Abstract:
A device and a method are for monitoring misalignment of a distance sensor on a vehicle which represents a combination of two individual procedures. The two individual procedures are selected in such a way that one procedure has advantages in areas in which the other procedure functions disadvantageously, so that the weaknesses of one procedure may be compensated for by the strengths of the other procedure. Furthermore, with the aid of this combination, it may be decided with far greater certainty whether a misalignment is present which may be removed using suitable correcting measures, or whether an extreme misalignment is present, based on which the system must be switched off.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for detecting and/or correcting misalignment of a distance sensor on a vehicle. 
     BACKGROUND INFORMATION 
     Some methods and devices for recognizing misalignment have been known to have the function of being able to self-correct their sensor viewing zone. 
     German Published Patent Application No. 197 46 524 describes a compensation device for compensating for the installation tolerances of a distance sensor in a vehicle. This is done by having the evaluation electronics registering the object distances and the object angles of the detected objects. These data are averaged over a specifiably long time, and-the thus ascertained object angle average is accepted as the new nominal directional angle of the preceding vehicle. Furthermore, a correcting angle is calculated from the difference of the nominal directional angle and the actual directional angle. The registered object angles are corrected with this difference angle. 
     European Published Patent Application No. 0 782 008 describes a device for calculating and correcting the deviation of the center axis of an obstacle recognition device on a vehicle and equipment for proximity-controlled cruise control based on a preceding vehicle. This system recognizes standing objects, and from the positional displacement of the standing object relative to the sensor in time, it calculates whether the standing object has a relative speed component which is orthogonal to the axis of symmetry of the sensor&#39;s viewing zone, also called the optical axis. In the case of an accurately adjusted sensor, a mean value generation in time of this lateral relative speed gives a value tending to zero. In the case of a sensor&#39;s viewing zone that is out of alignment, a value not equal to zero is obtained by the mean value generation in time, which, by its magnitude permits drawing a conclusion on the misalignment angle of the sensor. Using this method, a misalignment of the sensor can be recognized, and the sensor&#39;s misalignment can be corrected. 
     Both methods have a device and a method, respectively, for recognizing misalignment, and both methods correct, upon recognizing misalignment, in such a way that a correcting angle ascertained from the measured values is added to the measured object angle. The axis of symmetry of the sensor is tilted by calculation in such a way that it approximately coincides with the central axis of the vehicle. 
     SUMMARY 
     It is common to conventional methods and devices for recognizing misalignment of distance sensors that in specified travel situations they yield good results, and in other travel situations they yield results the errors of which cannot be neglected. Thus, advantageous and disadvantageous travel states exist for each system. 
     It is an object of the present invention to let two or more differently defined individual procedures for misalignment recognition proceed simultaneously in combination and monitor the travel state or operate a device which makes use of two or more individual procedures. These two individual procedures are developed in such a way that at least one procedure yields reliable values in each travel state. Thereby, the weakness of one procedure, namely that it yields unreliable values in this travel state, is compensated for by the strength of another method, since the latter yields reliable values in this travel state. For the evaluation of the results of the individual procedures, a quality factor is developed from the currently present travel state for each individual procedure, which are used for the weighting of the results of the individual procedures. A linked misalignment value may be determined, depending on the weighted results of the individual procedures, as well as the results of the value linkage, which is corrected as a function of these values, or as a result of which the system is switched off for safety reasons. Defects in the sensor hardware become apparent from error images which may be represented by special misalignment vectors. Such a misalignment vector is made up of a linear combination of the misalignment values of the individual procedures. By monitoring the up-to-date-misalignment vectors, some hardware functions of the sensor important to the operation may thus be monitored. The present invention is suitable for horizontal and also for vertical misalignment recognition and/or misalignment correction. In case a vertical misalignment recognition and/or correction is to be performed, the sensor may also be in a position to measure the elevation angle of reflecting objects. 
     If a travel situation occurs in which an individual procedure used yields unreliable measured values, the results of this procedure are weighted more lightly at this point in time than another individual procedure which is expected to yield more reliable values in this travel situation, with the aid of a quality factor. By such a combination of individual procedures, it is possible to compensate for the weak points of one procedure by the strength of another procedure. It is also possible to predict with greater probability, from a measured sensor misalignment, that there is misalignment. If both procedures determine that there is misalignment, but in different angular directions, this may be caused by too large single procedure errors. If misalignment values appear in the same angular direction and at approximately equal angles from the vehicle&#39;s center axis, one may assume with greater probability than when only one individual procedure is used, that the sensor&#39;s axis has been shifted. In this case, one may make a reliable correction or switch off the system at smaller misalignment values than is possible using a individual procedure. In accordance with the present invention, operation of a proximity-controlled vehicle under cruise control is much safer than operating a vehicle having a sensor which is monitored by only one individual procedure for misalignment. Furthermore, the error due to disadvantageous surroundings is held much smaller than with an individual procedure, since at the time of measuring, the more reliable procedure is treated as more dominant because of the lower weighting of the unreliable result. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of two vehicles traveling one behind the other in the same lane, the following vehicle being provided with a device according to the present invention. 
