Abstract:
A steering angle sensor detects a rotational angle of a steering system whose steering is assisted by driving of an electric motor via a reduction mechanism so as to calculate a steering angle of a steering wheel. The steering angle sensor includes: a sensor wheel operating simultaneously with rotation of a steering shaft; and an absolute angle calculating unit for calculating an absolute steering angle based on a steering angle signal from the sensor wheel. The sensor wheel includes a GMR element and a magnetizing portion which is arranged around the GMR element, and the rotational angle is detected by making two turns of the steering shaft as one cycle of the steering angle signal based on a change in resistance value of the GMR element obtained by changing a magnetic field direction according to a rotation of the magnetizing portion.

Description:
FIELD OF THE INVENTION 
   The present invention relates to improvement of a steering angle sensor for detecting a steering angle of a steering wheel in a steering apparatus. 
   BACKGROUND OF THE INVENTION 
   Such a kind of a steering angle sensor is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-29158, and  FIG. 1  is a diagram illustrating a steering system of a vehicle. In this diagram, a steering angle sensor  101  operates simultaneously with a steering wheel  102  in a rotational ratio of 1:1, and an output signal from the steering angle sensor  101  as well as a signal from a wheel velocity sensor  103  for detecting a wheel velocity, a signal from a yaw rate sensor  104  for detecting a yaw rate, a signal from a steering ratio sensor  105  for detecting a steering ratio, a signal from an ignition (IG.) switch  106  and the like is connected to a control unit  107 . A steering angle range which is covered by the output signal from the steering angle sensor  101  is divided into three zones: a neutral zone in a predetermined range including at least a neutral position; a right zone; and a left zone adjacent to the neutral zone where output signals from the steering angle sensor  101  show the same value. A determination is made which zone a current steering angle is in based on a velocity difference between right and left wheels, a yaw rate and a vehicle velocity. 
   The control unit  107  sets a steering angle of a steering wheel based on the signals from the respective sensors and controls an operation of a steering ratio variable mechanism  109  that controls a steering ratio of wheels  108 , so that a rear wheel steering mechanism  112  which is linked with a front wheel steering mechanism  111  via a transmission shaft  110  controls the steering angle of rear wheels. 
   In the steering system disclosed in Japanese Patent Application Laid-Open No. 8-29158, however, since information other than the steering angles, namely, information about the difference in wheel velocity, the yaw rate and the vehicle velocity is necessary in order to detect a rotational angle of the steering wheel, if these pieces of information have an error or abnormality, the determination as to the zone cannot be definitely made. Further, the determination as to the zones cannot be made unless initialization of the information is finished. 
   Therefore, it is an object of the present invention to provide a steering angle sensor in which a rotating cycle of a sensor wheel having a GMR element (Giant Magnetic Resistance) and a magnetizing portion arranged around the GMR element is improved and a range where a steering angle is detected using the GMR element is enlarged, so that accuracy of a steering angle estimation using the vehicle information can be low, a cost is low and an absolute steering angle can be detected quickly. 
   SUMMARY OF THE INVENTION 
   The above object of the present invention is achieved by a steering angle sensor, that detects a rotational angle of a steering system whose steering is assisted by driving of an electric motor via a reduction mechanism so as to calculate a steering angle of steering wheels, comprising: a sensor wheel operating simultaneously with rotation of a steering shaft; and an absolute angle calculating means for calculating an absolute steering angle based on a steering angle signal from the sensor wheel, wherein the sensor wheel includes a GMR element and a magnetizing portion arranged around the GMR element, wherein the rotational angle is detected with two turns of the steering shaft being one cycle based on a change in resistance value of the GMR element obtained by changing a magnetic field direction according to rotation of the magnetizing portion. 
   The object is achieved effectively by that the GMR element is composed of two GMR bridge circuits, and is arranged so that phases of output signals from the GMR bridge circuits shift by 90°. 
   Further, the object is achieved effectively by providing a turning direction determining means for determining a turning direction of the steering shaft, and a middle point specific value detecting means for storing a position of a middle point of the steering shaft and detecting a predetermined value near the middle point so as to calculate the absolute steering angle in an entire steering angle range based on the predetermined value. 
   Further, the object is achieved effectively by that the turning direction determining means can determine the steering angle with accuracy of at least 720°. 
   According to the steering angle sensor of the present invention, in the case where the steering angle of the steering shaft is detected, the two GMR bridge circuits, which are arranged in the steering angle sensor and composed of GMR element for detecting the angle from the magnetizing portion, are arranged so that their phases shift by 90°, and the steering angle is calculated based on the output signals from the GMR bridge circuits. In this sensor, when a reduction ration between the sensor wheel and the steering shaft is adjusted, the absolute angle can be detected with two turns of the steering shaft being one cycle. As a result, the accuracy of steering angle estimation utilizing wheel information such as a wheel velocity and estimated value of a self aligning torque (SAT) may be a level at which the turning direction can be determining. 
   Further, a conventional steering angle sensor with one rotational cycle cannot calculate an absolute steering angle without estimating a steering angle based on vehicle information or without a continuous operation at the time of disuse. The steering angle sensor of the present invention, however, detects a specific value of the steering angle near the middle point so as to be capable of calculating the absolute steering angle immediately without requiring the steering angle estimation. As a result, in comparison with the case of only the steering angle estimation, output timing of the absolute steering angle can be sped up, and thus the steering angle accuracy can be consist with the output timing of the absolute steering angle at low cost. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a steering system of a vehicle having a conventional steering angle sensor; 
       FIG. 2  is a sectional view illustrating a main section of an electric power steering apparatus according to the present invention; 
       FIG. 3  is a sectional view illustrating a main section of a reduction mechanism; 
       FIG. 4  is a sectional view illustrating a main section of a steering angle sensor; 
       FIG. 5  is a graph illustrating a change in signal between two GMR bridge circuits; 
       FIG. 6  is a graph illustrating a change in arctanθ obtained from signals in  FIG. 5 ; 
       FIG. 7  is a diagram illustrating a schematic constitution of a steering angle calculating system; 
       FIG. 8  is a flowchart illustrating algorithm of the steering angle calculation; 
       FIG. 9  is a diagram illustrating an allocation rule of signal data; 
       FIG. 10  is a graph illustrating a relationship between a sensor output value and a rotational angle of a steering shaft; 
       FIG. 11  is a graph illustrating a relationship between a sensor output θ out  and an actual steering angle; and 
       FIG. 12  is a graph illustrating a relationship between the sensor output θ out  and the actual steering angle in a lock-to-lock range. 
   

