FIELD OF THE INVENTION  
       [0001]     The present invention relates to a device for detecting objects in the blind spot on a side of a vehicle, a detection range of which device extends into the rear area of the vehicle and to the respective vehicle side, and which device includes an output unit for outputting a warning signal which indicates objects in the blind spot.  
       BACKGROUND INFORMATION  
       [0002]     A frequent cause of accidents in street traffic is that the driver overlooks another vehicle located in the blind spot during a lane change or when cornering. To reduce this danger, warning systems have been developed, which are capable of detecting objects in the blind spot with the aid of a ranging sensor, identified here as the main sensor, and outputting a warning indication to the driver, for example, in the form of a visual display in the outside mirror for the affected vehicle side. If necessary, an acoustic warning signal may also be output if the intention of the driver to change lanes or corner is recognized based on the status of the turn signal and/or the steering movements.  
         [0003]     Such warning systems are also referred to as BSD (blind spot detection) systems. In such systems, a short-range radar sensor (SRR), such as a pulse radar, a lidar sensor, or an ultrasonic sensor, is frequently used as the sensor. Typically, these sensors execute only a distance measurement and do not have any angular resolution capabilities, so that only restricted information is available about the precise position and the movement state of the detected object. Incorrect warnings may thus occur easily, which lower the driver&#39;s trust in the system.  
         [0004]     A warning system of this type is described in published German patent document DE 101 25 426, in which system, in addition to the main sensor which is located on the vehicle side to be monitored at the rear vehicle corner, a second ranging sensor is provided on the front vehicle corner, so that through the combined detection ranges of both sensors, the entire area of the neighboring lane is monitored at the side of the host vehicle, as well as slightly in front and behind. Through an analysis of the time curve of the distance values periodically measured by the two sensors, it may then be ascertained whether the detected object is moving in the same direction as the host vehicle or in the opposite direction, or whether the detected object is a stationary object. In this way, incorrect warnings which are caused by oncoming traffic or by stationary objects such as traffic signs, guide rails, and the like, may be prevented.  
         [0005]     Incorrect warnings may, however, also be triggered by objects which move in the same direction as the host vehicle, for example, by a following vehicle which follows the host vehicle in the same lane and approaches so closely that it reaches the detection range of the rear sensor. In order to avoid such incorrect warnings, until now the main sensor has been configured and positioned in such a way that its detection range is directed diagonally to the rear on the neighboring lane, so that following vehicles are normally not detected. However, in curves or in cases in which the following vehicle drives somewhat offset to the host vehicle, the following vehicle may still reach the detection range and thus trigger an incorrect warning.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention allows the probability of incorrect warnings in detecting objects in the blind spot to be reduced without deploying angle-resolving sensors. For this purpose, the device according to the present invention is implemented in such a way that, in addition to the distance signal of the main sensor which monitors the blind spot on one side of the vehicle, it additionally analyzes the signal of a ranging auxiliary sensor, whose detection range extends angularly offset to the detection range of the main sensor into the rear area of the vehicle, so that it only detects following vehicles, but not objects which are actually located in the blind spot on the neighboring lane. If an object is detected by the main sensor, it may be recognized by analyzing the signal of the auxiliary sensor whether the detected object is a relevant object in the blind spot or merely a following vehicle which should not trigger a warning. If an object is only detected by the main sensor but not by the auxiliary sensor, it is a relevant object in the blind spot. However, if an object is also detected by the auxiliary sensor and a specific correlation exists between the distance data of the main sensor and the auxiliary sensor, for example, in that the distance data and/or its time derivatives (relative velocities) correspond within certain limits, it may be concluded that both sensors are detecting the same object and therefore this object is a following vehicle, which is not actually in the blind spot.  
         [0007]     The sensor which is referred to here as an “auxiliary sensor” may be a sensor which is already present in the vehicle, so that the present invention may be implemented without additional or more complex sensor components having to be installed on the vehicle.  
