Patent Publication Number: US-2023136119-A1

Title: Optical position measuring device and method for operating an optical position measuring device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Application No. 10 2021 212 224.8, filed in the German Patent and Trademark Office (Deutsche Patent- und Markenamt (DPMA)) on Oct. 29, 2021, which is expressly incorporated herein in its entirety by reference thereto. 
     FIELD OF THE INVENTION 
     The present invention relates to an optical position measuring device and to a method for operating an optical position measuring device, e.g., to determine the position of a first object relative to a second object. The position measuring device may include, for example, a reflective measuring scale connected to the first object and a scanning unit connected to the second object. The scanning distance between the measuring scale and the scanning unit may be determined by a signal processing unit. 
     BACKGROUND INFORMATION 
     Particularly for optical position measuring devices that work with incident light and include a measuring device having a reflective measuring scale, it is of interest to capture the scanning distance between the measuring scale and the scanning unit in addition to the actual position information. For modularly constructed position measuring devices, this information can be used during installation in the particular application, for example, in order to correctly set the scanning distance. During measuring operation, it is possible, for example, for a corresponding, rotational variant of such a position measuring device, to draw conclusions from the continuous monitoring of the scanning distance about shifting of the rotating shaft or with respect to thermal influences. 
     A series of approaches are conventional for determining the scanning distance in such position measuring devices. 
     Japanese Patent Document No. 2001-174287 describes that the light emitted by an additional light source for measuring the scanning distance is guided onto a reflector track disposed between two measuring scale tracks on a measurement scale. The reflected light impinges on a detector in the scanning unit, in which the beam diameter of the of the beam bundle impinging on the detector changes as a function of the scanning distance. The scanning distance can be estimated by an intensity measurement. With this approach, in addition to the components for measuring position, a further light source, a separate reflector track, and an additional detector are required accordingly in order to obtain the information relating to the scanning distance. 
     The approaches described in Japanese Patent Document Nos. 2013-113634 and 2016-050886 do not require such additional components. In this regard, the grating structure of the measuring scale is illuminated for measuring each position, and the grating self-images being formed periodically along the scanning distance direction are evaluated according to the Talbot effect. The amplitudes of the grating self-images represent the measurement of the scanning distance of interest. A disadvantage is that periodic grating structures on the measuring scale are necessary, that is, determining the scanning distance is not possible in conjunction with an aperiodic code structure. Light sources meeting particular coherence requirements are also required for these measurement methods. 
     A further approach is described in German Patent Document No. 10 2018 104 280, which describes using the detector both for position measurement and for determining changes in the scanning distance in reflective position measuring devices. As a measure of the change of the scanning distance, the changes in intensity and/or the location of the light reflected back from the measuring scale, as captured by the detector, are evaluated. Changes in the scanning distance can be determined at a resolution of about 0.1 mm. For monitoring the corresponding position measuring device, particularly during measuring operation, however, such a resolution for determining the scanning distance is too low. Furthermore, evaluating the light intensity for determining the distance cannot be combined well with typical methods for stabilizing the illumination of the measuring scale in a manner that is acceptable for operating the position measuring device. 
     SUMMARY 
     Example embodiments of the present invention provide an optical position measuring device and a method for operating an optical position measuring device, in which the scanning distance can be determined as precisely as possible without additional components being necessary. 
     According to an example embodiment of the present invention, an optical position measuring device is configured to determine the position of a first object displaceable relative to a second object along a measuring direction. A measuring standard having a reflective measuring scale extending along the measuring direction and including scale regions having different reflectivities is connected to the first object. A scanning unit is connected to the second object and is disposed at a scanning distance relative to the measuring standard. The scanning unit includes at least one light source and a detector arrangement having a plurality of optoelectronic detector elements disposed periodically along the measuring direction. The scanning unit is further associated with a signal processing unit adapted to generate position signals relating to the position of the first object relative to the second object from the photocurrents generated by the detector elements, to determine a total photocurrent in a middle region of the detector and in at least one edge region of the detector, and to determine the scanning distance from the photocurrent ratio of the total photocurrents formed in the middle region of the detector and in the edge region of the detector. 
     The signal processing unit may be adapted to forming the photocurrent ratio by using values of the photocurrents used to generate the position signals. 
     The signal processing unit may furthermore be adapted to determine a plurality of photocurrent ratios during the measuring operation, to form an average value thereof, and to determine the scanning distance from the averaged photocurrent ratio. 
     The signal processing unit may be adapted to determine the scanning distance from an analytical relationship and/or to determine the scanning distance from a table stored in the signal processing unit that describes the relationship between the determined photocurrent ratio and the scanning distance. 
