Abstract:
A linear position sensing system with improved accuracy is shown. The sensor system has a magnetic field detector which senses the position of a target object. The target object has a magnet which emits a magnetic field. The position of the target object corresponds to the angle of the detected magnetic field. The linearity of the change in magnetic field angle is roughly proportional to the change in position of the target object. The magnet is magnetized on a slope which is determined by the path of traversal of the target object. In this manner, greater linearity in change of field angle relative to target object travel is achieved by altering the slope of the division between the magnetic poles.

Description:
FIELD OF INVENTION 
     This invention relates to magnet based position sensor. More specifically, this invention relates to a position sensor which uses a magnet which is magnetized such that the angle of the magnetic field varies linearly when rotated or translated. 
     BACKGROUND 
     It is desirable in many applications to determine the precise position of various objects which rotate. This is accomplished using magnetically based sensors. Such sensors function by measuring either the change in magnetic field intensity such as by a Hall effect sensor or the change in the angle of the fields such as an anisotropic magneto-resistence (AMR) sensor. A magnet is placed on the axis of a rotating element and rotation angle is sensed by the sensor according to the field angle. Such sensors provide cost effective and accurate measurement of the rotational angle of the object through the measurement of the field intensity or field angle. 
     Unfortunately, there are numerous applications in which a magnet cannot be placed on the axis of rotation. For example, a sensor for steering wheel position must be mounted on the outer surface of the steering wheel shaft. In such-applications, the magnet is installed on the periphery of the device. However, since the magnet is not on the axis of rotation, the path transcribed by the magnet does not produce an easily quantifiable relationship between input angle and the output field angle. In such applications, some form of field shaping must be employed by the magnetic field sensor in order to modify the field gradient from the magnet such that a relationship may be generated between the rotation and the output voltage of the sensed magnetic field. Such devices also must be used for a magnetic sensor which senses the location of objects which linearly traverse a defined path, since the magnitude of the field changes disproportionally with the distance. 
     Such present methods suffer from several problems. Most significantly, the magnetic field strength nor the angle of the magnetic field are exactly linear which introduces error in the position measurement. This inaccuracy increases as the magnetic field reach further distances away from the transducers for measurement of the fields. 
     Additional solutions have included using a linear magneto-resistive transducer in conjunction with a moving magnet. The magnetic field sensed by the transducers is an indication of the position of the magnet. However, the non-linear nature of the magnetic field results in distortions near the ends of the traversal of the magnet. Such distortions may be corrected, but such corrections require extra circuitry or processing which increase the complexity and cost of the device. Additionally, it requires complex shaping of a magnet in order to insure proper magnetic field output over the range of movement of a measured object. Such shaping is difficult to achieve and adds to the cost of the sensor. 
     Thus, there exists a need for a magnetic sensor which allows placement of a magnet in the target object without reliance on placement on the rotational axis. There is also a further need for a position sensor which uses magnet which can be compensated for a non-axial location on the target object. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a position sensor system for determining the position of a moveable object. The system has a magnet coupled to the object. A first magnetic field transducer detects the generated magnetic field and outputs a first sinusoidal signal representative of the magnetic field direction. A second magnetic field transducer detects the generated magnetic field and outputs a second sinusoidal signal representative of the magnetic field direction. A signal processor unit is coupled to the first and second magnetic field transducers, the signal processor unit outputs a signal which is a function of the sinusoidal signals representative of the position of the object relative to the first and second transducers. The magnet is magnetized such that the change of the angle of the generated magnetic field detected by the first and second magnetic field transducers is linear to the position of the magnet 
     The invention is also embodied in a method of determining the position of an object. A magnet is fixed on the object. The magnetic field direction produced by the magnets is detected. The magnetic field direction is converted into sinusoidal signals. The position of the object is determined based on the sinusoidal signals. The magnet is magnetized such that the change of the angle of the generated magnetic field detected is linear to the position of the magnet. 
     It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a front view of a magnetic field position sensor and a magnet according to one embodiment of the present invention which is mounted on a target object. 
