Abstract:
An electric power assist steering system is controlled by sensing torque in a steering shaft at a point along said steering shaft between a hand wheel and a mechanical connection to an electric motor, wherein the sensing includes sensing a magnetic field direction and intensity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part and claims benefit of U.S. patent application Ser. No. 09/825,794 filed Apr. 4, 2001, now U.S. Pat. No. 6,655,493. 
    
    
     BACKGROUND 
     In a typical electric power steering (EPS) system, a hand wheel is connected to a shaft, which comprises an upper shaft and a lower shaft connected by a torsion bar. The upper shaft connects to the hand wheel and the lower shaft connects to an intermediate shaft that ultimately connects to the rack and pinion gear of a vehicle. When the hand wheel is turned, the upper shaft rotates and a torque sensor measures the angular displacement of the torsion bar. The torque sensor is typically located at the interface between the upper and the lower shaft, which is also the location of the torsion bar. The type of torque sensor typically used has been a contacting type, which requires use of a torsion bar to measure the amount of twist on the torsion bar. The torque sensor sends a signal to the controller, which then sends a signal to the motor to begin operating. The motor powers a gear mechanism, which provides assistance in turning the lower shaft and ultimately the road wheels. 
     A drawback of such torque sensors that rely on the relative rotational displacement of an upper and lower shaft is that they generate hysteresis, which is a lagging effect, and torque ripple, both effects being detrimental to the feel of the power assist steering system. Hysteresis is generated, e.g., from the sensor, the torsion bar itself, bearings on the upper and lower shafts, and any misalignment of the shafts. The amount of hysteresis of the sensor, torsion bar, and bearings can be 0.5 Nm or larger. Hysteresis in these elements generate a torque ripple effect which can be felt at the handwheel as an uneven resistance or periodic pulling effect. 
     SUMMARY 
     Disclosed is a method for controlling an electric power assist steering system with low hysteresis and torque ripple by sensing torque in a steering shaft at a point along said steering shaft between a hand wheel and a mechanical connection to an electric motor, wherein the sensing includes sensing a magnetic field direction and intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 is a schematic perspective view of a steering system of a vehicle; 
     FIG. 2 is a top view of an EPS system with a motor; 
     FIG. 3 is a cross-section view of an EPS system with a single shaft and single housing unit; and 
     FIG. 4 is a schematic perspective view of a prior art non-compliant torque sensor. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the steering system  20  comprises an EPS system, which is connected at a hand wheel  24  through a shaft  26  and a housing  28 . The EPS system provides a driver with assistance in turning a vehicle&#39;s road wheels  22 . The driver turns the hand wheel  24 , which is mechanically connected to a shaft  26 . The rotational force of the hand wheel  24  is transmitted to the shaft  26 , which is detected by a non-compliant torque sensor  30 . The non-compliant torque sensor  30  is located at the shaft  26  from about a midpoint  29  at the shaft  26  to an upper end  27  of the shaft  26 . The non-compliant torque sensor  30  measures the torque applied to the shaft  26  and sends a signal to a controller  38 , which may be a column electronics module. The controller  38  then sends a signal to the motor  32  to begin operation. The motor  32 , which is in mechanical communication with a worm  34  and a worm gear  36 , rotates the worm  34  and the worm gear  36 , which provide turning assistance to the shaft  26 . As the shaft  26  turns, an intermediate shaft  33 , connected through a universal joint  31  rotates a pinion gear (not shown) located under a gear housing  35 . Rotation of the pinion gear (not shown) moves a rack  41 , which moves a tie rod  37 . When the tie rod  37  moves, it turns a steering knuckle  39 , which turns a road wheel  22 . 
     Referring to FIGS. 2 and 3, the EPS system and shaft  26  are mounted to a vehicle by a housing  28 , which may be a single cast unit. The EPS system, shaft  26 , and housing  28  collectively may be referred to as the steering column  60 . Referring to FIG. 3, an upper bearing  44  and a bearing  46  support the shaft  26 . The upper bearing  44  is secured to the shaft  26  by a retaining ring  42 . A bearing lash eliminator  48  is pressed between the upper bearing  44  and the retaining ring  42 . 