   DESCRIPTION OF REFERENCE SYMBOLS 
   
       
         1 : steering shaft 
         7 : reduction mechanism 
         19 : steering angle sensor 
         23 : sensor wheel 
         24 : North pole magnet 
         25 : South pole magnet 
         26 : magnetizing portion 
         27 : GMR bridge 
         27   a:  first magnetic resistance element (first GMR bridge circuit) 
         27   b:  second magnetic resistance element (second GMR bridge circuit) 
         30 : absolute angle calculating means 
         31 : turning direction determination means 
         32 : middle point specific value detecting means 
     
  
   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of the present invention is explained in detail below with reference to the drawings. 
     FIG. 2  illustrates a schematic constitution of a general steering system, and a steering shaft  1  is supported by a bearing  2  and freely rotates based on an operation of a steering wheel. An input shaft  4  and an approximately cylindrical output shaft  5  are connected to a base end side (left side in  FIG. 2 ) of the steering shaft  1  via a torsion bar  3 . The torsion bar  3  is inserted into an output shaft  5 , and its one end is press fitted and fixed to the input shaft  4 , and the other end is fixed to the output shaft  5  by a pin  6 . 
   A reduction mechanism  7  is supported to an outer periphery of the output shaft  5  by a pair of bearings  8  and  9 , and a torque sensor  10  is arranged at a forward end side (right side in  FIG. 2 ) of the reduction mechanism  7 . The torque sensor  10  has the torsion bar  3  and an electromagnetic yoke  13  which is arranged on an outer periphery of a spline groove  11  formed at the forward end of the output shaft  5  and houses a coil winding wire  12 . The torque sensor  10  generates torsion of the torsion bar  3  according to a steering torque generated on the steering shaft  1 , so that the coil winding wire  12  in the electromagnetic yoke  13  detects a magnetic variation. 
   Further, the reduction mechanism  7  is composed of a worm wheel  14  which is attached to the outer periphery of the output shaft  5  in a fixed manner by press fitting, a worm  15  which engages with the worm wheel  14 , and a motor in which the worm  15  is attached to a driving shaft  16 . The reduction mechanism  7  reduces rotation of the motor via the worm  15  and the worm wheel  14 , so as to transmit an auxiliary torque to the output shaft  5 . An approximately circular concave groove portion  17  is formed on a side surface of the worm wheel  14 , and as shown in  FIG. 3 , a gear  18  is formed on an inner periphery of the concave groove portion  17  so as to engage with a sensor pinion  21  at a forward end of a sensor bar  20  of a steering angle sensor  19 . 
   As shown in  FIG. 4 , the steering angle sensor  19  contains a sensor wheel  23  which engages with a sensor worm  22  formed on a base end side(left side in  FIG. 4 ) of the sensor bar  20 , and rotation of the steering shaft  1  is transmitted from the worm wheel  14  via the sensor pinion  21  and the sensor bar  20  to the sensor wheel  23 . Further, the sensor wheel  23  is provided with a magnetizing portion  26  composed of a north pole magnet  24  and a south pole magnet  25 , and the magnets  24  and  25  rotate in constant cycle so as to synchronize with the rotation with the steering shaft  1 . In this case, a gear ratio is set so that the sensor wheel  23  caracoles with respect to the rotation of the worm wheel  14 . 
   A magnetic resistance element section (hereinafter, GMR bridge circuit section)  27  that detects a change in a magnetic field from the magnetizing portion  26  is arranged in the sensor wheel  23 , and the GMR bridge circuit section  27  is fixed to a part of a case of the steering angle sensor  19 . A pair of first and second magnetic resistance elements (hereinafter, GMR bridge circuits)  27   a  and  27   b  are arranged in the GMR bridge circuit section  27 , so that a phase is shifted by 90° according to an angle of the sensor wheel  23 . That is to say, when a waveform to be output from the first GMR bridge circuit  27   a  is designated by sinθ and a waveform to be output from the second GMR bridge circuit  27   b  is designated by cosθ, the waveforms sinθ and cosθ with respect to the steering angle of the steering shaft  1  are as shown in  FIG. 5 , and a change in arctanθ as shown in  FIG. 6  can be obtained according to an calculation in  FIG. 8 , mentioned later. 
   Further, the first and second GMR bridge circuits  27   a  and  27   b  are connected serially and are arranged orthogonally so as to operate as a three-terminal circuit element. A rotational angle of a body of rotation, namely, the sensor wheel  23  is detected by utilizing a change in resistance value of the element according to a change in the magnetic field strength. That is to say, since the first and second GMR bridge circuits  27   a  and  27   b  are arranged orthogonally to each other, shifts of output signals from the first and second GMR bridge circuits  27   a  and  27   b  shift by 90° as shown in  FIG. 5 . 
