Abstract:
A method for validating a signal indicative of a speed of a vehicle includes receiving a plurality of signals indicative of vehicle speed; correlating at least a first of the received signals with at least a second of the received signals to determine if either signal is invalid; and signaling a rejection of any signal found to be invalid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/309,434, filed Aug. 1, 2001, the contents of which are incorporated by reference herein in their entirety. 
     
    
     
       BACKGROUND  
         [0002]    In vehicular applications, a typical four-wheel steering system steers the front and the rear wheels of a vehicle. More specifically, a rear-wheel steering portion of the system may produce a desired rear-wheel steering angle to improve maneuverability at low speeds. The low speed steering method may achieve tight turning radii at low speeds by steering the front and rear wheels in different directions to thereby reduce the effective turning radius of the vehicle.  
           [0003]    In the rear-wheel steering portion of a four-wheel steering system, an electric motor is typically employed to steer the rear wheels. The position of the rear steering mechanism may be determined with a sensor and, as aforementioned, is positioned with reference to the vehicle speed. It may be desirable to provide a vehicle speed diagnostic mode that retains the many benefits of four-wheel steering while compensating for the presence of any irregularities in vehicle speed signals than can cause an unintended steering in the vehicle.  
         SUMMARY  
         [0004]    The above described and other features are exemplified by the following Figures and Description in which a vehicular speed diagnostic algorithm is disclosed that validates a signal indicative of a speed of a vehicle by receiving a plurality of signals indicative of vehicle speed; correlating at least a first of the received signals with at least a second of the received signals to determine if either signal is invalid; and signaling a rejection of any signal found to be invalid.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    Exemplary embodiments will now be described by way of example with reference to the accompanying drawings wherein like reference numerals designate like features in the several figures, in which:  
         [0006]    [0006]FIG. 1 is a schematic diagram of a vehicular four-wheel steering system having a controller;  
         [0007]    [0007]FIG. 2 is a signal flow diagram for the controller of FIG. 1 showing various vehicle speed inputs and controller output;  
         [0008]    [0008]FIG. 3 is a flow diagram illustrating a method for validating a rate of change in a discrete vehicle speed signal; and  
         [0009]    [0009]FIG. 4 is a flow diagram illustrating a method for validating a vehicle speed signal from a speed sensor (i.e., a transmission shown in FIG. 2); and  
         [0010]    [0010]FIG. 5 is a flow diagram illustrating a method for validating a correlation between the discrete vehicle speed signal and the vehicle speed signal from the speed sensor. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]    As shown in FIG. 1, a vehicular four-wheel steering system is indicated generally by the reference numeral  10 . The system  10  includes a controller  18  coupled to various sensors and interfaces for performing a variety of processes prescribed by the desired controlling functions.  
         [0012]    The system  10  is incorporable into a vehicle (not shown) to provide enhanced steering and directional control of the vehicle. The system  10  comprises a front steering mechanism, shown generally at  14 , a rear steering mechanism, shown generally at  16 , and a controller  18  in signal communication with the front steering mechanism  14  and the rear steering mechanism  16 . Although the system  10  is described as being applicable to a rack and pinion steering arrangement, the system  10  is adaptable to other steering arrangements including, for example, integral gear steering systems.  
         [0013]    The front steering mechanism  14  comprises a rack shaft  20 , a rack  22  disposed intermediately between opposing ends of the rack shaft  20 , a tie rod  24  disposed on each opposing end of the rack shaft  20 , a knuckle arm  26  connected to each tie rod  24 , and a front steerable wheel  28  rotatably disposed on each knuckle arm  26 . The rack shaft  20 , tie rods  24 , and knuckle arms  26  are configured such that the front steerable wheels  28  can pivot in unison relative to the vehicle to steer or to effect a change in the direction of travel while the vehicle is moving.  
