Patent Publication Number: US-2021190951-A1

Title: Method for operating a lidar system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of German patent application 10 2019 135 570.2 filed on Dec. 20, 2019. The entire disclosure of this earlier patent application is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for operating a LIDAR system. The LIDAR system can be used for determining distances both of moving objects and of stationary objects and, in particular, for determining the topography or shape of a spatially extended three-dimensional object. 
     Prior Art 
     For the purposes of measuring the distance of objects by optical means, a measurement principle also referred to as LIDAR is known in which an optical signal is emitted to the relevant object and evaluated after back-reflection has taken place at the object. In practice, use is made both of time-of-flight-based measurement systems (TOF-LIDAR measurement systems, TOF=time of flight), in which the time of flight of the laser light to the respective object and back is measured directly, and FMCW-LIDAR measurement systems using a frequency-modulated FMCW laser (FMCW=“frequency-modulated continuous wave”). 
     One problem that occurs in practice is that a LIDAR system is exposed to considerable environmental pollution depending on the use scenario. In the case of application in road traffic, for example, the LIDAR system can be protected as such against environmental pollution by a front shield that is sufficiently transparent to the operating wavelength (e.g. a headlight lens in the case of accommodation in the headlight). However, soiling or damage of said front shield itself as a result of dirt particles or stone chips or as a result of precipitation such as rain, snow or ice can occur with the consequence that a correct signal detection and a reliable determination of the distance of objects are no longer provided. If applicable, in phases in which no reflected measurement signal is detected by the LIDAR apparatus, it is not possible to distinguish whether this is attributable to the absence of objects to be measured with regard to their distance, or else to soiling of the front shield. 
     Corresponding remedial measures e.g. in the form of cleaning or defrosting processes are generally accompanied by an impairment of the actual functionality of the LIDAR system and therefore typically require an interruption of the operation of the LIDAR system. 
     Possible approaches for continuous monitoring—which is also desirable with regard to minimizing such interruptions of operation—of the degree of soiling of the abovementioned front shield, for instance, include the use of additional suitable sensors (e.g. in the form of rain sensors or camera-based sensors), but are associated with an increase in the equipment outlay. Moreover, there is the risk, in principle, of disturbances or defects occurring on such additional sensors themselves, with the consequence that e.g. a cleaning process will not be initiated in a timely manner. 
     SUMMARY OF THE INVENTION 
     Against the background above, it is an object of the present invention to provide a method for operating a LIDAR system which enable early identification and optionally elimination of soiling functional disturbances with lower equipment outlay. 
     In an aspect of the invention, this object is achieved by a method for operating a LIDAR system comprising at least one spectrally tunable light source that emits a light beam having a temporally varying frequency. A transparent protective shield, which is arranged in a light path of the light beam, protects the LIDAR system against environmental pollution. The method comprises the step of determining distance values of the object on the basis of beat frequencies of beat signals resulting from a superposition of partial signals. The latter are obtained from partial reflection of the light beam at the object with reference signals not reflected at the object. Each distance value is determined from a peak in a signal spectrum obtained on the basis of a Fourier transformation of the beat signal. A degree of soiling of the protective shield is diagnosed by analyzing the signal spectrum in a predefined analysis frequency range. An upper limit frequency bounding said analysis frequency range is based on a distance of the protective shield. 
     In accordance with an embodiment, the upper limit frequency bounding the analysis frequency range is not greater than 2 MHz, in particular not greater than 1 MHz. In this case, the upper limit frequency bounding the analysis frequency range can be chosen in particular depending on the geometry given in the respective application, in particular the smallest possible object distance dictated by the geometry. Furthermore, the analysis frequency range can be predefined e.g. by the distance of a (front or protective) shield that for instance protects the LIDAR system against environmental pollution and that is sufficiently transparent to the operating wavelength. 
     The present invention is based on the concept, in particular, in the case of distance determination in an FMCW-LIDAR system using a frequency-modulated FMCW laser (FMCW=“frequency-modulated continuous wave”), of carrying out an automatic modification of the operation of the LIDAR system (for instance by way of implementing or planning a cleaning or defrosting measure) depending on an analysis of the signal spectrum itself that is obtained on the basis of a Fourier transformation of the beat signal. 
