Abstract:
A control system performs a method for controlling pitching and bouncing of a vehicle having an engine driving wheels through a transmission, and having a fuel control unit for supplying a variable amount of fuel to the engine in response to fuel control signals generated by an engine control unit. The method includes, from front and rear acceleration signals, generating vehicle pitch and bounce signals, converting the pitch and bounce signals to RMS pitch and bounce values, generating a fuel offset value as a function of the RMS pitch and bounce values, and modifying fuel delivered to the engine as a function of the fuel offset value. The fuel offset value is operate don by a bi-linear gain function wherein negative values are multiplied by a larger gain and positive values are multiplied by a smaller gain.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a system and method for controlling power hop in a vehicle such as an agricultural tractor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Power hop for agricultural tractors is a well known phenomena, as described by B. P. Volfson and M. Estrin in “The Slip-Stick Phenomenon in Vehicle Ride Simulation”, published by ASME “Computers in Engineering” 1983 Vol. 1. Power hop occurs most commonly with drawn implements in dry soils. Power hop can also occur on hard surfaces such as concrete or asphalt. It is believed that the power hop condition occurs when the traction force exceeds the stable limit of the tire-to-ground interface. When the traction load/pull reaches this limit, a stick-slip phenomena occurs at the tire-to-ground interface which excites the vehicle bounce and pitch natural frequencies. The resulting bounce and pitch motion is called power hop. Power hop can be pure pitch, pure bounce, or a mixture of bounce and pitch. In some cases, the bounce and pitch motions can become severe enough to cause operator discomfort, decreased implement performance, and equipment damage.  
         [0003]     Often when power hop develops on an agricultural tractor, the traction force must be brought to zero in order to stop power hop bounce and pitch oscillations. Simply reducing the traction force on the tires may not be enough to return to stable traction after the tractor bounce and pitch resonant frequencies have been excited. An operator is often required to raise the implement or stop the tractor to stop power hop oscillations. An automatic means to reduce traction force and stop power hop bounce and pitch tractor oscillations without lowering productivity is needed.  
         [0004]     An experienced operator may manually reduce the traction force by slowing down the tractor or raising the implement, since the traction force requirement is proportional to velocity and cut depth for many tillage applications. However, the operator has no way of knowing where to set the power level for stable traction. Usually a trial and error approach is used, causing the operator to experience power hop as field conditions vary. If the power level is set too high, the power hop condition will be present. An automatic means to lower the tractor steady state power level to maintain stable traction is needed.  
         [0005]     Attempts have been made to control power hop by adjusting the drawbar height, tire inflation pressure, and weight/ballast distribution. For example, European patent application EP 1 022 160, published on 26 Jul. 2000 and assigned to the assignee of this Application, describes a wheel mounting disk which is intended to more accurately center a wheel and tire on a vehicle axle. However, such a wheel mounting disk has not completely eliminated road lope. These adjustments help by changing the traction limit slightly and/or by reducing the vehicle bounce and pitch motion amplitudes. However, the physical traction limit still exists, and when exceeded, the vehicle will power hop.  
         [0006]     It has been proposed to avoid power hop by limiting the traction force of the vehicle, such as with the traction control and anti-lock bake systems available on many automobiles today. In these systems, the traction force is reduced when wheel slide is detected by a wheel speed sensor. U.S. Pat. No. 6,401,853 describes a system which detects power hop using wheel speed sensors on accelerating over-the-road trucks and limits the traction force by controlling engine torque.  
         [0007]     However, the torsional compliance of a pneumatic agricultural tractor tire is large when compared to that of an automobile tire. For this reason, the automotive and over-the-road truck approach of measuring wheel stick-slip characteristics with a wheel speed sensor is not practical for an agricultural tractor. When stick-slip is occurring at the tire-to-ground interface, the tractor axle speed may be constant. Some other suitable means for detecting power hop on an agricultural tractor is needed.  
         [0008]     U.S. Pat. No. 6,035,827, issued 14 Mar. 2000 to Heinitz et al., discloses a system wherein an engine speed signal is fed back to a characteristic function, and wherein an engine fuel quantity signal is operated on by an inverse transmission function to generate a compensated fuel quantity signal. However, the practicality of such a system is doubtful because it is believed to be difficult to extract the needed information from the feed back engine speed signal.  
