Abstract:
A method for detecting a sensor degradation in distance sensors, having the following: a) sending out at least one transmit pulse; b) acquisition of a sensor signal at least in a decay interval; and c) determination of a degree of degradation based on a frequency response of the acquired sensor signal. Also described is a computer program product, a distance sensor unit, and a driver assistance system for carrying out such methods.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a method for detecting sensor degradation in distance sensors. The present invention further relates to a distance sensor unit, a driver assistance system, and a computer program for carrying out the method. 
       BACKGROUND INFORMATION 
       [0002]    Driver assistance systems are auxiliary devices in a vehicle that support the driver when driving the vehicle. Such driver assistance systems typically include various subsystems such as driver information systems or predictive safety systems. Some of these subsystems require an environmental sensor system that monitors the surrounding environment of the vehicle in order for example to detect objects that may present obstacles on the roadway. 
         [0003]    Typical methods for environmental recognition in a vehicle make use of distance sensors, in particular ultrasound sensors, that, based on a transmission pulse echo method, measure the distance from objects in the environment surrounding the vehicle, and whose measurement data are used as a basis for a reaction that is to be generated by the driver assistance system. The quality of the provided measurement data therefore plays a decisive role in the provision of a driver assistance function. Thus, sensor degradations or environmental influences such as snow or ice may cause disturbances that result in the complete loss of function of the ultrasound sensors. The ultrasound sensors are then quasi-blind. In order to make diagnoses of the functioning of the ultrasound sensors that are as reliable as possible, various methods are known. 
         [0004]    In the simplest case, the functioning of the ultrasound sensor is measured by measuring whether the sensor is detecting a signal at all. From DE 10 103 936 A1, an ultrasound sonar system is known for detecting an obstacle, in which an ultrasound oscillator produces a post-oscillation or decay oscillation frequency. By changing from a first transmission frequency to a second transmission frequency different from the post-oscillation frequency, the presence of an echo, and thus of an obstacle, within a specified distance can be inferred. However, such methods cannot provide a reliable assessment of whether the sensor is impaired or nonfunctional. 
         [0005]    A second group of methods measures a transfer function at various excitation frequencies, and is capable of extracting information therefrom concerning the frequency characteristic and the signal quality. DE 10 2010 003 624 A1 describes a method for acquiring disturbances of an ultrasound transducer. The ultrasound transducer is excited with two different frequencies, and the resulting different release time durations are compared. However, the measurement, which is a function of frequency, requires a good deal of time, and during this time the sensor cannot be used for other purposes. In addition, additional electronics are required that make the configuration of the sensor more complex and more expensive. 
         [0006]    The third group of methods makes use of the die-out characteristic of the sensor in order to monitor its functionality. From EP 0 816 865 A2, a method is discussed for self-testing a device for ultrasound runtime measurements, in which the die-out characteristic of the ultrasound sensor is evaluated after a transmission process. In “diagnostic” mode, the die-out characteristic is evaluated with regard to at least one signal shape parameter, and is monitored to check the observance of an error criterion. Here, the die-out characteristic can be measured with each send-receive cycle. However, it is difficult to make reliable assessments of the functionality of the sensor on the basis of the die-out characteristic. 
         [0007]    A further disadvantage of the known methods is that a total failure of ultrasound sensors is difficult to recognize. In particular when used in driver assistance systems, such a total failure means that the system is at least partially “blind.” Therefore, there is an ongoing interest in reliably recognizing possible disturbances, in particular those that result in complete loss of functionality. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the present invention, a method is proposed for detecting a sensor degradation in distance sensors, including the following steps:
   a) sending out at least one transmit pulse;   b) acquiring a sensor signal at least in a decay interval; and   c) determining a degree of degradation based on a frequency response of the acquired sensor signal.   