     In this case, the invention proceeds from the consideration that the position and the distance e.g. of the abovementioned front shield or some other component present e.g. for protecting the LIDAR system are known, wherein in addition this distance is small in comparison with typical object distances to be detected. Particles such as e.g. dirt particles or precipitation present on said component or front shield can thus likewise be verified, in principle, as a peak in said signal spectrum. 
     Proceeding from this consideration, the invention then includes the concept, in particular, by way of analysis of the signal spectrum in a specific frequency range (typically with comparatively low frequencies of the order of magnitude of 1 MHz), of carrying out a diagnosis of the degree of soiling (or degree of icing, etc.) and, depending on this analysis, of automatically initiating corresponding remedial measures and/or an interruption of the operation of the LIDAR system. The invention here also makes use of the circumstance, in particular, that a plurality of distances or object distances can be determined simultaneously by means of an FMCW-LIDAR measurement system since each of these distances respectively corresponds to a dedicated peak in the signal spectrum obtained from a Fourier transformation of the beat signal. 
     Merely by way of example, for instance, the distance between a front or protective shield and the scanner of the LIDAR system may be of the order of magnitude of 10 cm. If it is further assumed that an object distance of 150 m corresponds to a beat frequency of 1 GHz, typical beat frequencies corresponding to the distance of the front shield or dirt particles situated thereon are of the order of magnitude of (1-2) MHz, such that the frequency range of the signal spectrum that is to be analyzed for the diagnosis of the degree of soiling (as analysis range) can be differentiated from the actual “search range” (i.e. the frequency range of the signal spectrum that is relevant to the actual measurement of the distance of objects) or can be separated therefrom during the evaluation. 
     By virtue of the fact that the signal spectrum that is to be determined anyway by Fourier transformation of the beat signal is used for the analysis according to the invention e.g. of the degree of soiling or the automatic initiation of suitable remedial measures, in particular the use of additional (soiling or icing) sensors can be dispensed with (and the costs associated with such use can be avoided). 
     However, the invention is not restricted to dispensing with the use of such sensors. In particular, the method according to the invention can also be realized in order to create additional redundancy in a LIDAR system already equipped with one or more sensors. 
     Furthermore, the invention is not restricted to the presence of a protective shield or to the use of such a protective shield for the method according to the invention. In this regard, it is also possible to utilize e.g. auxiliary surfaces (which can in turn be situated in proximity to a protective shield optionally present). Furthermore, in the context of the method according to the invention, it is also possible to utilize the effect of total internal reflection disturbed by surface wetting (e.g. on account of rain precipitation) given a suitable angle of incidence (wherein this effect can also be used to differentiate between rain precipitation and soiling). 
     In accordance with one embodiment, the step of analyzing comprises ascertaining the height of at least one peak in the predefined frequency range. 
     In accordance with one embodiment, the step of analyzing comprises ascertaining the total energy in the predefined frequency range of the signal spectrum. 
     In accordance with one embodiment, the step of analyzing comprises determining a temporal fluctuation in the predefined frequency range of the signal spectrum. 
     In accordance with one embodiment, the step of modifying operation of the LIDAR system comprises implementing or planning a cleaning or defrosting measure. 
     In accordance with one embodiment, the cleaning or defrosting measure is implemented on a shield that is transparent to the light beam emitted by the spectrally tunable light source. 
     In accordance with one embodiment, the cleaning or defrosting measure to be implemented is selected depending on the temporal fluctuation determined. 
     In accordance with one embodiment, the step of modifying operation of the LIDAR system comprises temporarily interrupting the operation of the light source and/or the distance determination. 
