Patent Publication Number: US-2007103343-A1

Title: Non-contact linear absolute position sensor

Description:
BACKGROUND OF THE INVENTION  
      1. Technical Field  
      The present invention relates generally to position sensors, and, more particularly, to a non-contact linear absolute position sensor.  
      2. Description of the Related Art  
      Angular and linear position sensors are widely used in automatic control systems as feedback-sensing devices in one or more control loops of the system. In the automotive industry, such position information may be used in substitution of more traditional, conventional control feedback provided by mechanical linkages, such as cables, rods, and the like.  
      For example, in the automotive field, it may be desirable to know the linear absolute position of a long travel mechanism, such as a rack and pinion mechanism (i.e., that moves when a driver of an automotive vehicle turns the steering wheel), or the position of a sliding door on a minivan. In the first example, a linear absolute position sensor can provide information as to the absolute linear position of the rack and pinion mechanism, which corresponds to the orientation of the front wheels (i.e., the steering wheels) of the automotive vehicle. In the second example, it may be desirable to know exactly where the sliding door is positioned within the long travel between a completely closed position and a completely open position. There are many other examples in and outside of the automotive industry. Non-contact linear absolute position sensing has conventionally been accomplished using a variety of technologies including inductive, optical, capacitive, and Hall Effect (i.e., magnetic flux intensity).  
      For example, inductive sensors are mechanically sturdy, but can be influenced by stray or externally-generated electromagnetic fields. Optical-based sensors are generally very accurate but require a relatively high degree of tolerancing on the parts, and are subject to strict sealing requirements in order to prevent or minimize dust from entering into the assembly, which can adversely influence an otherwise accurate measurement. Capacitive-based sensing technology generally provides satisfactory results but for conventional sized sensors the capacitance is generally relatively small and accordingly humidity and/or electromagnetic fields can also greatly influence an otherwise accurate measurement.  
      It is also known to use Hall Effect sensing technology for measuring absolute linear position, but such conventional approaches generally require very good material properties on the magnet and require flux concentrators made of a low hysteresis material. Additionally, these concentrators often require very accurate dimensioning and positioning. It is also very hard to achieve good temperature compensation using Hall Effect sensors alone. This problem is increased when you have to compensate for a component&#39;s position variation due to temperature.  
      U.S. Patent Application Publication No. 2004/0164727 A1 entitled “SINGLE MAGNET LINEAR POSITION SENSOR” discloses a sensor assembly for measuring linear position that includes a ferromagnetic flux concentrator, a magnet, and a galvanomagnetic sensing element such as a Hall Effect or magnetoresistive sensor.  
      In view of the foregoing, there is a need to provide a non-contact linear absolute position sensor that minimizes or eliminates one or more of the shortcomings referred to above.  
     SUMMARY OF THE INVENTION  
      Generally, the present invention fulfills the foregoing needs by providing, in one aspect thereof, a sensor system for measuring linear absolute position. The system comprises a first magnetic encoder, a second magnetic encoder, first and second magnetic flux intensity sensing units, and a processor. The first and second magnetic encoders have, respectively, n+1 pole pairs, and n pole pairs (where n is an integer greater than or equal to one). Each pole pair includes a north and south magnetic pole combination, as is known. The magnetic encoders are positioned proximate to each other and aligned with respect to each other so as to cover a linear distance. In one embodiment, the magnetic encoders are affixed to a traveling body whose linear absolute position is to be sensed. The first and second magnetic flux intensity sensing units are located next to and in sensing relation with the first and second magnetic encoders, respectively, and are configured to generate respective linear position signals. In one embodiment, the sensing units are fixed relative to the moving magnetic encoders. The linear position signals originating from the first and second sensing units exhibit a phase shift that is proportional to the linear absolute position (e.g., of the traveling body). The processor is responsive to the linear position signals (and thus the phase shift) and is configured to generate an output signal that is indicative of the absolute linear position.  
