Abstract:
The invention concerns a method for determining absolute coding represented by code elements of an optical code track, with illumination of the absolute coding with light, modulating of some of the illuminating light on code elements, determining of the absolute coding as modulated light and continuously varying modulation of the light on neighboring code elements.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for registering absolute coding of an optical code track. 
     BACKGROUND 
     An absolute encoder is a sensor for determining absolute positions of an object. The location of sensor and object in relation to one another is changeable. The sensor can register linear changes in location of the object and it can register rotating angular changes of the object. Sensors operating on contactless principles which determine the position of the object according to the optical or magnetic active principle are known. To this end, a sensor registers absolute coding of a code track and an evaluation unit evaluates the registered absolute coding and establishes the object position therefrom. Within the meaning of the invention, the absolute coding is a spatially resolved position specification. 
     Absolute encoders are used in multifaceted ways. In plant construction and engineering, they determine the positions of machine elements such as drives, swivel heads, rotary tables, etc. in relation to a reference system. In geodetic instruments such as theodolites, tachymeters, laser scanners, etc., they measure horizontal angles and vertical angles in relation to distant objects. In coordinate measuring machines, they register spatial alignments of robot arms, steering wheels, etc. 
     In the following, the special case of an optical code track is considered. An optical code track has a mechanical support in the form of a disk, a ribbon, etc. In this respect,  FIG. 1  shows an example from the prior art according to EP1890113A1. Many adjacent code elements are arranged on the mechanical support of the optical code track, which code elements embody the absolute coding  10 . Within the meaning of the invention, the code elements arranged in the track direction update the bijective position specification of the absolute coding from one code element to the next adjacent code element in a spatially resolved manner. 
     As a result of the presence of defined code elements, which respectively embody a discrete defined element of the code (and are then also considered code element by code element during the evaluation, wherein a state/value is established for each code element), it is possible to speak of a “digital” code here (in contrast to a continuous code, e.g. updated between 0 and 1, wherein any arbitrary intermediate value can be decoded into the respectively sought-after value such as the sought-after location specification on the basis of a defined conversion function; in this case, this is usually referred to as an “analog” code). 
     The code elements are e.g. light-transmissive rectangles which are arranged in an optically opaque residual region. The optical code track  1  is illuminated by light from a light source by way of the transmitted light principle. The code elements modulate the light. Light passed by the light-transmissive rectangles is registered by a sensor along the track direction; light not passed by the optically opaque residual regions is not registered by the sensor. The light-transmissive rectangles are imaged on the sensor as a cast shadow. The sensor generates state signals for registered light. In the case of relative motion of the optical code track and sensor, the sensor registers the absolute coding as a temporally discrete sequence of discontinuous bright/dark transitions. 
     The absolute coding has either a bijective position specification or a bijective code. Hence, the position specification is either established directly from the state signals or a position specification is assigned to the code of the state signal by way of look up in a table. Since the code elements and the sensor have a spatial extent, it is moreover possible to establish a centroid of the state signal in order to relate the established position specification to the centroid of the code element with sub-code element accuracy. Within the meaning of the invention, the width of the state signal in the track direction is referred to as signal width and the width of the sensor in the track direction is referred to as sensor width. A centroid of the code element is deduced from the centroid of the state signal. Moreover, the distance to a reference position in the track direction is determined from the centroid of the code element. Hence the state signal not only supplies a bijective position specification but also enables determination of the location of the code element in relation to a reference position. 
     However, as an alternative to determining the centroid, a person skilled in the art is also aware of different processes by means of which the precise position of the code element can be established on the basis of the registered code projection. 
     This is all carried out in order to determine the position of the object with high accuracy. Thus, positions of machine elements are determined with an accuracy of 1 μm and theodolites measure horizontal angles and vertical angles to objects at a distance of several hundred meters with an accuracy of 0.1 mgon. In order to be able to achieve such a high accuracy, systematic and non-systematic errors must be eliminated when determining the position of the object. 
     Highly accurate absolute encoders therefore comprise a plurality of sensors which are arranged with a fixed spatial relationship to one another and which redundantly register the absolute coding of the code track. By forming averages of the absolute coding registered redundantly it is possible to eliminate non-systematic errors when determining the position of the object. 