     FIG. 2 is a perspective view of the magnet in FIG. 1 located on the target object. 
     FIG. 3 is a side view of the magnet located FIG. 1 on the target object. 
     FIG. 4 is a block diagram of the magnetic field sensor detector unit of the sensor of FIG.  1 . 
     FIG. 5 is a block diagram of the decoding electronics of the sensor of FIG.  1 . 
     FIG. 6 is a cross sectional view of the magnetization in the magnet located on the target object in FIG.  1 . 
     FIG. 7 is a front view of the magnetization in the magnet located on the target object in FIG.  1 . 
     FIG. 8 is a front view of a magnetic field position sensor and magnet located on a target object according to another embodiment of the present invention. 
     FIG. 9 is a perspective view of the magnet in FIG. 8 located on the target object. 
     FIG. 10 is a side view of the magnet located FIG. 8 on the target object. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention is capable of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. 
     Referring now to the drawings and more particularly to FIGS. 1-3 which shows a front view, perspective view and a side view of a positioning-sensor system generally indicated at  10 . In general, the angular positioning sensor system  10  senses the position of a target object  12  such as a rotating flex spline  14  which has a support collar  16  which rotates around an axis  18 . The support collar  16  has an outer surface  20  which has a magnet  22  which is arcuately shaped to conform to the outer surface  20 . 
     The magnet  22  has two opposite end surfaces  24  and  26 , a pair of opposite side surfaces  28  and  30  and a top surface  32 . The top surface  32  of the magnet  22  is arcuately shaped in conformance with the outer surface  20  of the support collar  16 . The top surface  32  of the magnet  22  is placed in proximity to a sensor unit  40 . 
     The sensor unit  40  detects the rotational angle of the flex spline  14  via a magnetic transducer unit  42 . The output of the magnetic transducer unit  42  is coupled to a signal processing unit  44 . The position of the target object  12  is thus detected by the sensor unit  40 . The sensor unit  40  is mounted in a non-ferrous package to minimize magnetic interference. In this example, the object is a flex spline  14  but as may be understood, the principles of the sensor  10  and the magnet  22  may be applied to any rotational angle positioning application. 
     FIG. 4 is a block diagram of the magnetic transducer unit  42  and FIG. 5 is a block diagram of the signal processing unit  44 . In the preferred embodiment the magnetic transducer unit  42  is a transducer integrated circuit such as a KMZ 43  magnetic field sensor manufactured by Philips Electronics. The transducer unit  42  outputs an electronic signal responsive to the detected magnetic field direction from the magnet  22  in FIGS. 1-3. However any appropriate magnetic field sensor may be used. The transducer unit  42  has a pair of magnetic field transducers such as magneto-resistive elements  46  and  48  which detect the direction of the magnetic field generated by magnet  22 . 
     As will be explained below, the transducer unit  42  outputs a pair of varying sinusoidal signals from the magnetic field transducers such as magneto-resistive elements  46  and  48  which are representative of the angle direction of the detected magnetic field from the magnet  22 . The outputs of the transducer unit  42  are coupled to the signal processor unit  44 . The signal processor unit  44  reads the sinusoidal signals output from the transducer chip  42  and converts them into a digital linear output. The signal processor unit  44  in the preferred embodiment is a UZZ9000 sensor conditioning electronic unit manufactured by Philips Electronics. However any appropriate hardware or software configuration may be used to process the raw signals from the transducer chip  42  to output a linear signal. 
     With regard to FIG. 4, each sensing element  46  and  48  has a series of four magneto-resistive elements  62 ,  64 ,  66  and  68  and  70 ,  72 ,  74  and  76  respectively, arranged in two Wheatstone bridge circuits  78  and  80 . The magneto-resistive elements  62 - 76  are preferably thin film Permalloy. The Wheatstone bridge circuits  78  and  80  are separated galvanically. 
