Method and driver assistance system for detecting road conditions

Abstract

A method for detecting a road surface condition, wherein an ultrasonic sensor (12) receives noises caused by the rolling of a wheel (18) of a vehicle (1) on the road surface (17) and / or by a diffuse ground echo, and the intensity of the received noises is used to infer a weather-related condition of the road surface (17), wherein the intensity of a received noise is obtained from measured values ​​of the ultrasonic sensor (12) taking into account an ambient temperature, wherein a temperature is measured, a current ambient temperature is determined, and a previously determined temperature dependence of the transmission path (30, 32) of a received noise and a previously determined temperature dependence of the sound source of a received noise are taken into account when determining the intensity, characterized in that the temperature dependence of the sound source (31,37) of a received noise includes the influence of water temperature on the intensity of a noise caused by the rolling of a wheel (18) of a vehicle (1) on the roadway (17).

Description

The invention relates to a method for detecting a roadway condition, wherein rolling noises and / or ground echoes of a previously transmitted ultrasonic pulse caused by the rolling of a wheel of a vehicle on the roadway are measured with an ultrasonic sensor and a weather-related condition of the roadway is deduced from the received noises. A further aspect of the invention relates to a driver assistance system which is set up to carry out the method.Prior ArtWhen a vehicle is travelling on a roadway, rolling noises are produced which are caused by a wheel of the vehicle rolling on the roadway. These rolling noises are dependent on the weather-related state of the roadway, so that the rolling noises vary for a dry roadway, a moist roadway, a snowy roadway, an icy roadway and a wet roadway, wherein in the case of a wet roadway the rolling noises are also dependent on how high the water level is on the roadway. The rolling noises resemble noise and are also referred to as wet churns in the case of a wet roadway.In order to detect a weather-related condition of the roadway, it is desirable to use sensors already present on the vehicle, such as an ultrasonic sensor. Such an ultrasonic sensor comprises a sound transducer which is configured to emit ultrasonic pulses and to receive echoes reflected from objects in the environment again. Such a sound transducer of an ultrasonic sensor can also measure the rolling noises. However, it is problematic here that the level of both the rolling noise and the ground echoes can depend on the ambient temperature and the sensor temperature.When threshold values are used to classify the road condition on the basis of the noise level determined by an ultrasonic sensor, errors can thus occur. For example, at high ambient temperatures, a road may be incorrectly recognized as dry, although this is moist.DE 10 2006 037 591 A1 discloses a device for detecting a moving object. The apparatus has a plurality of ultrasonic sensors with different detection ranges. The sensitivity of the ultrasonic sensors can be adjusted. To set the sensitivity, a measurement of the current environmental conditions is carried out. The environmental conditions include, in particular, an attenuation of the amplitude of the ultrasound and a noise floor that is increased due to a wet road. For estimating the ambient noise, further sensors of the device are switched into a passive mode in which they do not emit sound waves, but rather only receive the external noise from the environment.The document DE 10 2019 203 190 A1 discloses a method and a driver assistance system for detecting a roadway state.Document DE 10 2018 108 791 A1 discloses an ultrasonic sensor device for detecting a state of a roadway taking into account an air temperature.The document DE 10 2018 203 924 A1 discloses a method for detecting a roadway surface condition.The document DE 10 2016 105 153 A1 discloses a method for operating an ultrasonic sensor of a motor vehicle with determination of the air humidity.The methods known from the prior art cannot distinguish between external noise from the environment and the background noise of the ultrasonic sensors.The object of the present invention is therefore to improve a method for detecting a roadway state compared to the prior art.Disclosure of the InventionAccording to the invention, the above object is achieved according to independent claims 1, 7 and 8.A method for detecting a roadway condition is proposed, wherein noise is received with an ultrasonic sensor, and a weather-related condition of the roadway is deduced from the intensity of the received noise. The noises which are received and evaluated with the method are preferably noises caused by the rolling of a wheel of a vehicle on the roadway and / or by a diffuse ground echo.The weather-related state of the roadway is in particular a thickness of a water film on the roadway, wherein in this case a wet churn occurs as rolling noise when a wheel of a vehicle rolls on a roadway. This