred embodiments of the invention, the acoustic actuator couples a periodic sound wave of fixed frequency into the cavity.

This measure has the advantage that the continuous wave excitation generates a stable sound wave in the cavity, which results in an equally stable signal of the sensor element. This is especially true when the frequency corresponds to the resonant frequency of the cavity.

Alternatively, however, the acoustic actuator can also couple a sound wave in the form of a single pressure jump into the cavity.

This measure has the advantage that in certain actuators, the response time can be reduced when the actuator is acted upon by a high jump signal. The actuator must then be in terms of its response at v

Arousal does not meet any special requirements. Then a single shock wave is generated in the cavity, which in turn generates echoes, in which case the distance of the echoes is a measure of the resonant frequency of the cavity. In a further group of embodiments of the invention, the acoustic actuator can successively couple sound waves of different frequencies into the cavity.

This measure has the advantage that a better differentiation of the type of interference is possible with a coupling of sound waves of different frequencies.

In a preferred embodiment of the invention, the acoustic actuator includes a heating element and the heating element is fed from a current source with an adjustable current.

This measure has the advantage that an effective acoustic actuator with simple and commercially available elements can be made available, which has a sufficient bandwidth and is able to generate a sound wave without moving elements.

For the reasons already mentioned, it is also preferred here if the current is a periodically sampled direct current.

This measure has the advantage that the sound wave in the cavity can be excited particularly effectively.

Preferably, the direct current is thereby sampled with a duty cycle of 50% and more preferably with a sampling frequency between about 10 and 300 Hz.

Alternatively, as already mentioned, the direct current may also be sampled once in a stepped manner.

Further alternatively, the power may also be an alternating current. In a preferred embodiment of the exemplary embodiment with a heating element, the heating element is flat, in particular as a carrier membrane with heating conductor tracks structured thereon.

This measure has the advantage that the thermal time constant is very small, for example, less than 1 ms, and thereby a correspondingly large temperature modulation is achieved with which effectively a sound wave can be excited in the cavity.

A particularly good effect is achieved when the heating element is arranged in a chamber, and the chamber is closed except for an outlet opening.

This measure has the advantage that within the chamber, a pulsating pressure is generated, which can be coupled via the outlet particularly well to the cavity.

It is further preferred if the chamber is elongated, in particular cylindrical, and the heating element is arranged along a longitudinal axis of the chamber.

This measure has the advantage that there is a design in which a very high sound pressure is generated in the cavity in a particularly effective manner and thus an intense sound wave can be excited.

In this case, a good effect is achieved in that the heating element has a radial distance from an inner wall of the chamber, wherein the distance is approximately equal to, but not smaller than the thickness of the temperature boundary layer around the heating element around.

In a further exemplary embodiment of the invention, the outlet opening is arranged in the region of a vibration node of a standing sound wave excited by the acoustic actuator in the cavity. This measure has the advantage that the sound wave is excited in the cavity with a particularly high efficiency.

This is especially true when the exit opening is located at a closed end of the cavity.

Furthermore, it is preferred if the sensor element and the acoustic actuator are arranged in a common housing.

This measure has the advantage that a small size is created and that a prefabrication of this module is possible, which facilitates later assembly, for example. In the motor vehicle.

In a first preferred form of signal evaluation in the collision sensor according to the invention, the evaluation unit determines the root mean square value of the output signal of the sensor element and compares the root mean square value with a predetermined desired value.

In a second preferred form of signal evaluation in the collision sensor according to the invention, the evaluation unit determines the quotient of the peak value and effective value of the output signal of the sensor element and compares the quotient with a predetermined desired value.

In a third preferred form of signal evaluation, a sound wave of a first frequency and then a double frequency sound wave are successively coupled into the cavity.

These alternative evaluation options make it possible to individually identify the respective incident with the highest possible accuracy for the various incidents (deformation, leakage). Against this background, the present invention also relates to collision sensors and methods of the type mentioned, in which the new forms of signal evaluation are used.

