Method and rain sensor arrangement for detecting rain on a vehicle's windshield

Abstract

To detect rain on the windshield (3) of a vehicle (1), light (5) is generated by at least one light source (4, 4') and coupled into the windshield (3). An optical detector (6) generates a time-dependent detector signal depending on a portion (5') of the light (5) reflected by the windshield (3). Depending on the time-dependent detector signal, a frequency-dependent signal (12) is generated. The rain on the windshield (3) is detected depending on the frequency-dependent signal (12).

Description

[0001] The present invention relates to a method for detecting rain on a vehicle's windshield, wherein light is generated by at least one light source and coupled into the windshield, and a time-dependent detector signal is generated by an optical detector depending on a portion of the light reflected from the windshield. The invention further relates to a corresponding rain sensor arrangement for a vehicle.

[0002] Rain detection methods and rain sensor arrays for vehicles, based on coupling light into the windshield and detecting the light components exiting the windshield, are well-established. These approaches evaluate the detector signal in the time domain. However, it is difficult to reliably distinguish between signatures indicating rain on the windshield and artifacts or interference caused, for example, by dirt on the windshield, wiper movements, external influences such as radar signals, and so on. More generally, the signatures in the detector signal that indicate rain on the windshield cannot always be reliably identified.

[0003] One objective of the present invention is to increase the reliability with which rain is detected on a vehicle's windshield.

[0004] This objective is achieved through the respective subject matter of the independent claims. Further embodiments and preferred configurations are the subject matter of the dependent claims.

[0005] The invention is based on the idea of ​​discarding the paradigm that the detector signal is evaluated in the time domain and instead generating a frequency-dependent signal based on the time-dependent detector signal and detecting the rain based on the frequency-dependent signal.

[0006] According to one aspect of the invention, a method for detecting rain on a vehicle's windshield is provided. Light is generated by at least one light source and coupled into the windshield, particularly into a pane of the windshield, especially to propagate within the windshield. A time-dependent detector signal is generated by an optical detector, depending on the portion of the light coupled into the windshield, reflected from the windshield, and, in particular, striking an active surface of the optical detector. A frequency-dependent signal, which is particularly time-independent, is generated, for example, by a processing circuit, depending on the time-dependent detector signal.The rain on the windshield is detected, particularly by the processing circuit, depending on the frequency-dependent signal.

[0007] The vehicle is in particular a motor vehicle, for example a car, a van, a truck, a motorcycle, etc.

[0008] Here and in the following, "light" can be understood, for example, to include electromagnetic waves in the visible, infrared, and / or ultraviolet ranges. Accordingly, the term "optical" can be understood to refer to light in this sense. Preferably, the light produced by the at least one light source is infrared light.

[0009] The at least one light source can be, for example, at least one light-emitting diode (LED), in particular at least one infrared LED. The optical detector can be, for example, a photodiode, such as an infrared-sensitive photodiode.

[0010] The at least one light source is arranged with respect to the windshield such that at least a portion of the light generated by the at least one light source enters the windshield, in particular the glass, and is at least partially reflected or reflected multiple times within the windshield, in particular the glass, for example, at surfaces of the windshield, in particular the glass. Total internal reflection is possible, but not mandatory. Similarly, the optical detector is arranged with respect to the windshield such that at least a portion of the light reflected from the windshield, for example the glass, strikes the active area of ​​the optical detector.

[0011] The portion of light coupled into the windshield and reflected from it is reflected, for example, at a surface of the windshield facing away from the at least one light source, and in particular the optical detector, and therefore exits the windshield on the side facing the optical detector. In other words, the portion of light coupled into the windshield and reflected from it corresponds to the light that is coupled into the windshield, reflected once or several times within the windshield, and then coupled out of the windshield on the side facing the optical detector.

[0012] Rain on the windshield affects the amount of light reflected from the windshield at any given time, but also as a function of time, especially when it is raining and therefore the amount and position of raindrops on the windshield is time-dependent.

[0013] Generating the frequency-dependent signal can involve applying a time-to-frequency transformation, such as a Fourier transform, to the time-dependent detector signal or to another time-dependent signal generated by processing the time-dependent detector signal. This processing can include, for example, filtering, amplification, modulation, demodulation, integration, converting a current signal to a voltage signal, and so on. The frequency-dependent signal can, for example, be generated as a real-valued, positive signal.

