Abstract:
A method of calibrating angle drift of a laser radar system is provided, in one aspect of the present invention. The method includes, providing a plurality of virtual fiducials into an xy scanner; and providing a plurality of auxiliary laser sources into the xy scanner. The method also includes, routing a plurality of auxiliary laser beams from the plurality of auxiliary laser sources into the xy scanner; and calibrating an angular position of a plurality of laser directing means. The methods provides creating a calibration signal for updating the angular position of a plurality of scanning mirrors.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     None  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to laser radar systems, and more particularly, to correcting the angular drift of laser radar systems.  
         [0004]     2. Background  
         [0005]     Laser Radar systems, among other emerging technologies, require an elevation, azimuth scanner. One of the common and inexpensive alternatives is the galvanometric x y scanner. This scanner is composed of a pair of mirrors each rotated by galvanometric motors about axes that are approximately horizontal and roughly perpendicular to each other. A light beam entering the scanner is reflected from a first mirror onto second mirror in such a way that rotation of the first mirror rotates the beam in azimuth and the second mirror rotates the beam in altitude.  
         [0006]     Measurement of the angle of the mirrors is currently done with a capacitive or optical sensor mounted on the galvanometric motor. Both of these sensors have significant calibration drifts with temperature. For a laser radar system operating at a large distance, the angular resolution of the scanner is the limiting factor in the three dimensional accuracy. For example, consider laser radar with a maximum range of 10 meters. If we want a position resolution of 25 microns, then the angular resolution must be 2.5 microradians. Typical drifts in the angular accuracy over time are many times this figure.  
         [0007]     This problem has been partially solved by the use of fiducial targets. Fiducial targets are objects mounted in the field of view of the xy scanner which are periodically measured to correct for the angular drift in the galvanometric motors. This solution is not suited for many applications of laser radar systems. For example, the solution would not be suited in measuring articles on a manufacturing floor or assembly line, because setting out the fiducial targets is an extra step in the measurement process and the targets would be in the way of the assembly technicians.  
         [0008]     Therefore, there is a need for a method and system for correcting the angular drift of radar systems using internal fiducials.  
       SUMMARY OF THE PRESENT INVENTION  
       [0009]     A method of calibrating angle drift of a laser radar system is provided in one aspect of the present invention. The method includes, providing a plurality of virtual fiducial targets into an xy scanner; and providing a plurality of auxiliary laser sources into the xy scanner. The method also includes, routing a plurality of auxiliary laser beams from the plurality of auxiliary laser sources into the xy scanner; and calibrating an angular position of a plurality of laser directing means.  
         [0010]     In another aspect of the present invention, a system for correcting angular drift of a laser radar system is provided. The system includes, a multidimensional laser scanner, the laser scanner including a plurality of motorized mirrors, the mirrors including a field of view, an input aperture, and an output aperture; a plurality of auxiliary laser sources; and a plurality of virtual fiducials.  
         [0011]     In another aspect of the present invention, provided are structures for supporting the plurality of auxiliary laser sources near the input aperture; and surfaces for supporting the plurality of virtual fiducials in a plane parallel to the plane of the output aperture.  
         [0012]     This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The foregoing features and other features of the present invention will now be described with reference to the drawings of a preferred embodiment. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate, but not to limit the invention. The drawings include the following figures:  
         [0014]      FIG. 1A  is a perspective view showing an xy scanner according to one aspect of the present invention.  
         [0015]      FIG. 1B  is a perspective view showing an xy scanner according to another aspect of the present invention.  
         [0016]      FIG. 2  is a block diagram showing a field of quad cells according to one aspect of the present invention.  
         [0017]      FIG. 3  shows a process flow diagram for calibrating angle drift of a laser radar, according to one aspect of the present invention.  
         [0018]      FIG. 4  is a block diagram showing a calibration apparatus for generating a calibration signal according one aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 1A  is a perspective view of a laser radar system  100  implementing the methods and systems in accordance with the adaptive aspects of the present invention. The system generally includes a main laser beam  150 , scanning mirrors,  118 ,  120 , galvanometer motors  124 ,  126 , a laser radar output lens  107 , a scanner input aperture  105 , a body  160 , auxiliary lasers  102 ,  104  and quad cells  140 ,  142 ,  144 ,  146 . The term ‘quad cell’ as used herein may be interchanged with the term ‘virtual fiducial’.  
         [0020]     The scanning mirrors  118 ,  120  generally include an azimuth mirror  118  for scanning in a horizontal direction, and an altitude mirror  120  for scanning in a vertical direction. The scanning mirrors  118 ,  120  may be coupled to galvanometric motors  124 ,  126  respectively. The galvanometric motor  124  provides rotation of the azimuth mirror  118  during the horizontal rotation of the main laser beam  150 . The galvanometric motor  126  provides rotation of the altitude mirror  120 , during the vertical rotation of the main laser beam  150 .  