The invention thus relates to a collision sensor in or on an object to be monitored, in particular a motor vehicle, having a deformable cavity, with at least one acoustic sensor element connected to the cavity, with an evaluation unit connected to the sensor element, and with an acoustic actuator connected to the cavity for generating preferably standing sound waves in the cavity, wherein the evaluation unit is designed for evaluating signals which are generated by the sound waves in the sensor element, wherein the collision sensor at predetermined operating states of the object to be monitored with regard to its function monitored, in particular the acoustic Actuator is operable for a predetermined period of time, wherein the acoustic actuator couples a standing sound wave in the cavity (12) and recorded in the cavity propagating sound event and with a predetermined setpoint (RMSo; CRo) verg is determined, and wherein the root mean square (RMS) of the output signal of a sound event sensor element detected and the root mean square (RMS) with the setpoint (RMSo) is compared and / or the quotient of peak and effective value (CR) of the output signal of the Sound sensor detected sensor element determines and the quotient (CR) is compared with the setpoint (CRo).

Another method according to the invention for checking the function of a collision sensor having a deformable cavity is characterized in that a preferably standing sound wave is coupled into the cavity (12), and that the sound event propagating in the cavity is detected and recorded at a predetermined setpoint value (FIG. RMSo; CRo) and that the root mean square (RMS) of the output signal of a sensor element detecting the sound event determines and the root mean square (RMS) with the setpoint (RMSo) and / or the quotient of peak and rms value (CR) of the output - Determined signal of a sound event detecting sensor element and the quotient (CR) with the setpoint (CR ₀ ) is compared.

In a further development of the collision sensor according to the invention, the predetermined operating state is the actuation of a starter of the motor vehicle.

This measure has the advantage that the checking of the functionality of the collision sensor takes place before the start of the journey, that is to say at a time when a collision can not be expected.

Alternatively or additionally, however, the predetermined operating state can also be a standstill of the motor vehicle.

This measure has the advantage that the review takes place at shorter intervals.

Further, in embodiments of the invention, it is preferred that the acoustic actuator couple a periodic acoustic wave into the cavity and that the predetermined period of time be shorter than about 20 periods of the acoustic wave.

This measure has the advantage that the check is completed in a minimum amount of time. For example, if the acoustic actuator emits sound waves in the frequency range between 5 and 500 Hz, then the duration is only a few milliseconds or seconds.

It follows from the above that the thermoacoustic actuator in the collision sensor according to the invention can be used either only for the self-test or for collision monitoring.

When expected high, short-term force, the collision sensor may preferably operate in pulsed mode. Here the sound wave becomes a sound impulse generated directly by the local deformation of the cavity and detected the associated air displacement at the sensor element. The thermoacoustic actuator is then intended only for the self-test.

With a rather low force and a correspondingly slow force build-up, continuous wave operation is preferred in which the thermoacoustic actuator continuously generates a standing sound wave. In the case of a local deformation, a temporary attenuation of the sound intensity corresponding to the time duration of the external force action is detected on the sensor element. The functional test, ie the self-test, is then carried out by the life-zero signal of the (still) undeformed cavity generated continuously by the thermoacoustic actuator on the sensor element, by which the functionality and in particular the continuity of the entire measuring path is signaled.

Further advantages will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Show it:

Figure 1 is an extremely schematic plan view of an embodiment of a collision sensor according to the invention;

Figure 2 is a representation, similar to Figure 1, but for a slightly modified embodiment; Figure 3 is another illustration, similar to Figure 1, but for another, slightly modified embodiment;

Figure 4 is a detail of an acoustic actuator, as in the

Collision sensor of Figures 1 to 3 can be used;

FIG. 5 shows a signal / time diagram for illustrating a continuous wave

Method, as it can be used in the collision sensor according to the invention;

Figure 6 is a signal / time diagram similar to Figure 5, but illustrating a pulse method;

FIG. 7 shows a signal / frequency diagram;

8 is a diagram showing the course of the signal and a crest

Value as a function of the axial coordinate of an elongate cavity in the collision sensor according to the invention, in the event of deformation of the cavity;

9 is a diagram showing the course of the signal and a crest

Value as a function of the axial coordinate of an elongate cavity in the collision sensor according to the invention, in the event of leakage in the cavity; and

FIG. 10 shows a decision matrix for different signal evaluation methods according to the invention in the case of different incidents.

