Abstract:
A method and apparatus for measuring ground height in front of a vehicle, such as a vehicle for clearing buried land mines, in which a scanning pulsed laser to used to scan along one or more lines in front of the vehicle, with such scanning being executed more slowly in certain regions where data is collected and more quickly in intermediate regions where data is not collected. The time of flight along those lines is measured, and using that measurement, the ground height calculated and stored, with the higher elevations being discarded as the representing the tops of vegetation and the lower elevations being representative of the ground height or elevation in front of blade. A dozer blade on the vehicle is then adjusted in response to the calculated height to position the blade for the proper depth of cut in order to effectively uncover and remove the buried mines.

Description:
VEHICLE REGIONAL SCANNER  
         [0001]    The present invention relates generally to a laser scanner that is mounted on a vehicle and used during the vehicle&#39;s forward motion to determine the elevation of the ground surface in front of the moving vehicle by non-contact means, even though the ground surface may be covered by vegetation.  
         BACKGROUND OF THE INVENTION  
         [0002]    On certain vehicles, such as bulldozers or land mine removing vehicles, it is important to know the elevation and contour of the ground with respect to the vehicle. Vegetation and rocks on the ground make such determinations difficult, because the sensor may sense the tops of vegetation or small rocks instead of the ground surface. As the blade of a bulldozer or land mine removing vehicle moves through the soil, the elevation immediately in front of the blade changes as the loose soil cut by the blade piles up against and is pushed forward by the blade. It is, therefore, necessary to determine the ground elevation of the undisturbed surface at a distance in front of the blade. Land mine removing vehicles have additional difficulties since physical sensors used to sense the elevation of the ground surface may be damaged or destroyed by rapidly expanding gases or flying debris creating by an exploding land mine. Knowing the ground elevation and contour with respect to the dozer blade is very important in order to maintain the blade at the proper depth and angle. If the blade is too shallow, the blade may pass over a land mine, permitting its subsequent detonation under the vehicle and thereby damaging or destroying the vehicle and possibly injuring personnel on board. If the blade is too deep, the forward progress of vehicle is slowed, lengthening the time required to clear the mine field, which could adversely affect the associated military operation, and in some cases may even cause the vehicle to stall. Knowing the ground height or elevation and contour allows for a safer and more efficient use of the vehicle. The “angle” to which reference is made is that angle between the blade and horizontal, and is achieved by raising or lowering one end of the blade to essentially rotate the blade about the longitudinal centerline of the vehicle. For clarity, that angle will be called the “tilt angle”, and movements of the blade to vary the tilt angle will be referred to as “tilt” or “tilting.” 
         BRIEF SUMMARY OF THE INVENTION  
         [0003]    It is an object of the invention to provide a vehicle with a laser scanner that is able to determine height or elevation of the ground surface and its contour in front of the moving vehicle.  
           [0004]    It is another object of the invention to provide a system that can measure ground height or surface elevation and is unaffected by the presence of vegetation.  
           [0005]    It is another object of the invention to provide a land mine removing vehicle that is able to detect ground surface elevation and contour by remote, non-contact sensing.  
           [0006]    The present invention provides a scanning laser system that is able to temporarily scan through vegetation to determine the elevation of the ground surface with respect to the vehicle. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a perspective view of a vehicle with a preferred embodiment of the invention.  
         [0008]    [0008]FIG. 2 is a schematic top view of the vehicle shown in FIG. 1 equipped with a preferred embodiment of the invention.  
         [0009]    [0009]FIG. 3 is a schematic top view of another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    [0010]FIG. 1 is a perspective view of a vehicle  10  equipped with a preferred embodiment of the invention. In this embodiment, a range finder assembly  11  comprising a laser  12 , a moving or rotating mirror assembly  13 , a sensor  14 , a processor  15  including volatile memory, and a controller  16  is mounted on a range finder support  18 . The range finder support  18  is mounted on the chassis  21  of the vehicle  10 , which, for example, may be a land mine clearing vehicle. The mine clearing vehicle  10  has a plowing or dozer blade  39  connected to the chassis  21  through a blade connection and position control system  22 . The blade connection system  22  includes a central lift cylinder  160  capable of raising and lowering the blade  39 , and left and right of pitch/tilt cylinders  162  and  164 . Simultaneous extension of both cylinders  162  and  164  will pitch the top of the blade forward and simultaneous retraction will pitch the top of the blade rearward. Tilt of the blade  39  is achieved by independent actuation of the cylinders  162  and  164 . For example, extension of the left cylinder  162  alone will cause the blade  39  to tilt toward the right and extension of the right cylinder  164  by itself will cause the blade  39  to tilt to the left. The range finder support  18  is mounted away from the blade  39  far enough to isolate the range finder assembly  11  from the deleterious effects of exploding land mines. In the embodiment illustrated in FIG. 1, the range finder support  18  is secured to the chassis  21  behind the plowing blade  39  with the assembly  11  mounted thereon at a height sufficient to allow the laser  12  to project its beam onto the ground in front of the blade at an oblique angle with respect to a horizontal surface. The chassis  21  is supported on a pair of tracks  25 , each of which is trained around a rear drive sprocket  166  and a front idler sprocket  168 . A plurality of road wheels  24  engage the lower run of each track  25  and are positioned intermediate to the sprockets  166  and  168  with each wheel  24  being mounted on a road arm connected to and suspended from the chassis  21 .  