     A position sensor  70 , which detects the angular position or displacement of hand wheel  24  (not shown in FIG.  3 ), is connected to a bracket switch mounting  68 , which is in operable communication with the controller  38 . The bracket switch mounting  68  is mounted to the face of the housing  28 . Both the position sensor  70  and the bracket switch mounting  68  are located adjacent to the hand wheel. 
     As stated above, the non-compliant torque sensor  30  is located anywhere from about a midpoint  29  at the shaft  26  to an upper end  27  of the shaft  26 . A spacer  50  may be used to locate the non-compliant torque sensor  30  on the shaft  26  in proximity to the end of the controller  38 . The non-compliant torque sensor  30  comprises a magnetometer housing  52 , which is secured to a bearing housing  54  by a fastener  56 . The bearing housing  54  contains a bearing  58  and a bushing  64 , which supports the magnetometer housing  52  and secures it to the shaft  26 . A snap ring  62  secures the bearing housing  54  to the shaft  26 . Preferably, there is a connection pathway  66  in the housing  28  to directly connect the non-compliant torque sensor  30  to the controller  38 , which is located on the face of the housing  28  adjacent to the hand wheel (not shown). 
     Referring to FIG. 4, the non-compliant torque sensor  30  comprises a transducer  202  and a magnetic field vector sensor  204 . The transducer  202  comprises one or more axially distinct, magnetically contiguous, oppositely polarized circumferential bands or regions  206 ,  208  solely defining the active or transducer region of the shaft. Region  210  of the shaft to the left of A and region  212  to the right of B are distinguishable from the active region only by the absence of any significant remanent magnetization. The shaft is typically formed of a ferromagnetic, magnetostrictive material having a particularly desirable crystalline structure. When the shaft of the non-compliant torque sensor  30  is the shaft  26  of the FIGS. 1-3. torque  214  is applied at one portion of the shaft  26  and is transmitted thereby to another portion of the shaft  26  where the motion of the shaft  26  due to torque  214  ultimately turns the road wheels (not shown) of the vehicle. Torque  214  is being shown as being in a clockwise direction looking at the visible end of the shaft  26 , but obviously can be applied to rotate in either direction depending on the direction the driver turns the hand wheel (not shown). 
     A magnetic field vector sensor  204  is a magnetic field vector sensing device located and oriented relative to the transducer  202  so as to sense the magnitude and polarity of the field arising in the space about the transducer  202  as a result of the reorientation of the polarized magnetization from the quiescent circumferential direction to a more or less steep helical direction. The magnetic field vector sensor  204  provides a signal output reflecting the magnitude of torque  214  and electrically connected to the controller (not shown). The non-compliant torque sensor  30  is more fully described in U.S. Pat. No. 6,145,387, which is incorporated in its entirety herein by reference. 
     Referring to FIGS. 2 and 3, when the controller  38  receives a signal from the non-compliant torque sensor  30  indicating steering effort by a driver against the hand wheel, the controller  38  then sends a signal to the motor  32  to turn on. When the motor  32  turns on it turns the shaft  26  through a worm  34  and worm gear  36  assembly. The worm  34  is rigidly connected to a motor  32  and engages worm gear  36 . Worm gear  36  is mounted to the shaft  26  on splines (not shown). A spring  74  is mounted between the splines (not shown). A nut  72  supports the worm gear  36  in place along the shaft  26 . A bearing  46  supports the worm gear  36  at the shaft  26 . 
     Referring to FIG. 2, a magnetorheological fluid stopper  40  is mounted on the motor  32 . The magnetorheological fluid stopper  40  is fully described in U.S. application Ser. No. 09/825,793, filed Apr. 4, 2001, entitled, “Magnetorheological Fluid Stopper At Electric Motor” under Attorney docket number DE3-/DP-303759, which is incorporated in its entirety herein by reference. 
     Hysteresis and torque ripple are virtually eliminated by sensing torque in shaft  26  without the use of a torsion bar and improving torque sensor accuracy and steering accuracy. The elimination of the torsion bar makes unnecessary additional supporting needle bearings, previously required to maintain the alignment of shaft portions connected by the torsion bar, further reducing hysteresis. 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the claims.