     The remaining systematic errors when determining the position of the object often have a harmonic nature. Such harmonic errors have multifaceted causes. Thus, they can be due to irregularly arranged code elements on the code track or be caused by thermal expansion of the code track, eccentricity of the mechanical support of the code track, mounting play of the absolute encoder, diffraction phenomena on code elements, etc. Moreover, the fixed spatial relationship between the sensors themselves and the regular arrangement of the code elements on the code track constitute periodic structures. The superposition of the periodic structures may form interfering moiré patterns in the case of optical absolute encoders. Moreover, according to the Nyquist-Shannon sampling theorem, information losses may occur when registering the absolute coding in the case where a selected sampling frequency of the absolute encoder is too small in relation to the maximum frequency of the code elements. 
     In this respect, WO2011/064317A1 describes a method for establishing error coefficients and a method for correcting the measured value of an absolute encoder using these error coefficients. The absolute encoder has at least two sensors and an optical code track. The sensors and the optical code track are movable relative to one another. The sensors register the absolute coding of the optical code track as a sequence of bright/dark transitions at different angular positions. The sensors are spaced apart from one another at an angle of at least 50 degrees. An evaluation unit establishes angle position values from the absolute coding registered by the sensors. By comparing the difference in angle position values of the sensors for a plurality of different angular positions, harmonic angular errors are represented as error coefficients in a Fourier series expansion. The angle position values are corrected by these harmonic angle errors. 
     SUMMARY 
     Some embodiments of the invention include providing an improved method for registering the absolute coding of an optical code track. 
     Some embodiments of the invention include providing an optical code track and an absolute encoder for registering the absolute coding of the optical code track, which require as few sensors and/or as little computational outlay as possible in order to determine the object in a highly precise way. 
     Some embodiments of the invention include providing a method for producing an optical code track, which method is compatible with existing and proven coating techniques in a cost-effective manner. 
     Some embodiments of the invention include providing an optical code track and an absolute encoder for registering the absolute coding of the optical code track, which have a high availability, even under rough usage conditions. 
     One aspect of the invention relates to a method for registering absolute coding, wherein the absolute coding is embodied by code elements of an optical code track and the individual code elements respectively form a main point or centroid, comprising illumination of the code elements with light; comprising modulation of part of the illuminating light at code elements; and comprising registration of the absolute coding as modulated light (i.e., modulated light is registered and state signals generated therefrom, for which state signals respectively one main point or centroid and the positions thereof are established). According to the invention, the modulation of the light at adjacent code elements in the direction of extent of the code track is effected in this case in a continuously varying manner. 
     Now, diffraction phenomena, which can lead to aliasing effects, may occur when modulating light on an optical code track from the prior art according to EP1890113A1 with rectangular code elements. These aliasing phenomena lead to quasi-stochastic errors when registering the modulated light. The quasi-stochastic errors also have an effect on the state signals which are generated for registered modulated light, and interfere with determination of the centroid of the state signals. The applicant discovered that such diffraction phenomena occur to reduced extent when registering light that is modulated at adjacent code elements with continuous variation. Adjacent code elements are a first and a second code element, which code elements adjoin one another and update the absolute coding from the first code element to the second code element in the track direction. The term “continuous” is always used in the mathematical sense; the updating of the absolute coding is brought about in a jump-less variation in the illuminating light. 
     Therefore, the invention relates to a digital absolute code, wherein the code track embodying the absolute coding is made up of defined code elements which respectively embody a discrete defined element of the code (and are then also considered code element by code element during the evaluation, wherein a state/value is established precisely for each code element). 
     By avoiding discontinuous bright/dark transitions, as occur at rectangular code elements of a digital code known from the prior art, diffraction phenomena are reduced, and so generated state signals also have fewer quasi-stochastic errors. Thus, according to the invention, there is a reduction in errors. Whereas harmonic errors in the prior art according to WO2011/064317A1 are expanded as error coefficients when registering a sequence of bright/dark transitions, and angle position values are subsequently corrected by these harmonic errors, the invention proceeds from the generation of the quasi-stochastic errors and reduces the sources or occurrence thereof. 