         [0014]    The front steering mechanism  14  further comprises a mechanism through which a vehicle operator can effectuate a desired change in the direction of travel of the vehicle. Such a mechanism comprises a steering column  30  disposed in operable communication at one end with the rack  22  through a pinion  32  and at an opposing end thereof with a steering device  34 . The steering device  34  may be a hand steering wheel, or “hand-wheel”. Manipulation of the steering device  34 , i.e., rotation of the hand-wheel, causes the axial rotation of the steering column  30 , which in turn causes the rotation of the pinion  32 . Rotation of the pinion  32 , through the engagement of the rack  22  and the pinion  32 , effectuates the lateral translation of the rack  22  relative to the vehicle. The lateral translation of the rack  22  causes the front steerable wheels  28  to angle relative to the vehicle, thereby altering the direction of travel while the vehicle is moving.  
         [0015]    The rear steering mechanism  16  comprises a rack shaft  36 , a rack  38  disposed intermediately between opposing ends of the rack shaft  36 , tie rods  40  disposed on each opposing end of the rack shaft  36 , a knuckle arm  42  connected to each tie rod  40 , and a rear steerable wheel  44  rotatably disposed on each knuckle arm  42 . The rack shaft  36 , tie rods  40 , and knuckle arms  42  are configured such that rear steerable wheels  44 , like the front steerable wheels  28 , can be pivoted in unison relative to the vehicle to steer the vehicle upon lateral translation of the rack  38 .  
         [0016]    The rear steering mechanism  16  further comprises a mechanism through which the rear steerable wheels  44  can similarly be pivoted. Such a mechanism comprises a motor  46  operably connected to the rack  38  through a drive mechanism  48 . The drive mechanism  48 , through a pinion  49 , transfers the rotational motion of a rotor shaft  47  of the motor  46  to linear motion of the rack  38 , which effectuates the lateral motion of the rack shaft  36  and, ultimately, the pivoting of the rear steerable wheels  44 .  
         [0017]    The vehicle is further provided with a steering sensor  50  for detecting an angular position of the steering column  30  or steering device  34 , a vehicle speed sensor  52 , and a rear rack shaft displacement sensor  58 . Vehicle speed sensor  52  includes a vehicle speed signal from the transmission and powertrain control module (PCM) (Not shown), while an anti-lock brake system (ABS) generally shown at  54  connected to at least one wheel  44  also produces a vehicle speed signal relative to wheel  44 . The rear rack shaft displacement sensor  58  detects the displacement of its corresponding rack shaft  36  from a reference position, which is the position in which each rear steerable wheel  44  is aligned and rotatable.  
         [0018]    The controller  18  is disposed in signal communication with vehicular systems. The controller  18  receives informational signals from each of the systems, quantifies the received information, and provides an output command signal in response thereto, such as in this instance, for example, to the rear steering mechanism  16  through the motor  46 .  
         [0019]    In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of the rear-wheel steering algorithms, and the like), the controller  18  may include, but need not be limited to, processors, computers, memory, storage, registers, timing devices, interrupts, communication interfaces, input/output signal interfaces, and the like, as well as combinations comprising at least one of the foregoing. For example, the controller  18  may include input signal filtering to enable accurate sampling and conversion or acquisition of such signals from communications interfaces.  
         [0020]    Turing to FIG. 2, a vehicle speed diagnostic algorithm for controller  18  of FIG. 1 is indicated generally by the reference numeral  60 . Transmission  62  generates a transmission vehicle speed signal  64  to a powertrain control module (PCM)  66  and to an anti-lock brake system (ABS) function  68 . ABS function  68  also receives signals  70  and  72  indicative of right and left wheel  44  speeds, respectively, and generates a Class 2 ABS vehicle speed signal  74 . A vehicle speed discrete signal used for a four-wheel steer algorithm is a buffered digital signal  76  generated from PCM  66 . Buffered digital signal  76  determines what ratio is to be used for a rear wheel command algorithm, thus signal  76  necessitates diagnostics to be implemented on this signal which determines rear wheel steering. PCM  66  also generates a Class 2 low-resolution (Class 2 Low Res) vehicle speed signal  78  and a C-bit information signal  80  indicative of whether the Class 2 vehicle speed signal  78  is corrupted. C-bit information is broadcast throughout the vehicle network and represents vehicle communication bus information, which is generally available information from other vehicular sensors and processes, and extracts signals indicative of throttle percentage, vehicle speed, brake application, transmission mode, selected gear, and diagnostics. In algorithm  60 , PCM  66  receives signal  64  indicative of raw vehicle speed from transmission  62  and produces signals indicative of a buffered vehicle speed and diagnostics. The ABS function  68  receives signal  64  indicative of raw vehicle speed along with signals  70  and  72  indicative of right and left wheel  44  speeds and produces a signal indicative of the state of the ABS system for diagnostic purposes discussed more fully herein. Controller  18  generates an output signal  84  in connection with algorithm  60 . Signal  84  is depicted as “F3” and is indicative of a vehicle speed diagnostic fault that causes the rear wheel angle (RWA) to ramp to zero and hold at zero to eliminate unintended steering effects as a result of one of the vehicle speed fault diagnostics. The diagnostics include a diagnostic to verify a value for a rate of change of the vehicle speed, a diagnostic to validate raw vehicle speed signal  64  between transmission  62  and PCM  66 , and a diagnostic to validate buffered vehicle speed digital signal  76  between PCM  66  and controller  18 .  