         [0008]     If the warning system is implemented for the purpose of monitoring the blind spot on both sides of the vehicle, the auxiliary sensor is the main sensor of the system in regard to one vehicle side, using which the other vehicle side is monitored. Therefore, to monitor the blind spot on both sides of the vehicle, two ranging sensors are still required in the rear of the vehicle, which function alternately as the main sensor and as the auxiliary sensor, depending on which vehicle side the analysis relates to. In this example embodiment, it is expedient to expand the angular detection ranges of both sensors in the direction of the longitudinal central axis of the vehicle, so that following vehicles may still be detected by both sensors even in the event of offset driving or in slight curves.  
         [0009]     According to another example embodiment, the auxiliary sensor is a sensor or a group of sensors which belong to another sensor system of the vehicle, such as ultrasonic sensors of an electronic parking aid, for example.  
         [0010]     If the following vehicle temporarily leaves the detection range of the auxiliary sensor, for example, when driving significantly offset or if the distance temporarily exceeds the detection depth of the ultrasonic sensors, the object, which is then still only detected by the main sensor, may also be qualified further as a following vehicle if the signal of the main sensor fulfills certain continuity and limiting conditions. These continuity and limiting conditions may be taken into consideration for the situations in which the previous following vehicle veers off to the neighboring lane and thus becomes a relevant obstruction in the blind spot or in which, in addition to the following vehicle, a neighboring lane object, such as a passing vehicle, drives into the blind spot. Both cases may be recognized in that the distance measured by the main sensor is reduced and/or falls below a specific threshold value. The threshold value may be selected in such a way that it is less than the smallest safety distance which a following vehicle would normally maintain, but is greater than the lateral distance between the main sensor and a vehicle driving in the neighboring lane.  
         [0011]     If the sensor system of the parking aid used as an auxiliary sensor has multiple ultrasonic sensors distributed over the rear of the vehicle, different modes of operation are possible for the auxiliary sensor system. For example, it may be expedient to only activate or analyze those sensors which are directed to the rear, approximately parallel to the vehicle longitudinal axis. It is also possible that only one of the sensors transmits an ultrasonic pulse, which is then received by multiple sensors. Vice versa, it is possible for multiple ultrasonic sensors to transmit a pulse simultaneously, which is only received by one single sensor (e.g., directed to the rear). This latter variation has the advantage that a higher overall sound pressure and therefore a greater position finding depth is achieved, which partially compensates for the generally shorter range of the ultrasonic sensors in comparison to radar sensors. In the analysis of the distance signals of the ultrasonic sensors, the different installation location of these sensors in comparison to the main sensor may be taken into consideration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a schematic block diagram of an example embodiment of a device according to the present invention for monitoring the blind spot on a vehicle side.  
         [0013]      FIG. 2  is a diagram illustrating an example traffic situtation in connection with the operation of the device shown in  FIG. 1 .  
         [0014]      FIG. 3  is a distance/time diagram for explaining the mode of operation of the device shown in  FIG. 1 .  
         [0015]      FIG. 4  shows a diagram illustrating another example traffic situtation in connection with the operation of the device shown in  FIG. 1 .  
         [0016]      FIG. 5  shows a diagram illustrating an example traffic situtation in connection with the operation of another example embodiment of a device according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  shows a block diagram of an example embodiment of a device according to the present invention for monitoing the blind spot on a side of a motor vehicle, such as the left vehicle side. A ranging main sensor  10 , such as an SRR pulse radar, provides distance data of the detected objects in the blind spot, e.g., in the rear area of the vehicle on the left neighboring lane, to an electronic analysis unit  12 . If at least one object is detected or if, in the event main sensor  10  has a greater range, the measured object distance or, in the case of multiple objects, the smallest of these distances is below a specific threshold value, it may generally be assumed that an object is located in the blind spot, and a warning signal is then output to the driver via an output unit  14  in the form of a visual display in the left outside mirror, for example.  