         [0011]    In operation, the vehicle  10  is propelled by the tracks  25  in a forward direction as they are simultaneously driven by the drive sprockets  166 , so that the blade  39  may engage and, if adjusted to do so, cut the ground in front of the vehicle. The narrow beam laser  12  in the range finder assembly  11  is projected toward the ground surface in a horizontal scan, i.e. transverse to the longitudinal axis of the vehicle  10 , by the rotating mirror assembly  13 , which causes the beam to follow an arcuate path  35 , which is essentially a very large circular arc, as illustrated in FIG. 2. The laser  12  may be of the pulse-echo type, which creates discrete pulses of coherent light during the scan so that the path or line  35  is defined by a sequence of dots, each dot being a pulse of light from the laser  12 . Each circular arc line passes through a first scan region  28 , a first intermediate region  29 , a second scan region  30 , a second intermediate region  31 , and a third scan region  32 , where a series of  1  to n parallel, circular arc lines comprise a scan region. As illustrated in FIG. 2, the single line presently being scanned is the last path in a scan region represented by the single line  35 . The two solid lines below the single line  35  represent the path of the laser beam during previous traverses of the aforementioned regions, the displacement between the paths represented by these two solid lines being a function of the vehicle&#39;s velocity. Together they form a 5 line scan region (n=5). The two broken lines above the single line  35  represent the anticipated paths of the laser beam as the vehicle moves forward.  
         [0012]    As the laser scans along an individual line across the first scan region  28 , the rotating mirror assembly  13  causes the laser beam to scan at a relatively slow rate of approximately one half of a degree per laser pulse (0.5°/pulse). At that rate, at least some of the pulses of light will penetrate through any vegetation to reach the surface of the ground and be reflected thereby. Light from each pulse of the pulse-echo beam is reflected by the ground surface or vegetation along the same path taken by the pulse of light to the rotating mirror  13  that then reflects the incident light to the sensor  14 . The time of flight for each pulse, i.e. the time elapsed between the time when the pulse is emitted to the time when the sensor  14  detects the light reflected from that pulse, is recorded and the distance to the surface creating the reflection is calculated by the processor  15 . The calculated distances are stored in memory until the path  35  is completely traversed. As the laser beam crosses the first intermediate region  29 , the rotating mirror assembly  13  causes the laser beam to move along the path as fast as possible, i.e. at a rate as fast as it is possible to move the mirror assembly, but in any event much greater than 0.5° per laser pulse. No data is collected as the laser crosses this intermediate region. As the laser scans across the second scan region  30 , the rotating mirror assembly  13  again causes the laser beam to move at a slow rate of approximately 0.5° per laser pulse. As with the first scan region  28 , light from each pulse of the laser beam is reflected from the ground or vegetation along the same path to the rotating mirror  13  and to the sensor  14 , the time of flight is determined and the distance to the ground is calculated and stored by the processor  15 . As the laser beam traverses the second intermediate region  31 , the rotating mirror assembly  13  causes the laser beam to move along the line as fast as possible, similar to the speed of traverse across the first intermediate region  29 . Again, no data is collected in this intermediate region. As the laser beam moves along the line in the third scan region  32 , the rotating mirror assembly  13  causes the laser beam to move at a slow rate of approximately 0.5° per laser pulse. Reflected light from each pulse of the laser beam is directed to the sensor  14  by the mirror  13  which permits the time of flight to be determined and the distance to the ground to be calculated and stored by the processor  16 . FIG. 2 shows the laser  12  having almost completed its traverse of the line  35 , with the broken line indicating that portion of the line  35  remaining to be scanned.  
