METHOD AND ANALYSIS SYSTEM FOR DETERMINING THE STATE OF A MEMBRANE OF AN ULTRASONIC SENSOR

Description

[0001] The present invention relates to a method for determining the state of a membrane of an ultrasonic sensor during operation of the ultrasonic sensor. Furthermore, this invention relates to an analysis system for a motor vehicle, which includes an ultrasonic sensor comprising a membrane.

[0002] Ultrasonic sensors are frequently used in motor vehicles to obtain information about the vehicle's surroundings. These sensors employ ultrasonic waves, which are often phase-modulated in various ways. One common phase modulation is binary phase shift, which involves a 180-degree phase shift. Because this allows for the encoding of exactly two states, it is often referred to as binary phase shift (BPSK). At any given time during the transmission process, the entire frequency spectrum is typically used. Therefore, it is not possible to assign a specific frequency to a particular point in time, as the transmitted spectrum contains a mixture of different frequencies.

[0003] Many ultrasonic sensors incorporate a membrane. The membrane's behavior is typically highly dependent on the frequency of an excitation signal. This behavior is also strongly influenced by various membrane conditions. These conditions can include, for example, the membrane being covered with ice or contaminants, as well as varying membrane temperatures. Additional sensors are often used to more accurately predict the membrane's behavior within the ultrasonic sensor. However, since these additional sensors are usually mounted in a different location than the ultrasonic sensors, the resulting sensor data can diverge geographically.

[0004] German patent application DE 10 2014 201 482 A1 describes a method and a device for detecting a malfunction of an ultrasonic transducer by evaluating an impedance envelope. The ultrasonic transducer is subjected to an excitation signal, and an impedance signal is determined that describes the transducer's impedance. An impedance envelope is generated from this signal and compared to a reference envelope. A malfunction is detected if the impedance envelope does not correspond to the reference envelope.

[0005] Patent DE 10 2009 040 992 B4 describes a method for detecting icing and contamination of ultrasonic sensors. In this method, a membrane of the ultrasonic sensor is excited by ultrasonic waves at a predetermined excitation frequency. Subsequently, a decay frequency is observed. By comparing the excitation frequency with the decay frequency, an indication of icing and / or contamination of the membrane is reached.

[0006] From DE 10 2012 216968 A1 a method in the field of environmental sensing using ultrasonic transducers is known, in which malfunctions are detected and quantified to adapt an evaluation of an ultrasonic transducer.

[0007] From DE 10 2017 105 043 a method for determining a functional state of an ultrasonic sensor of an ultrasonic sensor device for a motor vehicle is known, which is designed to emit an ultrasonic signal into an environment of the motor vehicle and / or to receive an echo signal of the ultrasonic signal, wherein an electrical test signal (P) is generated.

[0008] The object of the present invention is to determine a state of the membrane of the ultrasonic sensor more efficiently.

[0009] This task is solved by the independent patent claims. Meaningful further developments arise from the dependent claims.

[0010] The invention provides a method for determining the state of a membrane of an ultrasonic sensor during operation of the ultrasonic sensor. The term "operation of the ultrasonic sensor" refers specifically to transmission operation. The term "operating the ultrasonic sensor" thus preferably means the transmission of the ultrasonic sensor. The active emission of ultrasonic waves by the ultrasonic sensor is addressed by the operation of the ultrasonic sensor. In many cases, it is necessary to detect whether the ultrasonic sensor or its membrane is dirty or covered with ice. Such conditions are often also referred to as blocked states. This means that the present invention can be used, in particular, to detect blocked states of a membrane of an ultrasonic sensor.

[0011] Ideally, these states should not only be detected but also quantitatively determined. In step a), the membrane of the ultrasonic sensor is first subjected to an initial excitation signal with a predefined frequency profile. This predefined frequency profile is generally not constant. However, the frequency of the initial excitation signal is fixed at every point in time. For example, the initial frequency profile can start at a high frequency and end at a lower frequency. If this frequency range is traversed uniformly or linearly, it can be referred to as a down-chirp signal. Preferably, the initial excitation signal is chosen to have a frequency profile that increases or decreases monotonically.This means that the initial frequency response of the first excitation signal preferably exhibits a frequency change in only one direction. Ideally, the first excitation signal therefore does not include any change in the direction of the frequency change. This would be, for example, a frequency response that initially rises and later falls again.