[0014] In particular, the frequency-dependent signal can be generated based on the respective values ​​of the time-dependent detector signal within a predefined time interval. The length of the time interval defines or influences, in particular, the resolution of the generated frequency-dependent signal. The longer the time interval, the higher the achievable resolution of the frequency-dependent signal. In other words, by choosing the length of the time interval, a suitable compromise can be reached between the required or desired resolution and the measurement time.

[0015] Rain is detected primarily by evaluating the frequency-dependent signal, particularly its amplitude, for example, within one or more predefined frequency ranges. This evaluation can include, for instance, a comparison with one or more predefined threshold values. The processing circuit then determines, for example, whether the frequency-dependent signal indicates that rain might be present on the windshield. This can be interpreted as the detection of rain on the windshield. Since the frequency-dependent signal exhibits no temporal fluctuations, disturbances or artifacts not attributable to rain on the windshield are less significant in the frequency-dependent signal and / or can be more easily identified or filtered out than in the time domain.

[0016] According to several embodiments, the frequency-dependent signal is compared with at least one predefined threshold value, and the rain on the windshield is detected depending on the result of the comparison.

[0017] In particular, rain on the windshield can be detected if the frequency-dependent signal is greater than or, in some embodiments, at least equal to a respective threshold value of at least one threshold value, indicating that, for example, the light exiting the windshield occurs with at least a corresponding intensity.

[0018] The comparison of the frequency-dependent signal can be global, for example, by determining whether the signal is greater than or at least equal to the respective threshold for each frequency or within each available frequency range. However, it is also possible to perform a frequency-specific comparison, meaning that only a portion of the frequency-dependent signal within a predefined frequency range is compared to the respective threshold. This can also be done for two or more frequency ranges and corresponding thresholds, which can differ for different frequency ranges. Different frequency ranges could, for example, correspond to different rain characteristics such as "light rain," "drizzle," "heavy rain," etc.

[0019] According to several embodiments, the at least one predefined threshold includes a first threshold value that is assigned to a first predefined frequency range, and the rain on the windshield is detected when the frequency-dependent signal within the predefined first frequency range is greater than or, in some embodiments, at least equal to the first threshold value.

[0020] In other words, only the part of the frequency-dependent signal within the first frequency range is compared with the first threshold, and rain is detected, or considered to be detected, if the part of the frequency-dependent signal within the first frequency range is greater than or at least equal to the first threshold.

[0021] This has several advantages, including the ability to ignore frequency ranges or frequencies known to be characteristic of interference or false positives, for example, due to dirt on the windshield, external electromagnetic signals, and so on. The first frequency range, for instance, is chosen such that unwanted influences are either absent or insignificant within that range. It is noted that a particular advantage of frequency-dependent evaluation is the ability to easily ignore such unwanted influences by precisely defining the first frequency range and, in further embodiments, potentially other frequency ranges as well. In the temporal domain, complex and potentially error-prone analyses would be required.

[0022] The first frequency range may, for example, lie within the interval [1 Hz, 15 Hz]. It has been found that for the most relevant cases and types of rain, the characteristic frequencies lie within this interval. However, it is noted that the first frequency range may be significantly smaller than this interval; for example, in some embodiments, it may have a width of 0.5 Hz to 5 Hz or the like. In other embodiments, the first frequency range may have a width of 2 Hz to 10 Hz or the like.

[0023] According to several embodiments, the at least one predefined threshold includes a second threshold assigned to a second frequency range, and the rain on the windscreen is detected when the frequency-dependent signal within a predefined second frequency range is greater than or, in some embodiments, at least equal to the second threshold.

[0024] Consequently, rain detection is more robust against fluctuations in the type of rain, but unwanted influences can still be selectively suppressed.

[0025] In particular, in some embodiments, rain on the windshield can be detected if the frequency-dependent signal within the first frequency range is greater than or at least equal to the first threshold, or if the frequency-dependent signal within the second frequency range is greater than or at least equal to the second threshold.

[0026] The second frequency range differs from the first frequency range. In some embodiments, the first and second frequency ranges may overlap, while in others they may be disjoint. The second threshold may be the same as the first threshold or different from the first threshold.