         [0021]     The galvanometric motors  124 ,  126  may also include internal position sensors (not shown). The internal position sensors (not shown) communicate the current angular position of the azimuth mirror  118  and the altitude mirror  120  to a position detection section (not shown).  
         [0022]     The laser radar output lens  107  provides the focus needed for the main laser beam  150  before entering the xy scanner  100 . The laser radar output lens  107  occupies a plane parallel to the physical plane of an input aperture  105 .  
         [0023]     The body  160  provides structural support for the accompanying scanning mirrors  118 ,  120 , the galvanometric motors  124 ,  126 , the auxiliary lasers  102 ,  104  and the quad cells  140 ,  142 ,  144 ,  146 . Structural support for the auxiliary lasers  102 ,  104  and support for the quad cells  140 ,  142 ,  144 ,  146  will be explained in more detail elsewhere below.  
         [0024]     Still referring to  FIG. 1A  and  FIG. 1B , the auxiliary lasers  102 ,  104  comprise a first auxiliary laser  102  and a second auxiliary laser  104 . The first auxiliary laser  102  may be supported on a first structure  162 . The first structure  162  is also coupled to the body  160 . Further, the first structure  162  may comprise a material sufficient for a stationary positioning of the first auxiliary laser  102 . The positioning of the first auxiliary laser  102  is near a plane parallel to the plane occupied by the input aperture  105 .  
         [0025]     In a preferred embodiment, the first auxiliary laser  102  is positioned to one side of the input aperture  105 , so that a first auxiliary laser beam  106  passes through the input aperture  105 . In an alternative embodiment, the first auxiliary laser  102  may be positioned above the input aperture  105  so that the first auxiliary laser beam  106  passes through the input aperture  105 .  
         [0026]     The second auxiliary laser  104  is supported similarly as the first auxiliary laser  102 . A second structure  164  supports the second auxiliary laser  104  and is coupled to the body  160 . The positioning of the second auxiliary laser  104  is also near the plane of the laser radar input aperture  105 .  
         [0027]     In a preferred embodiment, the second auxiliary laser  104  is positioned on the opposite side of the input aperture  105  from laser  102 , so that a second auxiliary laser beam  108  passes through the input aperture  105 . In an alternative embodiment, the second auxiliary laser  104  may be positioned below the input aperture  105  so that the second auxiliary laser beam  108  passes through the input aperture  105 .  
         [0028]     As shown in  FIGS. 1A and 1B , the quad cells  140 ,  142 ,  144 , and  146  comprise a first horizontal quad cell  140 , a second horizontal quad cell  142 , a first vertical quad cell  144  and a second vertical quad cell  146 . The quad cells  140 ,  142 ,  144 , and  146  are a type commonly used by those of ordinary skill in the art to measure an angle of a laser beam. In a preferred embodiment, the quad cells  140 ,  142 ,  144 , and  146  may be segmented into four regions (not shown). During normal operation, the four regions (not shown) produce an electric current that is proportional to an amount of light sensed by any of the four regions. The centroid is the position on of a quad cell where all four regions (not shown) intersect each other. In normal operation, the four regions (not shown) produce an electric current that is equal to each other when a laser beam is focused on the centroid.  
         [0029]     The quad cells  140 ,  142 ,  144 , and  146  may be supported by a surface  130 . The surface  130  lies in a plane parallel to the output aperture (not shown) and the surface  130  is positioned slightly downstream from the altitude mirror  120 . Further, the surface  130  is affixed to the body  160 . The surface  130  may include an upper arm  134  and a lower arm  138 . In one embodiment, the upper arm  134  lies above the output aperture (not shown) and the upper arm  134  supports the horizontal quad cell  140  and the vertical quad cell  144 . Similarly, the lower arm  138  lies below the output aperture (not shown) and the lower arm  138  supports the horizontal quad cell  142  and the vertical quad cell  146 .  
         [0030]      FIG. 2 , with reference to  FIGS. 1A and 1B , shows a block diagram of a placement of quad cells  240 ,  242 ,  244 , and  246  according to one aspect of the present invention. The quad cells  240 ,  242 ,  244 , and  246  may be placed near the edges of an optical angular field of view (not shown). The optical angular field of view is typically double the range of a scanning mirror&#39;s moving range (not shown). In one aspect of the present invention, the optical angular field of view (not shown) for the azimuth mirror  118  and the altitude mirror  120  is plus and minus twenty degrees. The mirror moving range for the azimuth mirror  118  and the altitude mirror  120  is plus and minus ten degrees.  
