Patent Publication Number: US-2016223649-A1

Title: Systems and methods for radar vertical misalignment detection

Description:
BACKGROUND 
     Embodiments of the present invention relate to systems and methods for detecting a vertical misalignment of a radar sensor mounted on a vehicle. 
     SUMMARY 
     It is difficult to detect vertical misalignment of an automotive radar sensor. One reason for this is that the antennas of a set of radar sensors are in one line to give horizontal angular information. This antenna configuration provides no explicit information on reflections in the vertical direction. Adding extra antennas out of line with the other antennas or providing mechanically-operating scanning sensors with tilting mirrors increases the cost of a radar sensor. 
     Accordingly, embodiments of the present invention use only a horizontal radar beam to detect a vertical misalignment of a radar sensor. 
     In one embodiment, the invention provides a system for detecting vertical misalignment of a radar sensor mounted on a vehicle. The system includes a controller. The controller is configured to receive, from the radar sensor, a plurality of reflected radar signals from a target as one of the vehicle and the target moves and determine a plurality of data points, each of the plurality of data points corresponding to one of the plurality of reflected radar signals. The controller is also configured to determine a curve based on the plurality of data points, and determine a vertical alignment angle of the radar sensor by matching the curve to one of a plurality of pre-recorded curves and setting the vertical alignment angle of the radar sensor to an angle associated with the one of the plurality of pre-recorded curves. The controller can also be configured to compare the determined vertical alignment angle of the radar sensor to an operation range for the radar sensor and take corrective action if the angle is outside of the operation range. 
     In another embodiment, the invention provides a method for detecting vertical misalignment of a radar sensor mounted on a vehicle. The method includes receiving, from the radar sensor, a plurality of reflected radar signals from a target as one of the vehicle and the target moves and determining a plurality of data points, each of the plurality of data points corresponding to one of the plurality of reflected radar signals. The method also includes determining a curve based on the plurality of data points and determining a vertical alignment angle of the radar sensor by matching the curve to one of a plurality of pre-recorded curves and setting the vertical alignment angle of the radar sensor to an angle associated with the one of the plurality of pre-recorded curves. In addition, the method includes comparing the determined vertical alignment angle of the radar sensor to an operation range for the radar sensor, and, when the determined vertical alignment angle of the radar sensor is outside of the operation range, taking a corrective action to address misalignment of the radar sensor. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 a - d    schematically illustrate a vehicle approaching and passing a target. 
         FIG. 2  schematically illustrates a controller included in the vehicle of  FIG. 1 . 
         FIG. 3  is a flow chart illustrating a method performed by the controller of  FIG. 2  to determine a vertical alignment of a radar sensor. 
         FIG. 4  is a chart illustrating received power of a reflected radar signal associated with a vertically aligned sensor. 
         FIG. 5  is a chart illustrating received power of a reflected radar signal associated with a vertically misaligned sensor. 
         FIG. 6  is a chart illustrating a plurality of pre-recorded curves representing different alignment angles. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     It is also to be understood that while the term “sensor” implies a device that senses signals, as used herein, the term “sensor” includes devices capable of both transmitting and receiving or detecting signals, such as radar signals. 
     As noted above, embodiments of the invention determine a vertical alignment of a radar sensor. As described in more detail below, embodiments of the invention can use characteristics of reflected radar signals from a target to determine a vertical alignment of a radar sensor. 
