Abstract:
A method for controlling a four-wheel steering system of a vehicle includes recognizing a driver-selectable mode, receiving a signal preferably indicative of a front wheel steering-angle, and determining a rear-to-front steering-angle ratio in correspondence with the recognized mode and the received signal; and optionally includes receiving a signal indicative of vehicle speed, recognizing a current system state, determining a desired steering-angle, generating a command based on the desired and received steering-angles and the recognized system state, and diagnosing conditions in accordance with the received signals in order to enter a output disable mode.

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 porition of the system may produce a desired rear wheel steering angle to improve directional stability at high speeds and maneuverability at low speeds. The high-speed steering method may reduce vehicle yaw by steering the, front and rear wheels in the same general direction, and thereby improve vehicle stability. 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. 
     Use of a vehicle for the towing of a trailer, for example, may affect the optimal ratios between the front and rear wheel steering angles. For vehicles that are frequently used for towing, such as some trucks, for example, it may be desirable to provide a driver-selectable steering mode that retains the many benefits of four-wheel steering while compensating for the presence of a trailer. 
     SUMMARY 
     The above described and other features are exemplified by the following Figures and Description in which a method for controlling a four-wheel steering system of a vehicle is disclosed that includes recognizing a driver-selectable mode, receiving a signal preferably indicative of a front wheel steering-angle, and determining a rear-to-front steering-angle ratio in correspondence with the recognized mode and the received signal; and optionally includes receiving a signal indicative of vehicle speed, recognizing a current system state, determining a desired steering-angle, generating a command based on the desired and received steering-angles and the recognized system state, and diagnosing conditions in accordance with the received signals in order to enter an output disabled mode. 
    
    
     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 having a controller; 
     FIG. 2 is a signal flow diagram for the controller of FIG. 1; 
     FIG. 3 is a state-transition diagram for the controller of FIGS. 1 and 2; and 
     FIG. 4 is a symbol description table for the state-transition diagram of FIG.  3 . 
    
    
     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 sensors and interfaces for performing a variety of processes prescribed by the desired controlling functions. 
     The system  10  is incorporable into a vehicle 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 the 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 an actuating 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 the various systems of the vehicle. The controller  18  receives informational signals from the vehicular 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. 
     Turning to FIG. 2, a four-wheel steering algorithm for the controller  18  of FIG. 1 is indicated generally by the reference numeral  60 . In the algorithm  60 , a Hand-wheel Position Calculation function  62  receives signals indicative of instantaneous hand-wheel position, and produces signals indicative of hand-wheel acceleration, hand-wheel velocity, hand-wheel position, and diagnostics. A Vehicle Speed function  64  receives a signal indicative of raw vehicle speed and produces signals indicative of a filtered vehicle speed and diagnostics. A Driver Switch function  66  receives a signal indicative of a driver-selectable mode and produces signals indicative of the state of the driver&#39;s switch and diagnostics. A communication function  68  receives signals indicative of Vehicle Communication bus information, which is generally available information from other vehicular sensors and processes, and extracts the signals indicative of throttle percentage, vehicle speed, brake application, transmission mode, selected gear, and diagnostics. 
     A Rear Wheel Steering Angle function  70  receives raw signals indicative of rear wheel steering angle and produces signals indicative of actual rear wheel steering angle and diagnostics. A Driver Mode function  72  receives the signal indicative of the state of the driver&#39;s switch from the Driver Switch function  66 , receives the signal indicative of hand-wheel position from the Hand-wheel Position Calculation function  62 , and produces signals indicative of lamp command, Driver Mode, and diagnostics. A Gain function  74  receives the signal indicative of hand-wheel position from the Hand-wheel Position Calculation function  62  and produces a gain signal as input to a Rear to Front Ratio function  76 . The Rear/Front Ratio function  76  also receives the signal indicative of Driver Mode from the Driver Mode function  72 , the signal indicative of hand-wheel position from the Hand-wheel Position Calculation function  62 , the signal indicative of the filtered vehicle speed from the Vehicle Speed function  64 , a signal indicative of gear selection from the Vehicle Communication function  68 , and produces a signal indicative of the desired rear to front steering angle ratio for input to a Runtime Angle function  78 . 
