Abstract:
A method for implementing a vehicle stability enhancement (VSE) system for a vehicle is disclosed. In an exemplary embodiment, the method includes receiving a handwheel angle input and adjusting the handwheel angle input in response to a variable steering ratio generated in the vehicle. The adjusted handwheel angle input is inputted into a reference model, the reference model thereby outputting one or more desired vehicle handling aspects.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of: U.S. Provisional Patent Application No. 60/385,308, filed Jun. 03, 2002; U.S. Provisional Patent Application No. 60/385,329, filed Jun. 03, 2002; U.S. Provisional Patent Application No. 60/385,328, filed Jun. 03, 2002; and U.S. Provisional Patent Application No. 60/385,284, filed Jun. 03, 2002 the contents of which are incorporated by reference herein in their entirety. 
     
    
     
       BACKGROUND  
         [0002]    The present disclosure relates generally to automobile steering systems and, more particularly, to a method for implementing a vehicle stability enhancement (VSE) system reference model for compatibility with variable ratio (VR) steer control used in active front steering (AFS).  
           [0003]    Vehicle Stability Enhancement (VSE) systems currently in existence typically employ a “Reference Model” algorithm that calculates a desirable handling motion for the vehicle. Such reference model algorithms have traditionally assumed that the desirable handling motion of the vehicle is solely a function of handwheel angle and vehicle speed. However, with the advent of newer vehicle systems such as active front steering (AFS) and steer-by-wire (SBW), the desirable handling motion is no longer simply a function of handwheel angle and vehicle speed.  
           [0004]    In an AFS system, there is the ability to dynamically change the overall steer system ratio between the handwheel and the steered road wheels. The term “active steering” relates to a vehicular control system in which a generated output is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. In some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle.  
           [0005]    For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels. In other words, a motion of the handwheel may be supplemented by an additional motion, such as that from a differential steering actuator, which in turn translates into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the handwheel. Accordingly, in order to provide closed-loop stability control for an AFS system, the VSE reference model should take into account any changes as a result of the VR control.  
         SUMMARY  
         [0006]    The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for implementing a vehicle stability enhancement (VSE) system for a vehicle. In an exemplary embodiment, the method includes receiving a handwheel angle input and adjusting the handwheel angle input in response to a variable steering ratio generated in the vehicle. The adjusted handwheel angle input is inputted into a reference model, the reference model thereby outputting one or more desired vehicle handling aspects.  
           [0007]    Preferably, the variable steering ratio is generated by an active front steering system in which adjusting the handwheel angle input comprises summing an offset angle with the handwheel angle input to produce an effective handwheel angle input. The offset angle is a function of the variable steering ratio. In addition, the vehicle speed is also inputted into the reference model. The desired vehicle handling aspects include a desired yaw rate and a desired sideslip.  
           [0008]    In another embodiment, a vehicle stability enhancement system includes a vehicle reference model for determining one or more desired vehicle handling aspects in response to a handwheel angle input. The handwheel angle input is adjusted in response to a variable steering ratio generated in the vehicle. A control block receives the desired vehicle aspects and generates vehicle control outputs in response to the desired vehicle aspects, and in response to vehicle inputs thereto. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:  
         [0010]    [0010]FIG. 1 is a block diagram representative of an exemplary Vehicle Stability Enhancement (VSE) system;  
         [0011]    [0011]FIG. 2 is a block diagram that represents an existing VSE reference model algorithm used within a VSE system;  
         [0012]    [0012]FIG. 3 is a block diagram that represents a modified VSE reference model algorithm compatible with an active front steering (AFS) system, in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring initially to FIG. 1, there is shown a simplified block diagram of an exemplary Vehicle Stability Enhancement (VSE) system  10 . The VSE system  10  provides closed-loop, yaw rate control to achieve vehicle stability objectives. In addition to open-loop driver inputs, the system  10  also uses vehicle inputs to generate the control outputs applied therein. As is shown in FIG. 1, the driver inputs (e.g., handwheel position, vehicle speed) are sent through a vehicle reference model  12 . As will be described in further detail, the vehicle reference model  12  uses a reference model algorithm to generate desired vehicle handling aspects such as desired yaw rate and sideslip. The various vehicle inputs (e.g., sensed yaw rate, lateral acceleration) are sent through a vehicle state estimator  14  that, in addition to filtering sensed inputs, also provides other parameter estimation outputs that are not directly measured through sensing means. The outputs of both the vehicle reference model  12  and the vehicle state estimator  14  are inputted to vehicle control block  16  that ultimately provides the control outputs.  
