Abstract:
A steering angle sensor assembly  10  for determining absolute angular position of a steering wheel of a motor vehicle and includes a multi-axis detector  24  having an active operating mode and a standby operating mode and being adapted to measure speed and direction of rotation of the steering wheel. The steering angle sensor assembly  10  further includes at least one second detector  26  arranged to form a quadrature encoder to detect movement and direction of the steering wheel when the multi-axis detector  24  is in the standby or powered down operating mode.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims the benefit of and priority to U.S. Provisional Application No. 60/965,719, filed Aug. 22, 2007. 
    
    
     BACKGROUND 
     1. Field 
     The disclosure relates to a steering angle sensor for determining the absolute angular position of the steering wheel of a motor vehicle. More particularly, the disclosure relates to an absolute steering angle sensor that functions when the steering angle sensor is in a stand-by or powered-down mode. 
     2. Description of Related Art 
     The steering angle or the steering angle deflection in motor vehicles is, for example, required in driving dynamics control systems. Such a driving dynamics control system receives other measuring data in addition to the aforementioned steering angle values, e.g., the rotational speed of the wheels or the turning of the motor vehicle about its vertical axis. In this case, the absolute steering angle deflection as well as the steering speed are required for evaluating these values by the driving dynamics control system together with the other measured data, with the data subsequently being used for controlling actuators, e.g., the brakes and/or the engine management system. 
     SUMMARY 
     In one exemplary embodiment, a steering angle sensor for determining absolute angular position of a steering wheel of a motor vehicle is disclosed. The steering angle sensor includes a multi-axis detector having an active operating mode and being adapted to measure speed and direction of rotation of the steering wheel. The sensor further includes at least one second detector arranged to form a quadrature encoder to detect movement and direction of the steering wheel when the multi-axis detector is in a standby operating mode. 
     In another exemplary embodiment, a steering angle sensor for determining absolute angular position of a steering wheel of a motor vehicle, the steering angle sensor comprising: a first gear connected to a steering wheel shaft; a second gear configured to mesh with the first gear, the second gear including a split gear. 
     In another exemplary embodiment, a method for determining if a steering wheel of vehicle has moved when the vehicle ignition is off, includes the steps of:
         (a) measuring steering wheel position at a first instant of time when the vehicle ignition is on using a detector arranged to form a quadrature encoder to detect movement and direction of the steering;   (b) storing the steering wheel position obtained in step (a);   (c) measuring the position of the steering wheel at a second instant of time when the vehicle ignition is off using the detector arranged to form a quadrature encoder; and   (d) determining if a difference exists between the position obtained at the first instant of time and the position obtained at the second instant of time indicating a changed position of the steering wheel.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view showing an embodiment of the steering angle sensor of one exemplary embodiment. 
         FIG. 2  is a cross-sectional view of the sensor shown in  FIG. 1 . 
         FIG. 3  illustrates an embodiment of a magnet of the steering angle sensor of one exemplary embodiment. 
         FIG. 4  is a graph schematically illustrating the relationship between the steering wheel angle and the sensor angle. 
         FIG. 5  is an exploded view of a split gear of one exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present disclosure will be described with reference to the accompanying figures. It is to be noted that the same or similar reference numerals are applied to the same or similar parts or elements throughout the Figures, and the description of the same or similar parts and elements will be omitted or simplified. 
     In the following description, specific details are set forth, such as specific materials, process and equipment, in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In order instances, well-known manufacturing materials, processes, and equipment are not set forth in detail in order to now obscure the present invention. 
     Referring to  FIGS. 1 and 2 , a steering angle sensor  10  is generally shown which measures a steering angle of a steering shaft (rotor)  11  mounted in a vehicle. The steering angle sensor  10  includes a first gear or input gear  14  concentrically mounted to the steering shaft  11  and a second gear assembly comprised of a spur gear  16  and a split gear  18  in mesh with the input gear  14 . 
     In general, the second gear assembly includes the spur gear  16  and the split gear  18  is smaller in diameter than the input gear  14 . That is, the second gear assembly  16 ,  18  has less gear teeth than the input gear  14 . For example, the first gear has a number of gear teeth of “125”, and the second gear assembly has a number of gear teeth of “100”. Preferably, the gear ratio between the input gear  14  and the spur gear  16  ranges from about 1:1 to 2:1. More preferably, the gear ratio of the input gear  14  to the spur gear  16  is approximately 1.25:1. Referring to  FIG. 5 , the second gear assembly is comprised of the spur gear  16  and the split gear  18 , the split gear  18  is the same size as the spur gear  16  and also has the identical number of teeth as the spur gear  16 . The split gear  18  is attached to the spur gear  16  to allow the teeth of the respective gears  16 ,  18  to be biasingly offset with respect to each other. That is, the split gear  18  is attached to the spur gear  16  through a biasing mechanism, such as a springs  17  disposed in recesses  19 , that allow the teeth of the split gear  18  to be biased away from the teeth of the spur gear  16 . This ability to be biasingly offset, provides for improved gear mesh between the gear assembly  16 ,  18  and the input gear  14  as the biasing effect of the biasing mechanism on the split gear  18  provides enhanced engagement of the teeth of the gear assembly  16 ,  18  with the input gear  14 . The split gear  16 ,  18  also reduce hysteresis and gear lash. 