       FIGS. 1 a - d    illustrate a vehicle  10  including a radar sensor  12  and a controller  14 . As illustrated in  FIGS. 1 a - d   , as the vehicle  10  moves, the vehicle  10  approaches a target  16 . The target  16  represents any object that can be detected by the radar sensor  12 . In some configurations, the radar sensor  12  is mounted on a front portion of the vehicle  10  and is configured to transmit radar signals  18 . The transmitted radar signals  18  reflect from the target  16  and are received by the radar sensor  12  as reflected radar signals  20 . 
     As illustrated in  FIG. 2 , the controller  14  includes a processing unit  22  (e.g., a microprocessor, application specific integrated circuit, etc.), non-transitory computer-readable media  23 , and an input/output interface  24 . The computer-readable media  23  can include random access memory (“RAM”) and/or read-only memory (“ROM”). The input/output interface  24  transmits and receives information from devices external to the controller  14 , such as the radar sensor  12  (e.g., over one or more wired and/or wireless connections). The controller  14  can also use the input/output interface  24  to communicate with other controllers included in the vehicle  10 , such as console or dashboard controller that provides information to a driver of the vehicle  10 . 
     The processing unit  22  receives information (e.g., from the media  23  and/or the input/output interface  24 ) and processes the information by executing one or more instructions or modules. The instructions are stored in the computer-readable media  23 . The processing unit  22  also stores information (e.g., information received through the input/output interface  24  and/or information generated by instructions or modules executed by the processing unit  22 ) to the media  23 . It should be understood that although only a single processing unit, input/output interface, and computer-readable media module are illustrated in  FIG. 2 , the controller  14  can include multiple processing units, memory modules, and/or input/output interfaces. 
     The instructions stored in the computer-readable media  23  provide particular functionality when executed by the processing unit  22 . In general, the instructions use reflected radar signals from the target  16  to determine a vertical alignment of a radar sensor  12 . For example,  FIG. 3  illustrates a method  30  performed by the controller  14  to determine a vertical alignment for the radar sensor  12 . 
     As illustrated in  FIG. 3 , the radar sensor  12  receives reflected radar signals from the target  16 , which are provided to the controller  14  (at block  32 ). For example, returning to  FIGS. 1 a - c   , as the vehicle  10  approaches the target  16 , the radar sensor  12  emits a radar signal  18  and receives a reflected radar signal  20 . As the vehicle  10  passes the target  16  (see  FIG. 1 d   ), the radar signal  18  emitted by the radar sensor  12  no longer reaches the target  16  and, therefore, no corresponding reflected signal  20  is received by the radar sensor  12 . In some embodiments, as the vehicle  10  approaches and drives past the target  16 , a first reflected radar signal  20  associated with the target  16  has a horizontal angle of approximately zero degrees (this may vary in curves or for non-stationary objects) and the horizontal angle of reflected radar signals  20  associated with the target  16  increases until the target  16  is out of the field-of-view of the radar sensor  12  when the vehicle  10  passes target  16 . 
     For each reflected radar signal  20 , the controller  14  stores data regarding the signal  20  (e.g., to the computer-readable media  23 ) (at block  35 ), which the controller  14  uses to determine one or more data points for the signal  20  (at block  36 ). For example, in some embodiments, the controller  14  stores a compensated received power level and a horizontal angle for each reflected radar signal  20 . The compensated received power is the power that an antenna included in a radar sensor  12  receives for a reflected signal  20  with the distance-dependency and the effect of the antenna gain removed, which allows the reflected radar signals  20  to appear as if the target  16  is consistently positioned in directly in front of the vehicle  10  (e.g., at a zero degree horizon). 
     For example, the compensated received power (P r,comp ) can be measured in dB and can be calculated using the following equation: 
     