[0012] In step b), an initial voltage profile is measured as a function of the frequency of the first excitation signal, which is induced by the first excitation signal. Typically, the first excitation signal sets the diaphragm of the ultrasonic sensor into vibration. These diaphragm vibrations can be registered by a corresponding voltage drop or an induced voltage. Since a different voltage can be measured at each time, the first voltage profile is obtained by measuring the voltage signals. Thus, the first voltage profile corresponds, in particular, to many individual voltage measurements at different times. Therefore, in step b), the response of the diaphragm to the excitation signal is measured. The response of the diaphragm to this excitation signal can be expressed by the first voltage profile.Thus, the initial voltage curve can be used as an indicator of the membrane's reaction behavior.

[0013] In step c), the membrane of the ultrasonic sensor is subjected to a second excitation signal, which has a second frequency response that differs from the first. The first frequency response is thus different from the second. The explanations regarding step a) apply analogously to step c). Preferably, the first excitation signal differs from the second excitation signal. A first frequency range can coincide with a second frequency range. The frequency range is generally defined by a minimum frequency and a maximum frequency. However, it is particularly possible for a frequency range to be traversed differently. For example, the first frequency response can be a decreasing frequency response, and the second frequency response can be an increasing frequency response. This is particularly the case with chirp signals.A chirp signal is a term used in signal processing technology. A chirp signal typically describes a signal whose frequency changes over time. A distinction can be made between positive and negative chirp signals. In positive chirp signals, the signal's frequency increases over time; in negative chirp signals, the signal's frequency decreases over time. The frequency increase or decrease can be linear or exponential. A positive chirp signal, in which the frequency always increases over time, is often also called an up-chirp. Conversely, a negative chirp signal, whose frequency curve continuously decreases, is called a down-chirp. Preferably, the up-chirp signal is implemented in the opposite way to the down-chirp signal. In particular, the up-chirp signal has the same frequency points as the corresponding down-chirp signal.In particular, the up-chirp signal can be point-symmetrical to the down-chirp signal. In this case, the up-chirp signal would progress from a lower frequency to a higher frequency. The corresponding down-chirp signal could therefore progress from the same higher frequency to the same lower frequency. The respective frequency response is particularly similar to that of the corresponding up- or down-chirp signal, taking into account the different frequency changes.

[0014] In particular, the first frequency response of the first excitation signal differs from the second frequency response of the second excitation signal. Preferably, the two excitation signals are applied at different times. This means that the membrane of the ultrasonic sensor is preferably not stimulated by the first and second excitation signals simultaneously. Ideally, the first excitation signal is time-shifted relative to the second excitation signal. Preferably, the two excitation signals also exhibit further differences, primarily different frequency responses.

[0015] The term "sweep" is often used in the literature instead of "chirp signal." A sweep can be a signal with a constant amplitude whose frequency periodically and continuously sweeps through a predefined range. Unlike an up-chirp or down-chirp signal, a sweep signal can exhibit a change in the direction of the frequency change. For example, the frequency of a sweep signal can increase in one predefined subrange and decrease in another. The excitation signal can therefore be configured in various ways. It can be a sweep signal, an up-chirp signal, a down-chirp signal, or, in the simplest case, a constant excitation signal. Preferably, up-chirp or down-chirp signals are used within the scope of this invention.

[0016] The first and second excitation signals can be frequency-modulated. Both excitation signals can be periodic or sinusoidal. The frequency response of the first signal is not identical to that of the second. For example, the first excitation signal can be a stepped function. Each step can represent a different frequency. The stepped function can be ascending, descending, or a combination of both. The frequency response of the first excitation signal can be linear, quadratic, and / or exponential. For example, if the first excitation signal is an up-chirp, its frequency increases over time. However, this increase can be implemented in different ways. It can be linear, quadratic, exponential, and / or otherwise. The same applies to a down-chirp signal.However, it is possible that the first excitation signal is not a pure up-chirp or down-chirp signal. In this case, the frequency of the first excitation signal can increase over a given first time interval. In another given second time interval, the frequency of the first excitation signal can decrease. For example, the first excitation signal can exhibit a sequence of rising and falling frequency ranges. Thus, such a first excitation signal can enable triangular excitation. In the simplest case, the first excitation signal can be a signal with a constant frequency. Up-chirp and down-chirp signals are preferably used as the first and second excitation signals, respectively. The explanations regarding the first excitation signal mentioned in this paragraph apply analogously to the second excitation signal.

[0017] In step d), a second voltage waveform is measured as a function of the frequency of the second excitation signal, whereby the second voltage waveform is caused by the second excitation signal. The same principles apply to step d) as to step b). Thus, after performing step b), two different voltage waveforms are typically present. These two voltage waveforms are then evaluated to determine the state of the membrane.