     A further aspect of the invention relates to an optical code track embodying digital absolute coding; said optical code track comprises a mechanical support, preferably in the form of a disk or a ribbon; and said mechanical support, at least in regions, comprises code elements which respectively form a defined main point or centroid. Here, the code elements are once again embodied in such a way that adjacent code elements modulate illuminating light in the direction of extent of the code track in a continuously varying manner. 
     In a special embodiment, the absolute coding is embodied as an aperture stop with, in the direction of extent of the code track, continuous variation of code heights (i.e. with continuously varying stop opening sizes along the direction of extent of the code track). 
     It was discovered that an aperture stop can embody absolute coding with code elements that have continuously varying code heights. The aperture stop consists of individual code elements which are arranged in the track direction and have different widths. 
     Alternatively, the absolute coding can also be embodied as a point grid with continuous variation of point densities of adjacent code elements. 
     It was also discovered that a point grid can embody absolute coding with code elements that have continuously varying point densities. The point grid consists of a grid with a changeable density, in the track direction, of dark points on a bright background. 
     In accordance with a further special embodiment, the absolute coding is embodied as a polarizer with continuous variation of polarization efficiencies of adjacent code elements. 
     Moreover, it was discovered that a polarizer can embody absolute coding. A polarizer is an optical means which modifies a polarization state of light with a specific polarization efficiency. Here, a distinction is made between a polarization due to scattering, a polarization due to reflection, a polarization due to absorption and a polarization due to birefringence. 
     Another further aspect of the invention relates to a system for registering the absolute coding of the optical code track; which system comprises an absolute encoder with an optical sensor; which absolute encoder generates at least one state signal for registered modulated light; which state signal has a signal width, which signal width is less than a sensor width of the sensor. 
     It is known that the centroid of the state signal can only be established with minimum error if the signal width of the state signal is precisely an integer multiple of the sensor element width (i.e. of the pixel spacing) of the sensor. As soon as the signal width deviates therefrom, the centroid of the state signal can only be established with a larger or smaller error. The error is at maximum when the signal width of the state signal deviates from the integer multiple of the sensor element width by half a sensor element width. Now, from a practical and technical point of view, it is impossible to generate a signal width which is exactly a multiple of the sensor element width under all usage conditions. Thus, the signal width is influenced by varying factors such as the imaging scale, the surrounding temperature, production tolerances of the optical code track and absolute encoder, etc. The code element can be imaged on the sensor using a lens optical unit; each lens optical unit is afflicted by a distortion which images the code element with a different width onto the sensor depending on the location, the sensor generating a state signal for said image. 
     Another further aspect of the invention relates to a method for producing the optical code track, in which a mechanical support of the optical code track is provided; on which mechanical support a coating is applied, at least in regions; and into which coating code elements are structured, which code elements respectively form a main point or centroid and modulate illuminating light; the code elements are structured in such a way that illuminating light is modulated with a continuous variation at adjacent code elements. 
     Such a production method is cost-effective and compatible with existing and proven techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and features of the invention can, in conjunction with the attached figures, be gathered in an exemplary manner from the following description of currently preferred embodiments. 
