Abstract:
A method of determining wheel slippage in a work vehicle includes the steps of: sensing an absolute ground speed of the work vehicle; calculating a ground speed of the work vehicle using at least one drive train component; comparing the absolute ground speed with the calculated ground speed; generating a scaling factor based upon the comparison; and adjusting the calculated ground speed using the scaling factor. The method may also include the steps of scaling the absolute ground speed to a threshold value; comparing the calculated ground speed with the threshold value; and engaging a differential lock if the calculated ground speed is greater than the threshold value.

Description:
FIELD OF THE INVENTION 
     The present invention relates to work vehicles, and, more particularly, to methods of determining wheel slippage and engaging a differential lock in a work vehicle. 
     BACKGROUND OF THE INVENTION 
     Work vehicles, such as agricultural, construction or forestry work vehicles, typically include an internal combustion engine which drives a transmission, which in turn drives at least one axle through a differential lock. In the event that traction is lost and a wheel begins to slip, all power is applied to the slipping wheel through the differential and the opposite wheel receives little or no torque. In the case of a rear wheel drive work vehicle, an operator can typically lock the rear wheels together by operating a foot or hand lever to engage the differential lock. By engaging the differential lock, the wheels spin at the same speed and the wheel that is not spinning can be used to regain traction. 
     The assignee of the present invention, John Deere, also manufactures and sells a front wheel assist work vehicle used primarily in the construction and agricultural markets (also known as a mechanical front wheel drive, or MFWD). With an MFWD, the front wheels are typically locked together through a differential using an electric switch on the floor. 
     Certain types of work vehicles, such as front end loaders, may be used to push earth or the like, resulting in a load which could cause wheel slippage. Another example is an agricultural tractor pulling a pull-type implement through wet soil, in which event the drive wheels could also slip. Under such conditions, it may be desirable to automatically engage the differential lock of the rear and/or front differentials without operator intervention, thereby allowing the operator to focus on the work operation at hand. 
     What is needed in the art is a method of automatically engaging a differential lock in a work vehicle upon accurate detection of wheel slippage of one or more axles. 
     SUMMARY OF THE INVENTION 
     The invention in one form is directed to a method of determining wheel slippage in a work vehicle, including the steps of: sensing an absolute ground speed of the work vehicle; calculating a ground speed of the work vehicle using at least one drive train component; comparing the absolute ground speed with the calculated ground speed; generating a scaling factor based upon the comparison; and adjusting the calculated ground speed using the scaling factor. 
     The invention in another form is directed to a method of determining wheel slippage in a work vehicle, including the steps of: sensing an absolute ground speed of the work vehicle; calculating a ground speed of the work vehicle using at least one drive train component; scaling the absolute ground speed to a threshold value; comparing the calculated ground speed with the threshold value; and engaging a differential lock if the calculated ground speed is greater than the threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic system level view of a work machine configured for carrying out an embodiment of the method of the present invention for sensing wheel slippage and automatically engaging a differential lock; 
         FIG. 2  is a schematic illustration of various inputs used in the method of the present invention; 
         FIG. 3  is a schematic illustration of a portion of the logic of the method of the present invention; and 
         FIG. 4  is a schematic illustration of another portion of the logic of the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a schematic system level view of a work vehicle  10  which is configured for carrying out an embodiment of the method of the present invention for sensing wheel slippage and automatically engaging a differential lock (also referred to as “difflock” herein). Work vehicle  10  could be a construction vehicle, agricultural vehicle or other type of work vehicle. 
       FIG. 1  generally corresponds to the inputs, control logic and outputs for an electronic control unit (ECU)  12  onboard work vehicle  10 . As shown, ECU  12  generally includes control logic for carrying out transmission speed scaling (box  14 ), radar dropout detection (box  16 ), and difflock on/off logic (box  18 ). Inputs to ECU  12  are shown in box  20 , and outputs are shown in box  22 . The various inputs shown in box  20  and the radar dropout detection shown in box  16  will be described in more detail below with regard to  FIG. 2 . The transmission speed scaling shown in block  14  will be described in more detail below with regard to  FIG. 3 . The difflock on/off logic shown in box  18  will be described in more detail below with regard to  FIG. 4 . 