[0018] In step e), the first voltage waveform is shifted by a certain amount in a first direction. Similarly, the second voltage waveform is shifted by the same amount in a second direction, so that the respective positions of the maxima of the two voltage waveforms are aligned within a predefined frequency range. The term "aligned" does not necessarily mean that the two maxima must exactly coincide. It is sufficient if, after alignment in step e), the distance between the maxima does not exceed a predefined tolerance value. The distance between the two maxima can be expressed, in particular, as a Euclidean distance.By shifting the first and second voltage curves, step e) achieves the alignment of the first maximum of the first voltage curve in the first predefined frequency range with the second maximum of the second voltage curve in the second predefined frequency range. These two maxima are brought closer together, particularly through step e). Ideally, this alignment results in the merging of the two maxima into a single point.

[0019] In step f), a third stress profile is determined, which lies between the shifted first stress profile and the shifted second stress profile from step e). The first and second stress profiles from step f) onwards preferably refer to those stress profiles resulting from the shift in step e). The shifted first stress profile preferably results from the first stress profile being shifted by a certain amount in the first direction. Explanations of stress profiles relating to step f) refer to the shifted stress profiles resulting from step e). Similarly, the second shifted stress profile is obtained, in particular, by shifting or translating the second stress profile by the same amount in the second direction. It may be provided, in particular, that the two shifted stress profiles can be weighted differently.This can be achieved, for example, by using different weighting factors. Generally, it is not important whether the weighting factor is applied before or after shifting the stress curves. If, for example, the first stress curve is to be weighted more heavily than the second, a larger weighting factor could be chosen for the first stress curve than for the second. Ideally, both weighting factors for the two stress curves add up to 100 percent. In particular, it can be stipulated that the third stress curve corresponds to a median with respect to the first and second stress curves (shifted stress curve).

[0020] In step g), the state of the membrane is determined using the third voltage curve. This is done, for example, by determining at least one electrical parameter of a model for the continuous excitation of the membrane. Furthermore, this at least one electrical parameter is compared with at least one predefined reference parameter. The electrical parameter could be, for example, an effective resistance, an effective capacitance, or an effective inductance. The membrane behavior is described in step g) using the model for the continuous excitation of the membrane. This model assumes, in particular, that the membrane is excited by an excitation signal of constant frequency. This model can be described using an equivalent circuit diagram. In many cases, it is sufficient to compare only one parameter with its corresponding reference parameter.This is typically the parameter that exhibits the greatest deviation from its predefined reference parameter. The predefined reference parameter describes, in particular, a specific state of the membrane. These reference parameters are preferably obtained through reference measurements. These reference measurements are specifically performed using the membrane states that are to be determined later within this procedure. For example, a reference measurement may have been performed when the membrane was covered with ice.

[0021] The reference parameters obtained in this way can be used in step g) to determine the membrane's condition. Other reference measurements might include, for example, blocked membrane states. For instance, reference measurements may have been performed beforehand where the membrane exhibited a predetermined degree of contamination. The reference parameters obtained in this way can be used in step g) to determine whether the membrane is contaminated. Ideally, the comparison in step g) allows not only the contamination itself but also the degree of contamination to be determined. This means that the membrane's condition can be determined quantitatively in step g). Thus, it is possible in step g) not only to make binary distinctions but also to quantitatively determine the membrane's condition. This procedure can be applied separately for each individual ultrasonic sensor.This allows for the safe and reliable identification of which ultrasonic sensor is damaged or currently out of service.

[0022] Furthermore, the type of damage can be determined. For example, if it is detected that the membrane is covered with ice, it can be foreseen that the ultrasonic sensor can be put back into operation at higher temperatures in the future. However, if the comparison in step g) shows that the membrane is damaged or broken, it is clear that this particular sensor will no longer be available permanently. Ideally, the condition of the membrane can be determined so precisely that a failure prediction for the affected ultrasonic sensor becomes possible. This allows for the rapid identification of which ultrasonic sensor needs to be replaced, or which and how many ultrasonic sensors will need to be replaced in the future.