wet churn is dependent on the thickness of the water film on the roadway. Furthermore, the weather-related roadway condition can be an icing of the roadway, in which an ice layer is located on the roadway. Another possible weather-related state is a snowy roadway in which snow is located on the roadway. Sand or dirt which is swept or rinsed onto the roadway are also possible weather-related states of a roadway.The ultrasonic sensor used within the scope of the method is preferably an ultrasonic sensor which, in addition to detecting a roadway state, is also used for detecting objects in the environment of the vehicle. The proposed method is run through in particular when this ultrasonic sensor is not just required for detecting objects in the environment of the vehicle. Therefore, it is preferred if an ultrasonic sensor is used, which is part of a parking assistant, for example, so that it is not required for detecting objects in the surroundings of the vehicle during normal travel of the vehicle.The intensity or a level of the noises from the measurement values of the ultrasonic sensor can be obtained by determining a noise value or a signal level of the measured ambient noise. This can particularly preferably take place in a measurement window when echoes from ultrasonic pulses of the sensors have decayed, so that the noise value or the signal level can be measured as free of interference as possible. Furthermore, a background noise level can preferably be subtracted from the ascertained noise value or signal level, which is associated with the background noise of the ultrasonic sensor and is calculated in particular from the current sensor temperature of the ultrasonic sensor and the known temperature dependence of the ultrasonic sensor. In connection with the present method, in particular the intensity or level of sounds is evaluated, which arise as a result of the rolling of a tire and / or as a result of diffuse ground echoes.In order to infer the weather-related state of the roadway from the intensity of rolling noises determined using the ultrasonic sensor, a level is preferably determined from received rolling noises. The level is then compared with predetermined limit values or with predetermined level ranges. In this case, for example, a low level of the rolling noises determined is an indicator for a dry roadway, a level above a predefined limit value is an indicator for a wet roadway, it being possible to derive the water film thickness on the roadway from the strength of the level. Higher levels indicate thicker water films on the roadway. The levels or level ranges can be predefined in particular as a function of the current speed of the vehicle.During rolling, a part of the tire is deformed such that the tread blocks coming off the road are either bent or displaced. A very low coefficient of friction of the roadway due to snow or ice causes the tire tread blocks to shift during rolling, while a high coefficient of friction causes bending. As a rule, the rolling noises at a high coefficient of friction are greater when the tire profile blocks bend and snap back abruptly into the original shape. Loose snow also very well attenuates the sound, which is why a difference from dry road can also be measured here.A ground echo is understood to mean a noise which is caused by the reflection of emitted ultrasonic pulses through the ground, that is to say through the roadway. These floor echoes are generally diffuse. In contrast to rolling noises, a ground echo does not occur independently of the operation of the ultrasonic sensor or ultrasonic sensors of the vehicle, but in a temporal relationship with the emission of ultrasonic pulses by one of the ultrasonic sensors.In order to infer the weather-related state of the roadway from the intensity of ground echoes determined using the ultrasonic sensor, a level is preferably determined from the ground echo, which level is compared with predefined limit values or with predefined level ranges. In this case, for example, a high level of the ground echoes determined is an indicator for a dry roadway, a level below a predefined limit value is an indicator for a wet or icy roadway, wherein the water film thickness on the roadway can be derived from the strength of the level. Lower levels indicate thicker water films on the roadway. Loose snow also dampens the ground echoes even more strongly than water, which is why here too a difference with respect to dry and wet road can be measured.According to the invention, the intensity or level of a received noise is obtained from measured values of the ultrasonic sensor taking into account a temperature, wherein a current ambient temperature can preferably be measured directly as the temperature and / or an ambient temperature and a sensor temperature can be measured and on the basis of which a model of the current ambient temperature can be formed. An actual ambient temperature is thus determined and a predetermined temperature dependence of the transmission path of a received noise and a predetermined temperature dependence of the sound source of a received noise is taken