     The measuring scale may include measuring scale element cells in which an area ratio of summed areas of a category of scale regions to a total area of element cells is constant, and the following relationship is satisfied: 0 &lt; V F  = F TB1 /F GES  &lt; 1, V F  representing the area ratio, F TB1  representing the summed areas of the category of scale regions, and F GES  representing the total element cell area. 
     The measuring scale may be arranged as an incremental scale that includes a one-dimensional, alternating arrangement of rectangular or circular ring sector shaped scale regions having different reflectivities along the measuring direction. Circular ring sector shaped scale regions may also be referred to as annulus sector shaped scale regions. 
     Alternatively, the measuring scale may include a two-dimensional arrangement of scale regions having different reflectivities along the measuring direction and perpendicular to the measuring direction. 
     The measuring scale may be arranged as a pseudo random code and may include a one-dimensional, aperiodic arrangement of rectangular or circular ring sector shaped scale regions having different reflectivities along the measuring direction. 
     The detector arrangement may include a one-dimensional arrangement of rectangular or circular ring sector shaped detector elements disposed adjacent to each other along the measuring direction, in which the longitudinal axes of the detector elements are oriented perpendicular to the measuring direction, and/or a two-dimensional arrangement of detector elements disposed adjacent to each other along the measuring direction and perpendicular to the measuring direction. 
     The light source and the detector arrangement may be located in a plane parallel to the measuring scale. 
     According to an example embodiment of the present invention, a method for operating an optical position measuring device, by which the position of a first object relative to a second object displaceable along a measuring direction is determined, a measuring standard connected to the first object, having a reflective measuring scale extending along the measuring direction, and including scale regions having different reflectivities is provided. Furthermore, a scanning unit connected to the second object and arranged at a scanning distance relative to the measuring standard is provided, the scanning unit including at least one light source and a detector arrangement having a plurality of optoelectronic detector elements located periodically along the measuring direction. Position signals relating to the position of the first object relative to the second object are generated by a signal processing unit associated with the scanning unit from the photocurrents generated by the detector elements. A total photocurrent is further determined in a middle region of the detector and in at least one edge region of the detector, and the scanning distance is determined from the photocurrent ratio formed of the total photocurrents in the middle region of the detector and in the edge region of the detector. 
     The signal processing unit may form the photocurrent ratio by using values of the photocurrents used for generating the position signals. 
     During the measurement operation, the signal processing unit may determine a plurality of photocurrent ratios, determine an average thereof, and determine the scanning distance from the averaged photocurrent ratio. 
     The signal processing unit may determine the scanning distance from an analytical relationship and/or may determine the scanning distance from a table stored in the signal processing unit that describes the relationship between the determined photocurrent ratio and the scanning distance. 
     The signal processing unit may use of twice as many detector elements for forming the photocurrent ratio in the middle region of the detector as in two edge regions of the detector symmetrical about the middle of the detector. 
     An advantage of the approach described herein is that no additional components are required for determining the scanning distance. The components already used for measuring the position can be used, such as the light source, measuring scale, and detector arrangement. Furthermore, the device and method provide that the scanning distance can be determined with sufficient precision even during measuring operations. 
     The position measuring device may be arranged as both a linear measuring device and a rotational measuring device, for example, in the form of a rotary encoder. The measuring scale need not necessarily be periodic. 
     Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended schematic Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates the technique for determining the scanning distance. 
         FIG.  2    illustrates the relationship between the photocurrent ratio and the scanning distance. 
         FIG.  3    is a cross-sectional view of an optical position measuring device according to an example embodiment of the present invention. 
         FIGS.  4   a  to  4   c    illustrate portions of a measuring scale for an optical position measuring device arranged as a linear measuring device. 
         FIGS.  5   a  to  5   c    illustrate portions of a measuring scale for an optical position measuring device arranged as a rotational measuring device in the form of a rotary encoder. 
         FIGS.  6   a  and  6   b    illustrate detector arrangements for an optical position measuring device arranged as a linear measuring device. 
         FIGS.  7   a  and  7   b    illustrate detector arrangements for an optical position measuring device arranged as a rotational measuring device in the form of a rotary encoder. 
         FIG.  8    illustrates a method according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The technique for determining the scanning distance in an optical position measuring device illustrated in  FIGS.  1  and  2    and is based on calculating the distance Z from the distance-dependent change in the non-homogenous illumination of an optoelectronic detector DET by a divergent light source LQ. The illumination of a small partial detector area dA(x, y) depends on a spatial angle at which the partial detector area dA(x, y) is visible from the light source LQ. The illumination also depends on the emission characteristic of the light source LQ. For the example illustrated in  FIG.  1    of a light source LQ arranged as an LED having the emission characteristic of a Lambertian emitter, arranged at the location x = 0, y = 0, z = 0, for example, an illumination intensity dI results according to the following relationship: 
     