[0023] Another embodiment of the method provides that the first excitation signal is a down-chirp signal and the second excitation signal is an up-chirp signal. A chirp signal is defined as a signal whose frequency changes over time. In an up-chirp, the frequency increases over time. In a down-chirp, the frequency decreases over time. The first excitation signal is typically an electrical signal. Theoretically, the first excitation signal could also be an acoustic signal. However, an ultrasonic sensor or its diaphragm is usually controlled by electrical signals. For this purpose, an actuator based on piezoelectric technology can be used, for example. The piezoelectric element is often an integral part of the transducer element and is therefore also subject to diagnosis. This means that in most cases, the diaphragm is not considered in isolation.Associated components, such as the piezoelectric element, are usually also included in the analysis. The piezoelectric element can be understood as a component belonging to the membrane. A change in the state of the piezoelectric element can also be used to determine the state of the membrane. In most cases, the piezoelectric element cannot be considered an external excitation source. A piezoelectric element can excite the membrane using electrical signals. Electrical signals are preferred as excitation signals. The same applies to the second excitation signal. An up-chirp signal is usually a signal whose frequency increases over time. The increase in this frequency can be structured in different ways. For example, the frequency can increase monotonically over time. It can increase exponentially instead. A combination of different types of increase, i.e., a mixture of exponential and linear increases, is also possible.In other words, the first derivative of the frequency response for an up-chirp signal is preferably always greater than zero. For a down-chirp signal, the first derivative is preferably always less than zero. The behavior of the diaphragm depends in particular on the chirp direction. The chirp direction specifically represents the sign of the first derivative of the frequency response. Thus, the chirp direction indicates whether the frequency of the chirp signal increases or decreases over time. Since the diaphragm's decay behavior depends on the chirp direction, both an up-chirp signal and a down-chirp signal are preferably used. This allows the different decay behaviors of the diaphragm to be considered when determining its state. This enables a more precise analysis of the diaphragm's condition.

[0024] An alternative aspect of the invention provides that the state of the membrane is additionally or alternatively determined by comparing a resonance frequency of the third voltage waveform (the at least one parameter) with a predetermined resonance frequency (the predetermined reference parameter). This variant thus provides that, instead of determining at least one electrical parameter of the continuous excitation model, the resonance frequency of the third voltage waveform is compared with the predetermined reference resonance frequency. Based on this comparison, the state of the membrane can be determined. The predetermined resonance frequency represents a predetermined state of the membrane. If the determined resonance frequency deviates from this predetermined resonance frequency, the state of the membrane can be derived accordingly. Preferably, a quantitative state of the membrane can even be determined from the degree of deviation.For example, it is possible to determine how much ice is present on the membrane if it is covered with ice. The resonant frequency is defined, in particular, by a minimum in the third voltage waveform within a given frequency range.

[0025] Another embodiment of this invention provides that the model for continuous excitation of the membrane comprises a first resistance, a first inductance, and a first capacitance in a parallel circuit, as well as a second resistance, a second inductance, and a second capacitance in a series circuit, as the electrical parameters of the model. In particular, the following formula is used: Z = 1 i ⋅ w ⋅ L P + 1 R P + i ⋅ w ⋅ C P + 1 i ⋅ w ⋅ L S + 1 i ⋅ w ⋅ C S + R S − 1

[0026] In Formula 1, i represents the complex unit. Here, Z refers to the impedance of the membrane. wRp represents the angular frequency of the third voltage waveform. Rp represents the first resistance, Lp represents the first inductance, and the first capacitance is represented by the variable Cp. These parameters can be considered elements of a parallel circuit in an equivalent circuit diagram. The parameter Rs represents the second resistance, Ls represents the second inductance, and Cs represents the second capacitance. These parameters are preferably combined in a series circuit in an equivalent circuit diagram. Thus, Equation 1 includes exactly six electrical parameters. These electrical parameters can subsequently be compared with other predefined reference parameters. In particular, it can be provided that a Euclidean distance between the six electrical parameters and another six predefined electrical parameters is determined.This Euclidean distance can subsequently be used as a state variable to determine the state of the membrane. In particular, it can be investigated whether the Euclidean distance between the electrical parameters exceeds or falls below a predefined tolerance value. If the distance exceeds the tolerance value, a blocked state of the membrane can be detected. The temperature of the membrane can also be determined using the electrical parameters.

[0027] Another embodiment of this invention provides that reference parameters are specified for predetermined states of the membrane, and the electrical parameter of the model exhibiting the greatest deviation from its corresponding reference parameter is compared to determine the membrane state. In this case, only a single electrical parameter is compared with exactly one corresponding reference parameter. Specifically, the electrical parameter used for comparison is the one that shows the greatest deviation or distance from its corresponding reference parameter. This can accelerate the process by simplifying the comparison. This can be particularly useful, for example, when investigating specific predetermined states.For example, if the ultrasonic sensor is only to be examined for ice cover, it is possible that only certain or even just one electrical parameter of the model is affected. In this case, it is sufficient to focus solely on the relevant electrical parameter. Consequently, the comparison can be limited to this single electrical parameter. An analysis of ultrasonic sensors to determine their states can thus be simplified and accelerated.