The present invention takes advantage of the fact that in a gas volume with appropriate excitation sound waves can propagate. In the following and above is simplified for "air" spoken, it understands However, that in the context of the present invention, in particular in closed cavities, other gases can be used.

The properties of the sound waves are determined by the shape and dimensions of the cavity enclosing the air column. The cavity can be in a conventional manner, a separate component or a structural cavity of an object to be monitored, so for example. A vehicle, for example, the interior of a door of the vehicle. The cavity is preferably closed, but it may also be partially open, for example open on one side or it may have specifically provided outflow openings.

In the context of the present invention, a sound wave is generated in the cavity and detected by means of a sensor element, if the current propagation pattern of the sound wave corresponds to the propagation pattern of the undisturbed cavity, for which purpose special evaluation methods are provided. If the state of the cavity is disturbed, for example by a deformation (clamping, bottleneck) or by a leak (hole, crack), then this invention is individually recognized and displayed.

In order to check the condition of a resonant cavity, according to the invention, an acoustic pulse is coupled into the air column located in the cavity by means of an acoustic actuator. The actuator is preferably designed as a thermoacoustic actuator, which can also be referred to as a thermo-pneumatic actuator. In the cavity then a sound wave is generated, for which applies:

c = v λ [1]

where c is the velocity of sound in air, v the oscillation frequency and λ the wavelength in the cavity. The sound wave can form a standing wave, if additionally the mode condition X _n = 2 L / n (n = I ₁ 2, 3 ...) [2]

is satisfied, in which L is the effective length of the cavity. Therefore, a standing wave only forms in the cavity when the excitation frequency v corresponding to [1] and [2] with the resonance frequency

Vn = nc / 2L (n = 1, 2, 3 ...) ^' [3]

coincides. If the cavity has a disturbed state, that is, if it is deformed, for example, clamped at a certain point, in particular clamped, or there is a leak in the form of a hole or a crack at a certain point, then the effective resonance length L changes accordingly. The resonance frequency v _n from [3] then also changes. A shift in the resonant frequency thus signals a faulty state. This shift can be detected by means of a sound-sensitive sensor element and detected in an evaluation circuit.

Similarly, the conditions for a non-closed, eg unilaterally open, cavity can be derived by adapting equations [2] and [3] to open resonators.

The sensor element can be designed as a pressure sensor or as a flow sensor, with thermal flow sensors being preferred. Furthermore, it is preferred if the sensor element also serves as impact sensor element for the actual function of the collision sensor, namely the detection of a collision event.

In FIG. 1, 10 as a whole denotes an exemplary embodiment of a collision sensor according to the invention. The collision sensor 10 has a cavity 12, which in the illustrated example is elongate-cylindrical. The cavity 12 is a resonant structure which is in the frequency range of 5 to 500, preferably of 10 to 300 Hz is resonant. The cavity 12 has a quality which in practice may be between 100 and 1000.

A left in Figure 1, the rear end 14 of the cavity 12 is closed. A Y-shaped branch with a first sound-conducting channel 18 and a second sound-conducting channel 20 is connected to a right front end 16 in FIG.

The first channel 18 leads to a sensor element 22 and supplies an acoustic signal 23 to it. The sensor element 22 serves primarily to monitor the operability of the collision sensor 10 in the manner described in detail below. In addition, however, it can also detect signals which serve to detect a collision event, unless a separate sensor element is provided for this purpose. The second channel 20 leads to an acoustic actuator 24, which can generate and deliver a sound wave 25.