       
         
           
             d 
             I 
             ∼ 
             
               
                 
                   Z 
                   2 
                 
               
               
                 
                   
                     
                       
                         
                           x 
                           2 
                         
                         + 
                         
                           y 
                           2 
                         
                         + 
                         
                           Z 
                           2 
                         
                       
                     
                   
                   2 
                 
               
             
           
         
       
     
      in which dI represents the illumination intensity, x and y represent the coordinates of a partial detector area along the orthogonal coordinate directions x, y, and Z represents the distance of the light source from the detector. 
     The optoelectronic detector DET generates a photocurrent that is proportional to the integral of dI over the specifically illuminated detector area. From the above relationship, the photocurrent generated for a detector area in the middle region of the detector or center of the detector, i.e., for small values of the x- and y-coordinates, is greater than for an area in the edge region of the detector for large values of the x- y-coordinates. The light intensity is thus higher in the middle of the detector DET than in the edge regions. 
     When fixed detector areas in the middle region of the detector and in the edge region of the detector are selected for measuring photocurrents, the ratio V I  of the photocurrents of the detector areas depends only on the distance Z between the light source LQ and the detector DET. Influencing factors such as the absolute brightness of the light source LQ are eliminated by forming the ratio. 
     The relationship between a photocurrent ratio of photocurrents in a middle region of the detector and in an edge region of the detector and the distance Z between the light source and detector is illustrated in  FIG.  2    for an exemplary arrangement of a light source and detector. The ratio V I  of the photocurrent in the middle region of the detector to the photocurrent in the edge region of the detector is illustrated along the ordinate, and the distance Z between the detector and the light source along the Z-direction is illustrated along the abscissa, indicated in mm. As illustrated in  FIG.  2   , for a greater value of the photocurrent ratio V I , i.e., for greater differences in intensity between the middle region of the detector and the edge region of the detector, a lesser distance Z is present. Conversely, for lesser differences in intensity between the middle region of the detector and the edge region of the detector, i.e., a lesser value of V I , a greater distance Z is present. The lesser the distance Z, conversely, the greater the difference in intensity between the middle region of the detector and the edge region of the detector. The distance Z of interest between the light source and the detector can thus be determined by the difference in intensity between the middle region of the detector and the edge region of the detector. 
     For defined, specified detector areas and a known emission characteristic of the light source (such as the Lambertian emitter characteristic mentioned above), analytic relationships can be derived for the photocurrent ratio V I  as a function of the distance Z and can be solved for Z using analytical or numerical methods. 
     For the example illustrated in  FIG.  1   , the detected intensity I i  at the detector DET within a detection area A i  having the center coordinates x i , y i  can be stated approximately as follows: 
     
       
         
           
             
               I 
               i 
             
             = 
             
               
                 
                   A 
                   i 
                 
                 
                   Z 
                   2 
                 
               
               
                 
                   
                     
                       
                         
                           x 
                           i 
                           2 
                         
                         + 
                         
                           y 
                           i 
                           2 
                         
                         + 
                         
                           Z 
                           2 
                         
                       
                     
                   
                   2 
                 
               
             
           
         
       
     
      in which I i  represents the detected intensity at the detector, A i  represents the detection area, x i  and y i  represent the coordinates of the detection area, and Z represents the distance of the light source from the detector. 
     For a measured photocurrent ratio  
     
       
         
           
             
               V 
               I 
             
             = 
             
               
                 
                   I 
                   1 
                 
               
               
                 
                   I 
                   2 
                 
               
             
             &gt; 
             1 
           
         
       
     
     of detected intensities I 1 , I 2  on the basis of two detection areas of equal size in the middle of the detector and at the edge of the detector, the desired distance Z is thus derived based on the following relationship: 
     
       
         
           
             Z =  
             
               1 
               2 
             
             
               
                 
                   
                     
                       
                         
                           x 
                           2 
                           2 
                         
                         + 
                         
                           y 
                           2 
                           2 
                         
                         − 
                         
                           
                             
                               V 
                               I 
                             
                           
                         
                         
                           
                             
                               x 
                               1 
                               2 
                             
                             + 
                             
                               y 
                               1 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           V 
                           I 
                         
                       
                     
                     − 
                     1 
                   
                 
               