[0028] Another embodiment of this invention provides that the predetermined states of the membrane signify a covering of the membrane with ice or a contaminant. In this case, the predetermined states are defined. In particular, they have been predefined or specified. This means, in particular, that reference data or reference measurements exist for these defined states. Based on these reference measurements or reference parameters, a comparison can be made with the electrical parameters determined in step g). This means, for example, that several reference parameters can be available for different degrees of membrane coverage with ice. If electrical parameters are determined in step g), they can be compared with the existing predetermined reference parameters. This comparison makes it possible to determine whether the membrane is covered with ice.If several reference parameters for different degrees of ice coverage are available, it is even possible to determine the extent of the membrane icing. An analogous procedure is also possible for membranes containing contaminants. This allows not only the determination of whether the membrane is contaminated, but also the degree of contamination. Thus, a degree of contamination can be determined. Similarly, the degree of ice coverage of the membrane can be determined analogously.

[0029] Another embodiment of the invention provides that the predetermined states of the membrane correspond to a membrane temperature. Instead of analyzing blocked membrane states, this embodiment can determine the membrane temperature. For example, if the ultrasonic sensor is operated outside its permissible temperature range, its measurement results may be considered unreliable. A corresponding warning message can be sent to the driver to alert them to unreliable measurements from the ultrasonic sensor.

[0030] Another embodiment of this invention provides that, based on at least one determined electrical parameter, the degree of membrane coverage and / or the degree of contamination can be determined to quantitatively ascertain the condition of the membrane. This is achieved in particular because several reference parameters can be available for several predefined conditions. Thus, in step g), it is possible not only to determine whether the membrane is contaminated, damaged, or covered with ice, but also to determine the extent of the damage or contamination. In the case of an ice cover, the thickness of the ice layer on the membrane can be specified. Likewise, the thickness of a layer of contaminants on the membrane can be determined. Under certain circumstances, even a profile of an ice layer on the membrane can be determined.

[0031] Another embodiment of this invention provides for the generation of a warning signal in the event of a damaged or dirty diaphragm. If the diaphragm of the ultrasonic sensor is damaged or dirty, the ultrasonic sensor's function is generally impaired. This means that its measurements or measurement results may be unreliable. In this case, it is advantageous to warn the driver of a motor vehicle about unreliable ultrasonic sensors. The warning signal can be visual, haptic, and / or audible. This allows the driver to be informed which ultrasonic sensors are not functioning correctly or which ultrasonic sensors have failed and need to be replaced. This can facilitate faster repairs, as there is no need to search for defective ultrasonic sensors.

[0032] Another embodiment of this invention provides that the third voltage waveform in step f) is determined by averaging the first and second voltage waveforms. Preferably, the averaging is performed using the shifted voltage waveforms from step e). This means that step f) refers primarily to the respective voltage waveforms resulting from shifting the first and second voltage waveforms. The third voltage waveform preferably lies between the shifted first and the shifted second voltage waveforms. In this embodiment, however, the voltage waveform is positioned exactly midway between the two shifted voltage waveforms. This ensures that the up-chirp signal and the down-chirp signal are weighted equally.However, it is also possible to weight these two different excitation signals differently using different weighting factors. This variant therefore also includes averaging, which incorporates weighting factors. Preferably, however, no weighting factor should be zero.

[0033] According to the invention, in step e), the first direction is opposite to the second direction when shifting the voltage profiles. The first direction is, in particular, diametrically opposed to the second direction. The first direction can be converted into the second direction by a 180-degree rotation. Specifically, the first and second directions can be horizontal. This ensures that no frequency distortion occurs during the further course of the process. It thus enables a more reliable determination of the ultrasonic sensor's state.

[0034] The present invention also provides an analysis system for a motor vehicle. This analysis system comprises an ultrasonic sensor with a diaphragm. The analysis system also includes a signal generation unit for generating a first excitation signal and a second excitation signal to excite the diaphragm of the ultrasonic sensor. Furthermore, the analysis system includes an evaluation unit configured to execute a method according to one of the preceding variants or examples. The variants, examples, and advantages described and mentioned above also apply analogously to the analysis system.

[0035] Another embodiment provides an analysis system in which the first excitation signal is a down-chirp signal and the second excitation signal is an up-chirp signal. The examples and definitions mentioned above regarding the down-chirp signal and the up-chirp signal apply analogously to this variant of the invention. These two excitation signals are preferably generated by the signal generation unit. This allows the membrane of the ultrasonic sensor to be excited differently. Depending on which chirp signal is used, a different state of the membrane can be detected. For example, a specific chirp signal can be provided for a temperature analysis of the membrane. Thus, specific states of the membrane can be investigated in a targeted manner. A customized analysis system for the membrane can therefore be created and used.

