Method for identifying road surface conditions and driver assistance system

Abstract

The invention relates to a method for identifying a road surface condition, wherein noise is received with an ultrasonic sensor and a weather-related condition of the road surface is inferred on the basis of the intensity of the received noise. The noise which is received and evaluated by means of the method is preferably noise which is caused by the rolling of a vehicle wheel on the road surface and / or noise which is caused by diffuse ground echoes. According to the invention, the intensity or level of the received noise is derived from the measured values of the ultrasonic sensor, taking into account the ambient temperature, wherein the temperature is measured, the current ambient temperature is determined and the previously determined temperature dependency of the transmission path (30, 32) of the received noise and / or the previously determined temperature dependency of the sound source of the received noise are taken into account when determining the intensity. A further aspect of the invention relates to a driver assistance system which is designed to implement the method.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for identifying a road surface condition, wherein rolling noise caused by the rolling of a wheel of a vehicle on a road surface and / or a ground echo of previously emitted ultrasound pulses is measured with an ultrasound sensor, and a weather-dependent condition of the road surface is inferred on the basis of the received noise. A further aspect of the invention relates to a driver assistance system which is designed to implement the method. BACKGROUND

[0002] Rolling noise caused by the rolling of a wheel of a vehicle on a road surface occurs when the vehicle is driving on the road surface. These rolling noises depend on the weather-dependent condition of the road surface, so that the rolling noise changes for dry road surfaces, wet road surfaces, snow-covered road surfaces, icy road surfaces and wet road surfaces, wherein in the case of a wet road surface the rolling noise also depends on how high the water level on the road surface is. The rolling noise is similar to noise and is also referred to as wet hissing (Nasszischen) in the case of a wet road surface.

[0003] In order to identify the weather-dependent condition of the road surface, it is desirable to use sensors which are already present on the vehicle, for example ultrasound sensors. Such ultrasound sensors comprise a sound transducer which is designed to emit ultrasound pulses and to receive echoes reflected by objects in the surroundings again. The sound transducer of such an ultrasound sensor can also measure the rolling noise. However, it is problematic here that not only the level of the rolling noise but also the level of the ground echo can be related to the ambient temperature and the sensor temperature.

[0004] Therefore, errors can occur when using threshold values for classifying the road surface condition on the basis of the noise level determined by the ultrasound sensor. For example, the following situation can occur in the case of a high ambient temperature: a road is incorrectly identified as dry, although the road is wet.

[0005] A device for sensing moving objects is known from DE 10 2006 037 591 A1. The device has a plurality of ultrasound sensors with different sensing ranges. The ultrasound sensors can be adjusted in their sensitivity. In order to adjust the sensitivity, the current environmental conditions are measured. The environmental conditions include, inter alia, the damping of the ultrasound amplitude and the increased background noise due to the wetness of the road. In order to estimate the environmental noise, further sensors of the device are switched to a passive mode in which they do not emit sound waves, but only receive extraneous noise from the environment.

[0006] The method known from the prior art cannot distinguish between extraneous noise from the environment and background noise of the ultrasound sensor. SUMMARY

[0007] A method for recognizing a road surface condition is proposed, in which noise is received with an ultrasonic sensor and a weather-dependent condition of the road surface is inferred from the intensity of the received noise. The noise received and evaluated with the method is preferably noise due to the rolling of a wheel of a vehicle on the road surface and / or noise due to a diffuse ground echo.

[0008] The weather-dependent condition of the road surface is in particular the thickness of a water film on the road surface, wherein in this case a wet hissing sound occurs as rolling noise when a wheel of a vehicle rolls on the road surface. Here, this wet hissing sound depends on the thickness of the water film on the road surface. Furthermore, the weather-dependent road surface condition can be an icy road surface, in the case of which an ice layer is present on the road surface. Another possible condition that is weather-dependent is a snow-covered road surface, in the case of which snow is present on the road surface. Sand or pollutants that are blown onto or washed onto the road surface are also possible conditions of the road surface that are weather-dependent.