[0027] The second frequency range can, for example, lie within the interval [1 Hz, 15 Hz]. The second frequency range can, for example, have a width of 0.5 Hz to 5 Hz or a width of 2 Hz to 10 Hz, or the like.

[0028] According to several embodiments, generating the frequency-dependent signal includes filtering out frequencies that are lower than, or in some embodiments at most equal to, a predefined lower cutoff frequency, and / or filtering out frequencies that are higher than, or in some embodiments at least equal to, a predefined upper cutoff frequency.

[0029] Consequently, undesirable effects due to interference and / or false positive results are more reliably avoided.

[0030] Filtering out frequencies lower than or equal to the lower cutoff frequency, and / or frequencies higher than or equal to the upper cutoff frequency, involves, for example, applying one or more frequency-dependent filters, such as low-pass, high-pass, or band-pass filters, to the time-dependent detector signal or to another time-dependent signal generated by processing the time-dependent detector signal. This processing may include, for example, further filtering, amplification, modulation, demodulation, integration, conversion from a current signal to a voltage signal, and so on. The resulting frequency-dependent signal is then generated as a function of, or dependent on, the filtered time-dependent detector signal or the filtered additional time-dependent detector signal.In particular, the filtered time-dependent detector signal or the filtered further time-dependent detector signal does not include any frequency components below the lower cutoff frequency and / or does not include any frequency components above the upper cutoff frequency, or frequencies below the lower cutoff frequency and / or above the upper cutoff frequency are suppressed in the filtered time-dependent detector signal or the filtered further time-dependent detector signal.

[0031] In embodiments that utilize the first and / or second frequency range as described above, the lower cutoff frequency is equal to or less than a minimum frequency of the first frequency range and / or a minimum frequency of the second frequency range. Alternatively or additionally, the upper cutoff frequency is equal to or greater than a maximum frequency of the first frequency range and / or a maximum frequency of the second frequency range.

[0032] Filtering out frequencies lower than or equal to the lower cutoff frequency includes, for example, applying a high-pass or band-pass filter to the time-dependent detector signal or to the other time-dependent signal. Filtering out frequencies higher than or equal to the upper cutoff frequency includes, for example, applying a low-pass or band-pass filter to the time-dependent detector signal or to the other time-dependent signal.

[0033] According to several embodiments, generating the frequency-dependent signal includes filtering out frequencies within one or more frequency subranges that are within or partially within the interval [f lc , f uc ] lie, where f lc the lower cutoff frequency is denoted and f uc the upper cutoff frequency.

[0034] In this way, unwanted influences in the relevant frequency range, for example from windshield wipers, etc., can be suppressed.

[0035] This additional filtering may also include the application of respective low-pass and / or high-pass and / or band-pass filters.

[0036] According to several embodiments, the light is generated alternately as a first light component, which is generated by a first light source of the at least one light source, and as a second light component, which is generated by a second light source of the at least one light source.

[0037] Consequently, the light detected by the optical detector also originates alternately from the first and second light sources. By introducing such a level of redundancy, abnormal behavior, artifacts, or false alarms can be more easily identified, since the light detected by the first and second light sources should behave nominally identically or very similarly when there is no rain or other objects on the windshield.

[0038] For example, the first and second light sources can be controlled by a driver circuit such that each emits its respective light component periodically during the respective emission periods, and, for example, so that at any given time only one of the first and second light sources emits light. For example, the first and second light components can be coupled into the windshield at different positions, or, in other words, the first and second light sources can be positioned at different locations on the windshield.

[0039] The emission periods can be the same or nearly the same for the first and second light sources. However, it is possible that, for example, due to unavoidable fluctuations and tolerances or different aging behavior of the light sources, the emission periods for the first and second light sources may differ, particularly to compensate for these effects and / or to achieve the same or nearly the same optical output power of the two light sources during their respective emission periods.

[0040] According to several embodiments, a time-dependent difference signal is generated depending on the time-dependent detector signal, wherein the time-dependent difference signal corresponds to a difference between a portion of the first light component reflected by the windshield and a portion of the second light component reflected by the windshield. The frequency-dependent signal is generated depending on the time-dependent difference signal.