[0036] Another embodiment of this invention provides a vehicle assistance system with an analysis system. This allows the vehicle assistance system to determine the respective states of the ultrasonic sensors. This information can be combined within the vehicle assistance system with other information from other analysis systems. Thus, the analysis system can determine important information regarding the states of the ultrasonic sensors and make it available to the vehicle assistance system.

[0037] Within the scope of this invention, a motor vehicle with a vehicle assistance system is also proposed. Based on knowledge of the states of the respective ultrasonic sensors, the operation of the motor vehicle can be designed to be safer and more efficient.

[0038] This invention also proposes a computer program product with program code stored in a computer-readable medium for carrying out the method according to one of the previous variants when the computer program product is executed on a processor of an electronic evaluation unit. The computer program product can be integrated into the on-board electronics of a motor vehicle. Thus, it is not absolutely necessary to provide separate digital resources for determining the state of the membrane.

[0039] The signal generation unit can be implemented as a piezo actuator or piezo element. Typically, the first excitation signal differs from the second. In particular, the frequency of the first excitation signal can differ from the frequency of the second. This can also apply to the amplitude or phase. Thus, the first excitation signal can have a different amplitude profile than the second excitation signal. The same can also apply to the phase. This invention specifically provides that the membrane is excited with excitation signals whose frequency is not constant over time. However, in the further course of the process, a so-called constant-wave model is preferably used. The constant-wave model corresponds to the model for continuous excitation of the membrane.

[0040] Further features of the invention will become apparent from the claims, the figures, and the description of the figures. The features and combinations of features mentioned above in the description, as well as those subsequently mentioned in the description of the figures and / or shown in the figures alone, are not only usable in the combinations specified, but also in other combinations without departing from the scope of the invention. Thus, embodiments that are not explicitly shown and explained in the figures, but which can be derived and generated from the explained embodiments by separate combinations of features, are also to be considered as encompassed and disclosed by the invention.

[0041] This shows: Fig. 1 a motor vehicle with an analysis system; Fig. 2 a schematic drawing of an ultrasonic sensor with a diaphragm; Fig. 3 a diagram showing a first voltage profile and a second voltage profile, as well as partially corresponding shifted voltage profiles; Fig. 4 a diagram showing a third voltage profile; and Fig. 5 an exemplary flowchart for better illustration of process steps;

[0042] Fig. 1 Figure 1 shows a motor vehicle 16 equipped with an analysis system. This analysis system comprises several components, including an ultrasonic sensor 10, an evaluation unit 14, and a signal generation unit 12. The motor vehicle 16 can be configured as a passenger car. The ultrasonic sensors 10 can be arranged not only in the front area of ​​the motor vehicle 16, as shown, but also in the rear area (trunk) of the motor vehicle 16. The ultrasonic sensors 10 can be configured as parking sensors. Such parking sensors are preferably used for the safe parking of the motor vehicle 16.

[0043] Fig. 2 Figure 10 shows the ultrasonic sensor 10 schematically in a larger view. The ultrasonic sensor 10 is specifically designed to emit ultrasonic waves 22. These ultrasonic waves 22 are in Fig. 2 The ultrasonic sensor 10 is schematically represented by indicated wavy lines. It has a diaphragm 20, which can be excited by the signal generation unit 12. This means that the signal generation unit 12 can cause the diaphragm 20 to vibrate. The vibrations of the diaphragm 20 then generate ultrasonic waves 22. Specifically, the signal generation unit 12 can generate the first excitation signal and the second excitation signal, thus exciting the diaphragm 20. Preferably, up-chirp or down-chirp signals are used to excite the diaphragm. An evaluation unit 14 is specifically designed to measure the displacements of the diaphragm 20. These diaphragm displacements are often registered as a voltage or a voltage signal. Fig. 3 Figure 1 shows an example frequency response for the first excitation signal 29 and a second frequency response for a second excitation signal 31. These two excitation signals are represented as voltage waveforms as a function of a frequency F. Fig. 3 The first voltage waveform 29 is, in this case, the one generated by the down-chirp signal. The second excitation signal is represented by the second voltage waveform 31. The x-axis represents the frequency F, and the y-axis represents a voltage U. For a given frequency range F', a first maximum M1 and a second maximum M2 of the first and second voltage waveforms, respectively, are plotted. These two maxima M1 and M2 are each shifted. The first maximum M1 is shifted along a direction R1, and the second maximum M2 along a direction R2. The magnitude of the respective shifts is identical. This means that the shift direction R1 has the same length as the shift direction R2.