 
         [0012]    The distance sensor can be part of the environmental sensor system of a driver assistance system having various subsystems, for example having a parking assistant, a side monitoring assistant, or a lane departure warning system. The environmental sensor system of the driver assistance system is used to monitor the environment around the vehicle, using for example ultrasound sensors, radar sensors, infrared sensors, lidar sensors, optical sensors, or combinations thereof as environmental sensors. For the distance measurement, in particular those sensors are used that determine the distance from objects in the environment around the vehicle using a pulse-echo method. Ultrasound sensors may be suitable for this. 
         [0013]    In the context of the present invention, the distance sensor can both send out a transmit pulse and also receive an echo. However, this configuration is advantageously in no way compulsory. It is equally possible for the transmitter and receiver to be made separate. In the case of separate units, at least two distance sensors are used, one distance sensor acting as “transmitter” when it emits a transmit pulse, and one distance sensor acting as “receiver” when it receives a signal. 
         [0014]    The method according to the present invention can be carried out in the context of a runtime measurement that is carried out with the aid of at least one distance sensor. The distance sensor is first controlled to send out a transmit pulse having a transmit pulse length. The controlling can take place in a driver assistance system, for example being accomplished centrally by a control device (electronic control unit, ECU), or by an electronics device assigned to the distance sensor, such as an application-specific integrated circuit (ASIC). Here, the sensor parameters, such as the transmit current, the frequency, the amplitude, the transmit pulse length, or the modulation of a transmit pulse or of successive transmit pulses, can be variable and can be adapted to the respective situation. The transmit interval is followed by a decay interval in which for example a membrane of the distance sensor post-oscillates. Subsequently, the sent-out transmit pulses are detected by one or more distance sensors as echo signals when reflected by objects. From the runtime of a transmit pulse, i.e. the time between the sending out of the transmit pulse and the reception of the echo signal, the distance between the object and the distance sensor then results, taking into account the speed of the signal and possibly the speed of the vehicle. 
         [0015]    In an implementation of the method according to the present invention, the sensor signal is acquired in each decay interval that follows the transmit interval. The decay interval may be followed by a receive interval in which echo signals are received before a subsequent transmit pulse is sent out. The sensor signal may be acquired in a decay interval and in a transmit interval. 
         [0016]    In a further implementation of the method according to the present invention, the frequency response of the sensor signal is determined from intervals of at least a quarter wavelength. The frequency response may be determined from intervals of a half wavelength. 
         [0017]    The frequency response of the acquired sensor signal in the decay interval can include at least three intervals, a first and a third interval containing at least one extremum, and the second interval being situated between the last extremum of the first interval and the first extremum of the third interval. 
         [0018]    The degree of degradation may be determined on the basis of the frequency response in the second interval. A mean value can be determined from the frequency response in the second interval, and the degree of degradation can be determined from the mean value and a specified target value. The target value can for example correspond to the inherent frequency of the distance sensor in a fully functional state. 
         [0019]    In a further implementation of the method according to the present invention, the degree of degradation can result from the envelope of the frequency response. The envelope of the frequency response can be determined for this purpose and can be compared to a specified curve, in particular to the curve of a fully functional distance sensor. The specified curve can be stored in the electronics device allocated to the distance sensor, or can be stored in a central control device. The difference between the curve of the frequency response and the specified curve can be determined and characterized using standard methods such as mean deviation. Due to the simplicity of the method, this can be realized as hardware circuitry, for example as circuitry having comparators and counters, or as software in the electronics device allocated to the distance sensor or to the central control device. 
         [0020]    Objects in the near field can generate echoes that interfere with the frequency response in the decay interval, so that in this way differences also result from the “normal” frequency response, and there is the false appearance of a degradation. In a further implementation of the method according to the present invention, in an additional step the degree of degradation is therefore validated by a variation of the transmit pulse length, which may be a transmit pulse length that is made longer relative to the normal operation of the distance sensor. Through the modified transmit pulse length, a near field echo interferes with the frequency response in the decay interval at a different point in time. This also results in different frequency curves. If the frequency response in the decay interval remains independent of the transmit pulse duration, then the distance sensor is blind. 
