Patent Publication Number: US-2021173043-A1

Title: Method for identifying static radar targets using a radar sensor for motor vehicles

Description:
FIELD 
     The present invention relates to a method for identifying and classifying static radar targets with the aid of a radar sensor of a motor vehicle, including identifying an object as a static radar target based on the received radar signals reflected by the object. 
     BACKGROUND INFORMATION 
     In driver assistance systems for motor vehicles, for example, in systems for automatic adaptive cruise control or in collision warning systems, radar sensors are frequently used for detecting the traffic surroundings. In addition to dynamic objects, driver assistance systems must also reliably detect static objects as potentially collision-threatening obstacles. However, it is not possible in a single radar measurement to distinguish actual static obstacles from objects able to be driven over such as, for example, manhole covers, empty chip bags or cola cans. Until now, it has been common to observe and classify the radar signature of a static object over time for the purpose of classifying objects as actual static obstacles or as objects able to be driven over. The objects in this case are viewed as punctiform. 
     Recently, there is an increasing interest in radar sensors that are able to detect significantly more radar reflections for an object with a higher degree of accuracy. 
     SUMMARY 
     An object of the present invention is to provide a method for a radar sensor, which allows it to detect static obstacles more reliably. 
     This object may be achieved according to example embodiments of the present invention. 
     In accordance with an example embodiment of the present invention, for a static radar target, which is tracked over time and is classified on the basis of radar signatures of the radar reflections assigned to the radar target, an occupancy pattern is also entered into an occupancy grid, the assignment of the occupancy pattern to the radar target being stored. In the process, a radar target is assigned an occupancy pattern that corresponds to one or to multiple radar reflections assigned to the radar target. Thus, the occupancy pattern is linked to the classification of the radar target on the basis of radar signatures of the radar reflections. 
     The occupancy pattern may specify an occupancy probability for respective grid positions (grid cells). The occupancy grid is stored and updated over time. The radar reflections in the occupancy pattern may be accumulated over time. The expansion of the occupancy pattern allows for an estimation of the expansion of a static obstacle. If a radar target is classified as being able to be driven over, the occupancy pattern may be removed from the occupancy grid in accordance with a setting of the occupancy probability at a value that indicates “not occupied.” A similar approach may be taken with vehicles that have driven off. The method may supplement a conventional method for classifying point targets and for detecting actual obstacles. 
     In accordance with an example embodiment of the present invention, it is particularly advantageous that by assigning an occupancy pattern to an object, it is possible to subsequently carry out for occupancy probabilities entered in an occupancy grid a classification of the underlying object and to link the pieces of information about the occupancy probabilities in the occupancy grid and about the classification to one another, for example, in the course of an approach to the corresponding object. 
     A multitude of radar reflections assigned to the object may result in the course of an approach to an object, for example, on the order of 100 or more reflections. The assignment of an occupancy pattern to an object, even in the case of a multitude of radar reflections assigned to the object, may enable a simple evaluation of the occupancy pattern for estimating a position of the object and/or for estimating an object size. 
     The distinction between various groups of static radar targets may further enable radar targets adjacent to one another in the occupancy grid to be distinguished from one another. This enables a better mapping of the surroundings of the motor vehicle. One example is a possible distinction between a guardrail and a neighboring tree. A further example is the distinction between stationary vehicles and other static radar targets. 
     One particular advantage of the assignment of the occupancy pattern to the object is that with a later estimation of the object as being able to be driven over, it is possible to remove the occupancy pattern from the occupancy grid, since the occupancies in the occupancy grid belonging to the object are known due to the assignment. Thus, a safe removal of occupancy probabilities in the occupancy grid may be enabled. Thus, if in the course of an approach to an obstacle, the obstacle proves able to be driven over, the corresponding occupancy in the occupancy grid may be deleted again. One example is a railway track, for example, in a road, for example, a tramway rail or a railway track at a railway crossing. The railway track represents a static radar target with a wide expansion which, however, may be deleted in the occupancy grid after being classified as being able to be driven over. 
     Similarly, the occupancy pattern may also be deleted from the occupancy grid in the case of a radar target that has initially been recorded as a static radar target in the occupancy grid, and which is identified as a vehicle beginning to move. 
     In accordance with an example embodiment of the present invention, it is also advantageous that, based on the occupancy grid, an additional estimation of the expansion of objects is enabled, in particular, of objects that have conventionally been treated as point targets. This enables a reduction of the rate of the false-positive identifications of obstacles. 
     In the case of larger objects as well having a high number of radar reflections over time, an additional estimation of the expansion based on the occupancy pattern is advantageous, since the occupancy pattern may be much more easily evaluated than the parameters of a large number of radar reflections. 