     The Wheatstone bridge circuits  78  and  80  are each coupled to a power source  82  and  83  respectively and ground leads. The Wheatstone bridge circuits  78  and  80  have a positive output  84  and  86  respectively and a negative output  88  and  90  respectively. The four outputs  84 ,  86 ,  88  and  90  are coupled to the processing circuit  80 . The Wheatstone bridge circuit  78  outputs the sinusoidal signal representing the sine of the magnetic field direction sensed from the magnets  14  and  16 . The Wheatstone bridge circuit  80  is oriented at a 45 degree angle to the Wheatstone bridge circuit  78  and thus outputs a sinusoidal signal representing the cosine of the magnetic field direction sensed from the magnet  22 . 
     The Wheatstone bridge circuits  78  and  80  are arranged in order to determine the angle of the magnetic field relative to the circuits  78  and  80 . The angle degree of the detected magnetic field is approximately sinusoidally in proportion to the location of the magnet  22  (shown in FIGS. 1-3) in relation to the transducer unit  42 . As the magnet  22  moves, the arctangent of the magnetic field angle degree changes in an approximately linear fashion. As will be explained below, the magnetic field in the magnet  22  is shaped such that the angle degree will change linearly despite the fact that the rotation of the magnet  22  does not rotate about its axis. 
     FIG. 5 is a block diagram of the processing circuit unit  44 . The positive output signal  84  of the Wheatstone bridge circuit  78  (shown in FIG. 4) is coupled to an analog to digital converter circuit  92 . Similarly, the positive output signal  86  of the Wheatstone bridge circuit  80  is coupled to an analog to digital converter circuit  94 . The digital output of the analog to digital converter circuits  92  and  94  are coupled to a circuit processor block  96 . The signal processor block  96  converts the signals into a digital signal using a CORDIC algorithm. The CORDIC algorithm converts the sine and cosine values received from the sinusoidal signals into an arctangent value. 
     The output of the signal processor block  96  is coupled to an output curve characteristic processor  98 . The output curve characteristic processor  98  shapes the signal to output the desired span and offset angle. The output of the output curve characteristic processor  98  is coupled to a digital to analog converter  100  which outputs a signal which is buffered by a buffer circuit  102 . 
     As noted above, the signal processor  44  calculates the arctangent of the angle based on the sine and cosine of the magnetic field direction determined from the sensing elements  46  and  48  which measure the magnetic field angle of the magnet  22  and thus the position of the target object  12 . 
     The magnet  22  is typically a rare earth magnet which are preferably made of SmCo material although any suitable magnetic material such as Alnico or ceramic may be used. Alternatively electromagnets may bemused when stronger magnetic fields are desired. FIG. 6 shows a perspective view of the magnet  22  and FIG. 7 shows a cross section view of the magnet  22  showing the magnetization which produces a linear response over a limited rotation of the target object  12 . The magnet  22  has a north pole  110  and south pole  112 . The north and south poles  110  and  112  are areas created according to a curve equation which will be explained below to produce a linear response over limited rotation of the magnet  22 . 
     The north pole  110  and the south pole  112  are presented on the face  32  of the magnet  22 . The end surfaces  24  and  26  are likewise bisected by the north and south poles  110  and  112 . The north pole  10  forms the side surface  28  while the south pole  112  forms the side surface  30 . As may be seen in FIG. 7, the field directions are angled depending on the position of the magnet. Due to the changing angles of the field depending on the movement of the magnet  22 , an approximate linear field measurement is obtained when the magnet  22  is rotated with the support collar  16  relative to the transducer unit  42 . 
     The magnetization is created via shaping the application of high current on the magnetic material of the magnet  22 . Of course other methods may be used to achieve a sufficiently shaped magnetization in the magnet. The magnetization is at a sufficient level to achieve an acceptable signal to noise ratio in the transducer unit  42 . A border  114  is shaped as will be explained below to insure that shaped magnetization will occur as the magnet  22  is rotated relative to the transducer unit  42 . 