An evaluation unit 26 is connected with a line 27 to the sensor element 22 and a line 28 to the actuator 24. An output 29 of the evaluation unit 26 leads to a trip unit, not shown, for a safety system, which contains, for example, airbags, belt tensioners, a roll bar or the like.

The sensor element 22, the actuator 24 and the evaluation unit 26 and possibly also the Y-shaped branching can be arranged in a common housing 30, which is shown in FIG. The housing 30 is arranged there at the front end 16. Alternatively, the housing 30 'may also be connected to a center position 31 of the cavity 12', as shown in FIG. In any case, it is preferred that the actuator 24 is connected to a vibration node of the sound wave in the cavity 12, because then there is an optimal coupling.

In FIG. 1, 32 denotes an optional further sensor element, which is likewise connected to the cavity 12. The further sensor element 32 detects a for a collision event characteristic signal. If this function is already performed by the sensor element 22, as described above, no further sensor element 32 is required. Of course, the further sensor element can also be arranged elsewhere and work according to any other suitable physical principles of action.

In the context of the present invention, perturbations of concern to the state of the cavity 12 are of interest. In the undisturbed state, the cavity 12 has a predetermined, for example, cylindrical shape. As a result of external influences, the cavity can now be deformed, as indicated in FIGS. 1 and 34 and 36. This may be a bulge or a clamping defect 34, in which the cavity 12 is narrowed over a substantial part of its cross section, as indicated in FIG. Another disturbance is formed by a leak, which is indicated in Figure 1 as a hole defect 36, but which may also be a crack or a porous spot. The defects 34, 36 are located in the example shown at a position z, where z = 0 is located at the front end 16.

Figure 4 shows details of the acoustic actuator 24. The actuator 24 has a housing 40 in which a chamber 42 is located. The housing 40 and the chamber 42 are closed except for a common outlet opening 43. The chamber 42 is at least approximately cylindrical in the illustrated embodiment.

In an interior space 44 of the chamber 42 is located along the longitudinal axis of the chamber 42, a heating element 46. The heating element 46 is for example a filament in which a thin hot wire, such as a platinum wire of 20 microns in diameter, meandered or stretched in a plane or as Spiral is arranged around a longitudinal axis. Preferred is a so-called hotplate. This consists of a flat membrane chip made of silicon with a corresponding heating structure. Such filament heating elements have a very short response time and are operable in a wide frequency range. Therefore, in the context of the present invention, such heating elements are used as thermal-pneumatic or thermoacoustic See actuators used instead of conventional mechanical acoustic actuators, such as piezoelectric elements, which are usually only at a fixed frequency and in a much higher frequency range, in particular in the ultrasonic range, operable.

The heating element 46 is connected directly or inductively to a current source 50, to which a heating voltage UH is supplied via a first input 52 and a control voltage Uc via a second input 54. The power supplied by the power source 50 to the heating element 46 may be an alternating current. Preferably, however, a sampled DC, i. a rectangular time profile, in which by means of the control voltage Uc of the heating current with a duty cycle of 50% and a sampling frequency in the range between about 5 to 500 Hz, preferably from about 10 and 300 Hz, is adjustable. The current supplied to the heating element 46 then also has a rectangular time profile, which switches between the value 0 and a predetermined current value.

In the chamber 42 is in the example shown atmospheric air. This is heated with the heating element 46 to the Heizfilament around. Due to the periodic heating, a movement of the air takes place, which causes an exchange of air at the surface of the filaments. Due to this, non-heated air is periodically transported to the heating filaments, so that the temperature difference between the heating filaments and the air is high and the response time is low. With a in Figure 4, the thickness of the so-called temperature boundary layer is designated. The thickness of the temperature boundary layer is to be understood as meaning the distance from the heating element 46 at which the temperature has dropped to 1/100 times the amount in static ambient air.

The distance of the Heizfilamente to the inner wall of the chamber 42 is preferably about a. For a thermal crosstalk and associated heat losses in the wall of the chamber 42 are avoided. Due to the sampled heat generation, the air expands in the interior 44 of the chamber 42 periodically and also keyed. The air blasts are blown through the outlet opening 43 to the outside in the cavity 12, whereby in the cavity 12 a vibration of the air column therein and at a suitable sampling frequency a standing sound wave is excited.