         [0021]    A further quantity that can be used to provide assistance here is the decay time itself. In particular in the case of longer transmit pulse durations (and thus also echo durations), the decay time will significantly increase. If, in contrast, the decay time remains the same independent of the transmit pulse length, then it can be assumed that no objects are situated in the near field. In a further implementation of the method according to the present invention, additional echo signals are therefore recognized in the decay interval based on a decay time. For this purpose, the transmit pulse length can be selected to be longer than the decay time, or a modification of the echo signal can be used as a basis. If the decay time is the same in both variants, then the distance sensor is blind. 
         [0022]    According to the present invention, in addition a computer program is proposed for carrying out one of the methods described herein when the computer program is run on a programmable device. The computer program can be stored on a machine-readable storage medium, such as a permanent or rewritable storage medium, or in allocation to a computer device, or on a removable CD-ROM, DVD, or USB stick. In addition or alternatively, the computer program can be provided on a computer device such as a server for downloading, e.g. via a data network such as the Internet or cloud, or via a communication connection such as a wireless connection. 
         [0023]    According to the present invention, in addition a distance sensor unit is proposed having a receive unit that is fashioned to carry out the method described above. The subject matter of the present invention also includes a driver assistance system for carrying out the method described above, having the following components:
       i) at least one distance sensor for sending out at least one transmit pulse;   ii) at least one component, in particular a receive unit, for acquiring a sensor signal in at least one decay interval; and   iii) at least one component for determining a degree of degradation based on a frequency response of the acquired sensor signal.       
 
         [0027]    In addition, the distance sensor unit according to the present invention and the driver assistance system according to the present invention can include a component that validates the degree of degradation through a variation of the transmit pulse length, which may be a transmit pulse length that is made longer relative to the normal operation of the distance sensor. 
         [0028]    Here, the individual components represent functional components or routines that are executed for example in the context of a computer program on an electronic device such as a programmable computer device. The computer device can for example be a control device (ECU) for implementing a driver assistance system, or a subsystem thereof, or can be electronic devices (ASIC) allocated to the distance sensors. In this way, in particular the receive unit can be fashioned as a central control device or as a receive unit allocated to the distance sensors. 
         [0029]    The electronic device can communicate with the distance sensor via control signals, as a central device or as a sensor-individual device. Thus, control signals can be generated that trigger the distance sensor to send out transmit pulses having a defined transmit spectrum. Conversely, the distance sensor can forward received signals to the electronic device for signal processing. In this way, the determination of the degree of degradation takes place in the context of the signal processing. 
         [0030]    The present invention makes it possible to reliably monitor the functional capacity of a distance sensor. In particular on the basis of the frequency response in the decay interval, a reliable assessment can be made concerning the degree of degradation of the distance sensor. From this it results whether the distance sensor is fully functional, partially blind, or completely blind. Such a monitoring is advantageous in particular for distance sensors in a driver assistance system, because in this way incorrect assessments and false reactions due to blind distance sensors are avoided. 
         [0031]    The determination of the degree of degradation is accomplished through a simple measurement that can be carried out in each transmit-receive cycle without influencing the functionality of the distance sensor. In addition, the signal curve in the decay interval provides a very stable signal that in turn increases the reliability of the method according to the present invention. In addition, simple monitoring mechanisms can be integrated into the method according to the present invention that avoid possible misclassifications of a distance sensor. 
         [0032]    It is also the case that no special requirements have to be made of the electronic components in order to realize a unit for determining the degree of degradation, and already-existing receive units can be used. In particular, the method can be realized very economically in the distance sensor, because only comparators and counters have to be used for the determination of period durations. A further advantage is that no special operating mode is required for the degradation; i.e., the information is obtained in normal measurement operation for each transmit-receive cycle. As a result, no additional latencies arise. 