         [0014]    [0014]FIG. 2 is a block diagram that represents an existing VSE reference model algorithm  20  found in the vehicle reference model  12 . The existing algorithm  20  uses lookup tables to determine four aspects of the desirable handling motion, specifically (1) a desired yaw rate; (2) a desired sideslip, (3) a desired damping ratio, and (4) a desired natural frequency. As shown in FIG. 2, algorithm  20  inputs the handwheel angle (HWA)  22  to lookup tables  24  and  26 . In addition, the vehicle speed  28  is also inputted into lookup tables  24  and  26 . The output  30  of lookup table  24  is a desired, steady state yaw rate. The output  32  of lookup table  26  is a desired, steady state sideslip. In addition, the vehicle speed  28  is further inputted into lookup tables  34  and  36 . The output  38  from lookup table  34  yields the desired damping ratio, while the output  40  from lookup table  36  yields the desired natural frequency. Then, each of the lookup table outputs  30 ,  32 ,  38  and  40 , are inputted into a filter  42  to produce a desired, dynamic yaw rate  44  and a desired, dynamic sideslip  46 . These dynamic handling aspects are, among other inputs, used by the control block  16  in FIG. 1 to ultimately generate control outputs.  
         [0015]    As stated previously, however, the desirable handling motion of a vehicle is no longer simply a function of the actual handwheel angle, since the AFS will change the physical relationship between the handwheel and the steered road wheels. Thus, to effectively provide stability enhancement, the VSE reference model needs to take into account the effect of the variable ratio aspect of an AFS system.  
         [0016]    Therefore, in accordance with an embodiment of the present invention, there is disclosed a method for configuring a vehicle stability enhancement (VSE) system reference model for compatibility with variable ratio (VR) steer control used in active front steering (AFS). Referring now to FIG. 3, there is shown block diagram that represents a modified VSE reference model algorithm  50 . As can be seen, the handwheel angle input  22  has been replaced by an “effective handwheel angle” input  52 . The effective handwheel angle input  52 , provided to lookup tables  24  and  26 , is essentially representative of a modified handwheel angle generated by the output of a summing block  54  that has the actual handwheel angle  56  and an offset angle  58  as inputs thereto.  
         [0017]    The offset angle input  58  is generated as a result of the VR control aspect of an active front steering system. An understanding of the application of the effective handwheel angle input to the VSE system may be obtained upon consideration of the following example. It will first be assumed that a vehicle without AFS has a fixed steering ratio of 15:1; that is, for every 15 degrees of handwheel angle input, the steering system causes the steerable wheels (e.g., the front wheels) to be turned 1 degree of steering angle. Thus, if the handwheel is turned 90 degrees to the right of center, the front wheels are turned 6 degrees to the right of center. Since this ratio is fixed, the handwheel angle itself is used as an input to the reference model to determine driver intent in the form of desired yaw rate and sideslip.  
         [0018]    On the other hand, an AFS system having VR control will create a varied steering ratio. Assuming further, for example, that at a certain speed the steering ratio due to VR control is decreased from 15:1 to 7.5:1. In this case, a 90 degree handwheel input results in the wheels being turned 12 degrees to the right of center. Thus, in order to compensate for this ratio change with regard to the VSE system, an offset angle is added (or subtracted) from the actual handwheel angle. Thereby, the reference model receives an input indicative of the driver&#39;s intent without the change in steering ratio.  
         [0019]    It will thus be appreciated that by using the “effective handwheel angle” as an input, a VSE reference model will generate the correct desired states for a closed loop stability function, thereby allowing both VR control and stability control to coexist without conflicts therebetween. Although the embodiments depicted are in the context of a steering-based VSE system, the principles herein are equally applicable to other stability systems employing reference models, such as a brake-based stability system.  
         [0020]    In addition, the disclosed invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or as a data signal transmitted whether a modulated carrier wave or not, over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.  
         [0021]    While the invention has been described with reference to a preferred embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.