In the context of the present invention, it is on the one hand preferred if the air surges are coupled into the cavity 12 as a continuous wave signal of constant frequency, preferably at the resonant frequency of the cavity 12. Then, the sound signal S detected by the sensor element 22 has a profile 62 shown in FIG. 5 in a first diagram 60 as a function of the time t. As will be explained, it is also possible to excite the cavity 12 in chronological succession with a first frequency and with a second, preferably twice the frequency.

Alternatively, it is also possible to excite the sound wave in the cavity 12 with a jump function in which the current supplied to the heating element 46 jumps from 0 to a finite value at a certain time and remains there. This has the consequence that a pulse-shaped sound wave is generated in the cavity, which generates echoes and decays slowly, depending on the quality of the cavity. Thereafter, the heating current is switched off again. This sequence can be repeated as often as desired.

Then, the sound signal detected by the sensor element 22 has the profile 68 shown in a second diagram 66 in FIG. 6, which shows echoes decaying in the sound amplitude. The envelope of the echoes reflects the quality of the cavity 12 and the distance of the echoes the resonance frequency again.

In the preferred case of Figure 5, however, the heating element 46 is driven in the manner described above with a fixed frequency of the heating current, which is so dimensioned that the air jets emitted by the actuator 24 in the resonant Cavity 12 excite a standing sound wave, which corresponds to the fundamental vibration of the cavity 12 (n = 1, v = vi). The sensor element 22 then generates a signal S with the frequency vi and maximum amplitude. This signal S then indicates that the cavity 12 is unchanged and the function of the collision sensor 10 is not impaired. This form of monitoring can be carried out continuously or at predetermined time monitoring intervals.

If, in an accident, the fundamental frequency vi of the cavity 12 is changed, for example by a clamping defect 34 and / or a hole defect 36, then the frequency vi of the air pulses no longer corresponds to the fundamental vibration of the changed cavity 12. The signal S of the sensor element 22 changes then. The greater the quality, i. the edge steepness of the transmission curve of the cavity 12, the greater the signal change.

In the case of a sudden excitation, the frequency change is the measure of an existing fault.

If the acoustic actuator 24 is driven at a fixed frequency in the preferred continuous wave mode, then frequency selective signal processing per se is not required. If, on the other hand, the actuator 24 is operated with a jump function, then it is expedient to connect a narrow bandpass filter downstream of the sensor element 22, in which the frequency vi lies in the passband, preferably in its middle.

It is now possible to define a limit value in the evaluation unit 26 which corresponds to a permissible deviation from the nominal value (undisturbed state of the cavity 12). If the signal S now exceeds or exceeds the limit value, this is displayed as a fault. This will be explained in more detail below with reference to FIGS. 8 to 10. In order to determine the fundamental frequency vi of the cavity 12 before the practical use of the collision sensor 10 for a specific collision sensor 10, the cavity 12 is excited once by the actuator 24 with sound waves 25, the frequency of which is then tuned in a wide frequency band.

As a result, FIG. 7 shows a third diagram 70 with a profile 72 of the signal S at the sensor element 22 as a function of the excitation frequency v of the sound waves 25 for an exemplary embodiment of a cavity 12 with an acoustic length of 150 cm. The curve 72 shows a first maximum 74 at the fundamental frequency vi of approximately 100 Hz and a second maximum 76 at the second harmonic vz of approximately 200 Hz. The signal S is determined as the root mean square RMS over several oscillation periods.

Below will now be explained how to determine from the sensor signal, which type of fault (clamping defect 34 or hole defect 36) is present.

FIG. 8 shows a fourth diagram 80 for different axial positions of a clamping defect 34 in a 150 cm long cavity 12. The diagram 80 contains a first profile 82, which represents the RMS value of the signal S as a function of the distance z. The associated limit value RSo is indicated at 84. A second curve 86 represents the so-called crest value CR. This is understood in the specialist world to be the quotient of the peak value and the effective value of the signal S. The associated limit value CRo is indicated at 88.

