Abstract:
A method for controlling the rear steering angle of a vehicle includes receiving a plurality of signals indicative of rear steering angle; checking at least one of the plurality of signals to determine if it falls within a valid range; correlating at least a first signal of the plurality of signals with at least a second signal of the plurality of signals to determine if either the first signal or the second signal is invalid; and rejecting any signals found to be invalid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     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 
     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. 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. 
     SUMMARY 
     The above described and other features are exemplified by the following Figures and Description in which a vehicular diagnostic system is disclosed that controls the rear steering angle of a vehicle by receiving a plurality of signals indicative of rear steering angle; checking at least one of the plurality of signals to determine if it falls within a valid range; correlating at least a first signal of the plurality of signals with at least a second signal of the plurality of signals to determine if either the first signal or the second signal is invalid; and rejecting any signals found to be invalid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of a vehicular four-wheel steering system; 
         FIG. 2  is a sensor signal correlation plot for the four-wheel steering system of  FIG. 1 ; 
         FIG. 3  is a sensor signal correlation table for the four-wheel steering system of  FIG. 1 ; and 
         FIG. 4  is a flow diagram illustrating a method for controlling the rear steering angle of the four-wheel steering system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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 . 
     The vehicle is further provided with a steering sensor  50  for detecting an angular position of the steering column  30 , a vehicle speed sensor  52 , and a rear rack shaft displacement sensor  58 . 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. 
     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 . 
     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. 
     Referring now to  FIG. 2 , the rear-wheel angle is measured by a rear-wheel-angle sensor that produces output signals according to the plot  60 , including a first signal  61  and a second signal  62 . The first signal  61  is an absolute signal versus rotation angle, also called “signal- 1 ” and “RWA 1 ”. The second signal  62  is a relative signal versus rotation angle, also called “signal- 2 ” and “RWA 2 ”. The diagnostics implemented in this algorithm determine whether signal- 1  and signal- 2  are each in a specified valid range. This protects the system  10  from using potentially invalid sensor signals that are shorted to battery, shorted to ground, or open. The algorithm also checks the correlation between signal- 1  and signal- 2  to determine whether the signals are shorted to each other or otherwise incongruent. As shown, signal- 2  can be of the same value as signal- 1  at a single distinct angle in each of the five window ranges, which span uniquely resolvable rear-wheel steering angles between about negative 405 degrees and positive 405 degrees. 
     As shown in  FIG. 3 , a table  66  indicates exemplary values for signal- 1  and signal- 2  for rear-wheel angles between about negative 540 degrees and positive 540 degrees. The rear-wheel angle may be resolved to the accuracy of the potentially higher resolution signal (signal- 2 ) by selection of the window range corresponding to the value of the potentially lower resolution signal (signal- 1 ). Thus, this strategy protects the system from providing an unintended rear steer for conditions of the sensor&#39;s signal- 1  and signal- 2  being shorted to each other, ground or battery, and from open circuit conditions. 
     Turning now to  FIG. 4 , a rear sensor correlation function is indicated generally by the reference numeral  70 . The inputs from the sensor are signal- 1  and signal- 2 , which typically vary with time. The function  70  includes a decision block  71  for determining whether the value of the second rear-wheel angle signal (signal- 2 ) is within a valid range by checking that signal- 2  is less than a valid maximum value of 4.925V and greater than a valid minimum value of 0.075V. It may be understood by those of ordinary skill in the pertinent art, that this and other valid ranges may be adjusted in order to meet design criteria. For example, an alternate exemplary embodiment may have a valid range for signal- 1  of 0.25 Volts to 4.75 Volts, and a valid range for signal- 2  of 0.5 Volts to 4.5 Volts. 
     If signal- 2  is not within the valid range, typically indicating that a short to battery or a grounded output has occurred, a function  73  produces a signal indicative of a rear sensor signal- 2  out-of-range fault. If, on the other hand, signal- 2  is determined to be within a valid range, control is passed to a decision block  72  for determining whether the value of the first rear-wheel angle signal (signal- 1 ) is less than its value at a first inflection point of the second rear-wheel angle signal. 
     The first inflection point is the first and lowest of four consecutively enumerated inflection points between the uniquely resolvable rear-wheel angles of +/−405 degrees. In this exemplary embodiment, the four inflection points correspond to RWA 1  signal voltage values of 1.0V, 2.0V, 3.0V and 4.0V, respectively. If the first value of signal- 1  is less than its value at the inflection point, then the position corresponding to the second rear-wheel angle signal (signal- 2 ) is determined by function  74  by subtracting a second rear-wheel angle center value from the second rear-wheel angle signal (signal- 2 ), multiplying the difference by a scale factor to convert volts to degrees, and adding a second rear-wheel angle offset corresponding to the first inflection point. 
     The second rear-wheel angle center value is determined during an initial wheel alignment, at which time both signal- 1  and signal- 2  will typically approach 2.5 volts. Accordingly, the center value will typically approach zero degrees or 2.5 volts, but may be some value other than zero degrees due, for example, to tolerances in the build and/or mechanical installation of a given vehicle. If, on the other hand, the condition of decision block  72  is false, then a decision block  76  determines whether the value of the first rear-wheel angle signal (signal- 1 ) is less than its value at a second inflection point, −90 degrees here, of the second rear-wheel angle signal. If the first value of signal- 1  is less than its value at the second inflection point, then the position corresponding to the second rear-wheel angle signal (signal- 2 ) is determined by function  78  by subtracting a second rear-wheel angle center value from the second rear-wheel angle value, multiplying the difference by a scale factor, and adding a second rear-wheel angle offset corresponding to the second inflection point. The sign of the scale factor alternates for adjacent windows, and is about −180/4.5 for windows −2, 0, and 2; and +180/4.5 for windows −1 and 1 in this exemplary embodiment. 