             
           
         
       
     
      in which V I  represents the measured photocurrent ratio, x 1 , y 1 , x 2 , and y 2  represent the coordinates of the detector areas, and Z represents the distance of the light source from the detector. 
     As an alternative to the foregoing, a table determined by a calibration and describing the functional relationship between the distance Z and the photocurrent ratio V I  can also be saved. Such a calibration takes place prior to the actual measurement operation of the position measuring device. During the measurement operation, the distance Z in the saved table can be determined for a measured photocurrent ratio V I . The distance Z corresponding to the nearest entry for the ratio V I  can be read using a nearest-neighbor technique, for example, or an interpolation between the nearest entries can be performed. 
     Both methodologies can thus be used for determining the desired distance Z from the measurement of the photocurrent ratio V I . This is exploited in the optical position measuring device for determining the scanning distance. 
       FIG.  3    is a schematic cross-sectional view of an optical position measuring device according to an example embodiment of the present invention. The corresponding position measuring device is implemented as a linear measuring device and is adapted to determine the position of a first object  01  relative to a second object  02 . The first and second objects  01 ,  02  are disposed displaceably relative to each other along the linear measuring direction x. The objects  01 ,  02  may be machine components displaceable relative to each other along the measuring direction x, for example, for positioning in a defined manner by a machine controller, by the position signals S P  generated by the position measuring device. 
     A measuring standard  10  of the position measuring device is connected to the first object  01 . The measuring standard  10  includes a measuring scale  11  extending along the measuring direction x and having scale regions  11 . 1 ,  11 . 2  having different reflectivities disposed alternately along the measuring direction x. A wide variety of arrangements exist with respect to the measuring scale  11 , both with respect to the measuring scale structure and with respect to the measuring scale configuration. The measuring scale  11  may be arranged as an incremental scale and/or as an absolute measuring scale in the form of a pseudo random code. With respect to the measuring scale configuration, it is possible to arrange the measuring scale  11  as an amplitude grating and/or as a phase grating. In the case of an amplitude grating, the scale regions  11 . 1 ,  11 . 2  may be arranged as highly reflective and slightly reflective (or non-reflective). If the measuring scale  11  is implemented as a phase grating, different phase shifting effects on the reflected beam bundle result in the different scale regions  11 . 1 ,  11 . 2 , i.e., the different scale regions have different phase deviations Δ1, Δ2. Further details of the measuring scale  11  are described below. 
     A scanning unit  20  associated with further components of the position measuring device is connected to the second object  02 . At least one light source  21  and a detector arrangement  22  are provided. The light source  21  and the detector arrangement  22  are disposed on a common support  23  in the scanning unit  20 , for example. As illustrated in  FIG.  3   , the light source  21  and the detector arrangement  22  are jointly arranged in the scanning unit  20  together in a plane E parallel to the measuring scale  11 . The detection plane of the detector arrangement  22  and the emission plane of the light source  21  are disposed in the same plane E. The light source  21  and the detector arrangement thus have same distance from the measuring scale  11 , and the distance between the plane E and the measuring scale is defined as the scanning distance Z A . 
     An LED (light emitting diode) is provided in the scanning unit  20  as the light source  21 , for example, and emits radiation at a wavelength of approximately 850 nm. The light source  21  has no upstream collimation optics, i.e., the measuring scale  11  is illuminated divergently by the light source  21 . 
     The detector arrangement  22  includes a plurality of optoelectronic detector elements such as photodiodes arranged at least periodically along the measuring direction x. Modulated photocurrents I i  are generated by the detector elements from the scanning of a pattern resulting from the imaging of the measuring scale  11  in the detection plane during the relative motion of the measuring standard  10  and the scanning unit  20 , and are utilized for generating position signals S P  with respect to the position of the first object  01  relative to the second object  02 , for example. 
     A signal processing unit  24  is further associated with the scanning unit  20  and undertakes a series of functions that are described in more detail below, including generating position signals S P  from the photocurrents I i  of the detector elements and determining the scanning distance Z A  of interest. In the illustrated example, the signal processing unit  24  is provided in the scanning unit  20 , but this is not required, e.g., the signal processing unit  24  may also be integrated in a machine controller connected downstream of the position measuring device, for example. 