At the beginning of the cavity 12 (z "0), ie in the immediate vicinity of the actuator 24, the RMS value 82 can reach the setpoint value 84 and even exceed it, so that a clear diagnosis is not possible here. At the end of the cavity (z = 150 cm), however, the RMS value 82 is equal to the desired value 84, because the cavity 12 is closed there anyway. Therefore, with the exception of the initial range, a clamping defect 34 can be detected by the fact that the RMS value 82 of the signal S lies below the limit value RMSo indicated at 84. According to the invention, therefore, the second criterion is the CR value 86. The second course 86 shows that a clamping defect 34 over the entire length z of the cavity 12 results in the CR value 86 being above the limit value CRo indicated at 88.

FIG. 9 shows in a similar form to FIG. 8 a fifth diagram 90 in the case of a hole defect 36. A first curve 92 again represents the RMS value and 94 its limit value RMSo. A second curve 96 shows the CR value and 98 its limit value CRo.

As can easily be seen, a hole defect 36 is detected when the RMS value 94 is below the threshold 94. Only in the middle of the cavity 12 a clear diagnosis is not possible because there the RMS value 94 exceeds the limit 96 in some cases significantly. A similar problem arises for the CR value 96, which is only outside the center of the cavity 12 well above the limit value 98, but in the middle can assume the magnitude of the limit value.

Here, a third criterion must be considered, namely the second harmonic V2 (see FIG. A hole defect 36 at half the length of the cavity 12 corresponds to a unilaterally open resonator. In such a case, however, only odd-numbered harmonics (v ₃ , vs, V7...) In the sense of equation [2] can form. Therefore, if one excites the cavity 12 with the second harmonic V2 and there is no increased signal S in terms of the maximum 76 of Figure 7, then there is a hole defect 36.

FIG. 10 again shows the criteria for the cases "no disturbance", "clamping defect" and "hole defect" in the form of a decision matrix.

The checking of the collision sensor 10 is expediently carried out outside normal operation, because otherwise the sound waves 25 generated by the actuator 24 could interfere with the detection of a collision event. According to the invention, therefore, the check is made when starting the vehicle, possibly alternatively or additionally, when the vehicle is stationary or in the rest periods of the monitored object.

The verification process can be very fast. By "continuous wave operation" in the sense used above, it is to be understood that the actuator 24 emits the sound wave 25 only for a very limited number of periods, for example for less than 50, preferably less than 20 periods.

If, in the event of a collision of the monitored object to be detected, a brief force is to be expected, this collision can be detected via the one-time and short-term air displacement induced by this incident on the sensor elements 22 and / or 32. The thermoacoustic actuator 24 is then used only for the functional test.

If, however, lower force effects are expected, the thermoacoustic actuator 24 is operated continuously, so that in the event of an external force, a change in the signal permanently generated at the sensor element 22 is detected.

As already mentioned, the signal evaluation for the self-test described above can also be used advantageously when no thermoacoustic but an acoustic actuator is used, which is based on a different physical principle.

Claims

claims

1. Collision sensor with a deformable cavity (12), with at least one acoustic sensor element (22) connected to the cavity (12), with an evaluation unit (26) connected to the sensor element (22), and with one to the cavity (12) connected acoustic actuator (24) for coupling preferably standing sound waves (25) in the cavity (12), wherein the evaluation unit (26) for evaluating signals (23) is formed by the sound waves (25) in the sensor element (22 ), characterized in that the acoustic actuator (24) generates the sound waves (25) by means of the thermoacoustic effect.

2. Deformation collision sensor according to claim 1, characterized in that the acoustic actuator (24) emits sound waves (25) in the frequency range between 5 and 500 Hz.

3. collision sensor according to claim 1 or 2, characterized in that the sensor element (22) is further adapted to receive a collision signal generated by a on the collision sensor (10) acting collision event in the cavity (12).