     The offsets corresponding to the inflection points are −360, −180, 0, 180 and 360 degrees, respectively. For example, if the actual rear-wheel angle is negative 180 degrees, signal- 1  should be about 1.5V (see  FIG. 2 ) corresponding to signal window(−1) (see  FIG. 3 ) and signal- 2  (RWA 2 ) should be about 2.5V (see  FIG. 2 ), so block  78  would calculate the rear-wheel angle position based on the RWA 2  signal to be (RWA 2 −RWACenterValue)*Scale+RWA_OFFSET 2 =(2.5−2.5)*(180/4.5)+(−180)=−180 degrees. For an actual rear-wheel angle of negative 200 degrees, signal- 1  should be about 1.4V (see  FIG. 2 ) corresponding to signal window(−1) (see  FIG. 3 ) and signal- 2  (RWA 2 ) should be about 2V (see  FIG. 2 ), so block  78  would calculate the rear-wheel angle position based on the RWA 2  signal to be (RWA 2 −RWACenterValue)*Scale+RWA_OFFSET 2 =(2−2.5)*(40)+(−180)=−200 degrees. 
     If the condition of decision block  76  is false, then a decision block  80  determines whether the value of the first rear-wheel angle signal (signal- 1 ) is less than its value at a third inflection point of the second rear-wheel angle signal. If the first value of signal- 1  is less than its value at the inflection point, then the position corresponding to the second rear-wheel angle signal (signal- 2 ) is determined by function  82  by subtracting a second rear-wheel angle center value from the second rear-wheel angle value, multiplying the difference by a scale factor, and adding a second rear-wheel angle offset corresponding to the third inflection point. 
     If the condition of decision block  80  is false, then a decision block  84  determines whether the value of the first rear-wheel angle signal (signal- 1 ) is less than its value at a fourth inflection point of the second rear-wheel angle signal. If the first value of signal- 1  is less than its value at the inflection point, then the position corresponding to the second rear-wheel angle signal (signal- 2 ) is determined by function  86  by subtracting a second rear-wheel angle center value from the second rear-wheel angle value, multiplying the difference by a scale factor, and adding a second rear-wheel angle offset corresponding to the fourth inflection point. 
     If, on the other hand, the condition of decision block  84  is false, then the position corresponding to the second rear-wheel angle signal is determined by function  88  by subtracting a second rear-wheel angle center value from the second rear-wheel angle value, multiplying the difference by a scale factor, and adding a second rear-wheel angle offset corresponding to the fifth inflection point. 
     A decision block  90  is used for determining whether the value of the first rear-wheel angle signal (signal- 1 ) is within a valid range by checking that signal- 1  is less than a valid maximum value of 4.925V and greater than a valid minimum value of 0.075V. If signal- 1  is not within the valid range, typically indicating that a short to battery or a grounded output has occurred, a function  92  produces a signal indicative of a rear sensor signal- 1  out-of-range fault. If, on the other hand, signal- 1  is determined to be within a valid range, the position corresponding to the first rear-wheel angle signal is determined by function  94  by subtracting a first rear-wheel angle center value from the first rear-wheel angle value, and multiplying the difference by a scale factor and control is passed to the decision block  72 , described above. 
     Thus, the rear-wheel signal correlation diagnostics compare the first signal (“signal- 1 ”) with the second signal (“signal- 2 ”). Since signal- 2  is a saw tooth waveform, a particular value of signal- 2  does not absolutely determine the exact angle value. Depending on the value of rear sensor signal- 1 , five regions for the rear sensor signal- 2  are defined by four inflection points, as follows: RWA 2 _INFLECT_PT 1 =1.00 V; RWA 2 _INFLECT_PT 2 =2.00 V; RWA 2 _INFLECT_PT 3 =3.00 V; and RWA 2 _INFLECT_PT 4 =4.00 V, respectively, for the first through fourth inflection points. 
     An angle offset corresponding to each region defined by the inflection points has to be added to the relative value of signal- 2 , and is determined based on each region as follows: RWA 2 _OFFSET 1 =−360 sensor degrees; RWA 2 _OFFSET 2 =−180 sensor degrees; RWA 2 _OFFSET 3 =0 sensor degrees; RWA 2 _OFFSET 4 =180 sensor degrees; and RWA 2 _OFFSET 5 =360 sensor degrees, respectively, for the first through fifth regions. 
     Once the rear position value is calculated for each of signal- 1  and signal- 2 , as discussed above, the rear sensor correlation diagnostic determines whether the two signals correlate by comparing the signals and determining that they each correlate or fall within a predetermined threshold tolerance of each other, such as, for example, within 40 degrees of each other. Thus, the diagnostic will be triggered when the comparison value is above the threshold tolerance, or above 40 degrees in this exemplary embodiment. 
     Accordingly, the four-wheel steering system  10  uses the rear sensor  58  mounted on the pinion to measure the absolute position of the rear wheels. The rear sensor provides two signals; signal- 1  may be used for initialization, and signal- 2  may be used for more accurate measurement. If the sensor signals were intermittent, shorted to ground, shorted to battery, shorted to each other or open, it might cause the reading to be corrupted and hence the rear-wheel angle estimation to be incorrect. This diagnostic algorithm will detect any of these conditions, thus preventing an unintended steer. 
     The above-described methodology provides a diagnostic algorithm for detecting erroneous rear-wheel steering angle 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. 
     The teachings of the present disclosure can be embodied in the form of computer program code, for example, whether stored in a storage medium  100  communicated with controller  18 , 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. 
     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.