     The optical position measuring device is arranged, for example, as an incident light system. The beam bundles emitted divergently by the light source  21  impinge on the reflective measuring scale  11  and are reflected back in the direction of the detector arrangement  22 , as illustrated in  FIG.  3   . Unlike the arrangement of the light source LQ and detector DET illustrated in  FIG.  1   , the detector arrangement  22  is not directly illuminated, but rather the light is reflected back onto the detector arrangement  22  by the structures of the measuring scale  11 . For such scanning, the divergent illumination of the measuring scale  11  and the associated projection effect results in a magnified image of the structures of the measuring scale  11  on the detector arrangement  22 . Because the light source  21  and the detector  22  in the position measuring device are jointly disposed in a plane E parallel to the measuring scale  11  in the scanning unit  20  and thus have the same scanning distance Z A  from the measuring scale  11 , an image of the scale structures results having a magnification factor of 2, independently of the scanning distance Z A  in the detection plane. It is further noted that, due to the back reflection of the light by the measuring scale  11  onto the detector arrangement  22 , unlike that illustrated in  FIGS.  1  and  2   , double the scanning distance Z A  is first determined from the determined photocurrent ratio between the middle region of the detector and at least one edge region of the detector. 
     In order to utilize the technique described in connection with  FIGS.  1  and  2   , in the optical position measuring device for determining the scanning distance Z A  from the photocurrent ratio V I  between the center of the detector and at least one edge region of the detector, it may be provided that certain prerequisites are met by the measuring scale  11  and/or by the detector arrangement  22 , and that particular measures are taken. This is explained in greater detail below with reference to  FIGS.  4   a  to  7   b   . 
     For example, the measuring scale used in the optical position measuring device is first described.  FIGS.  4   a  to  4   c    are partial views of exemplary embodiments of measuring scales  111 ,  211 ,  311  for position measuring devices arranged as a linear measuring device.  FIGS.  5   a  to  5   c    are partial views of measuring scales  411 ,  511 ,  611  for use in rotational or angular position measuring devices, i.e., in rotary encoders, for example. 
     The various measuring scales are implemented as amplitude gratings, in which higher-reflecting scale regions are illustrated as dark regions, and less-reflecting scale regions are illustrated as light regions. As mentioned above, it is also possible to arrange the measuring scales as phase gratings. In this instance, the light and dark regions represent scale regions having different phase-shifting effects. 
       FIGS.  4   a  and  5   a    illustrate measuring scales  111  and  411  arranged as an incremental scale and including a one-dimensional, alternating arrangement of rectangular or circular ring sector shaped scale regions  111 . 1 ,  111 . 2  or  411 . 1 ,  411 . 2  having different reflectivities along the measuring direction x. For the measuring scale  111  illustrated in  FIG.  4   a    for a linear measuring device, the measuring scale x extends linearly along the direction along which the measuring scale  111  and the scanning unit are displaceable relative to each other. For the measuring scale  411  of a rotary encoder illustrated in  FIG.  5   a   , the measuring direction x extends, as illustrated, annularly about an axis of rotation, about which the measuring scale  411  and scanning unit are rotatable relative to each other. 
       FIGS.  4   b  and  5   b    illustrate further measuring scales  211  and  511  that may be utilized for linear measuring devices and rotary encoders. A two-dimensional arrangement of scale regions  211 . 1 ,  211 . 2  and  511 . 1 ,  511 . 2  having different reflectivities along the measuring direction x and perpendicular to the measuring direction x is provided. The scale regions  211 . 1 ,  211 . 2  and  511 . 1 ,  511 . 2  of the measuring scales  211 ,  511  are arranged approximately in a diamond shape. The corresponding contours of the scale regions  211 . 1 ,  211 . 2  are not necessarily linear, but may be curved in the shape of a cosine, whereby a reduction in undesired harmonics in the generated position signals is achieved. Such measuring scales  211 ,  511  and the scanning of such measuring scales  211 ,  511  are described, for example, in European Patent Document No. 3 511 680 and U.S. Pat. No. 11,073,410, each of which is expressly incorporated herein in its entirety by reference thereto. 
       FIGS.  4   c  and  5   c    illustrate measuring scales  311  and  611  for linear measuring devices and rotary encoders arranged as pseudo random codes and including a one-dimensional, aperiodic arrangement of rectangular or circular ring sector shaped scale regions  311 . 1 ,  311 . 2  and  611 . 1 ,  611 . 2  having different reflectivities along the measuring direction x. 