         [0009]     German Published, Non-Prosecuted Patent Application DE 195 37 787 A1 discloses a method for compensating bouncing oscillations. In that reference a signal expressing the wish of the driver is filtered through the use of a transmission element. The parameters of the transmission element, and thus its transmission behavior, are changed as a function of operating parameters while the internal combustion is operating. Furthermore, a subordinate rotational speed control is used for non-steady operation, which leads to a considerable number of application parameters.  
         [0010]     U.S. Pat. No. 6,589,135, issued to Miller and assigned to the assignee of the present application, describes a proposed system an method for reducing vehicle bouncing wherein the engine power or driveline torque is varied in order to counteract vehicle pitching. However, this system did not sense both vehicle bouncing and vehicle pitching, and it was found that this system would not completely eliminate vehicle bouncing.  
         [0011]     U.S. Pat. No. 5,819,866 is believed to describe a system for controlling pitching in a vehicle operating at transport speeds wherein pitching is detected by accelerometers.  
         [0012]     It would be desirable to have a system which controls both vehicle bouncing and vehicle pitching in an agricultural tractor operating at low speed and heavy load conditions.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, an object of this invention is to provide a method and system for reducing pitching and bouncing of a vehicle, such as an agricultural tractor.  
         [0014]     A further object of the invention is to provide such a method and system which does not require tire adjusting or manipulation.  
         [0015]     A further object of the invention is to provide such a method and system which does not require extracting information from an engine speed sensor or a wheel speed sensor.  
         [0016]     These and other objects are achieved by the present invention, which is a method and system for controlling road lope in a vehicle having an engine driving wheels through a transmission having multiple gear ratios, and having a fuel control unit for supplying a variable amount of fuel to the engine in response to fuel control signals generated by an engine control unit. The method includes sensing vehicle acceleration with an accelerometer mounted on the vehicle and generating an acceleration signal in response to motion of the vehicle, generating time constant and torque gain values as a function of the transmission gear, generating an output torque value as a function of the acceleration signal, an engine torque, the time constant and the torque gain, and modifying fuel delivered to the engine as a function of the output torque value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a simplified schematic block diagram of the present invention;  
         [0018]      FIGS. 2A and 2B  are comprise a flow chart illustrating an algorithm executed by the ECU of  FIG. 1 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     In  FIG. 1 , a vehicle  10 , such as a agricultural tractor, includes an engine  12  which supplies power to a transmission  14  which has multiple gear ratios, such as a production powershift transmission. The transmission  14  drives rear wheels  15 , (including tires  16 ) mounted on an axle  17 , and selectively drives front wheels  18  mounted on a front axle  19 . Engine control unit (ECU)  22 , which includes an electronic governor (not shown), controls pump  20  which supplies a variable amount of fuel to the engine  12  in response to fuel control signals generated by the ECU  22 .  
         [0020]     The ECU  22 , in addition to other signals it normally receives, such as a throttle signal from a conventional throttle control or speed command  24 . The ECU  22  also receives a status signal from diff lock switch  26  which controls a differential lock (not shown) in the transmission  14 .  
         [0021]     A front accelerometer  28  is preferably mounted on or near the front axle  19  and provides to ECU  22  a front acceleration signal Af representing an acceleration of the portion of the tractor  10  supported above the axle  19 . A rear accelerometer  30  is preferably mounted on or near the rear axle  17  and provides to ECU  22  a rear acceleration signal Ar representing an acceleration of the portion of the tractor  10  supported above the axle  17 . The accelerometers  28 ,  30  generate analog voltages Af, Ar which will vary from a minimum voltage which represents a maximum downward acceleration to a maximum voltage which represents a maximum upward acceleration. The acceleration signals will also typically be a time varying or oscillating signal with a frequency normally ranging from approximately 1.5 to approximately 3 Hz, depending on speed, weight, tire pressure, and upon whether and what implement (not shown) may be attached to the tractor  10 .  