[0009] The ultrasonic sensor used in the context of the method is preferably an ultrasonic sensor that is used to recognize objects in the vehicle surroundings in addition to recognizing the road surface condition. Here, the proposed method is carried out in particular when such an ultrasonic sensor is not necessarily required for sensing objects in the vehicle surroundings. It is therefore preferred if such an ultrasonic sensor is used that is, for example, part of a parking assistance device, so that this part is not required for sensing objects in the vehicle surroundings when the vehicle is driving normally.

[0010] The intensity or level of the noise can be derived from the measurement values of the ultrasonic sensor in such a way that the noise value or signal level of the measured ambient noise is determined. This can take place particularly preferably in a measurement window when the echo of the ultrasonic pulse of the sensor has subsided, with which the noise value or signal level can be measured as unobtrusively as possible. A background noise level that is attributed to the background noise of the ultrasonic sensor and is calculated in particular as a function of the current sensor temperature of the ultrasonic sensor and a known temperature dependency of the ultrasonic sensor can preferably also be subtracted from the noise value or signal level sought. With regard to the method, in particular the intensity or level of the noise produced by the rolling of the tire and / or the noise produced by a diffuse ground echo is analyzed and evaluated.

[0011] In order to infer the weather-related conditions of the road surface from the rolling noise intensity determined using ultrasonic sensors, a level is preferably determined from a plurality of received rolling noises. This level is then compared with a predetermined limit value or a predetermined level range. For example, a low level of the determined rolling noise is an indicator of a dry road surface, while a level above the predetermined limit value is an indicator of a wet road surface. The thickness of the water film on the road surface can be inferred from the intensity of the level. A higher level indicates a thicker water film on the road surface. The level or level range can be predetermined, in particular, based on the vehicle's instantaneous speed.

[0012] During rolling, a portion of the tire deforms so that the tread blocks, which are separated from the road, either bend or move. A very low coefficient of friction on the road surface, such as snow or ice, causes the tread blocks to move during rolling, while a high coefficient of friction causes them to bend. High coefficients of friction generally result in louder rolling noises when the tread blocks bend and suddenly snap back to their original shape. Furthermore, loose snow attenuates sound very well, so differences compared to dry roads can also be measured here.

[0013] Ground echo is understood to be the noise caused by reflections of transmitted ultrasonic pulses from the ground, i.e., the road surface. These ground echoes are generally diffuse. Unlike rolling noise, ground echoes do not occur independently of the operation of the vehicle's ultrasonic sensors, but rather occur in a temporal relationship with the emission of an ultrasonic pulse by one of the ultrasonic sensors.

[0014] In order to infer the weather-related condition of the road surface from the intensity of the ground echo determined using ultrasonic sensors, a level is preferably determined from the ground echo and compared to a predetermined limit value or a predetermined level range. For example, a high level of the determined ground echo is an indicator of a dry road surface, while a level below the predetermined limit value is an indicator of a wet or icy road surface. The thickness of the water film on the road surface can be inferred from the intensity of the level. A lower level indicates a thicker water film on the road surface. Furthermore, loose snow attenuates the ground echo more strongly than water, so the difference between dry and wet roads can also be measured here.

[0015] According to the present invention, the intensity or level of the received noise is determined from the measured values ​​of the ultrasonic sensor while taking into account the temperature. The temperature can preferably be the current ambient temperature measured directly and / or the ambient temperature and the sensor temperature measured, and a model of the current ambient temperature is determined based on the temperatures. Thus, the current ambient temperature is determined, and previously determined temperature dependencies of the transmission path of the received noise and / or previously determined temperature dependencies of the sound source of the received noise are taken into account when determining the intensity.

[0016] The ambient temperature is the temperature in the surroundings of the vehicle, in particular the temperature of the ambient air. This ambient temperature can be measured, for example, with a temperature sensor arranged on the vehicle.