     The structures present in the scanned measuring scale and necessary for determining the position fundamentally disturb the determination of the scanning distance Z A  according to the technique explained above. The reason for this is that in the case of a relative motion of the measuring scale and scanning unit, the photocurrent ratio V I  to be formed is also influenced, i.e., the photocurrent ratio V I  is not exclusively dependent on the scanning distance Z A . By a suitable configuration of the measuring scale, however, the interference with the determination of the scanning distance may be substantially reduced. It is thus provided that the area ratio V F  of the summed areas F TB1  of highly reflective scale regions to the total element cell area F GES  is constant in the measuring scale element cells continuously forming the particular measuring scale for the case of a measuring scale arranged as an amplitude grating.  FIGS.  4   a  to  4   c  and  5   a  to  5   c    illustrate, as a dashed line, a representative measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  of the corresponding measuring scale  111 ,  211 ,  311 ,  411 ,  511 ,  611  subject to this condition. 
     In the analogous case of a measuring scale arranged as a phase grating, for example, the area ratio of the summed areas F TB1  of measuring scales having the phase deviation Δ1 to the total element cell area F GES  is selected as constant, etc. 
     The area ratio V F  may satisfy the relationship, 0 &lt; V F  = F TB1 /F GES  &lt; 1, in which V F  represents the area ratio, F TB1  represents the summed areas of a category of scale regions, and F GES  represents the total element cell area. For example, V F  ≈ 0.5 may be provided. 
     Each measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  encloses only a small spatial angle as seen from the light source. Using the approximation-under the present circumstances for light sources arranged as LEDs-that the emission characteristic of the light source is constant over the spatial angle, the total amount of light reflected due to an arbitrarily structured measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  corresponds to the amount of light that the same measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  would reflect at an average, constant reflectivity without structuring. Assuming that all of the light of a measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  is captured by the detector arrangement, the initially interfering influence of the structuring averages out within a measuring scale element cell  112 ,  212 ,  312 ,  412 ,  512 ,  612  and the corresponding measuring scale  111 ,  211 ,  311 ,  411 ,  511 ,  611  acts like a mirror having reduced but constant reflectivity. The technique describe above can thus be used despite the structures present in the measuring scales  111 ,  211 ,  311 ,  411 ,  511 ,  611  in order to determine the scanning distance Z A  from the photocurrent ratio V I  between a middle region of the detector and at least one edge region of the detector. 
     It is explained below which measures may be provided for the detector arrangement  22  in the optical position measuring device in order to implement the technique described above for determining the scanning distance Z A .  FIGS.  6   a  and  6   b    illustrate exemplary configurations of detector arrangements  122 ,  222  for position measuring devices implemented as a linear measuring device.  FIGS.  7   a  and  7   b    illustrate detector arrangements  322 ,  422  for use in rotational or angular position measuring devices, such as rotary encoders. 
       FIGS.  6   a  and  7   a    are partial views of detector arrangements  122 ,  322  configured both for scanning incremental scales illustrated in  FIGS.  4   a  and  5   a    and for scanning measuring scales having pseudo random codes illustrated in  FIGS.  4   c  and  5   c   . The detector arrangement  122  illustrated in  FIG.  6   a    may be used in linear measuring devices having a linear measuring direction x, for example in order to scan the measuring scale illustrated in  FIGS.  4   a  or  4   c   . The detector arrangement  322  illustrated in  FIG.  7   a    may be used in rotary encoders, in which the measuring direction x extends annularly about an axis of rotation. That is, the detector arrangement  322  may be used for scanning the measuring scale illustrated in  FIG.  5   a   , for example. 
     The detector arrangement  122  configured for use in linear measuring devices illustrated in  FIG.  6   a    includes a one-dimensional arrangement of a plurality of identically configured, rectangular detector elements  122 . i  arranged adjacent to each other periodically along the linear measuring direction x. The longitudinal axes of the detector elements  122 . i  are oriented perpendicular to the measuring direction x, as illustrated. 
     The detector arrangement  322  configured for use in rotary encoders illustrated in  FIG.  7   a    includes a one-dimensional arrangement of a plurality of annular detector elements  322 .i located adjacent to each other periodically along the annular measuring direction x. The longitudinal axes of the detector elements  322 .i are oriented perpendicular to the measuring direction x, as illustrated. 
     The detector arrangements  222 ,  422  illustrated in  FIGS.  6   b  and  7   b    may be used for scanning the measuring scales  211 ,  511  illustrated in  FIGS.  4   b  and  5   b   . 
     The detector arrangement  222  configured for use in linear measuring devices illustrated in  FIG.  6   b    includes a two-dimensional arrangement of a plurality of detector elements  222 .i located adjacent to each other along the linear measuring direction x and perpendicular to the measuring direction x. The measuring scale illustrated in  FIG.  4   b    may be scanned by detector arrangement  222 . 
