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TECHNICAL FIELD 
       [0001]    This disclosure relates to compaction machines or compactors. More particularly, this disclosure relates to the identification and mapping of soft spots in the surface or material being compacted by compactors. 
       BACKGROUND 
       [0002]    Compactors are used for compacting various surfaces and materials associated with roads, landfills and construction sites. For example, road builders use compactors to compact asphalt, soil, gravel and other materials during road construction and repair. Thorough compaction of the underlying ground and each layer of a road provides the requisite structural stability for the road. Landfill operators use compactors to maximize the use of a landfill by compacting trash to create space for additional trash. Compacting the trash in a landfill also helps to increase the structural stability of the landfill. 
         [0003]    One type of compactor is a drum-type compactor having a heavy rotating drum for compacting or crushing the material over which the compactor is driven. In addition to the drum, such compactors may include two rear wheels and a frame. The weight of the compactor may provide the necessary weight and force for compaction. Additionally, the total weight of a compactor may be controlled by adding ballast, such as steel, concrete, calcium chloride, sand and/or water. 
         [0004]    Drum-type compactors may have a vibratory mechanism that includes movable weights arranged within the drum. Shifting the positions of the weights inside the drum induces vibrational forces that are imparted to the surface, aiding compaction. The amplitude and frequency of the vibrations may be controlled to increase the degree of compaction. Accelerometer-based measurement systems may sense the vibrational return and provide the operator with an indication of what conditions exist at or beneath the surface being compacted. Alternatively, soil conditions and stiffness can be measured using rolling resistance data from the drum and/or the wheels. 
         [0005]    Despite the use of a compactor, soft spots may occur in a surface that is being compacted. Soft spots may create safety hazards, reduce vehicle traction or preclude a stable foundation with a known and uniform density. Such soft spots are known to occur during road construction, landfill compaction or in almost any surface that is being compacted. As soft spots may not be visible to an operator of a compactor or other member of the crew, identifying and recording the location of soft spots is necessary for their timely and fast repair. These repairs may be accomplished by various means, including adding material to the soft spot, removing the soft material and replacing it with denser material, making additional passes over the soft spot with the compactor and/or digging up a foreign object that may be buried under the surface. 
         [0006]    However, because soft spots may not be clearly visible, precisely locating a soft spot that has been traversed by a compactor remains problematic because there is no convenient means for recording the precise location of a soft spot or transmitting the precise location of a soft spot to a repair crew. U.S. Pat. No. 8,073,609 (Koch) discloses a system for sensing slippage of a truck or vehicle travelling on a road or path and recording the location of the slippage on a map. The system of Koch uses the recorded location by adjusting a speed and/or steering angle of a truck subsequently travelling through the recorded location in response to an anticipated or actual slip. 
         [0007]    There is a need for an improved compactor capable of more accurately identifying and recording the location of soft spots so soft spots can be quickly and properly repaired. 
       SUMMARY OF THE DISCLOSURE 
       [0008]    In one aspect, a system is disclosed for identifying and mapping soft spots traversed by a vehicle. The vehicle may include a first wheel and a second wheel and the first and second wheels may be disposed on opposite sides of the vehicle. The system may include a first slip sensor for detecting the rotational speed of the first wheel. The first slip sensor may be linked to a mapping system for transmitting first wheel rotational speed data to the mapping system. The system may further include a GPS sensor for generating GPS data. The GPS sensor may be linked to the mapping system for transmitting the GPS data to the mapping system. Further, the mapping system may be configured to identify the location of the first wheel slip based on the first wheel rotational speed data and the GPS data. 
         [0009]    In another aspect, a compactor is disclosed. The compactor may include a first side including a first wheel and a second side including a second wheel. The compactor may further include a first slip sensor for detecting the rotational speed of the first wheel and a second slip sensor for detecting the rotational speed of the second wheel. The first and second wheel slip sensors may be linked to a mapping system for transmitting the first and second wheel rotational speed data to the mapping system. The compactor may further include a GPS sensor for gathering GPS data. The GPS sensor may be linked to the mapping system for transmitting GPS data to the mapping system. The mapping system may be configured to identify a location and a compactor side of a soft spot based on the first wheel rotational speed data, the second wheel rotational speed data and the GPS data. 