[0017] Before the measurement values of the ultrasonic sensor are further used for identifying the road surface state, the measurement values of the ultrasonic sensor can be corrected for this by means of the ambient temperature as follows:

[0018] The water temperature of the water on the wet road surface can be considered as a first influencing path. The intensity of the noise (wet hissing sound) caused by the rolling of the wheels of the vehicle on the road surface is related to the water temperature. In order to compensate for this temperature influence it can be assumed that the water temperature corresponds to the measured ambient temperature. For example, the measured level can be extrapolated to the case of a normal temperature, for example +20°C, by means of a temperature characteristic curve of the wet hissing sound level saved in the measuring system.

[0019] The air temperature in the surroundings of the vehicle can be considered as a second influencing path. The attenuation of the sound on the route from the sound source to the ultrasonic sensor is related to the ambient temperature and can amount to 2 dB / m and more. The attenuation of the sound on the route is also related to the air humidity. For this reason, in particular the attenuation of the sound can be calculated by additionally measuring the air humidity, for example by means of an air humidity sensor present on the vehicle. If no air humidity sensor is present, the air humidity range can instead be assumed as follows: If it can be concluded from the increased intensity of the received noise that the road is wet or damp, the air humidity is also assumed to be high (for example more than 70%). Instead, for an air humidity range of between 70% and 100% the air humidity can be assumed to be, for example, 85%.

[0020] In order to correct the measurement values of the ultrasonic sensor based on the air sound attenuation related to the temperature, which is based on the noise caused by the rolling of the wheels of the vehicle on the road surface, it can be assumed that the noise mainly originates from the tire closest to the ultrasonic sensor being measured. For the sound propagation path, the distance from the point of contact of the tire on the road surface to the ultrasonic sensor is considered.

[0021] In order to compensate for the diffuse ground echo based on the air sound attenuation related to the temperature, the route from the ultrasonic sensor to the ground and back to the ultrasonic sensor is preferably taken into account.

[0022] The absorption coefficient at the operating frequency of the ultrasonic sensor, for example 48 kHz, can be calculated from the measured current ambient temperature and the assumed average air humidity, for example 85%. In consideration of the sound propagation path, the absolute attenuation can be calculated and its ratio to the attenuation calculation at defined standard conditions, for example +20°C, 85%, 48 kHz, is determined. Now, the intensity of the received noise from the measurement of the ultrasonic sensor can be corrected in this ratio.

[0023] When measuring the road surface roughness by means of diffuse ground echo, a low air temperature leads to less attenuation and thus to a stronger intensity of the diffuse ground echo. Thus, in the case of a higher ambient temperature, the road surface roughness calculated by means of the diffuse ground echo can be corrected upwards. By means of the calculation of the ground surface roughness, it can be inferred whether the pores of the asphalt are filled with water or ice. Furthermore, it can be identified whether the road surface is snow-covered or contaminated.

[0024] In a preferred embodiment of the application, the membrane temperature of the ultrasonic sensor can be considered as a further influencing path: The membrane temperature is influenced by the ambient temperature and can be calculated, for example, by means of interpolation on the basis of the ambient temperature and the measured internal temperature of the ultrasonic sensor. The membrane temperature has an influence on the natural frequency of the membrane and on the acoustic damping and thus on the frequency response of the ultrasonic sensor. The efficiency of the emission and reception in a frequency range within a certain natural frequency range at standard conditions, for example 48 kHz, is generally highest at a membrane temperature of, for example, approximately +20°C and decreases with higher and lower temperatures. However, if the measurement is carried out in a frequency range whose mid-frequency is higher than the natural frequency at standard conditions, a decrease in temperature can lead to an increase in the natural frequency and thus to a higher efficiency. If the measurement is carried out in a frequency range whose mid-frequency is lower than the natural frequency at standard conditions, the opposite is the case. Here, a decrease in temperature can lead to a higher efficiency.

[0025] The influence of this effect over a temperature range of, for example, 0°C to +85°C can amount to, for example, 4.5 dB for the emission and reception combination and approximately 2.25 dB for the received signal alone. This influence can not only lead to a better coupling of the sound into the membrane and thus to a higher voltage at the same sound volume, but also to a poorer coupling of the sound and thus to a lower voltage in the ultrasonic sensor. The voltage in the reception circuit can preferably be saved as a temperature characteristic curve and can also be considered for the correction of the noise level or intensity.