         [0022]     The ECU  22  also receives a vehicle speed or wheel speed signal Sp from a speed sensor  25  and a powerhop detection on/off switch  23 . The ECU  22  also communicates with a hydraulic control valve  27  which controls a conventional hydraulic valve  29 .  
         [0023]     A transmission control unit (TCU)  32  controls the transmission  14 , is coupled to the ECU  22 , and receives a gear select signal from a gear select unit (not shown), such as such as described in U.S. Pat. No. 5,406,860.  
         [0024]     Referring to  FIGS. 2A and 2B , the ECU  22  repeatedly executes an algorithm  200 . The conversion of this flow chart into a standard language for implementing the algorithm described by the flow chart in a digital computer or microprocessor, will be evident to one with ordinary skill in the art. Referring now to  FIG. 2A , algorithm  200  begins at step  202  which filters the acceleration signals Af and Ar with high pass software filter to remove the 1 g bias caused by the earth&#39;s gravity and to generated front and rear filtered acceleration values F and R. Step  204  calculates bounce and pitch acceleration values from values F and R as follows:  
         [0025]     Bounce=(R+F)/2 and Pitch=(R−F)/2.  
         [0026]     Step  205  converts the bounce and pitch acceleration values to RMS values Bounce(RMS) and Pitch(RMS) for at least one period of the vehicle bounce frequency. For example, the bounce natural frequency of a large agricultural tractor is approximately 2 Hz. It should be understood that power hop may consist of pure pitch, pure bounce or a combination thereof.  
         [0027]     Step  206  determines whether or not a power hop detection enable flag is set. Preferably, this flag is set when the following conditions are satisfied: engine power is greater than an engine power threshold, the throttle position is at a max throttle position, a transmission differential lock (not shown) is engaged, the transmission  14  is in a forward gear and the tractor speed is between a minimum speed threshold and a maximum speed threshold, and switch  23  is on. The various thresholds are experimentally determined for each type of tractor. If power hop detection is enabled, then step  206  directs the algorithm to step  210 , else to step  208  which sets a power hop flag=false and directs control to step  230 .  
         [0028]     Step  210  determines if:  
         [0029]     Bounce(RMS)&lt;Bounce off threshold,  
         [0030]     Pitch(RMS)&lt;Pitch off threshold, and  
         [0031]     Bounce(RMS)+Pitch(RMS)&lt;Bounce pitch off threshold.  
         [0032]     If all these conditions are true step  210  directs control to step  214 , else to step  212 .  
         [0033]     Step  212  determines if:  
         [0034]     Bounce(RMS)&gt;Bounce off threshold,  
         [0035]     Pitch(RMS)&gt;Pitch off threshold, and  
         [0036]     Bounce(RMS)+Pitch(RMS)&gt;Bounce pitch off threshold.  
         [0037]     If any of these conditions are true, then the algorithm proceeds to step  218 , else if all are false, to step  208 .  
         [0038]     Step  214  sets a hop on time value=zero. After step  214 , step  216  sets the power hop flag=false and directs control to step  230 .  
         [0039]     Returning to step  218 , step  218  sets the hop on time value=hop on time value plus 1 and directs control to step  220 .  
         [0040]     In step  220 , if the hop on time value is greater than a hop on time threshold, the algorithm proceeds to step  222 , else to step  224 .  
         [0041]     Step  222  sets power hop flag=false and sets hop on time=zero, then directs control to step  230 .  
         [0042]     Step  224  sets power hop flag=true, then directs control to step  230 .  
         [0043]     In step  230 , if the power hop flag is false, step  230  sets a pair of gain values Gp=Gb=zero, and if power hop flag is true, sets Gp=Gb=C, where C is a predetermined constant. After step  230 , the algorithm proceeds to step  232 .  
         [0044]     Step  232  calculates a Fuel Offset value where Fuel Offset=(Pitch×Gp)+(Bounce×Gb).  
         [0045]     If Fuel Offset equals zero, then step  234  directs control to step  236 . Step  236  sets Total Fuel Command=Engine Governor Fuel Command, where Engine Governor Fuel Command is the normal fuel command generated by the ECU  22 , and then directs control to step  242 .  