         [0018]     The distance data is also relayed by analysis unit  12  to a comparison unit  16 , and compared there to distance data of ranging auxiliary sensor  18 , which monitors the rear area of the vehicle, for example. If the distance data measured by the two sensors is consistent in such a way that it may be assigned to the same object, this allows the conclusion that the object is still in the detection range of main sensor  10 , but is actually not in the blind spot; instead, the object is located behind the host vehicle, i.e., the object is a following vehicle which follows the host vehicle in the same driving lane at a relatively short distance. Under these circumstances, the output of the warning signal via warning unit  14  is suppressed, which is indicated in  FIG. 1  in that comparison unit  16  activates a blocking element  20  inserted between analysis unit  12  and output unit  14 . In practice, of course, analysis unit  12 , comparison unit  16 , and blocking element  20  may be implemented as software in a single electronic data processing unit. Because the output of the warning signal is suppressed when a following vehicle is recognized, incorrect warnings may be effectively avoided.  
         [0019]      FIG. 2  shows a vehicle  22  which travels in middle lane  24  of a multi-lane roadway and is equipped with two devices of the type shown in  FIG. 1 , namely one for each vehicle side. Main sensor  10  for monitoring the blind spot on the left vehicle side is installed at the left rear corner in vehicle  22  and has a detection range  26  shaped approximately like a circular sector, which covers the blind spot on left neighboring lane  28  and also a part of middle lane  24 , i.e., the lane of vehicle  22 , in the rear area of vehicle  22 . Auxiliary sensor  18  is installed at the right rear corner of vehicle  22  and simultaneously functions as the main sensor of the device for monitoring the blind spot on the right vehicle side. Detection range  30  of auxiliary sensor  18  is therefore a mirror image to detection range  26  of main sensor  10 .  
         [0020]     In  FIG. 2 , a following vehicle  32  travels in middle lane  24  at a short distance behind vehicle  22 , but offset somewhat to the left in relation to vehicle  22 . Because of the geometry of detection ranges  26  and  30  selected here, following vehicle  32  is detected by both main sensor  10  and also auxiliary sensor  18 . Both sensors also measure approximately equal vehicle distances at approximately the same points in time. It may be recognized from these detected conditions that the detected object is not actually in the blind spot on the left vehicle side, but rather the object is following vehicle  32  in middle lane  24 . Accordingly, the output of a warning signal is suppressed in this situation.  
         [0021]     For comparison purposes, boundaries  26 ′ and  30 ′ of the detection ranges of SRR sensors are drawn as dashed lines in  FIG. 2 , as they would be used in conventional systems for monitoring the blind spot on both vehicle sides. These two detection ranges are concentrated more strongly on the particular neighboring lane and leave a gap between the two detection ranges in the area of the vehicle longitudinal axis, in which a following vehicle not offset to the host vehicle would be normally located, so that the sensors would be “blind” to the following vehicle. Since, however, in the example shown in  FIG. 2 , following vehicle  32  drives offset somewhat to the left, it would nonetheless be detected by main sensor  10  (of the conventional systems) for the left vehicle side, but the following vehicle would lie outside boundary  30 ′ for the detection range of the conventional sensor on the right vehicle side. Since in this case only one of the two sensors would generate a signal, it may not be ascertained whether the detected object is a following vehicle or a natural obstruction in the blind spot. Through the configuration of detection ranges  26  and  30  shown in  FIG. 2 , such a decision is made possible in accordance with the present invention, even in a situation in which following vehicle  32  maintains a relatively long distance to vehicle  22 .  
         [0022]     In  FIG. 3 , distances d periodically measured by main sensor  10  and auxiliary sensor  18  in consecutive measuring cycles are plotted against time t. Measuring points  10 ′ in  FIG. 3  represent the measurements of main sensor  10 , and measuring points  18 ′ represent the measurements of auxiliary sensor  18 . The two sensors may operate at equal cycle time, but do not necessarily have to be synchronized, so that measuring points  10 ′ and  18 ′ may be offset in relation to one another on the time axis, as is shown in  FIG. 3 .  
