Abstract:
A method of measuring elevational misalignment of an automotive radar sensor in a factory or service setting utilizes two or more targets that can be discriminated by the radar system. The targets are positioned at different elevational angles with respect to the desired elevation axis, and the degree of elevational misalignment is determined according to the ratio or difference in return signal amplitude for the two targets. Discrimination of the targets may be ensured by differences in range, azimuth angle or Doppler. Since the amplitude difference is a measure of misalignment, the measurement may be used to verify proper alignment or to indicate the amount of adjustment required to achieve proper alignment.

Description:
TECHNICAL FIELD 
     The present invention relates to automotive radar sensors for adaptive cruise control and/or collision warning, for example, and more particularly to a method of detecting elevation mis-alignment of the radar sensor. 
     BACKGROUND OF THE INVENTION 
     Automotive radar sensors typically have a limited field of view and are scanned in a horizontal (azimuth) plane to detect objects of interest. Although the elevation of a detected object is ordinarily not important, proper elevational alignment of the radar sensor is important to enable reliable identification of objects at relatively long range, and to facilitate rejection of overhead objects such as road signs and bridges. For this reason, radar sensors usually include an adjustment mechanism such as a screw that is rotated to change the elevation alignment of the sensor with respect to a vehicle mounting bracket. 
     The elevational alignment of a radar sensor may be determined either mechanically by using a bubble level or the like, or electronically by monitoring the emitted signal strength in the center of the elevation field of view or the return signal strength from a test target in the center of the elevation field of view. The electronic techniques are more amenable to factory or dealer calibration, in which case the elevation mechanism is adjusted either manually or automatically until maximum signal strength (emitted or returned) is achieved. A similar technique can be used for azimuth alignment, as described for example, in the U.S. Pat. No. 6,087,995 to Grace et al., where azimuth and elevation alignment are achieved simultaneously by adjusting the alignment to maximize a summation of the azimuth and elevation signals impinging on a set of interferometers aligned with the thrust axis of the vehicle. However, the sensitivity of the emitted or returned signal strength to changes in elevational alignment becomes very low as the sensor approaches the desired alignment, making precise and accurate alignment of the sensor very difficult to achieve. Moreover, many adjustments and measurements are usually required to optimize the alignment since no single measurement yields a reliable measure of the degree or direction of misalignment. Accordingly, what is needed is a method of determining the elevational misalignment of an automotive radar sensor with a single high sensitivity measurement. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method of measuring elevational misalignment of an automotive radar sensor in a factory or service setting. According to the invention, two or more targets that can be discriminated by the radar system are positioned at different elevational angles with respect to the desired elevation angle, and the degree of elevational misalignment is determined according to the ratio or difference in return signal amplitude for the two targets. Discrimination of the targets may be ensured by Doppler (if the target is a transponder), or by differences in range or azimuth angle, provided of course, that signal strength variation due to the differences in range or azimuth angle are normalized. Since the amplitude difference is a measure of misalignment, the measurement may be used to verify proper alignment or to indicate the amount of adjustment required to achieve proper alignment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an automotive radar system including a radar sensor, a transceiver and a signal processor. 
     FIG. 2 is a graph depicting target amplitude as a function of elevational alignment for the system of FIG. 1, and a pair of targets positioned according to this invention. 
     FIG. 3 is a graph depicting a difference in return signal strength for the two targets depicted in FIG. 2 as a function of elevation misalignment. 
     FIG. 4 is a flow diagram illustrating the method of this invention as carried out with the system of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is disclosed in the context of an automotive forward-looking radar system designed to support vehicle control functions such as adaptive cruise control, collision warning, collision avoidance, and the like. FIG. 1 depicts a radar-equipped vehicle  10  traveling on a road surface  12  in the direction of arrow  14 . The depicted radar system components include a radar sensor or antenna  16 , a transceiver (TVR)  17  and a signal processor (SP)  18 . During vehicle operation, the transceiver  17  is activated to emit high frequency energy through antenna  16  in a characteristic beam pattern in a target detection zone forward of the vehicle. When the emitted energy impinges on a target such as an obstacle or another vehicle, energy is reflected back to the transceiver  17  through antenna  16 , and the signal processor  18  analyzes the reflected signal to discriminate individual targets and to determine their range and range-rate. 
     The emitted energy pattern includes a main lobe  20  and one or more principle side-lobes  22 ,  24 , and the radar antenna  16  is adjustably mounted in vehicle  10  so that the elevation angle of the main lobe  20  may be calibrated to a specified value, designated in FIG. 1 by the axis  26 . Proper elevational alignment of the antenna  16  will be checked initially in the vehicle assembly plant, and periodically thereafter to ensure proper operation of the system and associated vehicle control functions. Although the elevation of a detected object is ordinarily not important for vehicle control functions, proper elevational alignment of the antenna  16  is important to enable reliable identification of objects at relatively long range, and to facilitate rejection of overhead objects such as road signs and bridges. 