     FIG. 2 is a diagram illustrating the shutoff range of the system as a function of the two orthogonal individual procedures. 
     FIG. 3 is a block diagram of an example embodiment according to the present invention. 
     FIG. 4 is a block diagram of a second example embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a lane  1 , on which two vehicles  2  and  3  are traveling one behind the other, such that vehicle  3  follows vehicle  2 . Vehicle  3  is equipped with a sensor  4  for speed control and proximity-controlled cruise control, which includes the present invention. Ray  8  represents the center axis of vehicle  3 , which, in the case of an accurately adjusted sensor, is identical with the axis of symmetry of the sensor&#39;s viewing zone  7 . This is also the principal beam direction of the sensor&#39;s radiation characteristic. Rays  6  and  10  represent the right and left edges, respectively, of the sector-shaped sensor viewing zone, ray  8  being exactly the bisector of the two rays  6  and  10 . The lines marking an accurately adjusted sensor region ( 6 ,  8 ,  10 ) are indicated in FIG. 1 by solid lines. In the case of a horizontally misaligned sensor, i.e., the misalignment of the radiation characteristic was ascertained as being not equal to zero, the sensor&#39;s viewing zone is illustrated by broken lines  5 ,  7  and  9 . These rays differ from rays  6 ,  8  and  10  only in that they are rotated by the horizontal misalignment angle theta, as illustrated in FIG.  1 . In this regard, ray  5  represents the right edge of the sensor&#39;s viewing zone and ray  9  represents the left edge of the sensor&#39;s viewing zone. Ray  7  is the bisector between rays  5  and  9 , and thus the center axis of the sector-shaped viewing zone of the sensor. Angle theta  11  indicates the sensor misalignment rotation. This angle may be measured between the motor vehicle&#39;s center axis and the axis of symmetry of the sensor&#39;s viewing zone. Inside the viewing zone of the sensor, an electromagnetic wave is emitted, e.g., a radar or LIDAR signal. Objects present in this viewing zone of the sensor scatter back a reflected wave which is detected at the sensor with the corresponding propagation delay. The angle at which a reflected signal is radiated and received is designated as the object angle, and is processed further as object angle value. All objects within the sensor&#39;s viewing zone are known by object distance and object angle. From these objects, a target object is selected. The object being selected may closest in distance and closest to the vicinity of the longitudinal axis of the vehicle. 
     FIG. 2 is a two-dimensional diagram illustrating the switch-off range as well as the functional range. The two coordinate axes  12  and  13  form an orthogonal system. The instantaneous misalignment value of one of the two individual procedures is plotted on each axis, whereby the misalignment value combination of the instantaneous individual misalignment values may be represented by a point in two-dimensional error space. In this example embodiment, the normalized error of the long-term filtering of the target object&#39;s course displacement is plotted on coordinate axis  12 . The normalization occurs in such a way that the maximum tolerable error d_alpha_obj_max of this method is marked by point  14 . Analogously, the normalized misalignment value of the regression analysis of trajectories is plotted on coordinate axis  13 . Normalization occurs in the same manner, so that the maximum tolerable misalignment value of this method, d_alpha_traj_max is marked by point  15 . If both methods for misalignment recognition are evaluated separately from each other, the evaluations yield a rectangle the center of which corresponds to the coordinate&#39;s origin. If the instantaneous misalignment point is inside this rectangle, this indicates that both single errors simultaneously are below each of their limiting values. One may then assume that the misalignment values arise through non-optimal travel situations, and the sensor&#39;s viewing zone may be readjusted. One may combine both methods with each other. If both methods indicate errors having the same sign, this indicates that the error points are in quadrants I or III, and if possibly they also lie near straight line  16 , then both methods detect approximately the same misalignment value, and, with greater probability than with the use of one individual procedure, one may assume actual determination of the radiation characteristic compared to the vehicle&#39;s longitudinal axis. Because of this, in these areas, that is, in quadrants I and III, one may limit the functional region by removing a part of the functional region. This region, which now belongs to the switching off region, is thus called “broadened switching off region”. 
     It should be noted that the borders between functional region and switching off region  17 , as well as between functional region and broadened switching off region  18 , as well as between switching off region and broadened switching off region  19  are illustrated in all four quadrants in FIG. 2 as straight lines, for simplicity, but in practice they may be shaped as any desired curves. 
     One may introduce further “broadened switching off regions”, so as to be able to model the border of the functional region or the functional regions, as the case may be, as desired. 
     The functional manner of this combination method is illustrated in FIG.  3 . The two single procedures “long-term filtering of the target object&#39;s travel-path misalignment”  20  as well as “regression analysis of trajectories”  21  calculate in each case an instantaneous misalignment value d_alpha_obj or d_alpha_traj. These two values are passed on to function blocks  24 ,  25  and  26 , as in FIG.  3 . 
     The travel situation is simultaneously ascertained from measured travel dynamics of other systems and/or additional vehicle data. In this connection, it is determined whether the vehicle is traveling straight ahead or along a curve, whether it is going upwardly or downwardly, or whether further conditions impairing the measuring procedures are fulfilled, e.g., in that a yaw rate signal, a pitch signal or additional signals describing travel dynamics are used. From the ascertained travel situation, a quality factor is calculated for each procedure in function block  22 . The quality factor for the long-term filtering of the target object travel path displacement is passed on as q_obj, and the quality factor for the regression analysis of trajectories is passed on as q_traj. These quality factors are passed on to blocks  24 ,  25  and  26  in such a way that block  25  receives both quality factors q_obj and q_traj, block  24  only receives q_obj and block  26  only receives q_traj. In block  25 , the broadened switching off region is now formed using the function 
     