         [0010]    In another aspect, a method is disclosed for using wheel slip of a compactor to identify and locate soft spots. The disclosed method may include providing a compactor with a first side including a first wheel and a second side including a second wheel. The method may further include detecting a rotational speed of the first wheel and generating first wheel rotational speed data. The method may further include detecting a rotational speed of the second wheel and generating second wheel rotational speed data. The method may further include collecting GPS data for the compactor. The method may further include determining if the first wheel slips based on the first wheel rotational speed data exceeding a predetermined value and, if the first wheel slips, determining a first location along the first side of the compactor where the first wheel slips based on the GPS data. The method may further include determining if the second wheel slips based on the second wheel rotational speed data exceeding a predetermined value and, if the second wheel slips, determining a second location along the second side of the compactor where the second wheel slips based on the GPS data. The method may further include identifying a soft spot location that may encompass the first location, the second location or both the first and second locations. Finally, the method may include recording the soft spot location. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a side view of a compactor. 
           [0012]      FIG. 2  is a top view of a compactor with portions removed to show interior details. 
           [0013]      FIG. 3  is a top view of another compactor with portions removed to show interior details of an alternate embodiment from  FIG. 2 . 
           [0014]      FIG. 4  is a schematic map depicting elements which may be included in an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates a compactor  12 , which may include an engine  14 , a cab  16  and a drum  18 . In an exemplary embodiment, compactor  12  is a soil compactor. In alternative embodiments, compactor  12  may be any other compactor having wheels, such as a landfill compactor. The drum  18  is located forward of the cab  16  and rotates as the compactor  12  rolls over a surface  19 . The cab  16  may include various controls for an operator. As shown in  FIG. 2 , the compactor  12  may also include a first side  20  with a first wheel  22  and a second side  26  with a second wheel  28 . 
         [0016]    As further shown in  FIG. 2 , the compactor  12  may include a differential  32  coupled to an axle  34 , which may distribute power to one or both wheels  22 ,  28  from the differential  32 . The engine  14  may power a hydraulic pump  36  that drives a primary hydraulic motor  40  for transmitting hydraulic power to the differential  32  and one or both of the wheels  22 ,  28 . The engine  14  may also provide power to the differential  32  through mechanical means. 
         [0017]    Additionally, the compactor  12  may include a secondary hydraulic motor  42  and a tertiary hydraulic motor  43  that may be powered by a hydraulic pump  36 , as shown in  FIG. 3 . The secondary and tertiary hydraulic motors  42 ,  43  may each power one of the wheels  22 ,  28  independent of each other. Thus, engine  14  may power one or both of the wheels  22 ,  28  and the drum  18  mechanically or hydraulically and this disclosure is applicable to both types of drive systems. 
         [0018]    As the compactor  12  rolls, its mass, along with the shape of the drum  18 , compacts the surface  19  over which the compactor  12  traverses. However, as the compactor  12  travels over the surface  19 , the wheels  22 ,  28  may slip due to a lack of traction caused by a soft spot. The power delivered to the wheels  22 ,  28  by the engine  14  may exceed the capacity of one or both of the wheels  22 ,  28  to transfer power to the surface  19  without slipping because of the lower ground density of a soft spot in the surface  19 . For example, one wheel  22  may travel over a soft spot in the surface  19  while receiving power from the engine  14  and the other wheel  28  may travel over a dense area of the surface  19  while receiving power from the engine  14 . In this situation, the first wheel  22  may lose traction and slip while the second wheel  28  may transmit the power it receives to the surface  19  without slipping. In this example, detecting that the wheel  22  slips while detecting no slippage from the other wheel  28  may be used by a system  10  to isolate a soft spot to the first side  20  of the compactor  12  as opposed to both sides  20 ,  26  of the compactor  12 . Identifying and recording the location of soft spots allows for the quick repair of a soft spot. As soft spots may not be visible, locating a soft spot as precisely as possible allows repair work to proceed more quickly and accurately without having to search for the soft spot, saving time and expense. 