      In one embodiment, the first and second sensing units each include a pair of Hall Effect sensors. In each of the sensing units, one of the pair is oriented a pole&#39;s distance ahead of the other one of the pair so that each sensing unit produces dual signals 90° apart. This arrangement allows for temperature and target distance variation compensation, among other things. Accordingly, the inventive sensor system will be more robust to (i) variation of a material&#39;s magnetic properties, for example, between different lots of magnets, and variation with temperature; (ii) unpredictable target distance change due to temperature; and (iii) unpredictable target distance change due to application specific conditions (wear, pressure, etc.). For example, embodiments according to the invention may be made from plastic, which is characterized by a much less predictable change in dimension due to variation in temperature. The present invention can readily accommodate these uncertainties to provide an accurate output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will now be described by way of example, with reference to the accompanying drawings.  
       FIG. 1  is a plan view of a first embodiment of a non-contact linear absolute position sensor system including a pair of magnetic encoder tracks, corresponding sensing units and a processor.  
       FIG. 2  is a schematic and block diagram view showing, in greater detail, the processor (i.e., method processing block) of  FIG. 1 .  
       FIG. 3  is a plot of the sensing unit outputs as a function of position of a traveling body, showing in particular a phase difference between outputs indicative of absolute linear position.  
       FIG. 4  is a plot showing angle signals as a function of position that are output from a pair of single-angle determining units as shown in  FIG. 2 .  
       FIG. 5  is a plot showing a difference signal as a function of position produced from a dephaser block of the processor in  FIG. 2 .  
       FIG. 6  is a plot showing an absolute linear position (measured) signal as a function of position (actual) produced from the processor in  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION  
      Referring now to the figures wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  is a schematic and block diagram view of a sensor system  10  in accordance with the present invention. Sensor system  10  is configured to provide an output signal indicative of an absolute linear position of a traveling body, for example, traveling body  12  (shown in phantom line) in  FIG. 1 .  
      The present invention provides for a non-contact sensor system, which avoids wear and tear, providing like-new performance even after long use. As will be described in greater detail below, a sensor system according to the invention also provides for accurate absolute linear position measurements even over temperature variations.  
      As described in the Background, there are many uses for a sensor system according to the invention. Examples include, but are not limited to, those mentioned in the Background, namely measuring long linear travel parts such as a rack and pinion mechanism (e.g., steering) or the position of a sliding door. Exemplary applications are not limited to automotive applications and can be in many different industries as will be appreciated by one of ordinary sill in the art.  
      Before proceeding to a detailed description of the invention, it should be noted that the present invention has a configuration characterized by at least two low-cost magnetic encoders that share the same support (i.e., on traveling body  12 ). A pair of sensing units, each one near the face of a respective magnetic encoder, will produce a pair of output signals that will have a phase shift therebetween that is proportional to the position of the encoder strips relative to the sensing units, and thus the absolute linear position. Redundancy and/or increased resolution may be obtained by employing further encoders/sensing units, as will become apparent.  
      With continued reference to  FIG. 1 , sensor system  10  includes a first magnetic encoder  14 , a second magnetic encoder  16 , a first magnetic flux intensity sensing unit  18 , a second magnetic flux intensity sensing unit  20  and a processor  22 .  
      First encoder  14  may be provided in the form of a strip (e.g., flexible ferromagnetic materials of a desired thickness and width) and which includes n+1 pole pairs, where n is an integer greater than or equal to one. Each pole pair, as known, includes a “north” and a “south” magnetic pole. Second magnetic encoder  16  is provided with n pole pairs and is positioned proximate the first encoder  14  and aligned therewith at both a first end  26  and at a second, opposing end  28  and extending in a coextensive fashion so as to cover a linear distance  30 . Second encoder  16  may also come in the form of a strip as described above.  
       FIG. 1  further shows the first magnetic flux intensity sensing unit  18  as including a first Hall Effect sensor  32  and a second Hall Effect sensor  34 . The second magnetic flux intensity sensing unit  20  includes a third Hall Effect sensor  36  and a fourth Hall Effect sensor  38 . The first and second sensing units  18  and  20  are located near and in sensing relation to the first and second magnetic encoders  14  and  16 , respectively. The sensing units  18 ,  20  are configured to generate respective linear position signals. In this regard, the linear position signals originating from sensing unit  18 , on the one hand, and those originating from sensing unit  20 , on the other hand, are characterized by a phase shift that is proportional to the absolute linear position of the encoding strips  14 ,  16  relative to the sensing units.  