4. collision sensor according to one or more of claims 1 to 3, characterized in that a further sensor element (32) is connected to the cavity (12) which is adapted to receive a collision signal by a on the collision sensor (10). acting collision event in the cavity (12) was generated.

5. collision sensor according to one or more of claims 1 to 4, characterized in that the cavity (12) is filled with a fluid and that the sensor element (22) and / or the further sensor element (32) has a physical Kaichi parameters from the group: fluid pressure, fluid flow, detected fluid temperature.

6. collision sensor according to one or more of claims 1 to 5, characterized in that the acoustic actuator (24) a periodic sound wave (25) of fixed frequency (vi) couples into the cavity (12).

7. collision sensor according to one or more of claims 1 to 5, characterized in that the acoustic actuator (24) couples a sound wave (25) in the form of a single pressure jump in the cavity (12).

8. collision sensor according to one or more of claims 1 to 6, characterized in that the acoustic actuator (24) successively sound waves (25) of different frequency (vi, v ₂ ) coupled into the cavity (12).

9. collision sensor according to one or more of claims 1 to 8, characterized in that the acoustic actuator (24) includes a heating element (46) and that the heating element (46) from a power source (50) is fed with an adjustable current.

10. Collision sensor according to claim 6 and 9, characterized in that the current is a periodically sampled direct current.

11. collision sensor according to claim 10, characterized in that the direct current with a duty cycle of 50% is keyed.

12. collision sensor according to claim 10 or 11, characterized in that the direct current with a sampling frequency between 10 and 300 Hz in the heating element (46) is fed.

13. collision sensor according to claim 7 and 9, characterized in that the current is a one-time stepwise sampled direct current.

14. collision sensor according to claim 9, characterized in that the current is an alternating current.

15. collision sensor according to one or more of claims 9 to 14, characterized in that the heating element (46) is substantially stretched or coiled around a longitudinal axis of a chamber (42), in particular formed as a hot wire.

16. collision sensor according to one or more of claims 9 to 14, characterized in that the heating element (46) is flat, in particular as a support membrane with structured thereon Heizleiterbahnen formed.

17. collision sensor according to one or more of claims 9 to 16, characterized in that the heating element (46) in a chamber (42) is arranged, and that the chamber (42) except for an outlet opening (43) is closed.

18. collision sensor according to claim 17, characterized in that the chamber (42) is elongated, in particular cylindrical, and that the heating element (46) along a longitudinal axis of the chamber (42) is arranged.

19. A collision sensor according to claim 18, characterized in that the heating element (42) has a radial distance (a) from an inner wall of the chamber (42), wherein the distance (a) is equal to, but not smaller than the thickness of the temperature boundary layer around the heating element (42) around.

20. collision sensor according to one or more of claims 17 to 19, characterized in that the outlet opening (43) in the region of a vibration knot one of the acoustic actuator (24) in the cavity (12) excited, standing sound wave is arranged.

21. collision sensor according to claim 20, characterized in that the outlet opening (42) is arranged at a closed end of the cavity (12).

22. collision sensor according to one or more of claims 1 to 21, characterized in that the sensor element (22) and the acoustic actuator (24) are arranged in a common housing (30).

23. collision sensor according to one or more of claims 1 to 22, characterized in that the evaluation unit (26) determines the root mean square (RMS) of the output signal of the sensor element (22) and the root mean square (RMS) with a predetermined desired value (RMSo) compares.

24. collision sensor according to one or more of claims 1 to 22, characterized in that the evaluation unit (26) determines the quotient of the peak value and effective value (CR) of the output signal of the sensor element (22) and the quotient (CR) with a predetermined desired value ( CRo) compares.

25. A method for checking the function of a collision sensor (10) having a deformable cavity (12), in particular for checking the collision sensor (10) according to one of claims 1 to 24, characterized in that a thermoacoustically acting actuator is a preferably standing sound wave (25) is coupled into the cavity (12), and that the sound event propagating in the cavity (12) is detected and compared with a predetermined desired value (RMS ₀ ; CRo).