[0044] The two maxima M1 and M2 are therefore shifted relative to each other by the same amount. They are preferably shifted such that these two maxima merge into a single point. This preferably leads to shifted stress distributions whose maxima coincide at a single point. In this case, the first maximum M1 is shifted to the right and the second maximum M2 is shifted to the left. The two directions R1 and R2 are in the example of Fig. 3 horizontally formed. Due to these displacements, two new shifted stress distributions 29', 31' result. These two shifted stress distributions can be transformed into a new third stress distribution 30. The third stress distribution 30 lies between the shifted first stress distribution 29' and the shifted second stress distribution 31'. For clarity, the corresponding shifted stress distributions are shown in Fig. 3 not fully represented. In particular, the third voltage curve 30 is determined from these two shifted voltage curves by averaging.

[0045] This third voltage curve 30 is in Fig. 4 The third voltage curve 30 is shown as a voltage curve versus the frequency F. This third voltage curve 30 will be used in the following to determine the state of the membrane 20. Several possibilities exist for this purpose.

[0046] In the simplest case, a resonance frequency of the third voltage curve 30 is determined. This is specifically a minimum of the third voltage curve 30 within a predefined frequency range F'. This resonance frequency can be compared with a predefined resonance frequency. The predefined resonance frequency refers to predefined or defined states of the membrane 20. These predefined states of the membrane 20 could, for example, include a covering of the membrane 20 with ice or with contamination. However, it is also possible that these defined states represent a temperature of the membrane 20 or damage to the membrane 20. Instead of determining a resonance frequency, a model for the continuous excitation of the membrane 20 can also be used to determine the state of the membrane 20. For this purpose, for example, Formula 1 is used.The electrical parameters mentioned in Formula 1 are adjusted to replicate the third voltage curve, 30. Various optimization algorithms can be used for this purpose. For example, the Nelder-Mead or Gauss-Newton method can be used to determine the respective electrical parameters. Once the electrical parameters have been determined, they can be compared with predefined reference parameters. Exactly one reference parameter can be selected for comparison. This is usually the parameter that exhibits the greatest deviation from its predefined reference parameter.

[0047] However, it can also be provided that a Euclidean distance of all electrical parameters is determined relative to all predefined electrical parameters. In particular, it can be provided that a square of the distance is calculated in each case. This prevents opposing deviations from summing to zero, which could otherwise produce a distorted result. Ideally, a database exists that contains several reference parameters for different predefined states of the membrane. If the database is large enough, the state of the membrane can be determined quantitatively in addition to simply determining its state. For example, it can be determined how dirty the membrane 20 is or how much ice covers it. The thickness of an ice layer on the membrane 20 of the ultrasonic sensor can be determined.

[0048] Fig. 5 Figure 1 provides an overview of some process steps. In steps S1 and S2, the diaphragm 20 is excited with an up-chirp and down-chirp signal, respectively. The up-chirp signal is indicated by an upward-curving arrow, and the downward-curving arrow symbolizes the down-chirp signal. Based on these two excitation signals, the diaphragm 20 is excited to vibrate in step S3. It is specifically designed that the first excitation signal is generated before the second. Thus, the diaphragm 20 is preferably not excited by both excitation signals simultaneously. Due to these excitations of the diaphragm 20, two different voltage profiles can be measured in step S3.

[0049] In step S4, as in Fig. 3 The positions of the two maxima M1 and M2 are shown to be determined. This can be done using simple and common algorithms. These two maxima M1 and M2 are, in particular, those points where the derivative of the first stress curve 29 and the second stress curve 31, respectively, is zero.

[0050] In step S5, the positions of the two maxima are shifted by the same amount. The two directions of shift are opposite to each other. In particular, the first direction R1 can be horizontal and point to the right. The second direction R2 is therefore also horizontal and oriented to the left.

[0051] In step S6, the third voltage profile 30 is determined. This is preferably done by averaging the shifted voltage profiles 29' and 31'. It should be noted that the averaging does not refer to the original first voltage profile 29 and the original second voltage profile 31. The third voltage profile is preferably determined based on the shifted first voltage profile 29' and the shifted second voltage profile 31'. These two voltage profiles are in Fig. 3 The 29' and 31' markings are merely indicated to ensure clarity.