[0026] In contrast to sensing the tire noise, when transmitting and receiving ultrasonic signals for sensing the diffuse ground echo, the signal is influenced not only on the way from the ground to the ultrasonic sensor and in the ultrasonic sensor, but also when being converted into an acoustic signal and when being transmitted up to the ground. For this reason it can be preferred if not only the rolling noise of the tire is received, but also the diffuse ground echo and the rolling noise and the ground echo are evaluated for identifying the road surface state, then it is preferred to temperature-compensate for both cases by means of a characteristic curve pair, wherein a first characteristic curve for the rolling noise and a second characteristic curve for the diffuse ground echo form the characteristic curve pair. In a particularly advantageous extension, for simplifying the noise level compensation, the above-mentioned three action paths can be combined and a respective one of the composite temperature characteristic curve pairs based on the membrane temperature of the ultrasonic sensor and based on the ambient temperature can be saved for correcting the level or intensity of the wet hissing sound and for correcting the level or intensity of the diffuse ground echo. Furthermore, for further simplification, for example, a constant offset between the ambient temperature and the sensor-based membrane temperature based on self-heating can be calculated and thus two characteristic curves instead of the two characteristic curve pairs. Then the measured noise level only has to be corrected with these composite characteristic curves.

[0027] Preferably, in addition, the measured intensity of the received noise is corrected in relation to the vehicle speed.

[0028] The rolling noise caused by the wheels of the vehicle rolling on the road surface is also related to the speed of the vehicle, inter alia. Therefore, it is preferred to measure the speed of the vehicle and, before inferring the road surface state, to correct the rolling noise or the intensity assigned to the rolling noise in the light of the measured speed.

[0029] The intensity of the received ground echo is also related to the vehicle speed, inter alia. This is caused mainly by the Doppler shift based on the relative speed of the sensor and the frequency-dependent sensitivity. If the ground echo moves away from the sensor's own frequency due to the Doppler shift, the measured ground echo intensity decreases. For this reason, it is preferred to measure the speed of the vehicle and, before inferring the road surface state, to correct the ground echo or the intensity assigned to the ground echo in the light of the measured speed.

[0030] Preferably, in the method it is provided to check the measurement values of the ultrasonic sensor with respect to extraneous noise, since the weather-related state of the road surface is only inferred when the measurement values of the ultrasonic sensor are free of extraneous noise. Such extraneous noise is for example an ultrasonic pulse of an ultrasonic sensor of the own vehicle or of another vehicle in the surroundings. Furthermore, extraneous noise can be caused by other road users. Such extraneous noise can for example be identified by a high signal level or a defined frequency characteristic. That is for example the occurrence of ultrasonic pulses in a specific frequency range. Furthermore, the variance of the measurement values of the ultrasonic sensor can additionally or alternatively be analyzed. In the case of a variance below a predefined threshold, it is inferred that there is no extraneous noise.

[0031] If the vehicle has a plurality of ultrasonic sensors which can be used in connection with the proposed method, a temperature-dependent characteristic curve is determined for each individual ultrasonic sensor of these ultrasonic sensors accordingly.

[0032] A further aspect of the application relates to a driver assistance system, which comprises an ultrasonic sensor, a temperature sensor for sensing the ambient temperature and a controller. The driver assistance system is preferably provided for carrying out the method described. The features described in the context of one of the described methods accordingly apply to the driver assistance system and vice versa the features described in the context of the driver assistance system also apply to the described methods. In a preferred embodiment of the application, the driver assistance system further comprises an air humidity sensor.