     As discussed above, elevational alignment is ordinarily performed by placing interferometers or a reflector in the center of the elevation field of ice (that is, on the axis  26 ), measuring the strength of the emitted or reflected signal energy, and then adjusting the antenna elevation alignment until the signal energy is maximized. However, it is difficult to achieve accurate elevational alignment in this way due to low sensitivity, and the need to make numerous measurements and adjustments. This can be illustrated by the graph of FIG. 2, where the solid trace  28  depicts the reflected signal energy reduction in decibels as a function of the misalignment of a target from the elevational axis  26  for a radiated beam width of approximately 4 degrees. A factory-specified elevational alignment tolerance of +/− one degree is designated by the reference numeral  30 , and it can be seen that the reflected signal strength varies by only a small amount within the specified tolerance band. In fact, the sensitivity is essentially 0.0 dB per degree when the antenna  16  is in exact alignment and only 2.0 dB per degree when the alignment is barely within the specified tolerance band. Additionally, the signal strength measurement does not indicate misalignment per se, and the elevational orientation of the transceiver  17  has to be ad Lusted to determine if signal strength is at or near maximum. 
     The method of the present invention overcomes the aforementioned difficulties by measuring the actual elevational misalignment. If the misalignment is within the specified tolerance band, no adjustment of the antenna  16  is required; if the misalignment is outside the tolerance band, the alignment of the antenna  16  may be adjusted without making any further measurements, or in closed-loop fashion to bring the measured misalignment within the tolerance band or to zero. According to the invention, the misalignment is measured by placing two distinguishable targets at known angles relative to the desired elevational angle, and forming a ratio or difference of the signal strengths associated with the two targets. The ratio or difference will have a predetermined value based on the target angles when the antenna  16  is properly aligned, and will be above or below the predetermined value in relation to the degree of elevational misalignment when the antenna  16  is misaligned. Discrimination of the targets is most simply ensured by placing them at different ranges from antenna  16 , or at different azimuth angles, provided of course, that signal strength variation due to the differences in range or azimuth angle are normalized. Alternatively, the targets may be transponders that emit different frequencies when illuminated by the beam  20 , in which case the transceiver  17  will discriminate the targets by apparent Doppler frequency. 
     The targets are designated in FIG. 2 by the reference numerals  32  and  34 , and in the illustrated embodiment, are oppositely disposed about the axis  26  and separated in elevation by the beam width of antenna  16 . In the illustrated embodiment, for example, antenna  16  has a 3 dB main lobe beam width of approximately four degrees, and the targets  32  and  34  are located at +/− two degrees of elevation with respect to axis  26 , as shown in FIG.  2 . If the antenna  16  has no elevational misalignment, the reflected signal strengths from the targets  32  and  34  (SS 32  and SS 34 ) will be equal; in this case, the ratio (SS 34 /SS 32 ) will be have a value of one, or alternatively, the difference (SS 34 −SS 32 ) will have a value of zero. However, if the antenna  16  has a positive elevational misalignment, SS 34  will be greater than SS 32 , and the extent of misalignment will be indicated by the degree to which the ratio (SS 34 /SS 32 ) deviates from unity, or by the magnitude of the difference (SS 34 −SS 32 ). It will thus be recognized that the measurement indicates not only the magnitude of elevational misalignment, but also the direction of misalignment. FIG. 3 graphically depicts the difference (SS 34 −SS 32 ) in decibels as a function of the elevational misalignment of antenna  16 . Significantly, the sensitivity is several times higher than for the single target alignment technique, and approximately constant across the specified tolerance band  30  of +/− one degree of elevation. 
     Finally, FIG. 4 presents the measurement method of this invention as a series of steps  40 ,  42 ,  44 ,  46 ,  48  and  50 . First, the vehicle  10  and targets  32 ,  34  are positioned as described above. Then the transceiver  17  is activated, and the signal processor  18  discriminates the targets  32 ,  34  on one or more of the bases discussed above (range, azimuth angle, Doppler) and determines the signal strengths SS 32  and SS 34 . The processor  18  then computes the ratio (SS 34 /SS 32 ) or the difference (SS 34 −SS 32 ), and determines and outputs the corresponding elevational misalignment by simple table look-up. 
     In summary, the present invention provides a method of reliably and accurately measuring the elevational misalignment of an automotive radar sensor that has high sensitivity and that does not require adjustment of the transceiver  17 . It will be understood, of course, that while the method has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the elevational separation of targets  32 ,  34  may be greater or lesser than the antenna beam width. In general, the sensitivity increases with increasing elevational separation, but the antenna pattern becomes less repeatable as the distance from the beam center becomes large. Thus, it will be understood that methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.