[0041] In other words, the time-dependent difference signal can be considered a demodulated time-dependent detector signal. The time-dependent difference signal should have a low amplitude or zero amplitude when no rain is falling and no other object is on the windshield. If the amplitude of the time-dependent difference signal is not zero or differs significantly from zero, this indicates that rain or another object or effect on the windshield has been detected, which can be evaluated, refined, or confirmed in the frequency domain as described above. This further increases the robustness of the rain detection.

[0042] The time-dependent difference signal can be generated, for example, by subtracting the amplitudes of the time-dependent detector signal or of another time-dependent detector signal corresponding to the respective emission periods. The frequency-dependent signal can be generated by applying the time-to-frequency transformation to the time-dependent difference signal or to the signal generated by processing the time-dependent difference signal. This processing can include, for example, filtering, amplification, modulation, demodulation, integration, converting a current signal to a voltage signal, and the like.

[0043] According to several embodiments, an integrated signal is generated by temporal integration of the time-dependent difference signal, for example, using a sliding integration window. The frequency-dependent signal can be generated by applying the time-to-frequency transformation to the integrated signal.

[0044] According to several embodiments, generating the frequency-dependent signal involves applying a time-to-frequency transformation to the time-dependent difference signal.

[0045] According to several embodiments, generating the frequency-dependent signal includes integrating the time-dependent difference signal over time and applying a time-to-frequency transformation to the integrated time-dependent difference signal.

[0046] According to several embodiments, the time-dependent detector signal is a current signal and is converted into a time-dependent voltage signal, for example by a transimpedance amplifier. The time-dependent difference signal is generated by demodulating the time-dependent voltage signal. The integrated signal is generated by time integration of the time-dependent difference signal, and the frequency-dependent signal can be generated by applying the time-to-frequency transformation to the integrated signal.

[0047] According to several embodiments, the at least one light source consists of a single light source.

[0048] Consequently, the hardware costs for a corresponding rain sensor are reduced. This is possible primarily due to the frequency-domain evaluation. In particular, the evaluation of the frequency-dependent signal takes into account effects at different times or on different timescales.

[0049] According to several embodiments, in particular embodiments in which the at least one light source consists of the single light source, generating the frequency-dependent signal comprises applying the time-to-frequency transformation to the time-dependent detector signal or integrating the time-dependent detector signal over time and applying the time-to-frequency transformation to the integrated time-dependent detector signal.

[0050] According to a further aspect of the invention, a method for controlling a vehicle's windshield wiper is provided. This method includes detecting rain on the vehicle's windshield according to the invention, for example, by means of a rain sensor arrangement in the vehicle. The windshield wiper is controlled depending on the detected rain or depending on the frequency-dependent signal.

[0051] Further embodiments of the inventive method for controlling a windshield wiper arise directly from the various embodiments of the inventive method for detecting rain on a windshield and vice versa. In particular, individual features and corresponding explanations as well as advantages relating to the various implementations of the inventive method for detecting rain on a windshield can be transferred analogously to implementations of the inventive method for controlling a windshield wiper and vice versa.

[0052] According to a further aspect of the invention, a rain sensor arrangement for a vehicle is provided. The rain sensor arrangement comprises at least one light source configured to generate light that is coupled into a windshield of the vehicle, and an optical detector configured to generate a time-dependent detector signal depending on a portion of the light reflected from the windshield. The rain sensor arrangement further comprises a processing circuit configured to generate a frequency-dependent signal depending on the time-dependent detector signal and to detect the rain on the windshield depending on the frequency-dependent signal.

[0053] The processing circuitry may include analog circuits and / or a data processing system. The terms "data processing system" and "at least one data processing device" may be used interchangeably within the scope of this disclosure. For example, in this disclosure, a data processing device may be understood as a device with processing circuitry for processing data. A data processing device can thus perform arithmetic operations to process data. Indexed access to a data structure, such as a lookup table (LUT) or a database, may also be considered an arithmetic operation. Similarly, data processing that is partially or fully implemented in hardware may be considered an arithmetic operation.