[0033] The method of the application can take account of the received noise in the context of identifying the road surface state using an ultrasonic sensor. In the described method use is made of the fact that rolling noise related to the weather state of the road surface is generated when the wheels of the vehicle roll on the road surface. From this rolling noise it is then determined whether a water film is present on the road surface and how thick this water film is. Furthermore, from the rolling noise it can be inferred that ice is present and, if necessary, that snow is present. Alternatively or additionally, diffuse ground echoes can also be received and used for determining the weather state. By taking account of the ambient temperature when determining the intensity of the received noise from the measurement values of the ultrasonic sensor, an improvement in the accuracy in identifying the road surface state can be achieved. Furthermore, the road surface state (water, ice, snow, contamination...) can be determined more reliably by the described method and system. It is possible to distinguish more reliably whether the road surface is wet or dry and, furthermore, the degree of wetness of the road can be determined more precisely in the case of a wet road surface. BRIEF DESCRIPTION OF DRAWINGS

[0034] Embodiments of the application are explained in detail, by way of example, with reference to the accompanying drawings and the following description.

[0035] Figure 1Schematically showing a vehicle with a driver assistance system,

[0036] Figure 2 Schematically showing a part of a vehicle in a side view. DETAILED DESCRIPTION

[0037] Figure 1 A vehicle 1 with a driver assistance system 10 is shown. The driver assistance system 10 has an ultrasonic sensor 12 and a controller 16, and in addition, a temperature sensor 14 is provided, which is arranged for measuring the ambient temperature of the vehicle 1. Both the ultrasonic sensor 12 and the temperature sensor 14 are connected to the controller 16.

[0038] The driver assistance system 10 is arranged for determining the road surface condition as a function of the weather. Here, use is made of the fact that the wheels 18 of the vehicle 1, when rolling over the road surface, cause a noise. The rolling noise or the sound waves 20 generated here are measured by the ultrasonic sensor 12. The thus sensed rolling noise level can then be compared with a predefined threshold value or can be compared with a predefined value range. For example, a threshold value can be predefined, in which case a wet road surface is inferred when this threshold value is exceeded and a dry road surface is inferred when this threshold value is undershot. In addition, the ultrasonic sensor 12 can emit ultrasonic pulses 22 into the area in front of the vehicle 1. A small portion of the thus emitted sound is reflected back by the road surface and can be received by the ultrasonic sensor 12 as a diffuse ground echo. The level of the thus sensed ground echo can also be compared with a predefined threshold value or can be compared with a predefined value range. For example, a threshold value can be predefined, in which case a wet road surface is inferred when this threshold value is exceeded and a dry road surface is inferred when this threshold value is undershot. The controller 16 is arranged for determining the intensity of the received noise from the measurement values of the ultrasonic sensor 12, taking into account the ambient temperature T measured by the temperature sensor 14. To this end, for example, one or more characteristic curves are stored in the controller 16, which represent the temperature dependence of the transmission path of the received noise and / or the temperature dependence of the sound source of the received noise and are used in the determination of the intensity, so that, for example, the intensity of the received noise is obtained, which is corrected to a defined normal condition, for example T = 20°C. It is possible to distinguish between noise caused by the rolling of the wheels 18 of the vehicle 1 over the road surface and noise caused by the diffuse ground echo, for example, by taking into account the different propagation times and / or different frequency ranges of the received noise.

[0039] In Figure 2 different transmission paths and action paths are shown, which can be taken into account when identifying the road surface condition in accordance with a possible embodiment of the application. Figure 2A side view shows a vehicle 1 equipped with a driver assistance system 10 according to the present invention. Vehicle 1 is moving on a road surface 17, which in this embodiment is wet, i.e., has a thick film of water on its surface. Ultrasonic sensor 12 emits an ultrasonic pulse 22 into the area in front of vehicle 1. Ultrasonic pulse 22 strikes the road surface in area 37 and is reflected by it. A portion of the reflected sound is again received by ultrasonic sensor 12 as a diffuse ground echo.

[0040] The water temperature 19 of the water on the wet road surface 17 is considered as a first path of action. The intensity of the noise caused by the vehicle's wheels 18 rolling on the road surface 17 is dependent on the water temperature 19. To compensate for this temperature influence, it can be assumed that the water temperature corresponds to the measured ambient temperature. For example, the level measured by the ultrasonic sensor 12 can be extrapolated to a normal temperature of, for example, +20°C using the temperature characteristic curve for the wet hiss level stored in the control unit 16.