      In one embodiment, sensing units  18 ,  20  and processor  22  are fixed relative to magnetic encoding strips  14 ,  16  that are attached to and move with the traveling body  12 . The absolute linear position is this position of the traveling body  12  relative to the sensing units  18 ,  20 .  
      More particularly, however, sensing unit  18  produces linear position signals comprising a first alpha linear position signal (α 1 ), designated  40   1 , originating from Hall Effect sensor  32 , and a second alpha linear position signal (α 2 ), designated  40   2 , originating from Hall Effect sensor  34 . Hall Effect sensor  32  is positioned a predetermined distance from Hall Effect sensor  34 . The predetermined distance corresponds to one-half a pole distance taken with respect to the spacing in the corresponding magnetic encoder track  14 . This relative orientation between Hall Effect sensor  32  and Hall Effect sensor  34  (i.e., the one half pole spacing) results in the first and second alpha position signals  40   1  and  40   2  being offset, one relative to the other, by 90° (i.e., where a full pole pair corresponds to 360°). It should be noted that the relative spacing of one-half a pole distance between Hall Effect sensor  32  and Hall Effect sensor  34  is based on the actual spacing of the pole pairs in the corresponding magnetic encoder  14 , which corresponds to n+1 pole pairs. As will be described below, this Hall Effect sensor spacing is different, and slightly smaller, from the spacing between Hall Effect sensors  36  and  38 , since the corresponding magnetic encoder track  16  only includes n pole pairs over the same linear distance  30  (i.e., and hence results in a slightly wider magnetic pole pair spacing).  
      In this regard, sensing unit  20  thus also generates twin signals, a first beta linear position signal (β 1 ), designated  42   1 , originating from Hall Effect sensor  36 , and a second beta linear position signal (β 2 ), designated  42   2 , originating from Hall Effect sensor  38 . As with sensing unit  18 , sensing unit  20 , in a preferred embodiment, is constructed such that Hall Effect sensor  36  is offset from Hall Effect sensor  38  by a predetermined distance. This predetermined distance corresponds to one-half a pole distance taken with respect to the spacing in the corresponding magnetic encoder  16 . As alluded to above, magnetic encoder  16 , in accordance with the present invention, includes n pole pairs, and hence has slightly larger spacing than the encoder track  14 . As a result, first and second beta position signals  42   1  and  42   2  are offset, one from another by 90°.  
      Sensing units  18  and  20  are aligned, and as described above, are fixed relative to the moving encoder tracks  14 ,  16 . In  FIG. 1 , an absolute linear position  44  (also designated σ) corresponds to the actual, physical absolute linear position of the traveling body  12  relative to the fixed, sensing units  18  and  20 . As also shown in  FIG. 1 , the present invention employs a single angle parameter  46 , also designated alpha (α), which indicates the relative position of the sensing unit  18  (alpha sensor) within a pole pair on encoder track  14 . In a similar fashion, single angle parameter  48 , designated beta (β), indicates the relative position of sensing unit  20  (beta sensor) within a pole pair on magnetic encoder track  16 .  
      With continued reference to  FIG. 1 , processor  22  is responsive to the linear position signals  40   1 ,  40   2  and  42   1 ,  42   2  and is configured to generate an output signal  50  indicative of a measured absolute linear position (σ M ) of the traveling body  12 . Processor  22  may be implemented using conventional components known to those of ordinary skill in the art (e.g., hardware circuitry or programmed operation of a processor).  
       FIG. 2  is a simplified schematic and block diagram showing, in greater detail, processor  22  of  FIG. 1 . Processor  22  is configured to implement a method for solving for the distance indicative of the absolute linear position of the traveling body  12 . In this regard, block  22  is configured to generate an output signal  50  (σ M ) indicative of the absolute linear position of the traveling body  12 , which absolute position is also designated by reference numeral  44  in  FIG. 1 . Processor  22  includes a first single-angle determination unit  52  responsive to the alpha position signals  40   1  and  40   2  configured to generate a first angle signal  54 , a second single-angle determination unit  56  responsive to the beta position signals  42   1  and  42   2  configured to generate a second angle signal  58 , a dephaser  60  responsive to the first and second angle signals  54 ,  58  configured to generate a difference signal (φ)  62 , and a position generator  64  responsive to the difference signal  62  and the second angle signal  58  and configured to generate the previously mentioned output signal  50  (σ M ).  
      First single-angle determination unit  52  is configured to process the alpha linear position signals  40   1  and  40   2  to generate a composite, first-angle signal (α)  54 . In one embodiment, the first angle signal  54  may be defined as a function of 
 
