Abstract:
A simulation system ( 30 ) for simulating an operation of an automotive vehicle includes an input ( 34 ) providing vehicle information and path information and a controller ( 38 ) having a vehicle computer model therein. The controller ( 38 ) is programmed to determine a curvature of an intended path from the path information, determine a look ahead scale factor as a function of the intended path, determine a look ahead point as a function of the look ahead scale factor, determine a steering wheel angle input to the computer model by comparing the look ahead point and the intended path, operate the computer model with the steering wheel angle input, and generate an output in response to the vehicle model and the initial steering wheel input or the first steering wheel input.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention is related to U.S. applications (Attorney Docket No. 202-1479/FGT-1889) entitled “Method And Apparatus For Controlling A Vehicle Computer Model With Understeer”, and (Attorney Docket No. 202-1480/FGT-1838) entitled “Method And Apparatus For Controlling A Vehicle Computer Model With Oversteer”, filed simultaneously herewith. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     The present application relates generally to computer models that test the dynamics of an automotive vehicle, and more particularly, to a method for operating a vehicle model in an aggressive or limit-seeking manner with an adaptive look ahead.  
         [0003]     In the development of automotive vehicles, computer vehicle models are often used to test various designs. The various designs may be used to efficiently assess the handling of the vehicle using various parameters.  
         [0004]     Current algorithms for computer driving control of computer vehicle models are designed to efficiently follow a given path. That is, the given path is accurately followed to provide minimal loss of speed due to side slipping of the computer vehicle model. Typically, such systems use a simulated look ahead of a driver to determine whether the vehicle is on the desired path. This is illustrated in step  10 . In step  12  it is determined whether or not the vehicle is on target or on the desired path. If the computer vehicle model is on the desired path, step  14  is executed in which no change in the steering wheel angle is provided. The system then continues to step  16  in which the next time increment of the model is provided.  
         [0005]     Referring back to step  12 , if the vehicle is not on its intended path or on target step  18  is executed in which a new steering wheel angle (SWA) based on the size of the error between the look ahead path and the intended path is determined. In step  20  the computer model responds to the new steering wheel angle.  
         [0006]     One problem with current computer vehicle models is that they are not typically designed to test the limits of control of the vehicle.  
         [0007]     It would be desirable to be able to test the vehicle at aggressive or limit-seeking driving conditions. Typically, the computer model produces undesirable results that do not simulate real world driving when pushed to its limits. Typically, computer models generate undesirable steering wheel angles to compensate for variations in the desired path. The results are therefore not usable in the assessment of vehicle handling for such events. Therefore, it would be desirable to provide meaningful results when the vehicle model is driven aggressively, driven with understeer or oversteer.  
       SUMMARY OF INVENTION  
       [0008]     The present invention allows a vehicle computer model to be driven near its limits to allow vehicle designers to assess the vehicle handling.  
         [0009]     In one aspect of the invention, a method of operating a vehicle computer model having vehicle information and path information therein includes determining a curvature of an intended path from the path information, determining a look ahead scale factor as a function of the intended path, determining a look ahead point as a function of the look ahead scale factor when the vehicle is not on target, determining a steering wheel angle input to the computer model by comparing the look ahead point and the intended path, operating the computer model with the steering wheel angle input, and when the vehicle is on target, maintaining a previously determined steering wheel angle.  
         [0010]     In a further aspect of the invention, a simulation system for simulating an operation of an automotive vehicle includes an input providing vehicle information and path information and a controller having a vehicle computer model therein. The controller is programmed to determine a curvature of an intended path from the path information, determine a look ahead scale factor as a function of the intended path, determine a look ahead point as a function of the look ahead scale factor, determine a steering wheel angle input to the computer model by comparing the look ahead point and the intended path, operate the computer model with the steering wheel angle input, and generate an output in response to the vehicle model and the initial steering wheel input or the first steering wheel input.  
         [0011]     One advantage of the invention is that useful information may be obtained from vehicle models to allow vehicle designers to assess various vehicle designs in various limit-seeking an aggressive maneuvers. This, advantageously, will reduce the overall costs of development of the vehicle. That is, if more accurate information can be obtained using vehicle models, fewer prototypes will be built to test various designs.  
         [0012]     Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]      FIG. 1  is a flow chart illustrating the operation of a prior art vehicle computer model.  
         [0014]      FIG. 2  is a block diagrammatic view of a simulation system for operating a vehicle model on a computer driven path.  
         [0015]      FIG. 3  is a diagrammatic view of a vehicle on a simulated road illustrating various dynamic conditions.  
         [0016]      FIG. 4  is a flow chart of a vehicle computer model in an understeering condition.  