         FIG. 1  shows an example of an optical code track from the prior art; 
         FIG. 2  schematically shows an absolute encoder which registers an optical code track according to the invention with a sensor and communicates a state signal to an evaluation unit; 
         FIG. 3  shows a cross section of part of a first embodiment of an optical code track according to  FIG. 2 , with a constant thickness of the coating in the track direction; 
         FIG. 4  shows a cross section of part of the first embodiment of an optical code track according to  FIG. 3 , with a constant thickness of the coating obliquely to the track direction; 
         FIG. 5  shows a cross section of part of a second embodiment of an optical code track according to  FIG. 2 , with a varying thickness of the coating in the track direction; 
         FIG. 6  shows a cross section of part of the second embodiment of an optical code track according to  FIG. 2 , with a varying thickness of the coating obliquely to the track direction; 
         FIG. 7  shows a view of part of a third embodiment of an optical code track according to  FIG. 2 , with an aperture stop as absolute coding; 
         FIG. 8  shows a view of part of a fourth embodiment of an optical code track according to  FIG. 2 , with a point grid as absolute coding; 
         FIG. 9  shows a view of part of a fifth embodiment of an optical code track according to  FIG. 2 , with an aperture stop as absolute coding; 
         FIG. 10  shows a view of part of a sixth embodiment of an optical code track according to  FIG. 2 , with an aperture stop as absolute coding, during the process of registering light modulated at a first code element; 
         FIG. 11  shows a view of part of the optical code track according to  FIG. 10  during the process of registering light modulated at a first code element; 
         FIG. 12  shows a first embodiment of an individual rectangular state signal on the sensor of the absolute encoder according to  FIG. 2 ; 
         FIG. 13  shows the error when determining the centroid of the state signal according to  FIG. 12 ; 
         FIG. 14  shows a second embodiment of an individual bell-shaped state signal on the sensor of the absolute encoder according to  FIG. 2 ; 
         FIG. 15  shows the error when determining the centroid of the state signal according to  FIG. 14 ; 
         FIG. 16  shows a third embodiment of an individual rectangular state signal on the sensor of the absolute encoder according to  FIG. 2 ; 
         FIG. 17  shows the error when determining the centroid of the state signal according to  FIG. 16 ; 
         FIG. 18  shows a fourth embodiment of a multiple rectangular state signal on the sensor of the absolute encoder according to  FIG. 2 ; 
         FIG. 19  shows the error when determining the centroid of the state signal according to  FIG. 18 ; 
         FIG. 20  shows a fifth embodiment of a multiple bell-shaped state signal on the sensor of the absolute encoder according to  FIG. 2 ; 
         FIG. 21  shows the error when determining the centroid of the state signal according to  FIG. 20 ; 
         FIG. 22  schematically shows the registration of light modulated at code elements of the absolute coding according to  FIG. 2  by means of an absolute encoder according to  FIG. 2  and the determination of an object position by the evaluation unit according to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an absolute encoder  2  which registers an absolute coding  10  of an optical code track  1  using an optical sensor  20 . The optical code track  1  is fixedly arranged on an object  4 . Both the optical code track  1  and the object  4  may have any size, shape and form. By way of example, the optical code track  1  has the form of a disk, a ribbon, etc. By registering the absolute coding  10 , it is possible to determine an object position absolutely. In accordance with  FIG. 2 , the object  4  is schematically embodied as a cylinder and the optical code track  1  is arranged externally on the circumference of the object  4  in the form of a ribbon and completely surrounds the circumference. The absolute encoder  2  communicates with an evaluation unit  3 . The position between the absolute encoder  2  and the optical code track  1  is variable. 
     The absolute encoder  2  has a light source, said light source generating light  21 , said light  21  illuminating the optical code track  1 . The light  21  consists of electromagnetic waves such as radio waves, microwaves, visible light, etc. Light  21 ′ modulated at code elements  100 ,  100 ′ of the optical code track  1  is registered by the optical sensor  20 . Within the meaning of the invention, the phrase “modulation of light” is understood to mean a physical interaction between light  21  and code elements  100 ,  100 ′ of the optical code track  1 . Modulation of light comprises scattering of light, reflection of light, refraction of light, diffraction of light, absorption of light, polarization of light, etc. By way of example, the optical sensor  20  is a collection of sensor elements, such as a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS), etc. Here, the sensor has individual photoreception elements aligned next to one another, wherein the sensor width in the X-direction (and, optionally, in the Y-direction as well for a two-dimensional array) is specified in a number of individual photoreception elements (pixels). The X-direction is the sensor track direction. By way of example, in accordance with  FIGS. 12 to 22 , the optical sensor  20  is a line array with 1000 photoreception elements (pixel) in the X-direction. According to the invention, individual reception elements of the sensor are in this case not only illuminated either “not at all or completely”, but many different intermediate values (grayscale values) are generated for the illumination of the individual reception elements (pixels) as a result of the continuously varying modulation for the code projection generation, wherein the illuminance over the course of the individual pixels along the sensor line then corresponds to the continuously varying modulation within the scope of the code projection. 