26. The method according to claim 25, characterized in that the root mean square (RMS) of the output signal of a sound event detecting sensor element (22) determines and the root mean square (RMS) is compared with the desired value (RMSo).

27. Method according to claim 25, characterized in that the quotient of peak value and effective value (CR) of the output signal of a sensor element (22) detecting the sound event is determined and the quotient (CR) is compared with the desired value (CRo).

28. The method according to one or more of claims 25 to 27, characterized in that successively a sound wave (25) of a first frequency (vi) and then a sound wave (25) double frequency (v ₂ ) is coupled into the cavity (12) ,

29. The method according to one or more of claims 25 to 28, characterized in that the sound wave (25) with a frequency in the range between 5 and 500 Hz in the cavity (12) is coupled.

30. The method according to one or more of claims 25 to 29, characterized in that the sound wave (25) at predetermined Betriebszustän- of an object to be monitored, in particular a motor vehicle, for a predetermined period of time in the cavity (12) is coupled.

31. Collision sensor in or on an object to be monitored, in particular a motor vehicle, having a deformable cavity (12), with at least one acoustic sensor element (22) connected to the cavity (12), with an evaluation unit connected to the sensor element (22) ( 26), and with an acoustic actuator (24) connected to the cavity (12) for generating preferably standing sound waves (25) in the cavity (12), wherein the evaluation unit (26) for evaluating signals formed by the sound waves (25) in the sensor element (22), wherein the collision sensor (10) can be monitored with regard to its function under predetermined operating conditions of the object to be monitored, in particular the acoustic actuator (24). is operable for a predetermined period of time, wherein the acoustic actuator (24) couples a standing sound wave (25) in the cavity (12) and recorded in the cavity (12) propagating sound event and compared with a predetermined setpoint (RMSo; CRo) and the root mean square (RMS) of the output signal of a sensor element (22) detecting the sound event is determined and the root mean square (RMS) is compared with the target value (RMSo) and / or the quotient of the peak and effective value (CR) of the output signal the sensor element (22) detecting the sound event is determined and the quotient (CR) is compared with the desired value (CRo).

32. Collision sensor according to claim 31, characterized in that the predetermined operating state is the operation of a starter of the motor vehicle.

33. collision sensor according to claim 31 or 32, characterized in that the predetermined operating state is a stoppage of the motor vehicle.

34. collision sensor according to one or more of claims 31 to 33, characterized in that the acoustic actuator (24) couples a periodic sound wave (25) in the cavity (12) and that the predetermined period shorter than 20 periods of the sound wave (25) is.

35. collision sensor according to one or more of claims 31 to 34, characterized in that the acoustic actuator (24) emits sound waves in the frequency range between 5 and 500 Hz.

36. A method for checking the function of a collision sensor (10) having a deformable cavity (12), in particular for verification of the collision sensor (10) according to any one of claims 31 to 35, characterized in that a preferably standing sound wave (25) coupled into the cavity (12), and in the cavity (12) propagating sound event detected and with a predetermined setpoint ( RMSo; CRo) and that the root mean square (RMS) of the output signal of a sensor element (22) detecting the sound event is determined and the root mean square (RMS) with the target value (RMSo) and / or the quotient of peak value and effective value (CR ) of the output signal of a sound element detecting sensor element (22) and the quotient (CR) with the setpoint value (CRo) is compared.

37. The method according to claim 36, characterized in that successively a sound wave (25) of a first frequency (vi) and then a sound wave (25) of twice the frequency (v2) is coupled into the cavity (12).

38. The method according to claim 36 or 37, characterized in that the sound wave (25) with a frequency in the range between 5 and 500 Hz in the cavity (12) is coupled.

39. The method according to one or more of claims 36 to 38, characterized in that the sound wave (25) at predetermined Betriebszustän- the object, in particular of the motor vehicle for a predetermined period of time in the cavity (12) is coupled.