α= A TAN(α 2 /α 1 ) 
 
      where α is said first angle signal  54 , 
          α 1  is said first alpha linear position signal  40   1 ,     α 2  is said second alpha linear position signal  40   2 , and        

      wherein ATAN is the arctangent function.  
      A single Hall Effect sensor may have its output affected by temperature variations. However, the present invention, by using two Hall Effect sensors on the same encoder strip, and then processing both signals, as described above, is operative to minimize or eliminate the variation due to temperature dependence. Any variation of one Hall Effect sensor due to temperature is processed out when the twin signals  40   1  and  40   2 , each assumed to have a similar temperature based variation, are divided one by the other as described above. This is one advantage to using dual signals originating from dual Hall Effect sensors operating against the same magnetic encoder strip, such as strip  14 . Another advantage is that the twin signals can provide position information within a full 360° span of the pole pairs. The first angle signal (α)  54  is indicative of the position of the first sensing unit  18  within one of the n+1 pole pairs in the first magnetic encoder  14 .  
      Second single-angle determination unit  56  operates in the same way as determination unit  52  except that it processes the twin beta position signals  42   1  and  42   2  originating from the second sensing unit  20 . The second angle signal (β)  58  that is generated from unit  56 , in one embodiment, may be defined as a function of 
 
β= A TAN(β 2 /β 1 ) 
 
      where β is said second angle signal  58 , 
          β 1  is said first beta position signal  42   1 ,     β 2  is said second beta position signal  42   2 , and        

      wherein ATAN is the arctangent function.  
      The second angle signal (β)  58  is indicative of the position of the second sensing unit  20  within one of the n pole pairs in the second magnetic encoder  16 .  
      De-phaser  60  is configured to generate the difference signal  62 , which is representative of the difference in phase between α (signal  54 ) and β (signal  58 ), i.e., it is the phase difference between the two pole pairs calculated above. The phase difference is indicative of absolute linear position. As described above, the position of each sensing unit relative to its corresponding magnetic encoder strip is calculated by first and second determination units  52  and  56  to generate respective single-angle signals  54  and  58  (i.e., in one embodiment, the signal pairs  40   1  and  40   2  and,  42   1  and  42   2 , are de-phased 90°). In order to make an accurate calculation, however, each of the first and second single-angle signals  54  and  58  must first be put into the same scale since each was derived from encoders using (n+1) and (n) pole pairs, respectively. Accordingly, part of the processing in de-phaser  60  includes multiplying the first single-angle signal  54  by (n), and multiplying the second single-angle signal  58  by (n+1). In one embodiment, the de-phaser  60  is configured to generate the difference signal  62  as a function of 
 
φ=MOD( n/ 2*α−( n+ 1)/2*β+45, 90* n ) 
 
      where φ is the difference signal  62  and, 
          MOD is a function that returns the remainder after dividing (n/2*α−( n+ 1)/2*β+45) by (90*n),     α is the first angle signal  54 ,     and β is the second angle signal  58 .        