     The absolute encoder  2  generates as sensor output, dependent on the registered code projection, at least one state signal  200  for modulated light  21 ′ registered by the optical sensor  20  and communicates this state signal  200  to the evaluation unit  3 . Details in respect of the state signal  200  follow in  FIGS. 12 to 24 . The communication  23  of the state signal  200  can be effected in an arbitrary manner; in particular, it can be effected on the basis of cables, radio waves, etc. If a person skilled in the art knows of the present invention, he can embody the absolute encoder with an evaluation unit integrated in the housing as a single unit. 
       FIGS. 3 to 11  show a plurality of embodiments of an optical code track  1 , wherein a digital absolute code is embodied in each case. By way of example, the optical code track  1  comprises a mechanical support  11  made of glass, plastic, metal, etc.  FIGS. 3 to 6  show the optical code track  1  in a cross section along the track direction (X-direction) and in a cross section in the Y-direction across the track direction. By way of example, a coating  12 , made of chromium, silicon nitride and molybdenum, etc., is applied to the mechanical support  11 , at least in regions.  FIGS. 7 to 11  show a view of the optical code track  1 . By way of example, photolithographic, etched or electroplated code elements  100 ,  100 ′ are structured in the coating  12 , at least in regions. The regions of the mechanical support  11  uncovered after structuring, which no longer comprise a coating  12 , may be e.g. more than 90%, in particular more than 98%, light-transmissive for the light  21 . The regions of the mechanical support  11  covered after structuring, which still have a coating  12 , are e.g. more than 90%, in particular more than 98%, optically opaque, i.e. do not transmit light, for the light  21 . As shown in  FIGS. 3 and 4 , the thickness of the coating  12  of the optical code track  1  may, in regions, be constant in the X-direction and in the Y-direction. By way of example, the coating has a thickness of 1 μm, preferably a thickness of 0.5 μm, preferably 0.1 μm, etc., in both an edge region  107  and in a central region. As shown in  FIGS. 5 and 6 , the thickness of the coating  12  may, however, also be different for various regions of the optical code track  1 . By way of example, in accordance with  FIG. 5 , in the X-direction, the coating  12  is thicker in the edge region  107 , where it is 0.6 μm thick, than in central regions, where it is 0.2 μm thick. Moreover, in accordance with  FIG. 6 , the thickness of the coating  12  may vary continuously between 0.1 μm and 1.0 μm in the Y-direction. 
     The light  21 ′ modulated at the code elements  100 ,  100 ′ is registered by transmitted light or reflected light. In the transmitted light method, the optical sensor  20  registers the light  21 ′ modulated by uncovered regions; in the reflected light method, the optical sensor  20  registers the light  21 ′ modulated at covered regions. In the case of the optical code track  1  in the embodiment in accordance with  FIGS. 7, 9, 10 and 11 , modulated light  21 ′ is, in an exemplary manner, registered using the transmitted light method (wherein this—as is known to a person skilled in the art per se—can alternatively also be effected by the reflected light method). In the optical code track  1  in the embodiment as per  FIG. 8 , modulated light  21 ′ is, in an exemplary manner, registered using the reflected light method (wherein, alternatively, the transmitted light method can also be used here in turn—as is known to a person skilled in the art per se). 
     The optical sensor  20  registers the absolute coding  10  by changing the mutual position along the X-direction. The code elements  100 ,  100 ′ embodying the absolute coding  10  can have a constant code element width  104  in the X-direction and they have a constant maximum code element height  105  in the Y-direction. The sizes of the code element width  104  and the maximum code element height  105  lie in the range of a few micrometers to a few centimeters. Adjacent code elements  100 ,  100 ′ adjoin one another. In the Y-direction, the code elements  100 ,  100 ′ are arranged with the edge spacing  107  from an edge of the optical code track  1 . The size of the edge spacing  107  is constant and lies in the range of a few micrometers to a few millimeters. The dimensions of the optical sensor  20  (or of the spacing of the individual pixels of the sensor) and of the code elements  100 ,  100 ′ are matched to one another. A sensor width  24  of the optical sensor  20  along the X-direction is greater than two code element widths  104  of the code elements  100 ,  100 ′ such that the code projection generated on the sensor in each case contains a representation of in each case at least one whole code element  100 ,  100 ′ (optionally, the code element width can be selected in such a way in relation to the sensor line length (i.e. in relation to the sensor width) that the code projection contains many pulses which are generated by many code elements  100 ,  100 ′). A sensor height  25  of the optical sensor  20  (in the Y-direction, i.e., in other words, the extent in the direction transverse to the direction of extent of the sensor line) may in this case be selected to be greater than the maximum code element height  105  of the code elements  100 ,  100 ′. 