[0054] A data processing device may, in particular, comprise one or more computers, one or more microcontrollers, and / or one or more integrated circuits, for example, one or more application-specific integrated circuits (ASICs), one or more field-programmable gate arrays (FPGAs), and / or one or more systems-on-a-chip (SoCs). A data processing device may also comprise one or more processors, for example, one or more microprocessors, one or more central processing units (CPUs), one or more graphics processing units (GPUs), and / or one or more signal processors, in particular one or more digital signal processors (DSPs). The data processing device may also comprise a physical or virtual cluster of computers or other devices of the aforementioned type.

[0055] A data processing device may also include one or more hardware and / or software interfaces, for example for receiving and / or providing data.

[0056] A data processing device may also include one or more storage devices. A storage device may be implemented as volatile memory, such as dynamic random access memory (DRAM) or static random access memory (SRAM), or as non-volatile data storage, such as read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or flash EEPROM, ferroelectric random access memory (FRAM), magnetoresistive random access memory (MRAM), or phase-change random access memory (PCRAM).

[0057] According to several embodiments, the rain sensor arrangement includes the windshield.

[0058] According to several embodiments, the at least one light source is arranged in relation to the windshield in such a way that the generated light is coupled into the windshield.

[0059] According to several embodiments, the optical detector is arranged in relation to the windshield such that the part of the light reflected by the windshield hits an active area of ​​the optical detector.

[0060] Further embodiments of the rain sensor device according to the invention arise directly from the various embodiments of the method according to the invention, and vice versa. In particular, individual features and corresponding explanations, as well as advantages relating to the various embodiments of the method according to the invention, can be transferred analogously to corresponding embodiments of the rain sensor device according to the invention. In particular, the rain sensor device according to the invention is designed or programmed to carry out the method according to the invention. In particular, the rain sensor arrangement according to the invention carries out the method according to the invention.

[0061] According to another aspect of the invention, a vehicle is provided which includes a rain sensor arrangement according to the invention.

[0062] Further features of the invention are evident 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 mentioned below in the description of the figures and / or illustrated in the figures, may be encompassed by the invention not only in the combinations specified, but also in other combinations. In particular, embodiments and combinations of features that do not include all the features of an originally formulated claim may also be encompassed by the invention. Furthermore, embodiments and combinations of features that go beyond or deviate from the combinations of features mentioned in the claims may also include the invention.

[0063] The invention is explained in more detail below with reference to specific exemplary embodiments and corresponding schematic drawings. Identical or functionally equivalent elements in the drawings may be provided with the same reference numerals. The description of identical or functionally equivalent elements is not necessarily repeated with respect to the different figures. The figures show Fig. 1 schematically a vehicle with an exemplary embodiment of a rain sensor arrangement for a vehicle according to the invention; Fig. 2 a schematic block diagram of a further exemplary embodiment of a rain sensor arrangement for a vehicle according to the invention; Fig. 3 a schematic perspective view of a further exemplary embodiment of a rain sensor arrangement for a vehicle according to the invention; Fig. 4 a schematic block diagram of a further exemplary embodiment of a rain sensor arrangement for a vehicle according to the invention; and Fig. 5 schematically a frequency-dependent signal according to an exemplary embodiment of a method according to the invention for detecting rain on a windshield of a vehicle.

[0064] Fig. Figure 1 schematically shows a vehicle 1 with an exemplary embodiment of a rain sensor arrangement 2 according to the invention. Fig. Figure 2 shows a schematic block diagram of the rain sensor arrangement 2.

[0065] The rain sensor arrangement 2 comprises at least one light source 4, for example one or more LEDs, configured to generate light 5 that is coupled into a windshield 3 of the vehicle 1. In particular, the rain sensor arrangement 2 comprises a driver circuit 7 configured to control the at least one light source 4, for example, to generate the light 5. The at least one light source 4 is arranged, in particular, on or in the immediate vicinity of the windshield 3, so that at least part of the generated light 5 is coupled into the windshield 3 and propagates, for example, at least briefly within the windshield 3.

[0066] The rain sensor arrangement 2 comprises an optical detector 6, for example a photodiode, configured to generate a time-dependent detector signal depending on a portion 5' of the light 5 reflected from the windshield 3. In particular, the optical detector 6 is arranged on or near the windshield 3 so that the portion 5' can strike an active surface of the optical detector 6, causing the optical detector 6 to generate the time-dependent detector signal, for example as a current signal. The portion 5' of the light 5 can be reflected from the windshield 3, in particular from a surface of the windshield 3 facing away from the at least one light source 4 and the optical detector 6, for example due to rain or other objects on the surface of the windshield 3, which constitutes a disturbance for the reflection of the light 5 from the surface of the windshield 3.