      The difference signal  62  is indicative of the difference in phase between the first and second angle signals  54  and  58 .  
      Position generator  64  is configured to generate the output signal  50  (σ M ) as a function of both the difference signal (φ)  62 , and the second angle signal (β)  58 . In one embodiment, position generator  64  is configured to generate output signal  50  (σ M ) as follows:  
         σ   M     =         (       180   *     (     CYCLE   -   1     )       +   β     )     180     *       (     n   +   1     )     2           
          where σ M  is the output absolute linear position signal (measured),     β is the second angle signal  58 ,     and φ is the difference signal  62 , and  
       CYCLE   =         φ   +   45     90     .         
       

      Using the position of the second magnetic encoder strip  16  (i.e., the low frequency strip) and the difference signal (φ)  62 , the output signal  50  indicative of the absolute linear position can be determined with high accuracy. Providing further or more magnetic encoders can accomplish redundancy and/or can also provide for increased resolution.  
       FIGS. 3-6  show various plots of the signals at various stages of the processing.  
      Referring to  FIG. 3 , a plot  66  shows the outputs of the sensing unit  18 ,  20  as a function of the position of the traveling body  12 . More particularly,  FIG. 3  shows alpha position signal  40   2  plotted with beta position signal  422 . Note that at position zero, corresponding to the left-most end  26  of the linear distance  30  ( FIG. 1 ), the pole pairs on both encoders  14  and  16  are aligned. Accordingly, both signals in  FIG. 3  converge at position 0 mm. However, as the position of the traveling body  12  increases (i.e., the traveling body  12  moves to the left in  FIG. 1  wherein the encoder strips  14  and  16  move past the sensing units  18  and  20 ), the signals  40   2  and  42   2  begin to diverge and show a phase difference. This phase difference is due to the configuration of the encoder strips  14 ,  16  having (n+1) and (n) pole pairs, respectively.  
       FIG. 4  shows the outputs  54 ,  58  of the first and second single-angle determination units  52  and  56 , respectively. Plot  68  in  FIG. 4  shows first angle signal  54  and second angle signal  58 , each of which is a composite signal obtained by de-phasing of the respective signal pairs generated by sensing units  18 ,  20 . Note that  FIG. 4  also shows a phase difference between the outputs of sensing units  18 ,  20  as the position of traveling body  12  increases from 0 mm. This phase difference, as described above, is indicative of the absolute linear position.  
       FIG. 5  shows a plot  70 , which illustrates the difference signal  62  (diphase value) that is output from de-phaser  60  as a function of position (mm). The difference signal  62  represents the difference in phase between the two single-angle signals  54  and  58 .  
       FIG. 6  shows a total angle calculation plot  72 , which illustrates output signal  50  (measured position) as a function of actual position (mm). The output signal  50  (σ M ) is the measured absolute linear position that is indicative of the actual absolute linear position of the traveling body  12 .  
      While the particular non-contact linear absolute position sensor system  10  as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and thus, is representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled. For example, the present invention may be adapted to measure accurately the absolute angle of a rotational body (e.g., where there is no access to the center).  
      In one such alternate embodiment, tracks  14  and  16  are deployed to form a pair of concentric, closed, circle-shaped tracks, with one of the tracks having a greater diameter than the other one of the tracks. In this alternate embodiment, sensing units  18  and  20  are located near its corresponding track.  
      In a still further embodiment, tracks  14  and  16  are deployed to form a pair of closed, circle-shaped tracks but in which the tracks are offset axially from each other. For example, such tracks may be deployed on an outer surface of a rotating cylinder shaped component. The tracks  14 ,  16  would thus have about the same diameter (i.e., the diameter of the cylinder), but offset from each other. The sensing units  18 ,  20  are located near its corresponding track.  
      These two alternate examples show the use of the present invention to measure an absolute angle of a rotating body. It should be understood that the scope of the present invention is limited only by the appended claims.