     The magnet  22  is magnetized with proper co-ordinate transformations to vary the magnetic field angle according to the path followed by the magnet  22  as it is rotated relative to the transducer unit  42 . Since the magnet  22  follows an arc, then calculation of γ(θ)=mθ+b will produce a linear angle change with a rotation of θ, where γ is the angle of the normal. θ is the angular position of the magnet  22  relative to the transducer unit  42 , m is a constant which is the slope of the line determined by the application and b is an adjustment factor which is used to offset the initial angle measurement. This arrangement provides a linear response since the magnet  22  is not concentric with the axis of rotation  18  of the target object  14  in FIG.  1 . 
     By changing the angle function, γ(θ), a desired linear response may be created for many different inputs or output aside from linear or angular. For example, instead of a linear change in angle, one could require a parabolic change by setting γ(θ)=mx 2 +b. 
     FIGS. 8-10 show a front view, perspective view and a side view of a linear positioning system generally indicated at  200 . In general, the linear positioning sensor system  200  senses the position of a target object  202  such as an exhaust gas regulator (EGR) valve stem which moves in a linear motion relative a sensor unit  204 . The target object  202  which is sensed by the sensor  204  unit is typically moved or mounted on a fixed track or path to insure strict linear movement. 
     As in the above embodiment explained in FIGS. 1-3, the linear positioning sensor  204  senses the position of the target object  202 . The target object  202  has an outer surface  206  which has a magnet  208  which is shaped to conform to the outer surface  206 . 
     The magnet  208  has two opposite end surfaces  210  and  212 , a pair of opposite side surfaces  214  and  216  and a top surface  218 . The top surface  218  of the magnet  208  is placed in proximity to the sensor unit  204 . The sensor unit  204  and the sensor unit  40  in FIGS. 1-3 and operates in the same manner. 
     The sensor unit- 204  detects the linear position of the target object  202  via a magnetic transducer unit  220 . The output of the magnetic transducer unit  220  is coupled to a signal processing unit  222 . The position of the target object  202  is thus detected by the sensor unit  204 . The sensor unit  204  is mounted in a non-ferrous package to minimize magnetic interference. In this example, the target object  202  is an EGR valve stem but as may be understood, the principles of the sensor  204  and the magnet  218  may be applied to any linear position sensing application. 
     The magnet  218  is magnetized such that the direction of the magnitude of the magnetic field changes in a prescribed fashion along a path similar to that shown in FIG.  7 . The magnet  218  has a north pole  230  and a south pole  232 . The magnetization of the magnet  218  is formed along a border  234  in order to insure linear angle output even when the sensor is not exactly linear to the movement of the magnet  218 . 
     In order to insure a linear output, the magnetic field changes along a path for a linearly changing normal along a linear path. This may be described by 
     
       
         γ( x )= mx+b   
       
     
     where γ is the angle of the normal. x is the position of the magnet  218  relative to the target object  202 , m is a constant which is the slope of the line determined by the application and b is an adjustment factor which is used to offset the initial angle measurement. The normal of the function, s, is ds/dx=−1/(tan(mx+b)). The shape is then determined by: 
     
       
           s ( x )=−1/ m  ln(tan( mx+b ))+1/2 m  ln(1+tan( mx+b ) 2 ) where  ds/dx =γ( x ) 
       
     
     The shape may then be used to magnetize the magnet  218  by creating the border region between the north and south poles following the shape described by s(x). 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the present invention without departing from the spirit or scope of the invention. For example, it is to be understood that any linear or rotational sensor system which places a magnet outside of the axis of rotation or movement may utilize the above principles. Examples of rotational applications for vehicles include steering angle sensors, suspension position sensors, window position sensors, throttle plate position sensors and pedal position sensors. Other vehicles such as motorcycles may use the principles described above for applications such as a throttle grip position sensor. The invention is not limited to application to the motor vehicle art. Linear application can include suspension position sensors, seat track position sensors, pedal position sensors, window position sensors and intake and exhaust valve position sensors. Thus, the present invention is not limited by the foregoing descriptions but is intended to cover all modifications and variations that come within the scope of the spirit of the invention and the claims that follow.