[0067] The rain sensor arrangement 2 comprises a processing circuit 8 which is configured to generate a frequency-dependent signal 12 depending on the time-dependent detector signal, as schematically shown in Fig. Figure 5 shows how to generate the signal, for example, by applying a Fourier transform, in particular a fast Fourier transform (FFT), a discrete Fourier transform (DFT), or a sliding discrete Fourier transform (SDFT), etc. The processing circuit 8 is configured to detect the rain on the windshield 3 depending on the frequency-dependent signal 12.

[0068] For example, the processing circuit 8 can compare the frequency-dependent signal 12 with one or more predefined thresholds in one or more predefined frequency ranges. Depending on the result of the comparison, the processing circuit 8 can detect the rain on the windshield 3, for example, if the amplitude of the frequency-dependent signal 12 exceeds one of the thresholds in the respective frequency range.

[0069] Out of Fig. Figure 5 shows several distinct peaks in the frequency-dependent signal 12 in the range of 0 to 20 Hz, which may indicate rain on the windshield 3. The frequency-dependent signal 12 exhibits a relatively sharp rise towards zero. This can be avoided, for example, by filtering out low frequencies, such as before the Fourier transform. In this case, the frequency-dependent signal 12 can be evaluated even more easily.

[0070] In some embodiments, the at least one light source 4 is a single light source, as in Fig. Figure 1 shows. In other embodiments, the at least one light source comprises a first light source 4 and a second light source 4', as in the example of Figure 1. Fig. Figure 3 shows a schematic perspective view of a further exemplary embodiment of a rain sensor arrangement 2 or part thereof according to the invention, with a housing 9 for mounting on the windshield 3.

[0071] In such embodiments, the light 5 is generated, for example, alternately as a first light component produced by the first light source 4 and as a second light component produced by the second light source 4'. For example, a time-dependent difference signal can be generated depending on the time-dependent detector signal, wherein the time-dependent difference signal corresponds to a difference between a portion of the first light component reflected by the windshield 3 and a portion of the second light component reflected by the windshield 3. The frequency-dependent signal 12 is then generated depending on the time-dependent difference signal.

[0072] Fig. Figure 4 shows a schematic perspective view of another exemplary embodiment of a rain sensor arrangement 2 according to the invention. In this embodiment, the time-dependent detector signal is a current signal, and the processing circuit 8 comprises a conversion unit 12 configured to convert the time-dependent detector signal into a voltage signal. For this purpose, the conversion unit 12 may, for example, include a transimpedance amplifier (TIA). In some embodiments, the conversion unit 12 may also include one or more frequency-dependent filter circuits for filtering out unwanted or irrelevant frequency components. The frequency-dependent signal 12 is then generated depending on the voltage signal.

[0073] For example, the rain sensor arrangement 2 of the Fig.The rain sensor arrangement 2 comprises the first light source 4 and the second light source 4'. For example, the rain sensor arrangement 2 may include a clock generator 9 configured to generate a clock signal 10, which in particular has a duty cycle of 0.5, that is, it switches between a state in which it is on and a state in which it is off for the same duration. The driver circuit 7 receives the clock signal 10 and controls the first light source 4 and the second light source 4', for example, such that the first light source 4 emits light when it is on and the second light source 4' emits light when it is off, or vice versa. The processing circuit 8 includes a demodulation circuit 13, which generates a demodulated signal corresponding to the difference between the signal amplitudes of the voltage signal used to detect the light from the first light source 4 and the second light source 4', respectively.The frequency-dependent signal 12 is then generated depending on the demodulated signal.

[0074] For example, an evaluation circuit 11 of the processing circuit 8 can apply the Fourier transform to the demodulated signal. Alternatively, the processing circuit 8 can include a control circuit 14 configured to integrate the demodulated signal, and the evaluation circuit 11 can apply the Fourier transform to the integrated signal. The evaluation circuit 11 can, for example, be part of a system-on-a-chip (SoC) or a digital signal processor (DSP). Hardware acceleration for the Fourier transform is also possible.