       FIGS. 7, 9, 10 and 11  show various embodiments of an optical code track  1  with an aperture stop as absolute coding  10 . While the aperture stop in  FIG. 7  has a continuously varying aperture stop opening on both sides,  FIGS. 9, 10 and 11  show aperture stop openings varying on one side. Thus, the coating  12  is structured in such a way that it forms an aperture stop. Here, the mechanical support  11  in the edge region  107  of the code elements  100 ,  100 ′ is completely covered by the coating  12 ; here, an aperture stop opening is formed in the central region of the code elements  100 ,  100 ′ which, in terms of their opening size (i.e. the height of the opening measured across the direction of extent of the code track in the shown figure) varies without jumps, i.e. continuously, in the direction of extent of the code track. Here, the mechanical support  11  is thus only covered by the coating  12  in regions. In accordance with  FIGS. 7, 9, 10 and 11 , each code element  100 ,  100 ′ has a single contiguous region uncovered by the coating  12 . Code heights  101 ,  101 ′ of adjacent code elements  100 ,  100 ′ vary continuously, i.e. without jumps, in the direction of extent of the code track. As a result of the continuous variation in the code heights  101 ,  101 ′, illuminating light  21  is modulated with a continuous variation. The code height  101 ,  101 ′ is the extent of the coating  12  of a code element  100 ,  100 ′ in the Y-direction, as measured from the edge region  107 . The code height  101 ,  101 ′ therefore specifies the boundary of the coating  12  on the mechanical support  11 . In accordance with  FIGS. 7, 10 and 11 , the aperture stop has a boundary of the coating  12  in the form of curved aperture stop sections; in accordance with  FIG. 9 , the aperture stop has a boundary of the coating  12  in the form of straight-lined jagged sections. 
       FIG. 8  shows an embodiment of an optical code track  1  comprising a point grid as absolute coding  10 . The point density  102 ,  102 ′ of adjacent code elements  100 ,  100 ′ varies continuously. The mechanical support  11  is, both in the edge region  107  of the code elements  100 ,  100 ′ and in the central region of the code elements  100 ,  100 ′, only covered with a coating  12  in the form of points in regions. The region uncovered by the coating  12  may be contiguous, but it may also be non-contiguous. The point densities  102 ,  102 ′ of adjacent code elements  100 ,  100 ′ vary continuously. As a result of the continuous variation in the point densities  102 ,  102 ′, illuminating light  21  is modulated with a continuous variation. The point density  102 ,  102 ′ is the density of points measured per unit of area in the XY-coordinate system. The size of the points, such as 10 μm, 5 μm etc., may be constant for the whole optical code track  1 ; however, the size of the points may also be different for various regions of the optical code track  1 , such as 5 μm in a first region, 6 μm in a second region, etc. 
     The embodiments of an optical code track  1  in accordance with  FIGS. 3 to 11  can also be realized using a polarizer as absolute coding  10 , which polarizer modifies a polarization state of the light  21  with a specific polarization efficiency  103 ,  103 ′ such that the polarization efficiency  103 ,  103 ′ of adjacent code elements  100 ,  100 ′ varies continuously. As a result of the continuous variation of the polarization efficiency  103 ,  103 ′, illuminating light  21  is modulated with a continuous variation. Here, a distinction is made between polarization by scattering, polarization by reflection, polarization by absorption and polarization by birefringence. Polarization by scattering, absorption and birefringence can be realized with the embodiments in accordance with  FIGS. 7, 9, 10 and 11 . Polarization by reflection can be realized with the embodiment in accordance with  FIG. 8 . In accordance with  FIGS. 7, 9, 10 and 11 , the polarization efficiency  103 ,  103 ′ is a specific function of the code height  101 ,  101 ′; in accordance with  FIG. 8 , the polarization efficiency  103 ,  103 ′ is a specific function of the point density  102 ,  102 ′. Moreover, the polarization efficiency  103 ,  103 ′ can also be varied over the thickness of the coating  12 , the type of materials of the coating  12  and of the mechanical support  11 , etc. 