     Referring now to  FIG. 2 , there is shown a schematic illustration of the various inputs used in the method of the present invention illustrated in  FIG. 1 . A drive train component  24  provides an output signal “speed_trans_out” which is used to calculate a ground speed of work vehicle  10  by ECU  12 . In the embodiment shown, drive train component  24  is preferably in the form of a transmission output shaft and corresponding sensor providing the transmission speed output signal. 
     The box labeled gear position  26  is a sensor providing a sensor output signal with an indication of whether a gear shift lever in an operators cab is in a forward, neutral or reverse position. The differential lock may only be automatically engaged when the gear shift lever is in the forward or reverse positions, and thus this signal provides an input to the boolean logic in determining whether the difflock may be engaged. 
     The box labeled brake pedal position  28  provides an output signal indicating whether the service brakes of work vehicle  10  have been engaged. The difflock is not automatically engaged if the brake pedal has been depressed. 
     The box labeled radar  30  corresponds to a radar which is used to sense an absolute ground speed of work vehicle  10 . As shown within box  30 , the radar includes a front horn  32  and a rear horn  34  positioned at an angle α therebetween (e.g., 90°), which are positioned to reflect radar signals at an angle off of the ground surface. Front horn  32  is positioned at a forward angle relative to the direction of travel of work vehicle  10 , and provides an output signal “radar_machine_speed_front_horn” corresponding to the absolute ground speed sensed by front horn  32 . Similarly, rear horn  34  is positioned at a rearward angle relative to the direction of travel of work vehicle  10 , and provides an output signal “radar_machine_speed_rear_horn” corresponding to the absolute ground speed of work vehicle  10  sensed by rear horn  34 . The radar unit also provides a composite output signal labeled “radar_machine_speed”. 
     Referring now to  FIG. 3 , the transmission speed scaling shown at box  14  in  FIG. 1  will be described in greater detail. There are three primary inputs to this control logic, indicated at inputs  36 ,  38  and  40 . Input signal  36  is the unscaled absolute ground speed of work vehicle  10  which is sensed using the radar (composite radar signal), converted to appropriate units as shown in  FIG. 1 . Input  36  is transmitted via line  42  to box  44  for speed error correction, and is also transmitted via line  46  to boolean operators making sure that certain operating conditions exist. Boolean operators at boxes  48 ,  50  and  52  ensure that scaling of the radar signal only occurs if the ground speed of work vehicle  10  is greater than 20 kilometers per hour and less than 30 kilometers per hour. The output from box  52  is a yes (1) or no (0) which is transmitted as an input to boolean operator  54 . 
     Input  38  is a signal value indicating whether the gear shift is in the forward, neutral or reverse position, and is received from gear position  26  in  FIG. 2 . In the example shown, input  38  has a value of 0, 1 or 2 with the value 2 indicating that the gear shift is in the forward position. The value of the input signal is compared at boolean operator  56  with a constant value from box  58  (i.e., in this case, the value 2) and an output signal is provided to boolean operator  54 . In the example shown, the output signal from boolean operator  56  is true (1) if the gear shift lever is determined to be in a forward position, and false (0) if not. 
     Boolean operator  54  is basically a switch which passes the value of the top line or the bottom line, depending upon the value of the signal from boolean operator  56 . If the gear shift lever is not in a forward position, then a false (0) value is passed through boolean operator  54 . On the other hand, if the gear shift lever is determined to be in a forward position, then the value of the top line is passed through boolean operator  54 , in this instance a true (1) or false (0) representing whether scaling is to take place dependent upon the operating speed of work vehicle  10 . It will thus be apparent that an output signal of one from boolean operator  54  only occurs if the gear shift lever is in the forward position and the sensed operating speed is within a particular range. 