into account in the determination of the intensity.The ambient temperature is the temperature in the environment of the vehicle, in particular a temperature of the ambient air. This can be measured, for example, with a temperature sensor provided on the vehicle.For this purpose, the measured values of the ultrasonic sensor can be corrected as follows with the aid of the ambient temperature before they are used further to detect a roadway state:According to the invention, the water temperature of the water on the wet roadway is taken into account as the first active path. The intensity of the noise (wet churn) caused by the rolling of a wheel of the vehicle on the roadway is dependent on the temperature of the water. For the compensation of the temperature influence, it is assumed that the water temperature corresponds to the measured ambient temperature. For example, on the basis of a temperature characteristic curve of the wet-zisch level stored in the measurement system, the measured level can be calculated back to a normal temperature, e.g. +20° C.The air temperature in the environment of the vehicle can be taken into account as the second active path. The attenuation of the sound on the path from the sound source to the ultrasonic sensor is dependent on the ambient temperature and can be up to 2 dB / m and more. The attenuation is greatest at an ambient temperature of about 20° C. to 25° C. The attenuation of the sound over the path is also dependent on the air humidity. For this reason, the attenuation of the sound can be calculated in particular by an additional measurement of the air humidity, for example by means of an air humidity sensor present on the vehicle. Alternatively, if no humidity sensor is present, an assumption may be made for a humidity range: if a wet or wet road may be inferred from an increased intensity of the received sounds, then the humidity is also assumed to be high (e.g., greater than 70%). For an air humidity range between 70% and 100%, an air humidity of 85%, for example, can be assumed alternatively.For the correction of the measured values of the ultrasonic sensor based on noise caused by the rolling of a wheel of the vehicle on the roadway on account of the temperature-dependent airborne sound attenuation, it can be assumed that the noise originates predominantly from the tire which is closest to the measuring ultrasonic sensor. For the sound travel distance, the distance of the contact point of the tire on the roadway to the ultrasonic sensor is assumed.For the compensation of the diffuse ground echo due to the temperature-dependent airborne sound attenuation, the path from the ultrasonic sensor to the ground and back to the ultrasonic sensor is preferably used as a basis.From the measured current ambient temperature and an assumed mean air humidity of e.g. 85%, the absorption coefficient at the operating frequency of the ultrasonic sensor (e.g. 48 kHz) can be calculated. Taking into account the sound path, the absolute attenuation can then be calculated and set in relation to the attenuation under defined standard conditions (e.g. +20° C., 85%, 48 kHz). With this ratio, the intensities of the received noises can now be corrected from the measured values of the ultrasonic sensor.A low air temperature causes a lower attenuation and thus a higher intensity of the diffused ground echo when measuring the roughness of the road surface on the basis of a diffused ground echo. A roughness of the road surface calculated on the basis of the diffuse ground echo can therefore be corrected upward at a higher ambient temperature. By calculating the roughness of the ground surface, it can be deduced whether the pores of the asphalt are filled with water or ice. It can also be detected whether the road surface is snowy or soiled.In a preferred embodiment of the invention, the membrane temperature of the ultrasonic sensor can be taken into account as a further active path: the membrane temperature is influenced by the ambient temperature and can be calculated, for example, with the aid of interpolation from the ambient temperature and a measured temperature in the interior of the ultrasonic sensor. The temperature of the diaphragm has an influence on the natural frequency and the acoustic damping of the diaphragm and thus on the frequency response of the ultrasonic sensor. The efficiency for transmission and reception in frequency ranges in the range of the specified natural frequency under standard conditions (e.g. 48 kHz) is usually the highest at a membrane temperature of, for example, approximately +20° C. and decreases towards higher and lower temperatures. However, if measurement is carried out in a frequency range whose center frequencies are higher than the natural frequency under standard conditions, a reduction in the temperature can lead to an increase in the natural frequency and thus to a higher efficiency. The opposite is the case when measuring in a frequency range whose center frequency is lower than the natural frequency under standard conditions. Here, a decrease in temperature may result in higher efficiency.The influence that this