       
         
           
             
               P 
               r 
             
             = 
             
               
                 
                   P 
                   t 
                 
                  
                 
                   G 
                   2 
                 
                  
                 
                   λ 
                   2 
                 
                  
                 
                   σ 
                   2 
                 
               
               
                 
                   R 
                   4 
                 
                  
                 4 
                  
                 
                   π 
                   3 
                 
               
             
           
         
       
     
     where P r  is the received power of the reflected radar signal  20 , G is the antenna gain, λ is the wavelength of the reflected radar signal  20 , σ is the radar cross section of the target  16 , and R is the range of the radar sensor  12 . Based on the above equation, the compensated received power is proportional to the radar cross section of the target  16 : P r,comp  ∝σ 2    
     To compensate the effects of the distance term R 4  and the antenna gain G, the logarithm of both sides of the above equation can be taken. Thus, the above equation becomes: 
         P   r,comp[dB]   =P   r[dB] −20 log( G (α))+40 log( R )+ C  
 
     where C is a constant for the target  16 . The constant C can be determined based on the properties of the target  16  (e.g., the radar cross section), the wavelength, and internal losses for transmitting and receiving radar signals from the radar sensor  12 . As explained below, the method  30  tracks changes in compensated received power as compared to actual values. Accordingly, the change in the value of compensated received power values remains the same regardless of the value of C as long as C is a constant. 
     The equations above assume that the radar cross section (σ) of all detected targets  16  is the same (at least on average) even if the view angle is changed. For an ideal reflector, this would normally provide the same compensated received power for a target  16  no matter how far away or at what angle the target is detected, and the magnitude of the compensated received power would be proportional to the radar cross section of the target  16 . In some embodiments, all targets  16  are approximated as being point-like in the vertical direction. However, real-world targets  16  may not be point-like in the vertical direction. Accordingly, the method  30  can compensate for this assumption by restricting evaluation to targets  16  located beyond a predetermined distance from the vehicle  10 , where a height dimension of the target  16  is spread over a small angular range and, thus, makes the point-like assumption more value. Imposing this restriction can improve the resolution of the vertical alignment determined using the method  30 . 
     As noted above, the controller  14  uses the stored data regarding the reflected radar signals  20  to establish data points (at block  36 ). For example, in some embodiments, the controller  14  uses the stored received compensated power and horizontal angles for each reflected radar signal  20  received from a target  16  as data points. The controller  14  can be configured to categorize (e.g., assign to a bin) the data points according to the value of the horizontal angle (e.g., into bins or categories assigned 0.5 degree increments). It should be understood that categorizing the data points is optional and may not be used in some embodiments. Also, in some embodiments, the controller  14  uses different values for the categories than degrees and the increments. 
     In some embodiments, a minimum range of angular values must be collected for the controller  14  to consider a target  16  significant for evaluation. Accordingly, the controller can be configured to count the number of different horizontal angles represented by the stored data points and can compare the count to a predetermined threshold prior to using the data points to plot a curve as described below. 
     The controller  14  can also be configured to normalize the data points for each target  16  so that changes in the received compensated power from various targets can be meaningfully compared. In some embodiments, the controller  14  normalizes the data by interpolating or extrapolating each data set to determine the compensated received power value for the horizontal angle of zero. The controller  14  can then divide all data points by this compensated received power value to generate a normalized data set (i.e., normalized data points). Once the data points are normalized, if more than one compensated received power value exists in a category, the controller  14  can average the values in the category. 
     The controller  14  uses the data points to plot a curve (at block  38 ). For example, the controller  14  uses the data points (e.g., the normalized data points) to plot a compensated received power curve  40 , as illustrated in  FIGS. 4 and 5 . Curves  40   a  illustrated in  FIGS. 4 and 5  represent power curves associated with targets  16  with strong reflective characteristics, and curves  40   b  illustrated in  FIGS. 4 and 5  represent power curves associated with targets  16  with weak reflective characteristics. The controller  14  then evaluates the compensated received power curve  40  to determine an alignment angle for the radar sensor  12 . In particular, if a radar sensor  12  is vertically aligned, a constant compensated received power value is received for all angles, and the curve  40  is a straight line (see  FIG. 4 ). However, if the radar sensor  12  is vertically misaligned, the compensated received power decreases as the horizontal angle increases, and the curve  40  is not straight (see  FIG. 5 ). 