         [0033]    Further aspects and advantages of the present invention are now explained in more detail on the basis of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  shows a vehicle equipped with an ultrasound sensor according to the present invention, in a sample driving situation. 
           [0035]      FIG. 2  schematically shows a signal curve of the sensor signal for a measurement cycle in the transmit and decay interval. 
           [0036]      FIG. 3  schematically shows a frequency response of the sensor signal from  FIG. 2  for a measurement cycle in the transmit and decay interval. 
           [0037]      FIG. 4  schematically shows a further signal curve of the sensor signal for a measurement cycle in the transmit and decay interval. 
           [0038]      FIG. 5  schematically shows a further frequency response of the sensor signal from  FIG. 4  for a measurement cycle in the transmit and decay interval. 
           [0039]      FIG. 6  schematically shows a further signal curve of the sensor signal for a measurement cycle in the transmit and decay interval. 
           [0040]      FIG. 7  schematically shows a further frequency response of the sensor signal from  FIG. 6  for a measurement cycle in the transmit and decay interval. 
           [0041]      FIG. 8  shows, in the form of a flow diagram, a manner of operation of the ultrasound sensor according to the present invention from  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0042]      FIG. 1  schematically shows a driving situation with a vehicle  10  that for example is moving into a garage entrance or a bounded parking space  12 . 
         [0043]    Vehicle  10  is equipped with a driver assistance system  14  that includes an ultrasound-based sensor system for monitoring the surrounding environment, having ultrasound sensors  18 ,  19  at the front and at the rear on vehicle  10 . Ultrasound-based sensor system  18  is controlled by separate devices  20  assigned to ultrasound sensors  18 ,  19 , and by a higher-order unit  16 , which is typically a central control device of driver assistance system  12 . In addition to ultrasound sensors  18 ,  19  shown as examples in  FIG. 1 , other distance sensors, such as lidar infrared or radar sensors, can also be used for the runtime measurement. 
         [0044]    In the enlarged segment of  FIG. 1 , the configuration of an ultrasound unit  17  having a pulse generator  20  and a receive unit  28  is shown. Here, typically a piezoactuator  26 , which is connected to membrane  24  of ultrasound sensor  18 ,  19 , is controlled by pulse generator  20  in order to send out pulses. In addition, piezoactuator  26  is connected to a receive unit  28  in order for example to receive an echo signal or to detect a decay characteristic of membrane  24 . The signal processing here takes place in sensor-individual units  28  or in central electronics unit  16 , partial steps of the signal processing taking place in one of units  16 ,  28 , or in distributed fashion at units  16 ,  28 . 
         [0045]    When vehicle  10  approaches the bounded parking space or garage entrance  12 , front ultrasound sensors  18  send out ultrasound pulses in order to determine the distance between vehicle  10  and boundaries  12  from the runtime of the ultrasound pulses. Here, as is shown schematically in  FIG. 1 , a situation can occur in which for example an ultrasound sensor  18 , and in particular membrane  24  of the ultrasound sensor, is covered with snow or mud  22 , and as a result is limited in its functional capacity, or is even completely nonfunctional. Ultrasound sensor  18  and driver assistance system  14  are here, in the worst case, blind to obstacles  12 , which can lead to dangerous driving situations. 
         [0046]    In such situations, it is therefore essential to generate a reliable assessment of the functional capacity of ultrasound sensor  18 , so that driver assistance system  14  can react in a corresponding manner. 
         [0047]      FIG. 2  schematically shows a sample curve of sensor signal  30 , signal amplitude A measured by an ultrasound sensor  18 ,  19  being plotted over time t for a measurement cycle ΔT. 