       FIGS. 10 and 11  show an exemplary registration of the absolute coding  10  at two adjacent code elements  100 ,  100 ′. In accordance with  FIG. 10 , the optical sensor  20  registers modulated light  21 ′ from a first code element  100  and, in accordance with  FIG. 11 , the optical sensor  20  registers modulated light  21 ′ from a second code element  100 ′. By way of example, the optical code track  1  comprises a mechanical support  11  in the form of a disk, which disk has an eccentricity  16 ,  16 ′ in the Y-direction caused by the production process. The eccentricity  16 ,  16 ′ is expressed by virtue of a lower edge of the optical code track  1  in accordance with  FIG. 10  being further away from the optical sensor  20  than in accordance with  FIG. 11 ; accordingly, the eccentricity  16  in the first code element  100  is greater than the eccentricity  16 ′ in the second code element  100 ′. The optical sensor  20  has a sensor height  25  along the Y-direction which is greater than the code element height  105  of the code elements  100 ,  100 ′ in order thus to compensate eccentricities  16 ,  16 ′ of the optical code track  1  in respect of the Y-direction and to avoid code elements  100 ,  100 ′ being registered in an incomplete manner. 
     For the purposes of precise alignment of the code elements  100 ,  100 ′ in relation to the sensor  20 , it is possible to establish, for each state signal  200 , a position (e.g. in the form of a defined main point or of a centroid  301  of the state signal  200 ). A distance  302  of a code element  100  from a reference position  240  of the sensor  20  is determined with the aid of the position (e.g. of the main point or centroid  301 ). Details in this respect are depicted in  FIG. 22 . Here, the form of the code elements  100 ,  100 ′ and of the sensor  20  influence the accuracy when determining the centroids  301  of the state signals  200 .  FIGS. 12 to 23  show how an error F when determining the positions of the state signals  200  depends on the signal width  204  of the state signals  200  and on the pixel spacing of the individual registration elements of the sensor  20 .  FIGS. 12, 14, 16, 18, 20 and 22  show the state signals  200  and  FIGS. 13, 15, 17, 19, 21 and 23  show the error F. The ordinate in  FIGS. 12, 14, 16, 18, 20 and 22  is a signal strength S normalized to one, which signal strength is emitted by the individual registration elements (pixels) of the sensor registering the code projection; the abscissa in  FIGS. 12, 14, 16, 18, 20 and 22  is the X-direction (i.e. the direction of extent) of the sensor. By way of example, the sensor  20  consists of a line array with 1000 sensor elements arranged next to one another in the X-direction. All 1000 sensor elements form the sensor width  24 . The ordinate in  FIGS. 13, 15, 17, 19, 21 and 23  is the error F; the abscissa in  FIGS. 13, 15, 17, 19, 21 and 23  is the X-direction. The error F when determining the centroids  301  of the state signals  200  depends to a greater or lesser extent on a centroid shift V. The centroid shift V is caused or simulated by quasi-stochastic errors. 
       FIG. 12  shows an individual rectangular state signal  200 . A rectangular state signal  200  is generated for registered light which was modulated at rectangular code elements, as are known from the prior art. By way of example, the state signal  200  has a signal width  204  of fifteen and a half pixels. The signal width  204  is a non-integer multiple of the pixel spacing.  FIG. 13  shows the error F for such a selection of signal width  204  in relation to the pixel spacing for registering a rectangular state signal. The centroid shift V occurs in the range from zero to a whole pixel. As a result of the state signal being rectangular and the signal width not forming an integer multiple of the pixel spacing, the error F has a sawtooth-like profile in the range from −60 nm to +60 nm; the standard deviation of the error F is 36 nm. 