Claims

[1] Method for detecting rain on a windscreen (3) of a vehicle (1), wherein - Light (5) is generated by at least one light source (4, 4') and coupled into the windshield (3); - depending on a part (5') of the light (5) reflected by the windshield (3), a time-dependent detector signal is generated by an optical detector (6); - depending on the time-dependent detector signal, a frequency-dependent signal (12) is generated; and - the rain on the windshield (3) is detected depending on the frequency-dependent signal (12). [2] Method according to claim 1, wherein the frequency-dependent signal (12) is compared with at least one predefined threshold value and the rain on the windscreen (3) is detected depending on a result of the comparison. [3] Method according to claim 2, wherein the at least one predefined threshold includes a first threshold and the rain on the windscreen (3) is then detected when the frequency-dependent signal (12) within a predefined first frequency range is greater than the first threshold. [4] Method according to claim 3, wherein the first frequency range is in the interval [1 Hz, 15 Hz]. [5] Method according to one of claims 3 or 4, wherein the at least one predefined threshold includes a second threshold and the rain on the windscreen (3) is then detected when the frequency-dependent signal (12) within a predefined second frequency range is greater than the second threshold. [6] Method according to any of the preceding claims, wherein the generation of the frequency-dependent signal (12) comprises filtering out frequencies below a predefined lower cutoff frequency and / or filtering out frequencies above a predefined upper cutoff frequency. [7] Method according to one of the preceding claims, wherein the light (5) is alternately produced as a first light component generated by a first light source (4, 4') of the at least one light source (4, 4') and as a second light component generated by a second light source (4, 4') of the at least one light source (4, 4'). [8] Method according to claim 7, wherein the first light component and the second light component are coupled into the windshield (3) at different positions on the windshield (3). [9] Method according to one of claims 7 or 8, wherein - a time-dependent difference signal is generated depending on the time-dependent detector signal, wherein the time-dependent difference signal corresponds to a difference between a part of the first light component reflected by the windshield (3) and a part of the second light component reflected by the windshield (3); and - and the frequency-dependent signal (12) is generated depending on the time-dependent difference signal. [10] Method according to claim 9, wherein the generation of the frequency-dependent signal (12) - involves applying a time-to-frequency transformation to the time-dependent difference signal; or - includes a temporal integration of the time-dependent difference signal and an application of a time-to-frequency transformation to the integrated time-dependent difference signal. [11] Method according to any one of claims 1 to 6, wherein the at least one light source (4, 4') consists of a single light source (4, 4'). [12] Method according to claim 10, wherein the generation of the frequency-dependent signal (12) - involves applying a time-to-frequency transformation to the time-dependent detector signal; or - includes a temporal integration of the time-dependent detector signal and an application of a time-to-frequency transformation to the integrated time-dependent detector signal. [13] Method according to claim 10, wherein the time-dependent detector signal is a time-dependent current signal, the time-dependent current signal is converted into a time-dependent voltage signal, and the generation of the frequency-dependent signal (12) - involves applying a time-to-frequency transformation to the time-dependent voltage signal; or - includes a temporal integration of the time-dependent voltage signal and application of a time-to-frequency transformation to the integrated time-dependent voltage signal. [14] Method according to one of claims 10, 12 or 13, wherein the time-to-frequency transformation comprises a Fourier transformation. [15] comprising a rain sensor arrangement (2) for a vehicle (1) - at least one light source (4, 4') designed to generate light (5) for coupling into a windshield (3) of the vehicle (1); - an optical detector (6) configured to generate a time-dependent detector signal depending on a portion (5') of the light (5) reflected from the windscreen (3); and - a processing circuit (8) which is set up to generate a frequency-dependent signal (12) depending on the time-dependent detector signal and to detect the rain on the windshield (3) depending on the frequency-dependent signal (12). [16] Rain sensor arrangement (2) according to claim 15, which further includes the windshield (3), wherein the at least one light source (4, 4') is arranged in relation to the windshield (3) such that the generated light is coupled into the windshield (3) and the optical detector (6) is arranged in relation to the windshield (3) such that the part (5') of the light (5) which is reflected from the windshield (3) hits an active area of ​​the optical detector (6).

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