     The Runtime Angle function  78  also receives the signal indicative of hand-wheel position from the Hand-wheel Position Calculation function  62 , and produces a signal indicative of a desired rear wheel steering angle for input to a Zero Speed Estimation &amp; Swing Out Compensation Algorithm  80 . The Zero Speed Estimation &amp; Swing Out Compensation Algorithm  80  also receives the signal indicative of the filtered vehicle speed from the Vehicle Speed function  64 ; as well as the signals indicative of throttle percentage, brake application, and transmission mode from the Vehicle Communication function  68 . The Zero Speed Estimation &amp; Swing Out Compensation Algorithm  80  limits the desired rear wheel steering angle based on the vehicle status for input to a Servo-Error function  82 . The Servo-Error function  82  also receives the signal indicative of the actual rear wheel steering angle from the Rear Wheel Steering Angle function  70 , and produces signals indicative of a servo error and diagnostics. 
     A Diagnostics function  84  receives the signals indicative of diagnostics from the functions in the four-wheel steering algorithm  60 , such as the Rear Wheel Steering Angle function  70 , Hand-wheel Position Calculation function  62 , Vehicle speed processing function  64 , Driver switch process function  66 , Vehicle Communication function  68 , the Driver Mode function  72 , the Servo-Error function  82 , and a Motor Control function  90 , and produces signals indicative of vehicle recorder data, storable fault codes, and class of fault. 
     A System State function  86  receives the signal indicative of class of fault, a signal indicative of a battery signal, the signals produced by the Class  2  function  68 , and produces a signal indicative of a system state for input to a State Output Control function  88 . The State Output Control function  88  also receives the signal indicative of the servo error from the Servo-Error function  82 , and produces signals indicative of direction, pulse-width-modulation command, power relay enable, pulse-width-modulation enable, shorting relay enable, capacitor charging, and capacitor discharging. The Motor Control function  90  receives the signals indicative of direction, pulse-width-modulation command, power relay enable, pulse-width-modulation enable, and shorting relay enable from the State Output Control function  88 , and produces a signal indicative of motor position for input to the Rear Wheel Steering Angle function  70 , a signal indicative of diagnostics for input to the Diagnostics function  84 , and a signal indicative of the pulse-width-modulated command for the motor  46  of FIG.  1 . 
     Referring now to FIGS. 3 and 4, the System State function  86  of the four-wheel steering algorithm  60  of FIG. 2 has a state-transition diagram indicated generally by the reference numeral  92  of FIG. 3, with logical condition flags as listed in the table  108  of FIG.  4 . The state-transition diagram  92  includes an Initialize state  94  from which the controller  18  of FIG. 1 may transition to a Run Disable state  96  if the conditions {(I and F*) or (I and F 4 )} are met, an Output Disable state  98  if {F 1  or F 2  or F 3 }, or a Shutdown state  100  if {I*}. From the Run Disable state  96 , the controller  18  may transition to a Run state  102  if {(A* and E and I and and F*) or (A* and E and I and and F 4 )}, the Output Disable state  98  if {F 1  or F 2  or F 3 }, the Shutdown state  100  if {I*}, or remain in the Run Disable state  96  if {(E* and I and F*) or (A and I and F*)}. 
     From the Run state  102 , the controller  18  may transition to a Zero RWA state  104  if the conditions {(I and F 2 ) or (I and F 3 )} are met, the Output Disable state  98  if {I and F 1 }, the Shutdown state  100  if {I*}, or remain in the Run State  102  if {(A* and E and I and F*) or or (A* and E and I and and F 4 )}. From the Zero RWA state  104 , the controller  18  may transition to the Output Disable state  98  if {F 2  and Z}, the Shutdown state  100  if {I*}, or remain in the Zero RWA state  104  if {(F 3  and I) or (F 2  and I and Z*)}. From the Output Disable state  98 , the controller  18  may transition to the Shutdown state  100  if {I*}, or remain in the Output Disable state  98  if {(I and F 2 ) or (I and F 1 )}. From the Shutdown state  100 , the controller  18  may transition to the Initialize state  94  if {I}, or to an Off state  106  if {I*}. From the Off state  106 , the controller  18  may transition to the Initialize state  94  if {I}. 