         [0017]      FIG. 5  is a flow chart illustrating a vehicle computer model in an oversteering condition.  
         [0018]      FIG. 6  is a plot of a look ahead scale factor versus curvature.  
         [0019]      FIG. 7  is a flow chart of a vehicle computer model in an aggressive driving situation using the look ahead scale factor of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0020]     In the following figures the same reference numerals will be used to illustrate the same components.  
         [0021]     In the following figures a vehicle computer model is described. The computer model may be run on various types of computers, including main frames or personal computers. The present system, as described below, may be used in aggressive limit-seeking manners. The system may be used when the vehicle is in an understeering condition. Vehicle understeering occurs when the front of the vehicle is plowing. That is, understeering is when the vehicle does not respond to a change in the steering wheel angle.  
         [0022]     Oversteering is when the rear of the vehicle fishtails or slips out laterally relative to the front of the vehicle.  
         [0023]     Referring now to  FIG. 2 , a simulation system  30  is illustrated. Simulation system  30  has a computer  32  that has an input device  34  and an output  36  coupled thereto. Computer  32  may be various types of computers including a main frame computer or a personal computer or a network personal computer. Input device  34  may include various types of input devices for inputting various information such as a keyboard, a mouse or trackball, or other types of information such as complete files as in a CD-ROM or other information stored within a memory.  
         [0024]     Output device  36  may include various types of output devices including a screen display, printer output or file outputs such as a disk drive or CD-ROM drive. Of course, various combinations of input devices  34  and output devices  36  may be used in various commercial embodiments.  
         [0025]     Computer  32  includes a controller  38  that is used to control a simulation using vehicle model  40 . Vehicle model  40  may be manually input or selectively input using various input devices  34 . Information such as a desired path information  42  or vehicle information  44  such as dynamic control information may be input using at least one of the input devices  34 . The input device  34  may also initiate the operation of the vehicle simulation and input the desired path or changes in the desired path.  
         [0026]     The controller  38  generates an output that may be provided to output device  36 . Output  46  may include various limits, handling, reactions to double lane changes or the like. The various information provided by output  46  may be used to assess the vehicle&#39;s handling in aggressive driving and limit situations.  
         [0027]     In  FIG. 3 , a representation of a vehicle  50  on a road surface corresponding to a path  52  is illustrated.  FIG. 3  may represent a screen display. However, in an actual simulation a screen display may not actually be used.  FIG. 3  is intended to provide a visual representation of some of the variables used in the vehicle model as described below. For example, the vehicle has a steering wheel  54  that is controlled by the computer model to traverse the intended path. The steering wheel  54  thus has a steering wheel angle (SWA). Typically, the steering wheel angle is measured from zero to a plus or minus angular direction.  
         [0028]     The path  52  has a curvature with a radius represented by R. The vehicle model also includes a look ahead point  58  that has a look ahead distance which is the distance that the model looks ahead in order to determine the desired steering wheel angle of the vehicle as will be described below. Also, as further described below, the look ahead distance may be variable as opposed to fixed as in prior art vehicle models.  
         [0029]     Also illustrated is a longitudinal vehicle velocity V x  and a lateral vehicle velocity V y . The longitudinal vehicle velocity and lateral vehicle velocity may be measured at different points on the vehicle including the front and/or the rear. The side slip angle is the inverse tangent of the ratio of the lateral vehicle velocity and the longitudinal vehicle velocity.  
         [0030]     Referring now to  FIG. 4 , a method of controlling a vehicle model in understeer is illustrated. In step  70  the driver model looks ahead to find the vehicle path. If the vehicle is “on target”, step  72  is executed. In step  72  the vehicle is “on target” if the vehicle will follow a desired path within the look ahead range. If the vehicle will follow the desired path based upon the steering wheel input and various other inputs to the vehicle, no steering wheel change is provided in step  74 . In step  76  the time of the vehicle model is incremented and then step  70  is again executed. In step  78  a new current steering wheel angle (SWA-current) based on the size of the error between the look ahead point and the desired path is determined. In this step a previous or initial steering wheel angle is determined from a previous loop as will be described below. This step forms a plurality of periodically determined steering wheel angle values. If the vehicle is not understeering step  82  is executed. In step  82  the vehicle response is determined. The vehicle response in this embodiment is determined by the yaw acceleration which is normalized by the steering wheel angle. Of course, those skilled in the art will recognize various types of vehicle responses may be used. In step  84  the vehicle response, such as the normalized yaw acceleration, is compared to a threshold. If the normalized yaw acceleration is greater than a threshold and the absolute value of the steering wheel angle from step  78  is not increasing, step  86  is executed in which the steering wheel angle computed in step  78  is used and the plowing condition flag is set to false. Then, step  76  is executed.  