         [0048]    Measurement cycle ΔT begins with transmit pulse  32  for a transmit pulse length ΔT 1 , and ends after decay interval ΔT 2 . Transmit pulse  32  excites at least one ultrasound sensor  18 ,  19  to send an ultrasound pulse for a specified time period ΔT 1 . During decay duration ΔT 2 , membrane  24  of ultrasound sensor  18 ,  19  post-oscillates for approximately 0.7 to 0.9 ms, and the reception of echo signals is possible only to a limited extent. After decay duration ΔT 2 , the ultrasound sensor is ready to receive for a receive interval (not shown), the receive interval being a function of the desired range, for example 5 m. In this time window, ultrasound sensor  18 ,  19  can receive echo signals reflected by objects  12 . 
         [0049]    Here it is to be noted that possible saturations in signal curves  30  shown here originate from the amplifier stage used to carry out the trials. However, amplifier stages have long been familiar to those skilled in the art, so that the amplifier stage can easily be adapted according to the application. 
         [0050]      FIG. 3  schematically shows a frequency response  34  of the sensor signal from  FIG. 2  for a measurement cycle ΔT. 
         [0051]    Frequency response  34  of the sensor signal from  FIG. 2  results from half the wavelength  36  of the sensor signal. For this purpose, for each interval  36  of a half wavelength the frequency is determined and is plotted against a center time point t. During transmit pulse length ΔT 1 , frequency response  34  is determined by the electronic control unit of ultrasound sensor  18 ,  19 . For example, a transmit pulse  38  can be sent out having a pulse length ΔT 1  of approximately 1 ms and having a frequency of approximately 48 kHz. 
         [0052]    Measurement cycle ΔT 1  ends after decay interval ΔT 2 , which results from a post-oscillation or dying out of membrane  24  of ultrasound sensor  18 ,  19 . In decay interval ΔT 2 , in the example shown here there are contained three subintervals I, II, III. First, in subinterval I there occurs a minimum  38  that is followed by a rising passage  40  in subinterval II. In subinterval III, minima and maxima  42  are newly formed. 
         [0053]    Frequency response  34  in subinterval II characterizes the inherent frequency of ultrasound sensor  18 ,  19 . In order to determine the inherent frequency, the frequencies in this subinterval II are averaged. In contrast, frequency response  34  in subintervals I and II characterizes mechanical and electronic properties of ultrasound sensor  18 ,  19 . 
         [0054]      FIGS. 4 and 6  schematically show further signal curves  30  of the sensor signal for a measurement cycle ΔT 1  for distance measurement. 
         [0055]    In comparison to signal curves  30  shown in  FIG. 2 , signal curves  30  of  FIGS. 4 and 6  originate from an ultrasound sensor  18 ,  19  whose membrane  24  was covered with various quantities of mud and ice  22  during the measurement. In the example shown in  FIG. 4 , 50 mg of mud  22  are found on membrane  24 . In the example of  FIG. 6 , 100 mg of mud  22  are found on membrane  24 . 
         [0056]    From the signal curves of  FIGS. 4 and 6 , decay time T 2  results from the length of decay interval ΔT 2 . Here, decay time T 2  of signal curve  30  from  FIG. 4  is greater than decay time T 2  of signal curves  30  from  FIGS. 2 and 6 . In addition, decay time T 2  of signal curve  30  of  FIG. 6  is smaller than decay time T 2  of signal curve  30  shown in  FIG. 2 . From this, it can be seen that decay time T 2  is not correlated with the degree of degradation of ultrasound sensor  18 ,  19 . Thus, decay time T 2  does not give any indication as to whether ultrasound sensor  18 ,  19  is partially or completely blind. Here, the method according to the present invention can provide help. 
         [0057]      FIGS. 5 and 7  schematically show the corresponding frequency response  34 , which results, as described above, from signal curves  30  of  FIGS. 4 and 6 . 