       FIG. 14  shows an individual bell-shaped state signal  200 . A bell-shaped state signal  200  is generated for registered light  21 ′ which was modulated at code elements  100 ,  100 ′ according to the invention (i.e. modulated in a continuously varying manner), which continuous variation according to the invention leads to small diffraction phenomena and few quasi-stochastic errors in the state signal  200 . The state signal  200  has a signal width  204  of e.g. fifteen and a half pixels. The signal width  204  is a non-integer multiple of the pixel spacing.  FIG. 15  shows the error F for such a selection of signal widths  204  in nm in relation to the pixel spacing. The centroid shift V occurs in the range from zero to a whole pixel. As a result of the bell-shaped state signal, the error F is merely noise in the range from 0.7×10 −8  nm to 1.5×10 −8  nm; the standard deviation of the error F is 1.1×10 −8  nm. 
       FIG. 16  shows an individual rectangular state signal  200  with a signal width  204  of fifteen pixels. The signal width  204  is an integer multiple of the sensor elements.  FIG. 17  shows the error F for such a ratio of signal width  204  to pixel spacing. The centroid shift V occurs in the range from zero to a whole pixel. The error F is noise and varies in the range from −7.54×10 −8  nm to −7.44×10 −8  nm; the standard deviation of the error F is −7.49×10 −8  nm. 
       FIG. 18  shows a multiple rectangular state signal  200  with different signal widths  204  in the range from fourteen and a half pixels to fifteen and a half pixels. The error F for such a ratio of signal width  204  to pixel spacing can be seen in  FIG. 19 . The centroid shift V in this case is also in the range from zero to a whole pixel. The centroid shift V shows a rounded-off sawtooth-like profile and varies in the range from −11 nm to +11 nm; the standard deviation of the error F is 7.9 nm. 
       FIG. 20  shows a multiple bell-shaped state signal  200 , generated by the continuously varying modulation according to the invention, comprising different signal widths  204  in e.g. the range from fourteen and a half pixels to fifteen and a half pixels. The error F for such a configuration of the code (namely such that the signal width  204  has a varying width over the profile of the code in the code track direction and varies in an exemplary manner between fourteen and a half pixels and fifteen and a half pixels) can be seen in  FIG. 21 . The centroid shift V in this case is also in the range from zero to a whole pixel. The centroid shift V is noise and varies in the range from −7 nm to +9 nm; the standard deviation of the error F is 2.4 nm. 
       FIG. 22  shows the registration of light  21 ′, modulated at a code element  100 ′ of the optical code track  1 , by the absolute encoder  2 . The absolute encoder  2  generates a state signal  200  for registered modulated light  21 ′. Therefore, the code element  100 ′ is imaged as a state signal  200  (that is to say as code projection) on a sensor  20  of the absolute encoder  2 , and registered by the sensor  20 . The state signal  200  is communicated to the evaluation unit  3 . The evaluation unit  3  comprises a microprocessor and a computer-readable data storage medium. In order to evaluate the state signal  200 , a computer program product is loaded into the microprocessor of the evaluation unit  3  from the computer-readable data storage medium (which either can be situated locally in the evaluation unit  3  or else can be situated externally and connected via the Internet) and executed. The evaluation unit  3  may be a stationary computer such as a personal computer (PC), a printed circuit board with a microcomputer with a programmable logic or, or a mobile computer such as a laptop, smartphone, etc. Thus, the program code according to the invention can be present within the scope of a computer program product, which is stored in a machine readable medium, or a computer data signal embodied by an electromagnetic wave, wherein the program code is suitable for establishing the object position from absolute coding  10  of the optical code track  1  registered by the above-described system according to the invention when the program is loaded into, and executed on, the microprocessor of the evaluation unit  3 . The evaluation unit  3  establishes a position specification  300  in respect of the communicated state signal  200 . The state signal  200  has either a bijective position specification or a bijective code. Hence, the position specification is either established directly from the state signal  200  or a position specification is assigned to the code of the state signal  200  by way of look up in a digital table. The digital table is likewise stored in the computer-readable data storage medium and loadable into the microprocessor of the evaluation unit  3 . The evaluation unit  3  also determines a position (e.g. in the form of a centroid  301 ) of the communicated state signal  200  and determines a distance  302  between the centroid  301  and a reference position  240  of the absolute encoder  2 . Therefore, the object position is determined absolutely from the sum of the position specification  300  and the distance  302 . 
     It is understood that these depicted figures only schematically depict possible embodiments. The various approaches can likewise be combined with one another and with methods and instruments from the prior art.