         [0019]    The system  10  may include a first slip sensor  50  to detect slippage of the first wheel  22  and a second slip sensor  52  to detect slippage of the second wheel  28 . The slip sensors  50 ,  52  may be speed sensors located near each wheel  22 ,  28 , along the axle  34  or at various points along the powertrain, as shown schematically in  FIG. 2 . The slip sensors  50 ,  52  may mechanically or electrically detect rotation and sudden changes in the rotational velocity of the wheels  22 ,  28 , and accordingly measure for slip. Alternatively, the system  10  and compactor  12  may include one or more pressure sensors  54 ,  55  in communication with a hydraulic circuit  38  that includes at least the hydraulic pump  36  and the secondary and tertiary hydraulic motors  42 ,  43  ( FIG. 3 ). The pressure sensor  54  may sense a pressure at a point along the hydraulic circuit  38  between the hydraulic pump  36  and the secondary hydraulic motor  42  or between the secondary hydraulic motor  42  and the first wheel  22 . Similarly, the pressure sensor  55  may sense a pressure at a point along the hydraulic circuit  38  between the hydraulic pump  36  and the tertiary hydraulic motor  43  or between the tertiary hydraulic motor  43  and the second wheel  28 . Drops in the pressures measured by the pressure sensors  54 ,  55  can be indications of slippage at either of the wheels  22 ,  28 . The pressure sensors  54 ,  55  may function as to determine slip at each wheel  22 ,  28  independently by measuring the pressures at points within the hydraulic circuit  38  or at the secondary and tertiary hydraulic motors  42 ,  43 . 
         [0020]    Thus, the system  10  employs the first and second slip sensors  50 ,  52  (and/or pressure sensors  54 ,  55 ) to detect or recognize slip in each of the wheels  22 ,  28 . Slip, in the form of rotational speed of either wheel  22 ,  28  above a certain predetermined or threshold value, may be indicative of a soft spot in the surface  19 . The threshold value may be established by measuring and analyzing a variety of factors. These factors may include type, pressure and condition of the tires (not shown) mounted on the wheels  22 ,  28 , the weight of the compactor  12 , soil conditions and/or conditions of the surface  19 . These factors may also include environmental elements, such as temperature, pressure and humidity. Alternatively, the slip sensors  50 ,  52  may measure varying degrees of wheel slip independent of a threshold value. The slip sensors  50 ,  52  or pressure sensors  54 ,  55  may transmit the rotational speed data to the mapping system  64  as shown in  FIG. 4 . 
         [0021]    As shown in  FIGS. 2 and 3 , the system  10  may also include a GPS sensor  58 , an orientation sensor  62 , a mapping system  64  and a display  66 . As shown in  FIG. 4 , the mapping system  64  may be linked to the GPS sensor  58  for receiving GPS data and the mapping system  64  may also be linked to the orientation sensor  62  for receiving orientation data. Further, the mapping system  64  may be linked to the slip sensors  50 ,  52  for receiving rotational velocity data or slip data from slip sensors  50 ,  52  for each wheel  22 ,  28 . If pressure sensors  54 ,  55  are used, the mapping system  64  may be linked to the pressure sensors  54 ,  55  for receiving pressure data indicative of the traction or slippage experienced at the wheels  22 ,  28 . The system  10  may also communicate with a mapping network  68  for purposes of transmitting data indicative of a soft spot to a remote location or to another compactor or machine. 
         [0022]    The orientation sensor  62  may be located on the compactor  12  for detecting the directional orientation of the compactor  12  relative to a stationary ground reference and transmitting orientation data to the mapping system  64 . The orientation sensor  62  may employ magnetic components for generating orientation data for delivery to the mapping system  64 . This orientation data can be combined with wheel rotational speed data from the slip sensors  50 ,  52  or pressure data from the pressure sensors  54 ,  55  and GPS data from the GPS sensor  58  to determine the precise location of a soft spot under one or both of the wheels  22 ,  28 . Using this technique, the position, orientation and wheel slip data can be combined to isolate the soft spot to a first side  20  or second side  26  of the compactor  12 . The GPS sensor  58  may gather GPS data and either the mapping system  64  and/or the GPS sensor  58  may determine the global location of the compactor  12  continuously during the operation of the compactor  12 . This continuous locational data may also be used to determine the orientation of the compactor  12 . 