effect exerts over a temperature range of, for example, 0° C. to +85 ° C. can be, for example, up to 4.5 dB for the combination of transmission and reception and approximately 2.25 dB for the exclusive reception of signals. The influence can lead both to the fact that the sound can be better coupled into the membrane and thus higher voltages are generated at the same volume and to the fact that the sound is coupled in more poorly and thus lower voltages are generated in the ultrasonic sensor. The dependence of the voltage in the receiving circuit for the diffuse ground echo of the ultrasonic sensor can preferably be stored as a temperature characteristic curve and can likewise be used for correcting the noise level or the intensity.In contrast to detecting tire noise, when transmitting and receiving ultrasonic signals for detecting a diffused ground echo, the signals are influenced both in the path from the ground to the ultrasonic sensor and the conversion in the ultrasonic sensor, and in the conversion into an acoustic signal and the transmission to the ground. For this reason, if both rolling noises of a tire and a diffuse ground echo are received and evaluated to detect the roadway condition, the temperature compensation can preferably be carried out for both cases with the aid of a pair of characteristic curves, wherein a first characteristic curve for the rolling noise and a second characteristic curve for the diffuse ground echo form a pair of characteristic curves. In a particularly advantageous development, all three said effective paths can be combined to simplify the noise level compensation and a resulting temperature characteristic line pair can be stored on the basis of the temperature of the membrane of the ultrasonic sensor and the ambient temperature for the correction of the level or the intensity of the wet hissic and the correction of the level or the intensity of the diffuse ground echo. Furthermore, for further simplification, it is possible to count for example on a constant offset between ambient temperature and membrane temperature of the sensor due to self-heating and thus to replace the two pairs of characteristic curves by two characteristic curves. The measured noise levels then only have to be corrected with these resulting characteristic curves.Preferably, a correction of the measured intensity of a received noise is additionally carried out depending on the speed of the vehicle.The rolling noises caused when a wheel of the vehicle rolls on the roadway are also dependent, among other things, on the speed of the vehicle. Therefore, a speed of the vehicle is preferably measured and a correction of the rolling noises or the intensity assigned to the rolling noises is carried out in consideration of the measured speed before the condition of the roadway is deduced.The intensity of a received ground echo is also dependent, among other things, on the speed of the vehicle. This is due primarily to the Doppler shift due to the relative velocity and the frequency-dependent sensitivity of the sensors. If the ground echo is pushed far away from the natural frequency of the sensor by the Doppler shift, the measured intensity of the ground echo falls. Therefore, a speed of the vehicle is preferably measured and a correction of the ground echoes or the intensity assigned to the ground echo is carried out in consideration of the measured speed before the condition of the roadway is deduced.Preferably, the method provides that the measured values of the ultrasonic sensor are checked for extraneous noises, because the weather-related state of the roadway is only deduced if the measured values of the ultrasonic sensor are free of extraneous noises. Such extraneous noises are, for example, ultrasonic pulses from ultrasonic sensors, for example from the own vehicle or from other vehicles in the environment. Furthermore, extraneous noises can be caused by other road users. Such extraneous noises can be detected, for example, by means of a high signal level or specific frequency characteristics. For example, an ultrasonic pulse occurs in a specific frequency range. Furthermore, additionally or alternatively, a variance of the measured values of the ultrasonic sensor can be analyzed. If the variance is below a predefined threshold value, then it is concluded that extraneous noises are absent.If a vehicle has a plurality of ultrasonic sensors, which are to be used in conjunction with the proposed method, the temperature-dependent characteristic curve is determined accordingly for each individual one of the ultrasonic sensors.A further aspect of the invention relates to a driver assistance system which comprises an ultrasonic sensor, a temperature sensor for detecting the temperature of the environment and a control unit. The driver assistance system is preferably configured to carry out one of the methods described herein. Accordingly, features described within the scope of one of the methods apply to the driver assistance system and, conversely, the features described within the scope of the driver assistance system also apply to the methods. In a preferred embodiment