     The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings, in which 
         FIG. 1  shows a schematic illustration for explaining a possible basic sequence of a method according to the invention; 
         FIGS. 2 a -2 b    show diagrams for further elucidation of the method according to the invention; 
         FIG. 3  shows a schematic illustration for explaining a possible set-up with which the method according to the invention can be realized; and 
         FIGS. 4 a -4 b    show further schematic illustrations for explaining the set-up and manner of operation of a LIDAR system for distance determination in which the method according to the invention can be realized. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 4 a    firstly shows, merely in a schematic illustration, a basic set-up, known per se, in which a signal  411  with temporally varied frequency (also referred to as “chirp”), emitted by a spectrally tunable light source  410 , is split into two partial signals, this splitting being implemented, for example, by way of a beam splitter (e.g., a partly transmissive mirror or a fiber-optic splitter) that is not illustrated. The two partial signals are coupled by way of a signal coupler  445  and superposed at a detector  450 , with the first partial signal, as a reference signal  422 , reaching the signal coupler  445  and the detector  450  without a reflection at the object denoted by “ 440 ”. By contrast, the second partial signal arriving at the signal coupler  445  or at the detector  450 , as a measurement signal  421 , propagates to the object  440  via an optical circulator  420  and a scanner  430 , is reflected back by said object and consequently arrives at the signal coupler  445  and the detector  450  with a time delay and a correspondingly altered frequency in comparison with the reference signal  422 . 
     The detector signal supplied by the detector  450  is evaluated by means of an evaluation device  460 , wherein the difference frequency  431  between measurement signal  421  and reference signal  422 , said difference frequency being detected at a specific point in time and being illustrated in the diagram in  FIG. 4 b   , is characteristic of the distance between the object  440  and the measuring apparatus or the light source  410 . In accordance with  FIG. 4 b   , in this case, in order to obtain additional information with regard to the relative speed between the object  440  and the measuring apparatus or the light source  410 , the time-dependent frequency profile of the signal  411  emitted by the light source  410  can also be constituted such that there are two segments in which the time derivatives of the frequency generated by the light source  410  are opposite to one another. 
     If the light emanating from the light source  410  is split into a reference signal  422  and a measurement signal  421  which interfere at the detector  450 , a distance-dependent beat signal arises, the frequency of which can be determined from the chirp rate K. Given a distance d, the following holds true for the beat frequency: 
         f   beat ( d )=2*κ* d/c   (1)
 
     wherein κ denotes the chirp rate of the frequency tuning and c denotes the speed of light. 
     If the interference of a plurality of signals from different distances then occurs simultaneously at the detector  450 , a plurality of beat frequencies arise, which can be unambiguously ascertained by means of a Fourier transformation of the beat signal. 
     It is assumed hereinafter that the LIDAR system described above with reference to  FIG. 4 a    is protected against environmental influences by means of a front or protective shield or the like (e.g. a headlight lens in the case where the LIDAR system is accommodated in the housing of a vehicle headlight). 
     For monitoring and optionally eliminating contaminants such as weather-dictated precipitation, etc., situated on said protective shield, the invention makes use, then, of the circumstance that corresponding (dirt or precipitation) particles on the front or protective shield can also be regarded as objects, in principle, which for their part are verifiable in the form of a peak in the signal spectrum obtained on the basis of a Fourier transformation of the beat signal. 
     If it is furthermore taken into consideration that the corresponding peak caused by said particles occurs in a frequency range of the signal spectrum whose frequencies are significantly (e.g. by two to three orders of magnitude) lower than the frequencies corresponding to typical object distances to be determined in road traffic, according to the invention the desired soiling diagnosis can then be effected by way of an analysis of the signal spectrum in the relevant range of low frequencies (referred to hereinafter as “analysis range soiling”). 
       FIG. 1  shows a merely schematic illustration for elucidating the principle. In accordance with  FIG. 1 , the superposition signal generated from measurement signal and reference signal as described above with reference to  FIG. 4 a    passes firstly into a detector and amplifier unit, designated by “ 101 ”, and then into an analog-to-digital converter  102 . From the beat signal generated by said analog-to-digital converter  102 , a signal spectrum is calculated on the basis of a Fourier transformation in a manner known per se (block  103 ). 
     In said signal spectrum, then—as additionally illustrated in  FIG. 2 a    and  FIG. 2 b   —a “search range objects”  111  is differentiated from an “analysis range soiling”  121 . In this case, an upper limit frequency bounding the analysis frequency range is less than the maximum frequency that occurs in the signal spectrum evaluated by the evaluation device for determining distance values of the object. In embodiments, e.g. the upper limit frequency bounding the analysis frequency range can be chosen to be not greater than 2 MHz, in particular not greater than 1 MHz. In other words, the “analysis range soiling”  121  indicated in  FIG. 2 a    can be chosen merely by way of example such that it encompasses only frequencies up to a maximum frequency of 2 MHz, in particular up to a maximum frequency of 1 MHz. 