         [0046]     If, instep  234 , Fuel Offset is not equal to zero, then step  234  directs control to step  238 . Step  238 , if Fuel Offset is greater than zero, sets the Fuel Offset=Fuel Offset×G 1 , where G 1  is a first predetermined fuel offset gain value. If Fuel Offset is less than zero, then Fuel Offset=Fuel Offset×G 2 , where G 2  is a second predetermined fuel offset gain value which is preferably greater than G 1 . This results in an unsymmetrical Fuel Offset value, because the positive gain G 1  is less than the negative gain G 2 .  
         [0047]     Then, in step  240  a Total Fuel command value is calculated as follows:  
         [0048]     Total Fuel Command=Fuel Offset×Engine Governor Fuel Command, where Engine Governor Fuel Command is the normal fuel command generated by the ECU  22 .  
         [0049]     Next, step  242  sends the Total Fuel Command (from either step  236  or  240 ) to the injector pump  20  so that the amount of fuel supplied to the engine  12  is adjusted accordingly.  
         [0050]     Next, step  244  directs the algorithm back to step  202  if the power hop flag is true, else to step  246 .  
         [0051]     In order to determine if the tractor&#39;s steady state power level is to high, steps  246  and  248  calculate and monitor a cycle rate value of the fuel control algorithm. The cycle rate is calculated in step  246  by counting how often the power hop flag transitions from false to true over a fixed period of time. In step  248 , if cycle rate exceeds an experimentally determined cycle rate threshold, the vehicle power level is too high for stable traction, and step  250  shifts the transmission gear ratio to the next lower gear ratio. As a result of step  244 , this adjustment is made when the fuel control algorithm is off (power hop=false). The ECU sends a command message across the tractor electronic communication bus (CCD or CAN) for the transmission control unit (TCU) to shift the transmission (power shift or IVT) to a higher gear ratio (or lower gear number).  
         [0052]     When a tractor is in a pulling/loaded condition, the calculated RMS values of bounce and pitch provide a measurement of the level of power hop the vehicle is experiencing. The present invention defines a tractor as being in power hop condition when any of the RMS pitch and bounce accelerations exceed experimentally determined thresholds for a specified period of time (hop on time threshold). The present invention defines a tractor as being in stable traction condition when all of the values of the RMS pitch and bounce accelerations are below the experimentally determined thresholds for a specified period of time (hop off time threshold).  
         [0053]     It should be noted that a negative Fuel Offset value from step  238  causes the power level of the engine  12  to be reduced, thus reducing or eliminating the pitch and bounce accelerations and putting the tractor  10  back into a stable traction condition. The dynamic Fuel Offset value also helps to break up the bounce and pitch accelerations as the tractor returns to stable traction. Normally, when power hop occurs the engine will be operating on the rated torque curve (max power). In that condition only a small amount of additional fuel can be added before the absolute max torque curve of the engine is reached, and exceeding this fuel level can damage the engine and violates EPA regulations. Whereas, to reduce hop and bounce the amount of fuel can reduced by a larger amount without encountering such problems. Therefore, the Fuel Offset is unsymmetrical about a zero value, and the positive gain G 1  is less than the negative gain G 2 .  
         [0054]     When the tractor is experiencing power hop, the by gains Gb and Gp respectively. The resulting signals are summed and acted upon by a bilinear gain function (step  238 ). The output from the bilinear gain is summed with the engine governor (speed control) fuel command to form a total engine fuel command. By this approach, dynamic fuel commands with their associated engine torque are generated in a way that tends to cancel the power hop bounce and pitch dynamic accelerations. The bilinear gain function applies a low gain value to positive input signals, and a large gain to negative input signals. Since power hop bounce and pitch acceleration signals are symmetrical and centered about zero, the average value from the bilinear gain function is a negative fuel command. When this fuel command is summed with the engine governor fuel command, the total engine fuel command is reduced causing the tractor velocity to decrease thus lowering the traction force. The result is a system which controls both vehicle bouncing and vehicle pitching in an agricultural tractor operating at low speed and heavy load conditions.  
         [0055]     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. For example, the road lope control functions could be implemented by algorithms executed by a digital computer or microprocessor as part of an engine control unit. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.