     The spur gear  16  includes a multi-pole magnet  20  (north/south poles) which is concentrically mounted to the spur gear  16  as shown in  FIGS. 1 and 2 . A more detailed illustration of magnet  20  is shown in  FIG. 3 . The magnet  20  can be made of various materials well known to those in the art including ceramic materials, ferrous powder with ceramic or nylon binders, or the like. 
     The steering angle sensor  10  includes a first detector unit  24  (Melexis, Concord, N.H.) disposed on a printed circuit board  22  for measuring the speed and direction of steering wheel rotation. The first detector  24  is preferably a multi-axis sensor. The multi-axis detector  24  is preferably a Hall effect sensor stationarily mounted onto the printed circuit board  22  in opposition to the magnet  20  in an axial direction of the second gear assembly  16 ,  18  to detect an orientation of a magnetic field generated by the magnet  20 , which rotates above the detector  24  surface. The first detector  24  measures the absolute 0-360° angle of steering wheel shaft  11  based on the rotation of the input gear  14  and the spur gear  16  and their respective gear ratio. 
     At least one second detector  26  is stationarily disposed on the printed circuit board  22 . The detector(s)  26  is preferably a low-power, Hall-effect-type sensor, operably disposed in proximity to the magnet  20 . The detector(s)  26  can operate when the vehicle ignition is on or off. Preferably, the second detector  26  is arranged to create a quadrature encoder to detect when, and in what direction, the steering wheel is moved when the vehicle ignition is in either the on or the off positions. A quadrature encoder is a common type of incremental encoder that uses two output channels to sense position. By utilizing two sensors ( 24  and  26 ) that are 90° out of phase, the two output channels of the quadrature encoder  26  indicate both position and direction of rotation. As shown in  FIGS. 1 and 2 , two Hall detectors  26  are disposed approximately 90° apart from one another on the circuit board. The detectors  26  are also operably disposed opposite to the magnet  20  which rotates above the sensor surface to detect the orientation of a magnetic field generated by the magnet  20 . By being oriented to produce the quadrature encoder, the sensors  26  are able to track the number of turns or “turn number” of the steering wheel. By utilizing both the absolute 0-360° angle of steering wheel obtained from the first detector  24  and the turn number obtained from the second detectors  26 , the absolute steering angle of the steering wheel can be determined. A cover  28  which mates with the base  12  is utilized to enclose the steering angle sensor assembly  10 . 
     As shown in the chart in  FIG. 4 , the first detector  24  produces output angle signals (denoted multi-axis detector angle) which enable detection of a rotation angle of the spur gear  16 . Each detector  26  (denoted as Detector  1  and Detector  2 ) produces a signal processed by the quadrature encoder to give a step number which is indicative of the turn number of the steering wheel. 
     In operation, when the automobile ignition is in the “on” or powered position, the multi-axis detector  24  tracks the 0-360° position of the steering wheel as the south pole of the magnet  20  passes the detector  24 . An internal step counter disposed within an internal microprocessor (not shown) located on the printed circuit board  22  of the steering angle  10  is indexed up (incremented) or down (decremented) depending on the rotational direction of the steering wheel and the value is stored on the onboard memory associated with the steering angle sensor  10 . As shown in  FIG. 4 , a step number is assigned based on the position of the steering wheel. Each step number is unique and only correlates to a given steering wheel angle and turn number of the steering wheel. When the vehicle ignition is turned off, the steering angle sensor  10  enters a powered down or “standby mode”. At a predetermined time or frequency, a “wake up” signal is sent to each of the detectors  26  to determine if the steering wheel has been moved during the “standby mode”. 
     If the value of the step number obtained during the “wake-up mode” is the same as compared with the stored value stored when the ignition is on, then there is no change to the state of the detectors  26 , indicating that the steering wheel has not been moved in the “standby mode”, and the step counter is not adjusted. If the value of the step number obtained by the detectors  26  during the “standby mode” is different than the stored value compared with the value obtained when the ignition was on, this indicates that the steering wheel has moved in the “standby mode”, and the step counter is adjusted up or down, accordingly. 
     The “wake-up” signal can be generated by the internal microprocessor, the second detector  26 , or can be generated by an optional external activation mechanism (not shown). 
     By combining a multi-axis detector  24  with at least one low-power, Hall-effect-type detector  26  and a single magnet  20 , the steering angle sensor  10  provides a lower cost alternative to prior art steering angle sensors which utilize at least two of the more expensive, slower acting, and power consuming multi-axis detectors. 
     As set forth above, while steering angle sensors of the present disclosed embodiments have been described with reference to the devices and structures which are shown, no limitation is intended to such structures and, in the alternative, the structures of various component parts may be replaced with those of arbitrary structures with the same functions. 
     For instance, with reference to the steering shaft  11  mounted in the vehicle as a rotary member, the embodiments disclosed is not limited to such an application and may be applied to other orientations or applications. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the present invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency or the claims are therefore intended to be embraced therein.