of the invention, the driver assistance system also comprises an air humidity sensor.Advantages of the InventionThe method according to the invention can take into account received noise in the context of detecting a roadway state using an ultrasonic sensor. The method makes use of the fact that rolling noises are produced when a wheel of a vehicle rolls on a roadway, which noise are dependent on the weather condition of the roadway. Depending on the rolling noise, it is then determined whether a water film is on the roadway and how thick the water film is. Furthermore, the presence of ice and possibly snow can be deduced from the rolling noise. Alternatively or additionally, diffuse ground echoes can also be received and used for determining the weather condition. By taking into account the ambient temperature when determining the intensity of the received noise from measured values of the ultrasonic sensor, an improved accuracy of the detection of a roadway state can be achieved. In addition, the roadway condition (water, ice, snow, dirt, etc.) can be more reliably determined by the described method and system. Discrimination as to whether a road is wet or dry is possible more reliably, and also in the case of a wet road, the wet degree road can be more accurately determined.Brief Description of the DrawingsEmbodiments of the invention are explained in more detail with reference to the drawings and the following description.FIG. 1 schematically shows a vehicle having a driver assistance system.FIG. 2 schematically shows a detail of the vehicle in side view.Embodiments of the InventionFIG. 1 shows a vehicle 1 with a driver assistance system 10. the driver assistance system 10 has an ultrasonic sensor 12 and a control unit 16. Both the ultrasonic sensor 12 and the temperature sensor 14 are connected to the control unit 16.The driver assistance system 10 is configured to determine a weather-related roadway state. This makes use of the fact that the wheels 18 of the vehicle 1 cause noise when rolling on a roadway. Rolling noises or the sound waves 20 generated in this case are measured by the ultrasonic sensor 12. The level of the rolling noise detected in this way can then be compared with a predefined threshold value or can be compared with predefined value ranges. For example, a threshold value can be predefined, wherein a wet roadway is concluded if the threshold value is exceeded and a dry roadway is concluded if the 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 sound thus emitted is reflected back from the roadway and can be received as diffuse ground echo by the ultrasonic sensor 12. The level of the ground echo acquired in this way can likewise be compared with a predefined threshold value or can be compared with predefined value ranges. For example, a threshold value can be predefined, wherein a wet roadway is concluded if the threshold value is exceeded and a dry roadway is concluded if the threshold value is undershot. The control unit 16 is configured to determine the intensity of a received noise from measured values of the ultrasonic sensor 12 taking into account the ambient temperature T measured by the temperature sensor 14. For this purpose, for example, one or more characteristic curves are stored in the control unit 16, which represent a temperature dependence of the transmission path of a received noise and a temperature dependence of the sound source of a received noise and are used in the determination of the intensity, so that, for example, an intensity of the received noises corrected to defined normal conditions (e.g. T=20° C.) results. A distinction between noises caused by the rolling of a wheel 18 of a vehicle 1 on the roadway or by a diffuse ground echo is possible, for example, by considering different transit times and / or frequency ranges of the received noises.FIG. 2 shows the different transmission paths and active paths which can be taken into account when recognizing a roadway state according to a possible embodiment of the invention. FIG. 2 shows a section of a vehicle 1 having a driver assistance system 10 according to the invention in side view. The vehicle 1 moves on a roadway 17 which is wet in this example, i.e. has a water film of a certain thickness on its surface. Ultrasonic sensor 12 emits ultrasonic pulses 22 into the area in front of vehicle 1. Ultrasonic pulses 22 strike the roadway in region 37 and are reflected by the roadway. A portion of the reflected sound is received again as diffuse ground echo from the ultrasonic sensor 12.As a first active path, the water temperature of the water 19 on the wet roadway 17 will be taken into account. The intensity of the noise caused by the rolling of a wheel 18 of the vehicle on the roadway 17 is dependent on the temperature of the water 19. For example, on the basis of a temperature characteristic curve of the wet-zisch level stored in the control unit 16, a level measured by the ultrasonic sensor 12 can be calculated back to a normal temperature, e.g. +20° C.The air temperature in the environment of the vehicle 1 can be taken into account as the second active path. The