     FIG. 7  is a diagram illustrating a schematic constitution of a steering angle calculating system. In the drawing, in the steering angle sensor  19 , an integrated circuit (ASIC)  28  performs an calculation of arctanθ based on the signals sinθ and cosθ from the GMR bridge  27 , and the calculated results are converted into output signals according to a predetermined allocation rule as shown in  FIG. 9 , so that the output signals are serially output to a controller  29  of a steering assist device. The controller  29  includes an absolute angle calculating means  30 , a turning direction determining means  31  for determining a turning direction of the steering wheel based on vehicle information such as the wheel velocity and an SAT estimated value, and a middle point specific value detecting means  32  for storing a position where the rotational angle of the steering shaft  1  is 0°, namely, a middle point position and detecting a specific value near the middle point. 
   The allocation rule in  FIG. 9 , therefore, shows a corresponding relationship between the output value from the sensor and the rotational angle of the steering shaft  1 . Further, the output signal is according to an asynchronous method of a serial format, and the output value from the sensor and the rotational angle of the steering shaft  1  are displayed by three words. In the output signal, a plurality of words composed of a header (1 bit) and data (7 bits) are combined to be used. In this case, the output signal is composed of three words, the output value of the sensor is displayed by two words, and 16834 data are output with 14 bits. One word is used for horizontal parity check, and three words whose data are the same are output at the time of fail. 
   Steering angle calculating algorithm in the absolute angle calculating means  30  is explained below with reference to  FIG. 8 . 
   Levels of fs (sine wave detected by the first GMR bridge circuit  27   a  with respect to the steering angle) and fc (cosine wave detected by the second GMR bridge circuit  27   b  with respect to the steering angle) are compared using absolute values at step S 1 . When the absolute value of fc is larger than the absolute value of fs, namely, YES, the sequence goes to step S 2 . An arctan value (k) with respect to fs/fc is obtained at step S 2 , and a determination is made whether fc is not less than 0 at step S 3 . When fc is not less than 0, its value (k=k) is set for k at step S 4 , and when fc is smaller than 0, a value obtained by adding 180° to k (k=k+180) is set for k at step S 5 . 
   When the absolute value of fs is not less than the absolute value of fc, namely, NO at step S 1 , an arctan value (k) with respect to fc/fs is obtained at step S 6 , and a determination is made whether fs is not less than 0 at step S 7 . When fs is not less than 0, a value obtained by subtracting k from 90° (k=90−k) is set for k at step S 8 , and when fs is smaller than 0, a value obtained by subtracting k from 270° (k=270−k) is set for k at step S 9 . 
   A determination is made whether the values k set at steps S 4 , S 5 , S 8  and S 9  are not less than 0 at step S 10 . When k is not less than 0, its value (k=k) is set for k at step S 11 , and when k is smaller than 0, a value obtained by adding 360° to k (k=360+k) is set for k at step S 12 . 
   As a result, as shown in  FIG. 10 , a relationship between the output value of the sensor and the rotational angle of the steering shaft  1  is obtained. Therefore, when the steering angle is detected by a change in the resistance values of the first and second GMR bridge circuits  27   a  and  27   b,  its absolute steering angle can be obtained according to a change in the magnetic field of the magnetizing portion  26 . 
   The absolute steering angle is determined by the turning direction determining means  31  and the middle point specific value detecting means  32  based on the result of the absolute angle calculating means  30 . The turning direction determining means  31  determines the turning direction of the steering wheel based on an SAT estimated value derived by signals of a rotational angular velocity and assist power of the motor using the wheel velocity or inertia or static friction of the motor as a constant. The middle point specific value detecting means  32  stores a position of the middle point (rotational angle: 0°) of the steering shaft  1 , and detects a specific value near the middle point so as to determine the absolute steering angle. That is to say, the position of the middle point does not become constant at the time of assembling the steering shaft  1  or the like. For this reason, an output value θ 0  from the sensor is detected in a position where the rotational angle of the steering wheel is 0°, and this value is set as the middle point. 
   That is to say, when the sensor output value is designated by θ out , the absolute steering angle θ is θ out −θ 0 , and when a number of switching times from a maximum value into a minimum value of the sensor output can be counted n times, the absolute steering angle can be calculated according to the mathematical expression 1. 
   