     In accordance with an example embodiment of the present invention, it is also advantageous that the occupancy grid nevertheless enables a model-free representation of the static surroundings of the motor vehicle. As a result, the occupancy grid enables versatile applications, for example, for estimating guardrails or for searching for parking spaces. A modeling of the open space in the occupancy grid is also enabled. The modeling of the open space may also be updated by removing occupancy patterns of objects identified as being able to be driven over. The reliability of an occupancy grid may thus be improved. 
     An object may be identified as a static radar target, in particular, based on an estimation of an absolute velocity of zero, i.e., on a relative velocity that corresponds to the ego velocity of the motor vehicle with an opposite sign. 
     For example, the occupancy pattern assigned to a static radar target may be stored, for example, in the form of an index of the entries made in the occupancy grid that belong to the occupancy pattern. For example, an identification/identifier (ID) of the static radar target to which the occupancy pattern is assigned may be stored for each grid cell of the occupancy grid that belongs to an occupancy pattern of a static radar target. 
     An occupancy probability is preferably recorded in a respective grid cell of the occupancy grid. 
     The occupancy pattern may encompass one or multiple grid cells in the occupancy grid. 
     The received radar signals reflected by the object may be mapped in one or in multiple grid cells in the occupancy grid, the respective grid cell corresponding to a respective locating position of the received radar signals reflected by the object. 
     The occupancy grid preferably extends in at least two dimensions. The occupancy grid is preferably a two-dimensional grid, corresponding to an area extending horizontally in the surroundings of the motor vehicle. 
     For example, radar signatures of received radar signals may be compared with characteristic features and a static radar target may be classified based on the result of the comparison. 
     The characteristic features used for the classification of the static radar target may, for example, be: azimuth angle, elevation angle, radial distance, relative velocity and/or backscatter properties such as, for example, the radar cross section, as well as combinations of multiple of these features. 
     In tracking the static radar target over time, it is possible, for example, to update the classification of the static radar target. 
     Advantageous refinements and embodiments of the present invention are described herein. 
     Multiple radar reflections originating from different locating positions are combined to form a static radar target based on a spatial proximity of the locating positions, the generated occupancy pattern in the occupancy grid encompassing multiple grid cells, in which the multiple radar reflections are mapped. Thus, it is possible to record multiple radar reflections assigned to the radar target in respective grid cells of the occupancy pattern in the form of occupancy probabilities, the respective grid cells corresponding to respective locating positions of the radar reflections assigned to the radar target. 
     The present invention also includes a radar sensor for motor vehicles, in which one of the above described methods is implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment is explained in greater detail below with reference to the figures. 
         FIG. 1  shows a schematic diagram of a radar sensor, in which the present invention is applicable. 
         FIG. 2  schematically shows a representation of radar reflections, of an estimated point target and of an occupancy grid. 
         FIG. 3  shows a flowchart of a method for identifying and classifying static radar targets in accordance with an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The radar sensor shown in  FIG. 1  encompasses a sensor unit  10  including transceiver antennas connected to a control and evaluation unit  12 . The latter activates sensor unit  10  in respective measuring cycles for emitting radar signals and for receiving radar signals reflected by objects. The received radar signals are evaluated by control and evaluation unit  12  and parameters of individual radar reflections, i.e., received radar signals reflected on an object  14 , are estimated. The estimated parameters of the radar reflections as well as further data of the raw measurements, which may also be referred to collectively as radar signatures, are output to an object identification and tracking unit  16 . Object identification and tracking unit  16  combines the radar reflections based on a spatial proximity of locating positions of individual radar reflections at suitably closely coinciding relative velocities, and identifies them as a radar target. This is also referred to as forming clusters or clustering. The identified radar targets are tracked over time (object tracking). 
     In the event of a radar target identified as static, i.e., a radar target that corresponds to a stationary object, an object classification unit  18  connected to object identification and tracking unit  16  carries out in a first processing procedure a classification of the identified static radar targets based on characteristic features of their radar signatures. A classification may take place, for example, with the aid of a deep-learning approach, for example, with the aid of an artificial neural network. Parameters of the radar reflections used for the classification may be, for example, azimuth angle, elevation angle, radial distance, relative velocity and backscatter properties such as, for example, the radar cross section. Parameters based on multiple radar reflections may also be used for the classification such as, for example, number of radar reflections, arrangement of the reflections, patterns of the arrangement taking the respective backscatter properties of the radar reflections into account. A distinction may, for example, be made in the classification between: objects unable to be driven over (static obstacles) such as stationary vehicles, curbsides, guardrails, signs, traffic light poles, trees and, if necessary objects not further classified; as well as objects able to be driven over. 