[0052] In step S7, electrical parameters of the continuous membrane excitation model can be determined. For this purpose, formula 1 is used, for example. The respective electrical parameters can be determined using mathematical optimization algorithms. A comparison of these determined electrical parameters with predefined reference parameters enables a precise determination of the state of membrane 20. Thus, blocked states of membrane 20 can be identified. However, other states, such as the temperature of membrane 20 or the degree of ice coverage of membrane 20, can also be determined.

[0053] Using the electrical parameters, not only can different states of the membrane 20 be detected, but the model for the continuous excitation of the membrane 20 can also describe sensor behavior.

[0054] This sensor behavior can be used, for example, to compensate for level fluctuations or to enable temperature compensation of the ultrasonic sensor 10.

Claims

1. Method for determining a state of a membrane (20) of an ultrasonic sensor (10) during operation of the ultrasonic sensor (10) by executing the following method steps: a) Applying a first excitation signal (29) to the membrane (20) of the ultrasonic sensor (10) in a predetermined first frequency progression, b) Measuring a first voltage progression (29), which is caused by the first excitation signal, as a function of a frequency (F) of the first excitation signal, characterized by the further steps: c) Applying a second excitation signal to the membrane (20) of the ultrasonic sensor (10), which has a second frequency progression different from the first frequency progression, d) Measuring a second voltage progression (31), which is caused by the second excitation signal, as a function of a frequency (F) of the second excitation signal, e) Shifting the first voltage progression (29) by an amount in a first direction (R1) and shifting the second voltage progression (31) by the same amount in a second direction (R2), so that respective positions of maxima (M1, M2) of the two voltage progressions in a predetermined frequency range are aligned with each other, wherein a distance of the maxima does not exceed a predetermined tolerance value, and wherein the first direction (R1) is opposite to the second direction (R2) when shifting the voltage progressions (29, 31), f) Determining a third voltage progression (30) that runs between the shifted first voltage progression (29) and the shifted second voltage progression (31) from step e), g) Determining the state of the membrane (20) using the third voltage progression (30) by determining at least one electrical parameter of a model for continuous excitation of the membrane (20) and comparing the at least one electrical parameter with at least one predetermined reference parameter, and in particular determining the state of the membrane (20) based on a comparison of a resonance frequency of the third voltage progression (30) as the at least one electrical parameter with a predetermined resonance frequency as the predetermined reference parameter.

2. Method according to claim 1, wherein the first excitation signal (29) is designed as a down-chirp signal and the second excitation signal (31) is designed as an up-chirp signal.

3. Method according to claim 1 or 2, wherein the model for continuous excitation of the membrane (20) has a first resistor (Rp), a first inductance (Lp) and a first capacitance (Cp) in a parallel circuit as well as a second resistor (Rs), a second inductance (Ls) and a second capacitance (Cs) in a series circuit as the electrical parameters of the model.

4. Method according to any one of the preceding claims, wherein reference parameters are predetermined for predetermined states of the membrane (20) and exactly one electrical parameter of the model is compared with the associated reference parameter, which shows the greatest deviation from its associated reference parameter, in order to determine the state of the membrane (20).

5. Method according to claim 4, wherein the predetermined states of the membrane (20) indicate a covering of the membrane (20) with ice or a contamination.

6. Method according to claim 4, wherein the predetermined states of the membrane (20) indicate a temperature of the membrane (20).

7. Method according to any one of the preceding claims, wherein a degree of coverage and / or a degree of contamination of the membrane (20) is determined based on the at least one determined electrical parameter for a quantitative determination of the state of the membrane (20).

8. Method according to any one of the preceding claims, wherein in the case of a damaged or contaminated membrane (20), a warning signal is generated.

9. Method according to any one of the preceding claims, wherein the third voltage progression (30) in step f) is determined by averaging the first and second voltage progressions (29, 31).

10. Analysis system for a motor vehicle (16) with - an ultrasonic sensor (10) that has a membrane (20), - a signal generation unit (12) for generating a first excitation signal (29) and a second excitation signal (31) for exciting the membrane (20) of the ultrasonic sensor (20), and - an evaluation unit (14) that is designed to execute a method according to any one of the preceding claims.

11. Analysis system according to claim 10, wherein the first excitation signal (29) is designed as a down-chirp signal and the second excitation signal is designed as an up-chirp signal (31).

12. Vehicle assistance system with an analysis system according to any one of claims 10 to 11.

13. Computer program product with program code means stored in a computer-readable medium for carrying out the method according to any one of the preceding claims 1 to 9 when the computer program product is executed on a processor of an electronic evaluation unit.