     
       
         
           
             
               
                 θ 
                 = 
                 
                   
                     n 
                     * 
                     720 
                   
                   + 
                   
                     θ 
                     out 
                   
                   - 
                   
                     θ 
                     0 
                   
                 
               
             
             
               
                 ( 
                 
                   Mathematical 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
     
   
   Since n is determined by estimating a certain steering angle from the wheel velocity or the SAT estimated value at the first time after start-up and thereafter by counting the switching times from the maximum value into the minimum value of the sensor output, the absolute steering angle can be calculated based on the mathematical expression 1. 
   The actual rotational angle of the steering wheel is lock-to-lock, and narrower than ±720°. For this reason, the output from the steering angle sensor in the vehicle is less than two cycles, and as shown in  FIG. 11 , the same value is output from at most only two places, and these places are separated by 720°. The steering angle, therefore, may be estimated by the wheel velocity or the SAT estimated value with accuracy of 720°. That is to say, it is only necessary to be capable of determining the turning direction. 
   Further, the actual lock-to-lock range in the steering of the steering wheel is smaller than ±720°, and normally about 1.5 turn (540°). For this reason, as shown in  FIG. 12 , when the lock-to-lock range is, for example, ±600°, the specific value near the middle point is in a range designated by L in the drawing. The specific value near the middle point, therefore, can specify n in the mathematical expression 1 without detecting the vehicle information and estimating the turning direction. 
   For this reason, in the range of the specific value near the middle point, the number of switching times does not have to be counted, and thus the absolute steering angle of the steering shaft  1  can be detected quickly. Thereafter, the absolute steering angle can be calculated repeatedly according to the calculation in  FIG. 11 . 
   INDUSTRIAL APPLICABILITY OF THE INVENTION 
   The steering angle sensor of the present invention is suitable to be used as a means for detecting a steering angle in a steering apparatus, and is useful particularly for the case where the absolute angle is desired to be detected inexpensively and quickly.