attenuation of the sound on the path from the sound source to the ultrasonic sensor is dependent on the ambient temperature and can be up to 2 dB / m and more. The attenuation is greatest at an ambient temperature of about 20° C. to 25° C. Here, firstly, the transmission path 30 of the sound 20 from the sound source 31 to the ultrasonic sensor 12 can be taken into account and / or the transmission path 32 taken by the diffuse ground echo from the ultrasonic sensor 12 to the region 37 of the roadway 17 and back to the ultrasonic sensor 12. The path lengths are known since the distance from the point 31 at which the wheel 18 touches the roadway 17 to the installation location of the ultrasonic sensor 12 on the vehicle 1 is known and also the region 37 from which the diffuse ground echo is reflected back to the ultrasonic sensor 12 is known by the installation position of the ultrasonic sensor 12, the inclination and emission characteristic of the ultrasonic sensor 12.The attenuation of the sound via the transmission path 30 or 32 is also dependent on the air humidity. For this reason, the attenuation of the sound can be calculated in particular by an additional measurement of the air humidity, for example by means of an air humidity sensor 13 present on the vehicle.The invention is not limited to the exemplary embodiments described here and the aspects emphasized therein. Rather, within the scope of the claims, a variety of modifications are possible, which are within the scope of specialist activity.

Claims

Method for detecting a road condition, wherein with an ultrasonic sensor (12), noises are received which are - caused by a wheel (18) of a vehicle (1) rolling on the road (17) and / or - by a diffuse ground echo and are deduced from the intensity of the received noises from a weather-related condition of the road (17), wherein the intensity of a received noise is obtained from measurement values of the ultrasonic sensor (12) taking into account an ambient temperature, wherein a temperature is measured, a current ambient temperature is determined and a previously determined temperature dependence of the transmission path (30, 32) of a received noise and a previously determined temperature dependence of the sound source of a received noise is taken into account in the determination of the intensity, characterized in that the temperature dependence of the sound source (31, 31, 31, 31, 37) of a received noise comprises the influence of a water temperature on the intensity of a noise caused by the rolling of a wheel (18) of a vehicle (1) on the roadway (17).Method according to Claim 1, characterized in that a temperature-dependent transmission of a noise from a sound source until an amplified signal is provided in the ultrasonic sensor (12) is taken into account, wherein the temperature-dependent transmission is determined by determining a current ambient temperature and a previously determined temperature dependence of the transmission path (30, 32).Method according to one of the preceding claims, characterized in that the temperature dependence of the transmission path (30, 32) of a received noise is calculated with the aid of a first characteristic curve.Method according to one of the preceding claims, characterized in that a membrane temperature of the ultrasonic sensor (12) is calculated from a current ambient temperature and a measured temperature of the ultrasonic sensor (12), and an efficiency of the ultrasonic sensor is determined as a function of the membrane temperature of the ultrasonic sensor (12), and the efficiency thus determined is taken into account in the determination of the intensity.Method according to one of the preceding claims, characterized in that the temperature dependence of the transmission path (30, 32) of a received noise comprises attenuation of the received noise as a function of an air temperature and / or the air humidity on a route from the sound source to the ultrasonic sensor (12).Method according to one of the preceding claims, characterized in that the measured intensity of the received noises is corrected by means of a pair of characteristic curves, in particular depending on the current ambient temperature and a detected temperature of the ultrasonic sensor (12), and a weather-related state of the roadway (17) is recognized on the basis of the corrected intensity, wherein the pair of characteristic curves has a first characteristic curve for a temperature dependence of the noises caused by the rolling of a wheel (18) of a vehicle (1) on the roadway (17) and a second characteristic curve for a temperature dependence of the noises caused by a diffuse ground echo.Driver assistance system (10) comprising an ultrasonic sensor (12), a temperature sensor (14) for detecting the temperature of the ambient temperature of the ultrasonic sensor (12) and a control device (16), characterized in that the driver assistance system (10) is configured to carry out the method according to one of Claims 1 to 6.Driver assistance system (10) according to Claim 7, comprising an air humidity sensor (13).

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