         [0023]     If the object detected by main sensor  10  is a following vehicle, at every measuring point  10 ′ in a time interval which has an absolute value smaller than a predefined time interval Δt, there must be a measuring point  18 ′ having a distance value which differs only slightly from that of measuring point  10 ′, i.e., the distance differential must have an absolute value smaller than a specific value Δd. Comparison unit  16  thus checks for each measuring point  10 ′ whether there is a measuring point  18 ′ within time interval ±Δt around this point, whose distance value differs in absolute value from the distance value of measuring point  10 ′ by less than Δd. In order to rule out accidental correspondences of the distance values, it is additionally required that the above-mentioned condition be fulfilled not only for a single measuring point pair, but rather for all measuring point pairs within a time interval having at least a specific length τ. Through suitable selection of the parameters Δt, Δd, and τ, a suitable criterion for recognizing a following vehicle may thus be established.  
         [0024]     In the example shown in  FIG. 3 , Δt is somewhat greater than half of the measuring cycle time of the sensors. Even if the cycle times of the two sensors are not exactly equal, it is therefore ensured that there is at least one measuring point  18 ′ for each measuring point  10 ′ within time interval ±Δt if the object is also detected by auxiliary sensor  18 . When comparison unit  16  has found a measuring point pair once for which the distances correspond up to Δd, it is checked continuously for the following measuring point pairs whether the correspondence continues to be fulfilled, and if this is the case for a number of measuring points corresponding to time interval τ (if necessary, isolated outliers may be left out), the detected object is qualified as following vehicle  32 , and blocking element  20  is activated.  
         [0025]     If necessary, it may additionally be checked whether the increases or decreases in the distance values measured by main sensor  10  and auxiliary sensor  18  (i.e., the relative velocities of the positioned objects) correspond within specific tolerance limits.  
         [0026]     Sensors  10 ,  18  and the associated analysis electronics may be designed in such a way that the sensors are capable of detecting multiple objects simultaneously if these objects cause a sufficiently clear radar echo and their distances differ sufficiently from one another. For example, if main sensor  10  detects two objects simultaneously, two measuring points  10 ′ having different distances are obtained in each measuring cycle. If the second object is also detected by auxiliary sensor  18 , for example, if it originates from a roof structure of following vehicle  32 , two sequences of measuring point pairs  10 ′,  18 ′ are thus obtained, and the above-mentioned conditions must be fulfilled for each of these sequences so that blocking element  20  is activated. In this way, the case in which, in the situation shown in  FIG. 2 , a passing vehicle approaches on left neighboring lane  28 , which is then detected by main sensor  10  but not by auxiliary sensor  18 , may also be managed, for example.  
         [0027]     However, if the sensor system is configured in such a way that it outputs either no measured distance or only the smallest measured distance in each measuring cycle, such a passing vehicle may not be detected until it has passed following vehicle  32 , so that its distance becomes less than that of following vehicle  32 .  
         [0028]     The checks may also be continued by comparison unit  16  when the detected object has been classified as a following vehicle after expiration of time interval τ. For example, if previous following vehicle  32  begins passing and veers off onto left neighboring lane  28 , it will leave detection range  30  of auxiliary sensor  18 , with the result that measuring points  18 ′ are not registered. Simultaneously, the distances measured by main sensor  10  will decrease. In this case, blocking element  20  is deactivated again and a warning signal is output by output unit  14 .  
         [0029]     Blocking element  20  would also be deactivated if measuring points  18 ′ are still present, but no longer fulfill the required distance relationship. This case may occur, for example, if another vehicle travels on the right neighboring lane  34 , which is approximately at the same level as following vehicle  32  and has the same velocity. Auxiliary sensor  18  may then not make a distinction between following vehicle  32  and this vehicle on right neighboring lane  34 . However, if following vehicle  32  begins to pass and reduces its distance to vehicle  22 , the distance differential between measuring points  10 ′ originating from following vehicle  32  and measuring points  18 ′ originating from the vehicle in right neighboring lane  34  increases accordingly.  