         [0013]    Each pulse of light from the laser in the scan regions  28 ,  30  and  32  produces a calculated data point representative of the distance to the surface that reflected that pulse. These data points taken along successive paths in each of the scan regions  28 ,  30  and  32  are stored by the processor  15 . Once the line  35  is completely traversed, the distances between the lines in each individual scan region are processed to determine a range estimate to the ground surface for the scan region. The shorter ranges represent distances to the tops of vegetation or rocks, while the maximum ranges indicate the distance to the ground surface. Thus, a reasonably accurate determination of the height or elevation of the ground surface can be made. That determination is then sent to the controller  16  which causes hydraulic fluid to be directed to, or exhausted from, the lift cylinder  160  in order to position the blade  39  to cut through the ground at the desired depth below the surface. Also, by comparing the maximum distances for the first, second, and third scan regions  28 ,  30  and  32 , the lateral slope or contour of the ground can be determined, and the controller  16  can cause hydraulic fluid to be directed to the cylinders  162  and  164  to incline the blade  39  so that in conforms more closely to the slope or contour of the ground surface.  
         [0014]    When the rotating mirror assembly  13  commences the next horizontal scan, the vehicle  10  will have moved forward causing the laser beam to follow a path indicated by the broken line just above the line  35 .  
         [0015]    It has been shown experimentally that, even with dense vegetation, a relatively slow scan rate with a narrow beam laser allows enough data points to be recorded in each region so that at least some will be the result of the beam penetrating the vegetation to reach the surface of the ground and be reflected thereby. The technique described above is robust in that the results are not significantly affected by variations in vehicle speed, or by vibrations or other factors that may slightly alter the direction of the laser beam. Variations in the angle the beam makes with a horizontal surface, such as may be caused by extreme vibration or fore and aft pitching of the vehicle  10 , can affect the accuracy of the calculated distances, unless compensated. Thus, the scan speed, i.e. the speed of traversing each path, should be fast enough to be completed before any significant variation of the beam angle occurs.  
         [0016]    The shape of the footprint of scanned region is not critical since the only requirement is for the density of the dots in each region be great enough to obtain a reasonable number of vegetation penetrations. Further, three regions for active scanning are ideal since they correspond to the lines of travel of the middle and the two ends of the blade But more than three regions per scan is consistent with the spirit of this invention.  
         [0017]    [0017]FIG. 3 is a schematic illustration of another embodiment of the invention. A vehicle  110  has a blade  139  attached to, and mounted for movement relative to the vehicle, through a blade connection system  122 . A range finder assembly  111  is mounted on top of the vehicle  110  and provides four laser beams, either by using multiple lasers or by dividing the laser beam into separate beams.  
         [0018]    In operation, a first laser beam  143  creates a first scan line  144  which contacts the blade  139  and thus provides a means for determining the position of the blade. Separate laser beams, identified as second beam  146 , third beam  149  and fourth beam  152  simultaneously scan associated lines or paths  147 ,  150  and  153 , respectively. The scan lines  147 ,  150 ,  153  are all positioned to be in front of the blade  139 . As with the previous embodiment, there is a first scan region  128 , a first intermediate region  129 , a second scan region  130 , a second intermediate region  131 , and a third scan region  132 . The first, second, third, and fourth laser beams  143 ,  146 ,  149  and  152  scan slowly through the first, second, and third scan regions  128 ,  130 ,  132  and quickly through the first and second intermediate regions  129 ,  131  in a manner similar to the scanning pattern of the laser beam in the embodiment of FIG. 2. The light from the beam  143  is reflected from the blade  139 , sensed and the time of flight determined, which in turn permits the present position and orientation of the blade  139  to be determined. Thus, a separate sensor is not needed to determine present blade position, which information is needed to accurately position the blade at the desired depth. The range finder assembly  111  uses data from the second scan line  147  to determine the ground height close to the blade  139 . The range finder assembly  111  uses data from the third and fourth scan lines  150  and  153  to determine the ground configuration further ahead and thus, with the data from the second scan line, provide an indication of changes in the elevation of the ground surface. This provides an indication of what elevations of ground surface the vehicle will encounter in the immediate future and permits the controller  16  to begin adjustments in the blade height and tilt before those elevations are actually engaged, thus compensating for the time delays normally required to achieve fluid flow into or out of the proper ones of the cylinders  160 ,  162  and  164 . In this regard the embodiment of FIG. 3 may be characterized as predicative, while the embodiment of FIG. 2 is reactive in nature.  
         [0019]    In other embodiments of the invention, a different number of beams may be used and other means may be used to cause the laser beam to scan, such as a rotating prism, a changing refraction means, or a rotating laser. A multiple beam scan can also be accomplished using a single scan laser that scans far enough in front of the vehicle to allow retaining the results of several successive scans in memory. The laser system is able to accurately determine the height or elevation of the surface of the ground at a distance in front of the blade and is not influenced by soil piled up in front of the blade as a result of the blade cutting through the soil. Such determinations or measurements are difficult with mechanical sensors.  
         [0020]    While preferred embodiments of the present invention have been shown and described herein, it will be appreciated that various changes and modifications may be made herein without departing from the spirit of the invention as defined by the scope of the appended claims.