     The detector arrangement  422  illustrated in  FIG.  7   b    is configured for use in rotary encoders and includes a two-dimensional arrangement of a plurality of detector elements  422 .i located adjacent to each other along the annular measuring direction x and perpendicular to the measuring direction x. The measuring scale  511  illustrated in  FIG.  5   b    may be scanned by the detector arrangement  422 , for example. 
       FIGS.  6   a ,  6   b ,  7   a , and  7   b    illustrate for each of the detector arrangements  122 ,  222 ,  32 ,  422  which of the detector elements  122 .i are used in order to determine the total photocurrents I ges,MB , I ges,RB  in the middle region of the detector MB and in two edge regions of the detector RB required for determining the scanning distance Z A  as described above. The detector elements  122 . i  used to this end are indicated in  FIGS.  6   a ,  6   b ,  7   a ,  7   b    by different cross-hatching patterns relative to the remaining detector elements  122 . i . The detector arrangement  122  illustrated in  FIG.  6   a    and configured for use in a linear measuring device is described in more detail below. 
     It should be understood that it is not necessary to determine total photocurrents I ges,RB  in two edge regions of the detector. It is also possible to use only one edge region of the detector. 
     In the example illustrated in  FIG.  6   a   , eight adjacent detector elements  122 . i  in the detector arrangement  122  are used by the signal processing unit for determining the scanning distance Z A  in the middle region of the detector, and are arranged mirror-symmetrically about the axis of symmetry S of the detector arrangement  122 . The photocurrents I i,MB  of the eight detector elements  122 . i  are summed to form the total photocurrent I ges,MB  in the middle region of the detector MB. Four detector elements  122 . i  at each of the left and right edges of the detector arrangement  122  are used by the signal processing unit in the two corresponding edge regions RB, and the photocurrents I i,RB  thereof are summed to form a total photocurrent I ges,RB  in the edge region of the detector. As illustrated in  FIG.  6   a   , the four outermost detector elements  122 . i  in each of the left and right edge regions RB are selected. The photocurrent ratio V I  = I ges,MB/ I ges,RB  is formed from the total photocurrent I ges,MB  in the middle region of the detector MB and the total photocurrent I ges,RB  in the edge region of the detector RB, and the scanning distance Z A  is determined therefrom. 
     As indicated above, this can be achieved in that the relationship between the photocurrent ratio V I  and the scanning distance Z A  is saved in a table in the signal processing unit, according to the relationship illustrated in  FIG.  2   . 
     The number of detector elements  122 . i  used for determining the total photocurrents I ges,MB , I ges,RB  in the middle region of the detector MB and in the edge regions of the detector RB of the detector arrangement  122  is, for example, determined as a function of the imaging or projection of the scale structure in the detection plane. The number of detector elements  122 . i  in the middle region of the detector MB and in the edge region of the detector RB is determined so that whole-number multiples (n = 1, 2, 3 ...) of the measuring scale element cells projected into the detection plane are detected. Because four detector elements  122 . i  are provided per measuring scale element cell for generating position signals S P  in the form of four incremental signals at a phase offset of 90° due to the single field scanning provided, (n = 1) x 4 = 4 detector elements  122 . i  are used in each of the two edge regions of the detector RB for forming the total photocurrent I ges,RB , and (n = 2) x 4 = 8 detector elements  122 . i  are used in the middle region of the detector MB for forming the total photocurrent I ges,MB , as illustrated in  FIG.  6   a   . Consequently, altogether the same number of detector elements  122 . i  is used from the middle region of the detector MB as from the two edge regions of the detector RB for forming the photocurrent ratio V I  = I ges,MB/ I ges,RB  and thus for determining the scanning distance Z A . In other words, in the present example embodiment, photocurrents from the same number of scanned measuring scale element cells are used in the middle region of the detector MB and in the two edge regions of the detector RB for determining V I . It should be understood that using the same number of measuring scale element cells used in the middle region of the detector and in the edge region of the detector is not necessary. 
     Values of the photocurrents I i,MB , I i,RB  of selected detector elements  122 . i  are used accordingly by the signal processing unit for determining the photocurrent ratio V I , and the detector elements are also provided for generating the position signals S P  dependent on the displacement. Corresponding values, or copies thereof, can be generated, for example, in an analog manner by the current level or by a second voltage tap downstream of a combined transformer. It is also possible to first digitize the photocurrents I i,MB , I i,RB  and to use the corresponding values multiple times. 
     The position signals S P  in the form of a plurality of incremental signals are generated, e.g., in a conventional manner in the present example. Every measuring scale element cell projected onto the detector arrangement  122  is scanned by four detector elements  122 . i , resulting in four incremental signals, each phase offset by 90°, in the case of a relative motion of the measuring standard and the scanning unit. The photocurrents of detector elements  122 . i  generating incremental signals of identical phase are summed and are further processed by the signal processing unit, e.g., in a conventional manner in order to provide two incremental signals as position signals S P  for further processing on the output side, having a phase offset of 90°. 