         [0021]    Referring to FIG. 3, a diagnostic algorithm to monitor excessive rate of change of vehicle speed is shown generally at  90 . Diagnostic algorithm  90  is implemented by controller  18  to determine whether buffered vehicle speed or discrete vehicle speed signal  76  connection between PCM  66  and controller  18  is valid. Discrete vehicle speed  76  is low pass filtered to avoid triggering of the diagnostic for high frequency low amplitude noise in a low pass filter  92 . Low pass filter  92  generates a filtered vehicle speed signal  94  to a summer  96  and to a delay function  95  that results in vehicle speed acquisition every 4 ms. Delay function  95  generates a filtered delayed vehicle speed signal  98  input to summer  96 . Summer  96  compares signals  94  and  98  to determine a rate of change of vehicle speed and generates a vehicle speed rate of change signal  100  to function block  102 . If the rate of change in vehicle speed signal  100  exceeds a calibrated value, function block  102  triggers fault signal  84 . If the rate of change in vehicle speed signal  100  does not exceed the calibrated value, function block  102  indicates a diagnostic pass  106 . It may be understood by those of ordinary skill in the pertinent art, that the calibrated value may be adjusted in order to meet design criteria. Diagnostic algorithm  90  detects opens, shorts and excessive noise on the connection between PCM  66  and controller  18  providing buffered vehicle speed signal  76  for use in algorithms to determine rear-wheel angle (RWA) using vehicle speed as a parameter.  
         [0022]    Referring to FIG. 4, a diagnostic algorithm to determine an invalid vehicle speed signal  64  from transmission  62  is shown generally at  120 . Diagnostic algorithm  120  is implemented by controller  18  to determine whether the vehicle speed connection between transmission  62  and PCM  66  providing signal  64  from transmission  62  is valid. Controller  18  receives Class 2 Low Res Vehicle speed signal  78  in block  122 . If Class 2 Low Res C-Bit is set (e.g., Low Res C Bit=1), then block  122  triggers diagnostic fault signal  84 . If Class 2 Low Res C-Bit≠1, then block  124  determines whether discrete vehicle speed signal  76  and Class 2 Low Res speed signal  78  are both zero indicative that the vehicle speed is actually zero or that the vehicle speed connection between transmission  62  and PCM  66  is open or shorted. If both signals  76  and  78  are zero, then block  126 , if not, then diagnostic algorithm  120  indicates a diagnostic pass  128 . Block  126  determines whether Class 2 ABS Vehicle speed signal  74  is above a calibrated value. If signal  74  is above the calibrated value, a diagnostic fault  130  is triggered generating signal  84  indicative of a fault. If signal  74  is not above the calibrated value, diagnostic pass  128  results. It may be understood by those of ordinary skill in the pertinent art, that this calibrated value may be adjusted in order to meet design criteria.  