     After plotting a power curve  40 , the controller  14  compares the curve  40  to pre-recorded curves for vertically misaligned angles to identify a pre-recorded curve  26  that best fits the power curve  40  (at block  44 ). For example, the compensated received power curves for various vertical alignment angles (i.e., pre-recorded curves  46 , see, e.g.,  FIG. 6 ) can be learned and stored in the radar sensor  12  and/or the controller  14  (e.g., in the computer-readable media  23 ). In some embodiments, pre-recorded curves  46  are stored for 0.5 degree increments from a − degree alignment (representing a preferred alignment of the radar sensor  12 , which would be associated with a straight pre-recorded curve  46 ). The controller  14  can be configured to perform a least-squares match to find the best fit between a curve  40  and the pre-recorded curves  46 . Accordingly, the pre-recorded curve  46  best matching a curve  40  identifies the alignment angle of a radar sensor  12 . In some embodiments, the controller  14  collects compensated received power curves  40  from a quantity of targets  16  before determining an alignment angle for the radar sensor  12 , such that individual targets  16  cannot play a significant role in determining the alignment of the radar sensor  12 . Accordingly, the controller  14  can determine a best fit between each curve  40  and the pre-recorded curves  46  to identify multiple alignment angles of the radar sensor  12 . The controller  14  can average or perform another type of mathematical calculation on the multiple angles to identify a single alignment angle of the radar sensor  12 . 
     After identifying the alignment angle of the radar sensor  12  (at block  44 ), the controller  14  compares the alignment angle to an operation range associated with the radar sensor  12  (at block  46 ) (e.g., a range of alignment degrees that still provides normal or acceptable operation of the radar sensor  12  or a threshold alignment degree that indicates a maximum misalignment of the radar sensor  12  that still provides normal or acceptable operation). The operation range can be stored in the computer-readable media  23 . 
     If the alignment angle is outside of the operation range (at block  46 ), the controller  14  determines that the radar sensor  12  is misaligned and takes a corrective action (at block  48 ). The corrective action can include setting an error condition, issuing a warning on a human machine interface (e.g., an interior console or dashboard), issuing a command to disable one or more vehicle systems that rely on the radar sensor  12 , issuing a command to disable the radar sensor  12 , or combinations thereof. For example, if the controller  14  determines that the radar sensor  12  is misaligned, other vehicle functions such as collision mitigation, adaptive cruise control, blind spot detection, closing vehicle warning, cross traffic alert, and autonomous driving may need to be disabled to prevent erroneous operation. 
     It should be noted that the pre-recorded curves  26  can be collected based on real-world measurements. For example, the reflection of radar signals off of the ground causes received power and horizontal angle values for a target  16  to differ from those for the same target  16  in free space. This distinction causes curves to differ for positive (pointing up) and negative (pointing down) alignment angles. If the pre-recorded curves  46  are collected using real-world measurements, the curves  46  can be used to distinguish between positive and negative alignments of the radar sensor  12 . Accordingly, the determined vertical alignment angle for the radar sensor  12  can include both an angle and a direction (e.g., up or down), which aids correction of any misalignment. 
     The controller  14  can be configured to repeat the functionality described above to reliably determine an alignment angle (e.g., over the course of several minutes or a couple of hours). For example, each iteration of the method  30  can provide different results due to differences in the reflective characteristics of targets  16 , differences in environment conditions, and combinations thereof. The differences can be minimized over time given a variety of detected targets  16 . Also, as noted above, the controller  14  can repeat the method  30  to collect compensated received power curves for a sufficient number of targets  16  to ensure that no one target  16  plays a significant role in determining the alignment angle of the sensor  12 . For example, the controller  14  can be configured to identify an alignment angle of the sensor  12  when a predetermined number of curves  40  exhibit the same vertical alignment angle when compared to the pre-recorded curves  46 . 
     In some embodiments, the controller  14  can also be configured to deactivate vertical alignment detection when rain is detected (e.g., through use of the wipers or other detections) or when the radar sensor  12  is blind (e.g., to prevent errors due to rain, snow, or other water-films on the sensors  12 ). 
     The functionality described above can also be used to detect rotationally misaligned sensors by evaluating the left and right sides of a radar sensor  12  separately. For example, if the left side of a radar sensor  12  shows a positive alignment angle and while the right side shows a negative angle, the controller  14  can determine that the radar sensor  12  is rotated toward the right and can take appropriate corrective action. 
     It should be understood that the above alignment determination functionality can be used to determine a vertical alignment of a moving radar sensor  12  where the sensor  12  moves past a target  16  or a vertical alignment of a stationary radar sensor  12  where a target  16  moves past the sensor  12 . 
     Thus, the invention provides, among other things, systems and methods for determining a vertical alignment in a radar sensor. Various features and advantages of the invention are set forth in the following claims.