         [0031]      FIG. 2  shows another aspect of the placement of the quad cells  240 ,  242 ,  244 , and  246 . The placement of the quad cells  240 ,  242 ,  244 , and  246  may be governed by the size of the output aperture  220  and governed by orientation of the auxiliary lasers. The size of the output aperture  220  is defined by the physical dimensions of the distance A and the distance B. The distances C and D are governed by the orientation of the auxiliary laser, the spacing between the azimuth mirror  118  and the altitude mirror  120 , the spacing between the altitude mirror and the fiducial plane and the fixed calibration angles. The distance D is the optimum spacing either above or below the edge of the output aperture  220 . This optimum spacing result in placing the first vertical quad cell  244  and the second vertical quad cell  242  near the outer boundary of the vertical optical field of view of the altitude mirror  120 .  
         [0032]     In one embodiment, the output aperture  220  may comprise the distance ‘A’ of 153 mm and the distance ‘B’ of 117 mm. Further, the distance D may equal 45 mm and the distance D may equal 16 mm. Alternatively, other values for the distances A, B, C, and D may be used depending on the size of the output aperture  220  and depending on the optical angular field of view of the scanning mirrors  118 ,  120 .  
         [0033]      FIG. 3 , with reference to  FIGS. 1A and 1B , shows a process flow diagram for calibrating angle drift of a laser radar system  100 , according to one aspect of the present invention. The process begins in step  300 , wherein one of the two auxiliary lasers is aimed at one of the quad cells. The first auxiliary laser beam  106  is first directed through the input aperture  105  towards the azimuth mirror  118 . After striking the azimuth mirror  118 , the first auxiliary laser beam  106  is directed towards the altitude mirror  120 , where the first auxiliary laser beam  106  is reflected by the altitude mirror  120  to strike one of the virtual fiducials  140 ,  142 ,  144 , and  146 . The output of the quad cell is used in a feedback loop to ensure that the laser beam is centered on the quad cell. This aims the scanner system in a previously calibrated and well known direction. In step  310  the output of the altitude and azimuth sensors are measured. In step  320 , this sensor data is stored and associated with a known aim direction for this quad cell. This process is repeated for each of the quad cells.  
         [0034]     In a routing step  330 , the controller ( 410 ,  FIG. 4 ) takes all of the sensor measurements and the known aim directions to calculate new values for the span and offset of the altitude and azimuth sensors. In step  340  these values are updated in the calibration module.  
         [0035]     The process of steps  300 ,  310 ,  320 ,  330  and  340  are repeated for the output of the second auxiliary laser  104 . The second auxiliary laser  104  produces a second auxiliary laser beam  108  that is routed and calibrated in the similar manner as described above In one embodiment, the process in  FIG. 3  may be repeated until the auxiliary lasers  102 ,  104  have employed all four quad cells  140 ,  142 ,  144  and  146  in calibrating the xy scanner  100 .  
         [0036]      FIG. 4  shows a block diagram of a calibration apparatus  400 . The apparatus generally includes a scanner assembly  405  and a controller module  410 . The scanner module  405  may include position sensors  420 ,  430 , and a quad cell current sensor  440 . The position sensors  420 ,  430  reside internal to the galvanometric motors  124 ,  126  described elsewhere herein. The quad cell current sensor  440  may reside near the location of the quad cells  140 ,  142 ,  144 , and  146 .  
         [0037]     The controller module  410  generally includes a calibration module  450 . In normal operation, the position calibration module  450  receives position commands from the controller  410 . It converts these commands into electrical signals ( 475 ) sent to the scanner motors ( 420  A and  420 B). Subsequently, it receives sensor signals ( 455 ) from the azimuth and altitude position sensors ( 420  and  430 , respectively). The calibration module  450  converts these signals to azimuth and altitude angles. Periodically, the calibration procedure described in the previous section is performed.  
         [0038]     After the first auxiliary laser beam  106  strikes one of the virtual fiducials  140 ,  142 ,  144 , and  146 , the angular position of the scanning mirrors  118 ,  120  is sensed by the position sensors  420 ,  430 . Simultaneously, the scanning mirrors  118 ,  120  are positioned using a nulling technique. The nulling technique comprises focusing the first auxiliary laser beam  106  on the centroid (not shown) of an active quad cell until currents of the four regions equal each other as explained elsewhere herein.  
         [0039]     The process of steps  300 ,  310 ,  320 ,  330  and  340  are repeated for the output of the second auxiliary laser  104 . The second auxiliary laser  104  produces a second auxiliary laser beam  108  that is routed and calibrated in the similar manner as described above In one embodiment, the process in  FIG. 3  may be repeated until the auxiliary lasers  102 ,  104  have employed all four quad cells  140 ,  142 ,  144  and  146  in calibrating the xy scanner  100 .  
         [0040]     To complete the calibration process, the azimuth and altitude angles associated with each quad cell have been previously measured and are stored in the position calibration module  450 . A data set, composed of the measured sensor signals and the previously measured altitude and azimuth angles is assembled. These data are used to calculate new calibration values. The calibration constants in the calibration module  450  are updated and used thereafter to convert position commands to motor signals and sensor outputs to angles.  
         [0041]     While the present invention is described above with respect to what is currently considered its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.