Abstract:
An object detection system may be capable of sensing a buried object and providing an estimate of the object&#39;s depth. The object detection system may comprise a signal generator transmitting one or more signals. At least one of the signals may be directed toward the buried object and reflected off of the object back to the system. At least one of the signals may be transmitted along a variable length path. A correlator may then receive the signals and determine an offset between their arrival times at the correlator. The variable length path may then be adjusted over a range which includes a minimum offset indicating a distance to the object.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Pat. App. No. 61/678,377 filed on Aug. 1, 2012, which is incorporated herein by reference for all that it contains. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to Ground Penetrating Radar (GPR) systems, and more particularly, to timing within GPR systems. 
         [0003]    GPR systems often comprise at least one transmitter that transmits an electromagnetic impulse. The transmitted impulse may reflect off of a buried object to create a reflected electromagnetic waveform. At least one receiver may receive the reflected waveform to collect information about the object. Difficulty arises, however, in accurately scheduling the time when each transmitter transmits an impulse signal and when each receiver samples a received waveform. 
         [0004]    U.S. Pat. No. 6,496,137 to Johansson, which is herein incorporated by reference for all that it contains, describes a GPR timing system comprising a control circuit that receives a transmit timing input signal and a receive timing input signal. The system delays the transmit timing input signal and generates a number of intermediate transmit timing signals delayed with respect to each other by a delay time, selects either the transmit timing input signal or a corresponding one of the intermediate transmit timing signals as a corresponding output transmit timing signal, delays the receive timing input signal and generates a number of intermediate receive timing signals delayed with respect to each other by the delay time, adds the delay time to the intermediate receive timing signals, and selects either the receive timing input signal or a corresponding one of the intermediate receive timing signals as a corresponding output receive timing signal. 
         [0005]    U.S. Pat. No. 5,835,054 to Warhus, et al., which is herein incorporated by reference for all that it contains, describes how the accuracy of a calculation of distances using a GPR system depends on how precisely a time-delay of echoes can be measured. Ideally, each returned echo is a scaled and time-shifted version of the transmitted pulse. Thus, a reference point on the pulse can be used to calculate the time-delay, e.g., either the leading or trailing edge of the pulse. In the presence of noise, however, such reference points can be hard to determine. Correlation or inverse filtering of the incoming signal with the transmitted pulse can be used to produce a peak at the time-delay of the echo and provide a reference point that is much easier to locate. 
         [0006]    Despite these advances, simplified means for timing GPR systems is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    An object detection system may be capable of sensing a buried object and providing an estimate of the object&#39;s depth. The object detection system may comprise a signal generator transmitting one or more signals. At least one of the signals may be directed toward the buried object and reflected off of the object back to the system. At least one of the signals may be transmitted along a variable length path. A correlator may then receive the signals and determine an offset between their arrival times at the correlator. The variable length path may then be adjusted over a range which includes a minimum offset indicating a distance to the object. 
         [0008]    In various embodiments of the invention, the signals may be generated concurrently, as substantially identical and/or random signals. 
         [0009]    In order to direct a signal toward the buried object, the system may comprise a transmitter/receiver device for transmitting at least one of the signals toward the object and receiving a reflected signal from the object. The transmitter/receiver device may comprise a single antenna, or separate antennas may be used for transmission and reception. 
         [0010]    In order to adjust a length of the variable length path, the system may comprise a plurality of delays that may be individually incorporated into the variable length path. For example, each of the plurality of delays may comprise a length of wire or transmission line such as a coaxial cable, microstrip transmission line, stripline, or other types of transmission line known in the art. The lengths of wire or transmission lines may each be of equal length or of different lengths. A switch may incorporate one of the plurality of delays into the variable length path to alter the length of the variable length path. In such an embodiment, the plurality of delays may be connected in parallel. In other embodiments, each of the plurality of delays may comprise its own switch that may incorporate each delay into the variable length path. Such delays and switches may be connected in series. Also, groups of delays may be connected with other groups of delays with series or parallel connections between the groups. 
         [0011]    Another means to adjust the length of the variable length path may be a slidable link, or continuously-adjustable delay line, such as an adjustable coaxial delay line, or other adjustable-length transmission lines, often known as ‘trombones,’ capable of altering the length of the variable length path. 
         [0012]    Such various adjustment means may be controlled by an operator or automatically adjusted by a delay control. Alternatively, a delay control may adjust a time axis of a display. 
         [0013]    In various embodiments, the object detection system may be secured to a vehicle. The vehicle may further comprise a degradation drum that may comprise a plurality of picks disposed around an outer perimeter thereof and that may be rotated such that the plurality of picks engage and degrade a surface. A height of the drum may be adjusted to avoid engaging the object. The object detection system may also be secured to a handheld frame. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is an orthogonal view of an embodiment of a vehicle comprising an object detection system disposed thereon. 