         [0023]    Referring to FIG. 5, another diagnostic algorithm  140  is implemented to determine whether an excessive rate of change in the buffered vehicle speed signal  76  is indicated. Diagnostic algorithm  140  is implemented by controller  18  to determine whether the vehicle speed connection between PCM  66  and controller  18  providing buffered signal  76  from PCM  66  is valid. Controller  18  receives Class 2 Low Res Vehicle speed signal  78  in block  142 . If Class 2 Low Res C-Bit is set (e.g., Low Res C Bit=1), then block  142  indicates a diagnostic pass  128 . If Class 2 Low Res C-Bit≠1, then block  144  determines whether the absolute value of a difference between a discrete vehicle speed signal  76  value and a Class 2 Low Res speed signal  78  value is greater than a calibrated value. If the absolute value of the subtraction of signal  78  value from signal  76  value is greater than the calibrated value, diagnostic fault  130  is triggered generating signal  84  indicative of a fault. If the absolute value in the difference between signal  76 ,  78  values is not above the calibrated value, diagnostic pass  128  results indicative that signals  76  and  78  are correlated. It will be understood that the calibrated value is indicative of the tolerance for correlation between signals  76  and  78  before a fault is triggered commanding RWA to zero.  
         [0024]    It will be appreciated that diagnostic algorithm  90  performs a diagnostic to determine whether the buffered vehicle speed connection between PCM  66  and controller  18  providing signal  76  is valid. However, this diagnostic will not trigger a fault when this vehicle speed connection is open and shorted at initialization, since the rate of change of vehicle speed will indicate zero since the indicated speed is zero. In this manner, diagnostic algorithm  140  is implemented to check the correlation between discrete vehicle speed  76  and Class 2 Low Res vehicle speed  78  to make sure the two signals  76  and  78  are tracking within the calibrated value. If the absolute value of the difference between these two signal  76 ,  78  values is not within this calibrated value, a fault is indicated. For example, if the connection between PCM  66  and controller  18  is shorted or open indicating a signal  76  of zero and Class 2 Low Res Vehicle speed signal  78  is indicative of a speed greater than zero and above the calibration value, fault  130  is produced indicating a lack of correlation between signals  76  and  78 . The diagnostics implemented in algorithm  140  determine whether the difference between signal  76  and signal  78  are in a specified valid range. This protects the RWA system command algorithm from using potentially invalid sensor signals that are shorted to battery, shorted to ground, or open. The algorithm also checks the correlation between signals  76  and  78  to determine whether the signals  76  and  78  are shorted to each other or otherwise incongruent. Thus, this strategy protects the system from providing an unintended rear steer for conditions of the sensor&#39;s signal  76  and signal  78  being shorted to each other, ground or battery, and from open circuit conditions. It may be understood by those of ordinary skill in the pertinent art, that the specified valid range may be adjusted in order to meet design criteria.  
         [0025]    The diagnostic algorithms  90 ,  120 ,  140  receive signals indicative of vehicle speed from the functions in PCM  66  and ABS  68 , and produce signals indicative of flight recorder data, storable fault codes, and class of fault.  
         [0026]    In operation, controller  18  implements a four-wheel steering algorithm to control rear wheel steer to enhance the low speed manuverability and the high speed stability of a vehicle. The four-wheel steering algorithm uses the electric motor  46  of FIG. 1 to drive the rack  38  and pinion  49  rear wheel portion of the four-wheel steering system  10  of FIG. 1, which produces a desired rear wheel angle to improve the high speed directional stability and low speed turning ability of a vehicle. This disclosure provides features that may be used in automobiles, and that may be particularly desirable to ensure vehicle speed signal reliability.  
         [0027]    The four-wheel steering algorithm, in general, provides an out-of-phase rear wheel steering angle at low speed to reduce the turning radius of a vehicle and an in-phase steering angle at high-speed to enhance stability. The desired rear wheel steering angle is a function of vehicle speed and hand-wheel angle. If the vehicle speed signals are intermittent, shorted to ground, shorted to battery, shorted to each other or open, it may cause a vehicle speed reading to be corrupted and hence the rear-wheel angle estimation to be incorrect. The diagnostic algorithms disclosed herein detect any of these conditions, thus preventing an unintended steer.  
         [0028]    The above-described methodology provides diagnostic algorithms for detecting erroneous vehicle speed signals. In addition, the present teachings may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present teachings 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 media, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the teachings of the present disclosure.  
         [0029]    The teachings of the present disclosure can 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 data signal transmitted, whether by 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 teachings herein. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.  
         [0030]    While the disclosure has been made with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.