     Boolean operator  60  receives the output signal from boolean operator  54 . If the signal value is high (i.e., a value of 1) then boolean operator  60  basically acts as a switch to pass through the value of the top line corresponding to a corrected speed error signal. Otherwise, boolean operator  60  passes through a false (0) value from the bottom line. In the event the top line is passed through, the value is a difference between a sensed absolute ground speed of work vehicle  10  and a scaled, calculated ground speed of work vehicle  10 , as will be described in more detail hereinafter. 
     The output signal from boolean operator  60  is passed to an integrator  62 , and desirably is a small number indicating a small difference between the sensed and calculated ground speeds. Such errors between the sensed and calculated ground speeds can result, e.g., because of differing tire diameters caused by inflation pressures within tires, tread wear on tires, loading on work vehicle  10 , etc. In the embodiment shown, integrator  62  integrates the speed error signal over a specified time period (e.g., approximately 10 minutes) and can be used to detect an abrupt change in the error signal over time. 
     Integrator  62  provides an output signal in the form of a scaling factor which is transmitted to box  64 . The scaling factor is used as an adjustment to the calculated ground speed represented by input signal  40 , converted to appropriate units as shown in  FIG. 1 . The adjusted, calculated ground speed is fed back via line  66  in a closed loop fashion to an input of box  44 . This continual closed loop correction between the sensed and calculated ground speeds using the scaling factor from integrator  62  should result in the scaled, calculated ground speed being closer over time to the sensed ground speed at the input to box  44 , thus resulting in a smaller speed error correction output from box  44  over time. The output signal over time at output  68  thus should be close to the sensed ground speed of work vehicle  10 . 
     At shutdown of work vehicle  10 , the scaling factor from integrator  62  is stored to a flash memory as indicated by box  70 , and is restored as indicated by box  71  as in input to integrator  62  at machine startup. 
     As shown in  FIG. 1 , the scaled, calculated ground speed is transmitted to difflock on/off logic  18 , and also is transmitted as an input to radar dropout detection logic  16 . Input signals to radar dropout detection  16  are also received from front horn  32  and rear horn  34  of the radar unit ( FIG. 2 ). Generally, radar dropout detection  16  is used to detect a signal dropout resulting from radar transmission onto a highly reflective ground surface such as water. For example, as the work vehicle  10  moves across an area of standing water, the front horn  32  would first experience a signal dropout while the rear horn  34  would continue to provide a signal while moving into the water and then also lose signal when transmitting onto the water. If the scaled, calculated ground speed input signal to radar dropout detection  16  indicates that the wheels are still turning, while one or both of the radar horns have experienced signal dropout, then an output signal is provided from radar dropout detection  16  indicating such to difflock on/off logic  18 . 
     Referring now to  FIG. 4 , the difflock on/off logic  18  will be described in greater detail. The unscaled, composite radar signal indicating the absolute ground speed of work machine  10  is transmitted as an input for difflock latch conditions  72  and radar speed scaling  74 . Radar speed scaling  74  generally is in the form of a memory with a lookup table correlating the unscaled radar ground speed of work vehicle  10  with a corresponding threshold value at which the differential lock is to be automatically engaged. In other words, for a given sensed absolute ground speed using a radar, a corresponding threshold value is compared against a scaled, calculated ground speed. If the scaled, calculated ground speed is higher than the threshold value, then the difflock is automatically engaged. The scaling of the speed radar signal to the corresponding threshold value is shown as an approximate linear relationship in  FIG. 4 . However, it is to be understood that the scaling of the speed radar signal to a corresponding threshold value need not necessarily be a linear relationship across the range of input speed radar signals. The output from speed radar scaling  74  thus corresponds to a threshold value at which the differential lock is automatically engaged. 
     A block  76  labeled difflock engagement conditions is used to determine whether to automatically engage the difflock. If the scaled, calculated ground speed is less than a maximum value (i.e., 12 kph) and higher than the scaled radar signal (i.e., threshold value), and the brake is not applied, and radar dropout has not been detected, then the difflock engagement conditions are true (1) and a corresponding signal is output to difflock latch conditions  72 . The various latch conditions for automatically engaging the difflock are shown in difflock latch conditions block  72 . 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.