     Furthermore, the “analysis range soiling”  121 , just like the “search range objects”  111 , can be chosen in each case depending on the current beam direction, which makes it possible to take account of the circumstance that the outgoing measurement beam covers different distances to the front or protective shield depending on the beam or scanning direction. 
     The analysis of the signal spectrum within the “analysis range soiling” mentioned above can comprise, in particular, ascertaining the height of a peak detected in this frequency range. In this case, it is assumed that the peak height is proportional to the scattered light component and proportional to the degree of soiling.  FIG. 2 a    shows, in an exaggerated illustration not true to scale, exemplary scenarios of comparatively low soiling ( FIG. 2 a   ) and comparatively high soiling ( FIG. 2 b   ). As the degree of soiling increases, the peak that is to be assigned to the object to be measured with regard to its distance becomes smaller and the soiling-dictated peak occurring in the low-frequency “analysis range soiling” becomes larger. It is assumed here that the beam is comparatively large in relation to soiling that occurs (where a typical diameter of the beam can be 15 mm, for example) and the (front or protective) shield is arranged in proximity to the exit pupil. In this case, a higher degree of soiling results in a higher peak. 
     Alternatively or additionally, it is also possible to ascertain the total energy in the aforesaid frequency range of the signal spectrum (e.g. by integrating the squared signal level over the corresponding frequency range). 
     In further embodiments, additionally or alternatively, the temporal fluctuation of the peak intensity can also be evaluated. Since said temporal fluctuation of the peak intensity is significantly greater for instance in the case of rain or snow situated on the front or protective shield by comparison with the accumulation of dirt particles on the front or protective shield, by determining the temporal fluctuation of the peak intensity it is possible to differentiate between rain or snow, on the one hand, and soiling, on the other hand. 
     As soon as the degree of soiling determined as described above exceeds a specific measure, operation of the LIDAR system is automatically modified according to the invention. This can comprise the initiation of suitable remedial measures (e.g. cleaning with high-pressure water in order to eliminate dirt particles or heating in order to eliminate snow or ice) and/or a temporary shutdown or interruption of the operation of the LIDAR system. 
     If the (front or protective) shield is relatively far away from the exit pupil (e.g. at a distance of more than 100 mm given a diameter of the beam of 15 mm, for example), it is possible furthermore also to identify an angle range in which a sufficient signal is not to be expected or where measurement cannot reliably take place. The aforesaid angle range can be ascertained more accurately in the case of smaller beam diameters. 
     Depending on the degree of soiling, such remedial measures or interruptions of the operation of the LIDAR system can be effected immediately or as well (e.g. the next time the vehicle is stopped), optionally also preventively. 
     Said protective shield of the LIDAR system should preferably be designed in such a way that over the entire scanning range the measurement signal is not directly reflected into the detector or the receiver channel and only scattered light emanating from the dirt particles or the precipitation is detected. 
       FIG. 3  shows a merely schematic and greatly simplified illustration for explaining a possible set-up with which the method according to the invention can be realized. In this case, “ 300 ” denotes a LIDAR system, “ 320 ” denotes a (protective) shield that is transparent to light emitted by the light source of the LIDAR system  300 , and “ 325 ” denotes a mechanical mount of said shield  320 .  FIG. 3  likewise indicates particles (e.g. dirt particles or precipitation) situated on the shield  320 , said particles being designated by “ 330 ”, and various exemplary directions (each indicated by dashed arrows) of the scattered light emanating from said particles. 
     As indicated in  FIG. 3 , what can be achieved by means of a suitable geometry or an inclination of the shield  320  relative to the LIDAR system  300  is that only scattered light, but not the direct (specular) reflection of the measurement signal, enters the detector or the receiver channel of the LIDAR system. In further embodiments, however, a direct reflection occurring can also be accepted and optionally utilized for functional monitoring of the LIDAR system or the scanning process (since the direct reflection can only be observed during scanning operation, in contrast to a peak on account of scattered light). Furthermore, by means of a suitable coating e.g. of a front or protective shield, it is also possible to optimize the signal strength of the direct reflection with regard to the detector. 
     Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to the person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the appended patent claims and the equivalents thereof.