         [0031]     Referring back to step  84 , if the normalized yaw acceleration is greater than a threshold and the absolute value of the new steering wheel angle is increasing, the plowing flag set to true in step  88  and the steering wheel angle hold value (SWA_hold) is set to the current wheel value determined in block  78 . The system then continues to block  76 .  
         [0032]     Referring back to block  80 , if the plowing flag is set to be true from a previous step, step  90  is executed. In step  90  it is determined whether the error between the intended path and the desired path is converging or being reduced. The error is determined as a function of the normalized yaw rate and the current steering wheel angle. More specifically, in step  90  the normalized yaw acceleration is compared to a threshold. If the normalized yaw acceleration is greater than the threshold and the difference of the SWA_hold and the SWA_current is less than a tolerance and the absolute value of the SWA_current is decreasing, the error is being reduced. If the error is not being reduced, step  92  is executed in which the steering wheel angle is held at the previous time step value. In step  90 , if the error is being reduced as set by the above-mentioned conditions, the steering wheel computed for the current time step is used. Also, the plowing condition flag is set to false since the vehicle is no longer and minimally plowing at this point. That is, when the conditions in step  90  are true, the vehicle model is being brought back under control. As can be seen, the steering wheel angle is not allowed to be changed to provide an undesirable result as in previous models. Thus, the current SWA value is held (while being monitored in step  90 ) until the value when it is determined that the vehicle is plowing is reached.  
         [0033]     Referring now to  FIG. 5 , a method for operating the vehicle model during understeer is illustrated. In step  100  a rear side slip angle is determined. As mentioned above, the side slip angle is determined as function of the lateral vehicle velocity and the longitudinal vehicle velocity. The present system compares the rear side slip angle to a threshold such as 15°as is used in the present invention. If the rear side slip angle is not above 15°, step  102  is executed in which no change to the look ahead distance of the vehicle model is performed. That is, an unscaled look ahead factor is used. The system continues in step  104  in which the vehicle model is operated with the look ahead path. In step  106  if the vehicle is on the look ahead path no steering wheel angle change is performed in step  108 .  
         [0034]     Referring back to step  106 , if the vehicle is not on target, step  110  is executed in which a new steering wheel angle (SWA_current) is determined based on the size of the error between the look ahead point and the intended path. After step  108  and  110 , step  112  is executed in which the next time increment is provided to the vehicle model. Referring back to step  100 , if the rear side slip angle is greater than the threshold (which in this case is 15°), step  114  is executed. In step  114 , the look ahead distance is increased by a scale factor (SF). In the present example, a scale factor is determined that is exponential in value. That is, the absolute value of the rear side slip angle (SSR) is multiplied by a constant such as 0.02. This scale factor will be multiplied by the look ahead distance to increase the look ahead distance of the vehicle model. The new look ahead distance is used in step  104  to find the path. By providing the increased look ahead distance, the vehicle computer model generates useful results.  
         [0035]     Referring now to  FIGS. 6 and 7 , a method for controlling the vehicle model during aggressive maneuvers is illustrated. The look ahead scale factor is changed as the function illustrated in  FIG. 6 . That is, the look ahead scale factor is normally 1. However, during a straight driving condition, the look ahead scale factor is reduced to 62½ percent or 0.625 of the nominal look ahead scale factor. The scale factor increases as a function of the curvature of the road. Thus, as the curvature of the path increases, the look ahead scale factor also increases. In the present example, the look ahead scale factor is directly proportional (has a slope corresponding) to the curvature of the path. However, those skilled in the art will recognize that various curves may also be used depending on the vehicle. It should be noted that the curvature changes as a function of a negative and positive curvature of the path.  
         [0036]     In  FIG. 7 , step  120  computes the curvature of the intended path. In step  124 , the function of  FIG. 6  is used to compute the look ahead scale factor. In step  126  the driver model uses the look ahead scale factor to determine the path of the vehicle. In step  128  if the vehicle is on target no change in steering wheel angle is commanded for the computer model. After step  130 , step  132  increments the system to the next time.  
         [0037]     Referring back to step  128 , if the vehicle is on target, step  134  is executed in which the current steering wheel angle is based on the size of the error between the look ahead point and the intended path. This keeps in mind that the intended path may have been increased or decreased by the look ahead scale factor in step  124 . After step  134 , step  132  is again executed.  
         [0038]     As can be seen, the present invention allows the vehicle model to be controlled in various conditions such as understeering or oversteering and aggressive driving. This will allow vehicle designers to more quickly and readily determine how the vehicle handling reacts to various handling events.  
         [0039]     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.