         [0023]    If the mapping system  64  has isolated the soft spot to a particular location and side of the compactor  12 , this information may be sent from the mapping system  64  to a display  66 . The display  66  may be mounted in the cab  16  and may be visible to an operator. The information can be displayed visually, in conjunction with locational images, in conjunction with moving images or overlaid on a map. Accessibility to this information may allow an operator to quickly identify or avoid soft spots. Transmission of this information to the mapping network  68  ( FIG. 4 ) may allow a contractor or supervisor to institute a soft spot repair procedure in a timely manner. 
         [0024]    The mapping network  68  may include information from other displays, information for other vehicles or compactors, centralized information hubs and means for informational communication therebetween. The mapping network  68  could allow other vehicles or compactors to avoid a soft spot or communicate a soft spot location to repair crews. The mapping network  68  may also allow storage of the information away from the mapping system  64  for redundancy and security. The information could also be sent to a centralized information hub to convey the ground condition of a particular site. 
         [0025]    A method of using wheel slip to help identify and locate soft spots is also disclosed. The method may include providing a compactor  12 , wherein the compactor may have a first side  20  including a first wheel  22  and a second side  26  including a second wheel  28 . The method may further include detecting a rotational speed of the first wheel  22  and generating first wheel rotational speed data and detecting a rotational speed of the second wheel  28  and generating second wheel rotational speed data. The method may further include collecting GPS data for the compactor  12 . The method may further include determining if the first wheel  22  slips based on the first wheel rotational speed data exceeding a predetermined value and, if the first wheel  22  slips, determining a first location where the first wheel  22  slips based on the GPS data. The method may further include determining if the second wheel  28  slips based on the second wheel rotational speed data exceeding a predetermined value and, if the second wheel  28  slips, determining a second location where the second wheel  22  slips based on the GPS data. The method may further include identifying a soft spot location and on which side or sides of the compactor  12  the soft spot is located based on if the first  22  and/or second wheel  28  slips and where the first  22  and/or second wheel  28  slips. The method may also include recording the location of the soft spot. 
         [0026]    The disclosed method may also include enabling the mapping system  64  to send the information to a mapping network  68  including other displays, other vehicles or compactors and remote locations, and determining a threshold value to correspond to the slip a wheel  22 ,  28  would experience while traveling over a soft spot. 
       INDUSTRIAL APPLICABILITY 
       [0027]    When a compactor  12  traverses a soft spot in a surface  19 , a rear wheel  22 ,  28  may traverse the soft spot, while the other rear wheel does not. As a result, the rear wheel that traverses the soft spot will experience lower traction than the opposite wheel traveling over a properly compacted area, causing the wheel traversing the soft spot to slip. Monitoring and recording which individual wheel slips while traveling over a soft spot can help isolate the soft spot to a particular side of the compactor  12 . If both wheels  22 ,  28  slip, the soft spot is determined to extend across both sides of the compactor  12 . 
         [0028]    To alleviate this problem, a system  10  and the compactor  12  include slip sensors  50 ,  52  and/or pressure sensors  54 ,  55  associated with each wheel  22 ,  28  along with a GPS sensor  58  and a mapping system  64 . The system  10  and compactor  12  may include an orientation sensor  62 . The mapping system  64  may use wheel slip data from the slip sensors  50 ,  52  and/or pressure sensors  54 ,  55 , compactor  12  orientation data from the orientation sensor  62 , and GPS data from the GPS sensor  58  to determine a precise location of a soft spot under one or both wheels  22 ,  28 . Using this technique, compactor position, directional orientation and wheel slip data may be combined to isolate the soft spot to a first side  20  or a second side  26  of the compactor  12 . The precise location of a soft spot may be transmitted over a mapping network  68 , which may be used by a contractor or supervisor to dispatch equipment or personnel to repair the soft spot in a timely fashion. Identifying and recording the location of soft spots allows for their quick repair because workers no longer have to search for soft spots, thereby saving costs and time. The disclosed system  10  may be original equipment on new compactors  12  or added as a retrofit to existing compactors  12 .

Summary:
A compactor gathers GPS, orientation and wheel slip data to identify the location of a soft spot in a surface that is being compacted and to isolate the soft spot to a particular side of the compactor if the wheel slip data indicates that the soft spot is located beneath only one of the compactor wheels. The GPS, orientation and wheel slip data are displayed as location information to an operator and/or sent to a remote location to facilitate the fast and accurate repair of the soft spot.