     In operation, the four-wheel steering algorithm  60  of FIG. 2 controls rear wheel steer to enhance the low speed manuverability and the high speed stability of a vehicle. The four-wheel steering algorithm  60  uses the 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 in trucks used for towing. 
     The four-wheel steering algorithm  60 , 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. As introduced with reference to FIG. 2, the driver can select a desired mode of operation such as, for example, normal four-wheel steering and trailer modes, respectively. The selected driver mode will determine the desired rear-to-front ratio function  76  for optimum operation, which function may be implemented as a look-up table as known to those of ordinary skill in the pertinent art. The gain table function  74 , which may be non-linear, generally improves the on-center response of the four-wheel steering system  10 . 
     The Zero Speed Estimation &amp; Swing Out Compensation Algorithm  80  of FIG. 2 determines whether the rear wheel angle will be limited based on various vehicular inputs, such as, for example, signals indicative of transmission mode or “PRNDL”, throttle percentage, vehicle speed, and brake application. The limited desired rear-wheel-angle signal represents the variable compared against the actual rear-wheel-angle signal, which is based on motor position, to compute the servo error. Depending on the state that the system is in as shown in FIG. 3, the desired command to the motor is determined. If the controller is not in any diagnostic state, the servo-error command is used to determine a system gain, and hence, in this exemplary embodiment, a pulse-width-modulated (“PWM”) command to the motor  46  of FIG.  1 . 
     The state transition diagram  92  as shown in FIG. 3 may be implemented in software to control the operational state of the system  10 . Thus, the Off state  106  of FIG. 3 is the state in which the ignition is turned off and the controller  18  powers down all functions. The Off state  106  consumes as little power as possible and remains in this state until a valid ignition signal is received by the controller  18 . The Initialize state  94  performs all initialization functions for a typical start up process. Some of these functions are input and output (“I/O”) diagnostics, memory tests, variable initialization, and other standard tests as known to those of ordinary skill in the pertinent art. After completion of these tasks, the controller  18  will remain in the Run Disable state  96  until a valid state-transition is achieved. The Run Disable state  96  activates all outputs and processes all inputs, but the voltage command to the motor  46  remains at zero. In this Run Disable state  96 , the rear wheels will not steer under any circumstances. Having all output devices, such as, for example, field-effect transistors (“FETS”), remain open in this state is one way to ensure that no output to the motor  46  will occur in this state  96 . 
     In the Run state  102 , the rear-wheel steering angle is a function of hand-wheel position, vehicle speed, and the driver-selectable operating mode. The system  10  will ramp to the desired rear wheel steering angle so that the transition is generally undetected by the driver. The Run state  102  is the full operational state in which the system  10  normally operates. Full diagnostic and motor control function is enabled in this state  102 . 
     The Output Disable state  98  is the state the controller  18  reaches once a rapid shutdown or F 1  fault is detected by the diagnostic algorithms. In this state  98 , all outputs to the motor  46  are deactivated. The only way to leave the Output Disable state  98  is to cycle the ignition off-to-on. The specific actions in this state include that the output command to the motor is zeroed, the motor drive circuits are disabled, the commands to open the power relay and close the motor relay are issued no later than 4 msec of zeroing the output command, the fault lamp is commanded on, the appropriate fault code information is stored, the algorithms that calculate the output command are not executed, and the serial communications used to report fault information and some Vehicle Communication bus information are supported. The Shutdown state  100  performs all shutdown tasks and deactivates the system. For an F 2  fault or the controlled shutdown condition, the controller ramps the RWA to zero in state  104  and enters the Output Disable state  98  as described above. 
     The Zero RWA (rear wheel steering angle) state  104  is the state the controller  18  reaches once a F 3  type condition is detected by the diagnostic algorithms that are executed by the software. In the case of an F 3  fault, the controller will ramp the rear wheel steering angle (“RWA”) to zero and hold the rear wheels at zero RWA. For a Ramp to Zero and Hold, specific actions include that the output command to the motor is linearly ramped down to zero at a rate of 2 deg/sec, for example; that the command to the motor is continuously changed to maintain a substantially zero servo error command; that the fault lamp is commanded on; and that the appropriate fault code information is stored. 
     The above-described methodology provides a method for controlling a four-wheel steering system, thereby improving driver control of a vehicle. 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 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 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.