         [0058]    A comparison of frequency response  34  in the examples shown in  FIGS. 3 ,  5 , and  7  shows that frequency response  34  in decay interval ΔT 2  changes according to the degree of degradation. In particular in subinterval II, which is decisive for the inherent frequency of ultrasound sensor  18 ,  19 , the degree of degradation can be seen clearly. Thus, a determination of the inherent frequency by averaging the frequencies in subinterval II yields, in the example of  FIG. 3  with a fully functional ultrasound sensor  18 ,  19 , a higher inherent frequency than in the examples of  FIGS. 5 and 7 . In the example of  FIG. 5 , the average frequency in subinterval II is smaller than in the example of  FIG. 3 , and is larger than in the example of  FIG. 7 . Thus, ultrasound sensor  18 ,  19  is partially blind, and the degree of degradation is greater than in the case of the fully functional ultrasound sensor  18 ,  19  of  FIG. 3 . The example of  FIG. 7  shows a still greater degree of degradation, with an ultrasound sensor that is quasi-blind. 
         [0059]    In this way, the degree of degradation is determined from the inherent frequency in subinterval II of decay interval ΔT 2 . In order to indicate a concrete value for the degree of degradation of ultrasound sensor  18 ,  19 , the determined inherent frequency is compared to a stored target value that may correspond to the inherent frequency of fully functional ultrasound sensor  18 ,  19 , and the difference is formed. If the difference is &lt;1-2 kHz, the sensor is fully functional. If the difference is greater, the sensor is partially blind, and if the difference is greater than 5-8 kHz then the sensor is no longer functioning. 
         [0060]    In addition to the inherent frequency as a measure for the degree of degradation, the envelope  44  of frequency response  34  can also make it possible to infer the degree of degradation, as can be seen in  FIGS. 3 ,  5 , and  7 . Thus, envelope  44  of frequency response  34  of a fully functional ultrasound sensor  18 ,  19  differs from that of a blind or partially blind ultrasound sensor  18 ,  19 . In particular, extremum  34  in subinterval I shows a change from the minimum for fully functional ultrasound sensor  18 ,  19  to the maximum for the blind or partially blind ultrasound sensor  18 ,  19 . 
         [0061]    Envelope  44  of frequency response  34  in subintervals II, III also changes in particular with regard to the frequency level. In addition to the changes in envelope  44  of frequency response  34  in decay interval ΔT 2 , the changes in transmit interval ΔT 1  can also be used to determine the degree of degradation. The envelope of the frequency response can be determined for this purpose, and can be compared to a “normal” curve stored in the storage device of receiver device  28  of the sensor, i.e. can be compared to that of a fully functional distance sensor. The difference can be determined and characterized using standard methods, for example by determining the mean deviation. 
         [0062]    Due to the simplicity of the method, the method can also be realized as hardware circuitry in the receive device allocated to the distance sensor, or can be realized in software. 
         [0063]      FIG. 8  shows a manner of operation of the driver assistance system from  FIG. 1 , in the form of a flow diagram  100 . 
         [0064]    In a first step  102 , at least one ultrasound pulse having a determined pulse length is sent out. In a second step  104 , the sensor signal is acquired in decay interval ΔT 2  or in transmit interval ΔT 1  and decay interval ΔT 2 . In a third step  106 , the degree of degradation is determined on the basis of envelope  44  of the acquired sensor signal. The determination of the degree of degradation can take place as described above, on the basis of the inherent frequency or envelope  44 . 
         [0065]    In addition, it can be provided that echo signals are recognized in decay interval ΔT 2  based on a decay time, in order to avoid misclassifications of an ultrasound sensor  18 ,  19  in the context of the determination of the degree of degradation. For this purpose, transmit pulse length T 1  can be selected to be longer than decay time T 2 , or a change in the echo signal can be used as a basis, for example in the case of moving vehicles  10 . If in both variants the decay time remains equal, then ultrasound sensor  18 ,  19  is blind. 
         [0066]    The present invention is not limited to the exemplary embodiments described herein and the aspects thereof that are highlighted. Rather, within the scope indicated by the attached claims, a large number of modifications are possible that lie within the standard practice of those skilled in the art.