         [0030]      FIG. 4  illustrates an example of a situation in which following vehicle  32  temporarily leaves detection range  30  of auxiliary sensor  18 . This may occur, for example, if following vehicle  32  travels even further offset to the left to vehicle  22  or if, as in  FIG. 4 , vehicles  22 ,  32  enter a left curve, so that detection ranges  26 ,  30  are pivoted accordingly. In this case, however, in contrast to the situation described above, in which following vehicle  32  begins to pass, following vehicle  32  maintains its movement state essentially unchanged, i.e., the distances represented by measuring points  10 ′ remain essentially unchanged or become gradually larger, if following vehicle  32  falls back slightly. In this situation, measuring points  10 ′, which were assigned to following vehicle  32  on the basis of prior measurements, may still be identified with the following vehicle, and blocking element  20  remains active, so that an incorrect warning is suppressed.  
         [0031]     However, in this situation as well, blocking element  20  is deactivated again and a warning signal is output if following vehicle  32  accelerates (the distances represented by measuring points  10 ′ decrease) and/or if the distances measured by main sensor  10  fall below a specific minimum distance, which is symbolized in  FIG. 4  by a circle having radius R. This minimum distance R is selected in such a way that it is less than the safety distance which would be maintained by a following vehicle  32  in any case, so that falling below this minimum distance indicates that the object is located on neighboring lane  28 .  
         [0032]      FIG. 5  illustrates a modified exemplary embodiment in which the warning device of vehicle  22  only has main sensor  10  on the left vehicle side, while auxiliary sensor  18  is formed by a group of ultrasonic sensors  18   a ,  18   b , which are simultaneously part of a parking aid for vehicle  22 . Ultrasonic sensors  18   a ,  18   b  are installed in the rear bumper of vehicle  22 , for example, and have detection ranges  30   a  directed to the rear. In this case, blocking element  20  is activated when following vehicle  32  is detected by at least one of ultrasonic sensors  18   a ,  18   b , and the distance measured by this ultrasonic sensor or averaged over all ultrasonic sensors fulfills the criterion illustrated in  FIG. 3 . In this embodiment, auxiliary sensor  18  does not have to be continuously active, but rather it suffices to activate the ultrasonic sensors when a detection signal  10  is received by the main sensor.  
         [0033]     Since ultrasonic sensors generally have a shorter range than an SRR radar sensor, it is expedient to bundle the ultrasonic signals transmitted by all ultrasonic sensors  18   a ,  18   b , i.e., emit them synchronously, so that a greater signal strength and thus a greater detection are achieved. In this case, only one of the ultrasonic sensors has to be used to receive the reflected signal, for example, ultrasonic sensor  18   b  positioned in the middle of the vehicle. Since the installation position of ultrasonic sensors  18   a ,  18   b  generally differs from that of main sensor  10 , the distance data measured by the ultrasonic sensors possibly has to be corrected before it may be processed by comparison unit  16 .  
         [0034]     In a cornering situation, as is illustrated in  FIG. 4 , a distance correction may also be provided in both example embodiments, which takes the tilting of vehicle  22  in relation to following vehicle  32  into consideration. This tilting may be derived from the roadway curvature and the measured distance, and the roadway curvature may in turn be determined on the basis of the steering angle, the signal of a yaw rate sensor, or the like.
A device for detecting objects in the blind spot on a side of vehicle includes a ranging main sensor, an auxiliary sensor, a comparison unit, and an output unit for outputting a warning signal which indicates objects in the blind spot. The detection range of the main sensor extends in the rear area of the vehicle and toward the respective vehicle side. The auxiliary sensor has a detection range which extends angularly offset to the detection range of the main sensor in the rear area of the vehicle. The comparison unit ascertains whether the detected object is a following vehicle on the basis of predefined correlations between the detection signals of the main sensor and the auxiliary sensor, and the detection unit blocks the output unit from outputting a warning signal when a following vehicle is recognized.