     In a second processing procedure, the estimated parameters of the radar reflections of an identified static radar target are simultaneously entered into an occupancy grid  20  stored in a memory and accumulated over time. 
     This is illustrated, for example, in  FIG. 2 . Occupancy grid  20  maps the identified static surroundings of the motor vehicle.  FIG. 2( a )  shows locating positions of individual radar reflections.  FIG. 2( b )  shows a radar target  21  in the form of an estimated point target, which has been estimated based on the individual radar reflections.  FIG. 2( c )  illustrates how occupancy probabilities of corresponding grid cells  22  of occupancy grid  20  result from the locating positions of the individual radar reflections. The radar reflections are assigned corresponding grid cells, which are characterized as “probably occupied.” Various values of occupancy probabilities may be distinguished in the process. Different occupancy probabilities are represented by different markings of grid cells  22 .  FIG. 2( d )  shows obtained occupancy pattern  24  in occupancy grid  20 . Occupancy pattern  24  is thus generated by multiple radar reflections assigned to the radar target being recorded in the form of occupancy probabilities in respective grid cells  22  of occupancy pattern  24  corresponding to locating positions of the radar reflections. 
     An assignment between occupied grid cells  22  of occupancy pattern  24  and the relevant radar target is also stored. For example, the occupancy probability and an object identification (ID) that identifies the associated radar target may be stored in each relevant grid cell  22 . Alternatively or in addition, object identification and tracking unit  16  may store occupancy pattern  24  in the form of an index of grid cells  22 , which are assigned to the radar target. 
     Occupancy grid  20  is designed to store relevant occupancy patterns  24  for multiple detected static radar targets, the assignments in each case also being stored. 
     Object identification and tracking unit  16  is configured to estimate for a radar target the expansion of the radar target based on assigned occupancy pattern  24 . The expansion may, for example, be determined corresponding to occupied grid cells  22  of occupancy pattern  24 . The expansion thus estimated may be incorporated as an additional parameter of the radar target into the classification of the radar target by object classification unit  18 . 
     If, during the tracking of a static radar target, in particular, of a point target, the radar target proves not to be a relevant obstacle but is (re-)classified as being able to be driven over, then object identification and tracking unit  16  is configured to delete the assigned occupancy pattern  24  in occupancy grid  20  based on the assignment of occupancy pattern  24  to the static radar target. One example thereof is an object in the form of a manhole cover. Thus, occupancy grid  20  invariably maps only static radar targets classified as potential objects with no artefacts due to an eliminated classification of a static radar target as an obstacle remaining in occupancy grid  20 . 
     If during the tracking of a static radar target, the radar target proves to be set in motion, then object identification and tracking unit  16  is configured to delete assigned occupancy pattern  24  in occupancy grid  20  based on the assignment of occupancy pattern  24  to the radar target. Thus, occupancy grid  20  invariably maps only radar targets identified as static with no artefacts due to an eliminated classification of a radar target as a static radar target remaining in occupancy grid  20 . 
     If during the tracking of two or more static radar targets these prove to belong to one single object, then object identification and tracking unit  16  is configured to merge occupancy patterns  24  into one unified occupancy pattern  24  of a radar target based on the assignments of occupancy patterns  24  to the radar targets. 
     Object identification and tracking unit  16  including occupancy grid  20  and object classification unit  18  are, for example, part of a driver assistance system  30 . 
     The method shown in  FIG. 3  may be implemented in the described radar sensor and is described below. In a step S 10 , at least one object  14  is identified as a static radar target based on the received radar signals reflected by object  14 . In step S 12 , respective occupancy pattern  24  is generated in occupancy grid  20  based on the received radar signals reflected by respective object  14 . In step  14 , an assignment is stored, which assigns generated occupancy pattern  24  to the static radar target. In step S 16 , the above-described classification of the static radar target takes place. 
     The classification (step S 16 ) may take place in parallel to, prior to, or subsequently to the generation of the occupancy pattern (step S 14 ). 
     In step S 18 , it is checked whether a radar target has been classified as being able to be driven over. If yes, the occupancy pattern and the stored assignment are deleted in step S 22 . The method is then continued with step S 10  of identifying and tracking other radar targets. 
     If no, it is checked in step S 20  whether the radar target is still identified as a static radar target. If yes, the method is repeated, the radar target being further tracked in step S 10  and the occupancy pattern being updated in step S 12  via accumulation of the radar reflections assigned to the radar target. Otherwise, if, i.e., the radar target is identified as being in motion, the occupancy pattern and the stored assignment are deleted in step S 22 . The method is then repeated again with step S 10  of identifying and tracking other radar targets. 
     The described steps may be carried out in each case for all identified radar targets.