[0041] The air temperature in the surroundings of vehicle 1 can be considered as a second path of action. The attenuation of sound along the path from the sound source to the ultrasonic sensor is dependent on the ambient temperature and can reach 2 dB / m or higher. The attenuation is greatest at ambient temperatures of approximately 20°C to 25°C. Here, the transmission path 30 of sound 20 from sound source 31 to ultrasonic sensor 12 and / or the transmission path 32 of the diffuse ground echo from ultrasonic sensor 12 to area 37 of road surface 17 and back to ultrasonic sensor 12 can be considered. The lengths of these paths are known because the distance from point 31 where wheel 18 contacts road surface 17 to the mounting location of ultrasonic sensor 12 on vehicle 1 is known. Furthermore, the reflection of the diffuse ground echo back to starting area 37 of ultrasonic sensor 12 is known based on the mounting location, inclination, and radiation characteristics of ultrasonic sensor 12.

[0042] Furthermore, the attenuation of sound via the transmission path 30 or 32 is dependent on the air humidity. For this reason, the attenuation of sound can be calculated, in particular, by additionally measuring the air humidity, for example by means of an air humidity sensor 13 present on the vehicle.

[0043] The invention is not limited to the embodiments described herein and the aspects emphasized therein. Rather, numerous modifications are possible within the scope of the invention and are within the purview of a person skilled in the art.

Claims

1. A method for identifying road surface conditions, wherein: The ultrasonic sensor (12) receives noise, the noise - caused by the wheels (18) of the vehicle (1) rolling on the road surface (17), and / or - caused by diffuse ground echo, The invention relates to a method for determining the weather-related state of the road surface (17) from the intensity of the received noise, wherein the intensity of the received noise is obtained from the measured value of the ultrasonic sensor (12) taking into account the ambient temperature, wherein the temperature is measured, the current ambient temperature is determined, and when determining the intensity, a previously determined temperature dependency of the transmission path (30, 32) of the received noise and a previously determined temperature dependency of the sound source of the received noise are taken into account. wherein the temperature dependency of the sound source (31, 37) of the received noise comprises the influence of the water temperature on the intensity of the noise caused by the wheels (18) of the vehicle (1) rolling on the road surface (17), The measured intensity of the received noise is corrected with the aid of a characteristic curve pair depending on the current ambient temperature and the temperature sensed by the ultrasonic sensor (12), and the weather-related state of the road surface (17) is identified based on the corrected intensity, wherein the characteristic curve pair comprises a first characteristic curve for the temperature dependency of the noise caused by the wheels (18) of the vehicle (1) rolling on the road surface (17) and a second characteristic curve for the temperature dependency of the noise caused by diffuse ground echoes.

2. The method according to claim 1, characterized in that A temperature-dependent transmission of the noise from the sound source until an enhanced signal is provided in the ultrasonic sensor (12) is taken into account, wherein the temperature-dependent transmission is determined by determining the current ambient temperature and a previously determined temperature dependency of the transmission path (30, 32).

3. The method according to any one of the preceding claims, characterized in that The temperature dependency of the transmission path (30, 32) of the received noise is calculated using the first characteristic curve.

4. The method according to any one of the preceding claims, characterized in that The membrane temperature of the ultrasonic sensor (12) is calculated from the current ambient temperature and the measured temperature of the ultrasonic sensor (12), the efficiency of the ultrasonic sensor (12) is determined based on the membrane temperature of the ultrasonic sensor (12), and the efficiency determined in this way is taken into account when determining the intensity.

5. The method according to any one of the preceding claims, characterized in that The temperature dependency of the transmission path (30, 32) of the received noise includes an attenuation of the received noise that is dependent on the air temperature and / or air humidity on the path from the sound source to the ultrasonic sensor (12).

6. A driver assistance system (10), comprising an ultrasonic sensor (12), a temperature sensor (14) for sensing the ambient temperature of the ultrasonic sensor (12), and a controller (16), characterized in that: The driver assistance system (10) is configured to carry out a method according to any one of claims 1 to 5.

7. The driver assistance system (10) according to claim 6, comprising an air humidity sensor (13).

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