     The precision may be increased further when determining the scanning distance Z A  during a measuring operation, for example, if the photocurrent ratio V I  is determined multiple times by the signal processing unit. The average of the plurality of determined photocurrent ratio V I  is determined, and the scanning distance Z A  is determined from the averaged photocurrent ratio ØV I . In this manner, imprecisions in determining the scanning distance can be avoided, for example, potentially caused by local contamination of the measuring scale. Such an averaging can be performed over time, for example, in that the photocurrent ratios V I  are calculated periodically and a particular number of determined photocurrent ratios V I  are combined. It is also possible to obtain the average over a particular position range, in that the determined photocurrent ratios V I  are combined within specified position ranges of the position measuring device and averaged, thus outputting average values for the corresponding position ranges. 
     In analogous manner, the same procedure is used for forming the photocurrent ratio V I  for the detector arrangement  322  illustrated in  FIG.  7   a   , which is configured for scanning the measuring scale  411  illustrated in  FIG.  5   a    in a rotary encoder. 
     Similarly, detector elements  122 . i  in the middle region of the detector MB and the two edge regions of the detector RB are selected for use in the detector arrangements  222 ,  422  illustrated in  FIGS.  6   b  and  7   b    for scanning the measuring scales illustrated in  FIGS.  4   b  and  5   b   . As explained above with reference to  FIG.  4   b   , for example, the measuring scale element cell  212  is approximately square. In order to scan such a measuring scale element cell projected in a diamond shape onto the detection plane, a group of 3 x 4 detector elements  222 . i  is necessary in the detector arrangement  222  illustrated in  FIG.  6   b   , of which four detector elements  222 . i  are located adjacent to each other along the measuring direction x and three detector elements  222 .i are located perpendicular to the measuring direction x. Therefore, 12 detector elements  222 . i  are necessary for each group for scanning a measuring scale element cell. Six such groups of detector elements  222 . i  of the detector arrangement  222  are used in the middle region of the detector MB for forming the photocurrent ratio V I . Three such groups of detector elements  222 . i  are used in each of the edge regions RB. 
     For the detector arrangement  422  illustrated in  FIG.  7   b   , an analogous procedure is followed and may be implemented in a rotary encoder for scanning a measuring scale illustrated in  FIG.  5   b   . 
     A method for determining the scanning distance Z A  is described below with reference to  FIG.  8   , which illustrates the signal processing unit  24  of the optical position measuring device and certain function blocks thereof. 
     According to the function block S 100 , photocurrents I i  are generated form the light pattern projected into the detection plane by the detector arrangement of the position measuring device and are transferred to the signal processing unit  24 . 
     The photocurrents I i  are used by the signal processing unit  24  to generate position signals S P  therefrom relating to the motion of the measuring standard and the scanning unit (function block S 110 ). For example, the generated position signals S P  may be two sinusoidal incremental signals having a phase offset of 90° from each other. Alternatively, however, position signals S P  in the form of absolute position data may also be generated. The corresponding position signals S P  are transferred to subsequent electronics via a suitable interface  30 . 
     A portion of the photocurrents I i  provided by the detector arrangements is copied and used for determining total photocurrents I ges,MB , I ges,RB  from a middle region of the detector and edge regions of the detector, in that photocurrents of selected detector elements are summed for this purpose (function block S 120 ). 
     A photocurrent ratio V I  is formed from the total photocurrents I ges,MB , I ges,RB  according to V I  = I ges,MB/ I ges,RB  (function block S 130 ). 
     The scanning distance Z A  is determined from the photocurrent ratio V I  as explained above (function block S140). To this end, an analytical relationship describing the relationship between V I  and Z A  may be used, and/or the scanning distance Z A  may be determined from a table saved in the signal processing unit  24  that correlates the associated scanning distance Z A  for a plurality of photocurrent ratios V I . 
     The scanning distance Z A  thus determined may also be output by the interface  30  to subsequent electronics for further processing. 
     It is possible to visualize the determined scanning distance Z A  by a display unit in order to provide assistance for correct assembly when installing the position measuring device, for example. 
     It may further be provided that the determined scanning distance Z A  is evaluated by a suitable method and the result of such an evaluation is output to subsequent electronics, for example, in the form of an evaluation parameter, and/or is visualized by a display unit.