       
           F   3 ( d _alpha_obj,  q _obj,  d _alpha_traj,  q _traj)&gt; . . .  K   1 ( d _alpha_obj_max, _alpha_traj_max) 
       
     
     the two misalignment values d_alpha_obj and d_alpha_traj being weighted with the aid of quality factors q_obj and q_traj. If this equation is satisfied, then a greater error is present than is permissible, and a switching off request is passed on to block  27 . 
     In block  24 , using single misalignment value d_alpha_obj and the respective quality factor q_obj, a test is made whether 
     
       
           F   1 ( d _alpha_obj,  q _obj)&gt; q _alpha_obj_max 
       
     
     is satisfied. If yes, the error is greater than is permissible, and the switching off request is passed on to block  27 . In block  26 , using single misalignment value d_alpha_traj and the respective quality factor q_traj, a test is made whether the condition 
     
       
           F   2 ( d _alpha_traj,  q _traj)&gt; q _alpha_traj_max 
       
     
     is satisfied. If yes, the error is greater than is permissible, and the switching off request is passed on to block  27 . If block  27  receives at least one switching off request from one of blocks  24 ,  25  or  26 , it is passed on to block  29  that the proximity-control system and the cruise control may be switched off. 
     In FIG. 4 a further example embodiment of the present invention is illustrated. This example embodiment includes all the parts described in FIG. 3, but with additional supplementations. Block  24  has been added. Block  23  receives the two single misalignment values d_alpha_obj and d_alpha_traj as well as the pertaining quality factors q_obj and q_traj. In block  23 , a linked misalignment value d_alpha_comb is formed from these values, using the single misalignment values. The value thus created, d_alpha_comb is then passed on to likewise newly added block  28  where correction of the main radiation direction of the radiation characteristic is performed. If function block  28  is informed of a request to switch off, it causes deactivation of the correction and also deactivates the entire proximity-control system and cruise control system. 
     The values d_alpha_obj_max as well as d_alpha_traj_max may be constant values, but they may also be functions as illustrated in FIG. 2 as arbitrary straight lines, or they may look like arbitrarily shaped curves. 
     A plurality of variants are possible during system deactivation. Thus switching off the vehicle control may be kept up only so long as misalignment values are in a correctable region, that is, that block  27  receives at least one switching off request, or it may be switched off until the vehicle is started the next time and a negative self-diagnosis has been performed, or again, deactivation is kept up until this error message, which is stored in a nonvolatile memory, is reset in a garage. 
     The correction of the sensor&#39;s viewing zone may also be made in a different manner. One possibility is to add the determined linked misalignment angle value to all measured angle values, so that the new sensor viewing zone is tilted by calculation into the correct position. Another possibility provides for displacing the edge of the sensor viewing zone, which is on the side in the direction of which the axis of symmetry of the viewing zone has been displaced toward the center, until the axis of symmetry of the sensor&#39;s viewing zone is identical to the vehicle&#39;s center axis. This may have the disadvantage that the sensor&#39;s viewing zone becomes less at each correction, and after some operating period no longer exists. 
     In addition to monitoring the sensor adjustment, one may also monitor the sensor hardware. With certain combinations of the single misalignment values (d_alpha_obj; d_alpha_traj) one may conclude, on account of experiences gathered, that there are special defects in the sensor&#39;s hardware. If these combinations arise, the regulating system must be switched off due to possible hardware defects. 
     By linking a plurality of procedures, it is possible on the one hand to determine with greater probability that the ascertained correction value corresponds to the actual sensor malposition than when using a single procedure, whereby a robust monitoring procedure is ensured, and furthermore it is also possible to monitor parts of the sensor hardware for their functioning.