           [0015]      FIG. 2  is a cutaway view of an embodiment of an object detection system disposed on a vehicle. 
           [0016]      FIG. 3  is a schematic block diagram of an embodiment of an object detection system. 
           [0017]      FIG. 4  is another schematic block diagram of an embodiment of an object detection system. 
           [0018]      FIG. 5  is a graph of an embodiment of a plurality of signals. 
           [0019]      FIG. 6  is an orthogonal view of an embodiment of a handheld object detection system. 
           [0020]      FIG. 7  is an orthogonal view of an embodiment of a truck comprising an object detection system. 
           [0021]      FIG. 8  is a schematic block diagram of an object detection system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  discloses an embodiment of vehicle  100 , such as a milling machine, traveling over a paved surface  103 , such as asphalt or concrete. A degradation drum  101  comprising a plurality of picks  102  disposed around an outer perimeter thereof may be secured to an underside of the vehicle  100 . The degradation drum  101  may be rotated such that the plurality of picks  102  engage and degrade the paved surface  103 . 
         [0023]    Objects  104  such as rebar, railroad tracks and manhole covers may be buried beneath the paved surface  103  and may damage the picks  102  during degradation operations. To prevent damage to the picks  102 , an object detection system  105  comprising a ground penetrating radar unit may be disposed on the vehicle  100  to locate and collect information about the objects  104 . A height of the degradation drum  101  or the vehicle  100  may be adjusted to avoid engaging the objects  104  with the picks  102 . 
         [0024]      FIG. 2  discloses an embodiment of a vehicle  200  similar to the one referenced in the discussion of  FIG. 1  comprising a degradation drum  201  secured to an underside thereof. An object detection system  205  may be secured on the vehicle  200  forward of the degradation drum  201 . The object detection system  205  may locate buried objects  204  prior to them being engaged by the degradation drum  201  by transmitting radio waves  209  into a subsurface region  203  within which the buried objects  204  may be located and collecting reflected waves  254  that may reflect off of the buried objects  204 . The time required for the reflected waves  254  to return to the object detection system  205  may be used to estimate a distance to the buried objects  204 . 
         [0025]      FIG. 3  discloses an embodiment of an object detection system  305  in schematic form. Wire connections are shown between circuit elements, however, those of skill in the art will recognize that lower frequency radio signals may be carried on short wires while higher frequency radio signals may be carried on transmission lines such as microstrip, stripline, coaxial cables, or other types of transmission lines. In the embodiment shown, the object detection system  305  comprises a signal generator  306  that may transmit one or more signals  307 . The signals  307 , after being split by a power divider or splitter, may travel along two distinct paths to a correlator  311  that may compare the signals  307  to determine the relative delays between the paths traveled. 
         [0026]    For example, a first signal  308  may travel along a variable length path comprising a switch  314  and a plurality of switch contacts  387 . Each of the plurality of switch contacts  387  may correspond to a different length of path. As the switch  314  is actuated the length of the path may vary. 
         [0027]    A second signal  309  may travel along a path including a transmitter/receiver device  333 , such as an antenna, that may transmit the second signal  309  toward an object  304 . The second signal  309  may reflect off of the object  304  and be received again by the transmitter/receiver device  333 . 
         [0028]    If the first signal  308  and second signal  309  are substantially in phase when they reach the correlator  311  then they may have traveled a substantially related distance. If the first signal  308  and second signal  309  are not substantially in phase when they reach the correlator  311  then the switch  314  may be actuated to check a variety of preset distances for a better fit. For example, a first switch contact  316  may correspond to a short distance. If the first signal  308  and second signal  309  are not substantially in phase, the switch  314  may be changed from the first switch contact  316  to a second contact  317  that may correspond to a longer distance. If the first signal  308  and second signal  309  are now substantially in phase then a correlation value determined by the correlator will increase and a distance to the object  304  may be estimated by determining the delay difference between the two paths. 
         [0029]      FIG. 4  discloses an embodiment of another style of object detection system  405  in schematic form. The object detection system  405  may comprise a signal generator  406  that may transmit one or more signals  407 . Similar to the embodiment shown in  FIG. 3 , the signals  407  may travel along two distinct paths to a correlator  411  where they may be compared to determine relative time differences or differences in the lengths of the paths traveled. However, in this embodiment, a first signal  408  may travel along a variable length path made adjustable by a variable length delay  412 . The variable length delay  412  may comprise a plurality of individual delays  413  that may be uniquely controlled to create a desired path length. 
         [0030]    For example, by actuating a switch  414  the first signal  408  may be conducted through a first switch contact  416  and thus along a first wire or transmission line  418  of insignificant length or through a second switch contact  417  and thus along a second wire or transmission line  419  of significant length. The individual delays  413  may be connected in series such that each may further lengthen the path traveled by the first signal  408 . An increased number of delays  413  may improve the accuracy and range of the object detection system  405 . 
         [0031]    Similar to the embodiment shown in  FIG. 3 , a second signal  409  may travel along a path including a transmitter/receiver device  433  that may transmit the second signal  409  toward an object  404 . The transmitter/receiver device  433  may comprise a broadband antenna, a bowtie antenna, or other antennae known in the art. The second signal  409  may reflect off of the object  404  and be received again by the transmitter/receiver device  433 . The transmitter/receiver device  433  may comprise a single antenna which both transmits and receives or two antennas, one for transmitting and one for receiving. It should be appreciated that for simplicity, only the most essential devices are shown in the schematics. Additional circuit elements, such as RF amplifiers, filters, etc., may be added to the basic embodiments to improve performance. 
         [0032]    The first signal  408  and second signal  409  may have a time offset upon their arrival at the correlator  411  that may be due to the relative difference in distances and velocities of propagation over which the two signals traveled. A delay control  421  may automatically adjust the variable length delay  412  to find the closest fit allowing for a distance to the object  404  to be estimated. The delay control may also be used to control a time-axis of a display device that shows correlation or the return signal as a function of time. 
         [0033]      FIG. 5  discloses graphs representing embodiments of first and second signals as they may arrive at a correlator as described in relation to  FIGS. 3 and 4  above. In the graphs shown, time is represented on x-axes while signal amplitude is represented on y-axes. Additionally, t 0  represents a minimum additional delay while t m  represents a maximum additional delay. Times t 1 -t 3  represent various additional delays between the minimum and maximum additional delays. 
         [0034]    A second signal  509  may travel along a path including a round trip to and from a buried object. To estimate a distance to the buried object, the second signal  509  may be compared to a first signal  508  that may travel along a path of known length. Due to differences between the distance to the buried object and the known length, an offset  542  may exist between the first signal  508  and second signal  509  as they arrive at a correlator. A switch may actuate to divert the first signal  508  through a first distance of wire, transmission line, or first delay, that will alter the distance the first signal  508  travels before reaching the correlator. Due to the altered distance, the first signal through first delay  545  may comprise an offset  552  from the second signal  509  different from that of the first signal  508  without any additional delay. In a like manner, the switch may be further actuated to pass the first signal  508  through a plurality of delays of known distances or time delays. After each actuation of the switch, a delayed first signal may be compared to the second signal  509 . While the present embodiment shows three delays, any number of delays is contemplated. 
         [0035]    In this embodiment, an offset  562  of the first signal through second delay  546  from the second signal  509  is substantially smaller than an offset  552  of the first signal through first delay  545  or offset  572  of the first signal through third delay  547 . Thus, the distance to a buried object may be estimated based on the known distance of the second delay. 
         [0036]    The correlator may function by combining at least two signals, such as the first and second signals as described in relation to  FIGS. 3 and 4 , and measuring the correlation. A stronger correlation indicates a smaller offset and therefore a smaller difference between the two signals. The simplicity of the correlator, as compared to many of the timing systems currently used in the prior art, allows for a variety of signal types to be used with the present invention. Even random signals may be used as signals. 
         [0037]      FIG. 6  discloses an embodiment of a handheld object detection system  605 . An elongate frame  667  may be carried by an operator  668  by means of a handle  686 . The object detection system  605  may be disposed at an opposite end of the elongate frame  667  from the handle  686 . The object detection system  605  may locate buried objects  604  by transmitting radio waves and collecting reflected waves  609  that may reflect off of the buried objects  604 . The time required for the reflected waves  609  to return to the object detection system  605  may be used to estimate a distance to the buried objects  604 . A display  672  may also be disposed on the elongate frame  667  to show a distance to the objects  604 . 
         [0038]      FIG. 7  discloses an embodiment of an object detection system  705  attached to a vehicle  700 , such as a truck. The vehicle  700  may drive over a paved surface  703  to be degraded. The object detection system  705  may locate objects  704  buried beneath the paved surface  703 . A transmitter device  773  may be mounted to the vehicle  700 . The object detection system  705  may send data  764  containing locations of the objects  704  beneath the paved surface  703  to a computing device (not shown) via the transmitter device  773 . The computing device may record the data for a reference when degrading the paved surface  703 . 
         [0039]      FIG. 8  discloses an embodiment of an object detection system  805  in schematic form comprising a variable length between a signal generator  806  and a correlator  811 . The variable length may be formed by a telescoping wire or transmission line component comprising a slidable link  826 . The slidable link  826  may comprise a first part  828  that may slide with respect to, while maintaining contact with, a second part  829  thus changing the length of the slidable link  826  and a distance a signal may travel from the signal generator  806  to the correlator  811 . 
         [0040]    Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.