Abstract:
Two sensors scan a measuring scale, which can be displaced in relation to the sensors and comprises a plurality of equidistant measuring gradation, and deliver corresponding measuring signals. The measuring signals are periodic during a uniform relative displacement of the measuring scale, essentially sinusoidal and essentially phase-shifted by 90° in relation to one another. They have an essentially identical amplitude and a base frequency that corresponds to the relative displacement of the measuring scale. During a delivery period of measuring signals, the measuring scale carries out a relative displacement through one measuring gradation. Corrected signals are determined from the measuring signals using correction values. A signal of the position of the measuring scale in relation to the sensors is determined in turn using said correction signals. Fourier coefficients are determined in relation to the base frequency for the corrected signals or for at least one supplementary signal that is derived from the corrected signals, said coefficients being used in turn to update the correction values. Said correction values contain two shift correction values at least one amplitude correction value and at least one phase correction value for the measuring signals, or part of said values, in addition to at least one correction value for at least one higher frequency wave of the measuring signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the US National Stage of International Application No. PCT/EP2005/053681, filed Jul. 28, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2004 038 621.8 DE filed Aug. 9, 2004, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention relates to a determination method for a position signal, with which 
    two sensors scan a measuring scale which is moveable relative to the sensors, and has a plurality of equally-spaced scale divisions and thereby supply corresponding measuring signals,     for a uniform relative movement of the measuring scale the measuring signals are periodic, are essentially sinusoidal, have essentially the same amplitude, have a phase offset relative to one another which is essentially 90°, have a basic frequency which corresponds essentially with the relative movement of the measuring scale, and over the course of one period of the measuring signals the measuring scale executes a relative movement of one scale division,     by applying correction values, corrected signals are determined from the measuring signals,     using the corrected signals, a position signal of the measuring scale is determined relative to the sensors,     for the corrected signals, or for at least one supplementary signal derived from the corrected signals, Fourier coefficients are determined relative to the basic frequency,     the correction values are adjusted by reference to the Fourier coefficients,     the correction values include two offset correction values, at least one amplitude correction value and at least one phase correction value for the measuring signals or for some of these values.    
 
       BACKGROUND OF INVENTION  
       [0010]     Determination methods of this type are used in so-called incremental position sensors. With them, the measuring signals are generally referred to as the cosine and sine signals. By evaluation of the passages through zero of the measuring signals, a coarse position is determined—to an accuracy of one signal period. By evaluating in addition the values of the cosine and sine signals themselves, it is possible to determine—within a signal period, a fine position. For ideal measuring signals x, y, this then gives the position signal φ within the signal period concerned as 
 
φ=arctan ( y/x ) if x&gt;0  (1) 
 
φ=arctan ( y/x )+π if x&lt;0  (2) 
 
φ=π/2 sign ( y ) if x=0  (3) 
 
         [0011]     In practice, however, the measuring signals x, y are not ideal, but subject to error. With the state of the art, the formulation most commonly adopted for the erroneous measuring signals x, y is 
 
 x=a  cos (φ+Δ)+ x   0   (4) 
 
 y =(1 +m ) a  sin (φ)= y   0   (5) 
 
         [0012]     Here, x 0  and y 0  are offset errors in the measuring signals x and y, m is an amplitude error and Δ is a phase error. a is a signal amplitude. Methods for determining and compensating for these error quantities are generally known.  
         [0013]     Thus, for example, a determination method of the type mentioned in the introduction is known from DE-A-101 63 504.  
         [0014]     Determination methods for a position signal are known, from DE-A-100 34 733, from DE-A-101 63 528 and from the technical article “Erhöhung der Genauigkeit bei Wegsystemen durch selbstlemende Kompensation systematischer Fehler” [Increasing the precision of position measuring systems by self-learning compensation of systematic errors] by B. Höscheler, conference volume on SPS[PLC]/IPC/DRIVES, Elektrische Automatisierungstechnik—Systeme und Komponenten, Fachmesse und Kongress [Electrical Automation Technology—Systems and Components, Technical Fair and Congress] 23-25 Nov. 1999, Nuremberg, pages 617 to 626, by which: 
    two sensors scan a measuring scale, which is moveable relative to the sensors and has a plurality of equally-spaced scale divisions, and thereby supply corresponding measuring signals,     for a uniform relative movement of the measuring scale the measuring signals are periodic, are essentially sinusoidal, have essentially the same amplitude, have a phase offset relative to one another which is essentially 90°, have a basic frequency which corresponds essentially with the relative movement of the measuring scale, and over the course of one period of the measuring signals the measuring scale executes a relative movement of one scale division,     by applying correction values, corrected signals are determined from the measuring signals,     using the corrected signals, a position signal of the measuring scale is determined relative to the sensors,     the correction values are adjusted,     the correction values include two offset correction values, at least one amplitude correction value and at least one phase correction value for the measuring signals.    
 
         [0021]     With the determination method according to DE-A-100 34 733 and the technical article by B. Hoscheler, the position signal is then post-corrected by means of a fine correction method, to compensate for residual errors due to harmonics in the measuring signals. However, the fine correction method described there only works satisfactorily if any changes in speed which occur are sufficiently small.  
         [0022]     EP-A-1 046 884 discloses a method for determining a position signal with which two sensors scan a measuring scale, which is moveable relative to the sensors and has a plurality of equally-spaced scale divisions, and thereby supply corresponding measuring signals. For a uniform relative movement of the measuring scale the measuring signals are periodic, have essentially the same amplitude, are essentially sinusoidal, have a phase offset relative to one another which is essentially 90°, and have a basic frequency which corresponds essentially with the relative movement of the measuring scale. Over the course of one period of the measuring signals the measuring scale executes a relative movement of one scale division. The measuring signals are detected with a time displacement relative to one another. For one of the measuring signals, a corrected signal is determined from the measuring signals, using correction values. A position signal of the measuring scale is determined relative to the sensors by reference to the corrected signal and the other, uncorrected signal.  
       SUMMARY OF INVENTION  
       [0023]     An object of the present invention consists in specifying a method which is as simple as possible to carry out, giving as complete a correction as possible of the errors contained in the measuring signals, and which also works properly for larger speed changes.  
         [0024]     This object is achieved by a determination method with the features of claim  1 .  
         [0025]     The embodiment in accordance with the invention can be further simplified by determining the correction values only for those higher-frequency waves in the measuring signal, the frequency of which is an odd number multiple of the basic frequency. The components which are an even number multiple of the basic frequency are in many cases negligibly small.  
         [0026]     The determination method in accordance with the invention can be yet further simplified if the correction values are determined only for those higher-frequency waves in the measuring signal, the frequency of which is three or five times the basic frequency, and the correction values for the higher-frequency waves in the measuring signal, the frequency of which is five times that of the basic frequency, have a predetermined ratio to the correction values for the higher-frequency waves in the measuring signal, the frequency of which is three times that of the basic frequency. In particular it is even possible to determine only the correction values for those higher-frequency waves in the measuring signals, the frequency of which is three times that of the basic frequency, so that the ratio is zero.  
         [0027]     Implementation of the determination method in accordance with the invention is particularly simple if 
    for the purpose of determining the Fourier coefficients the corrected signals, or the at least one supplementary signal as applicable, are saved into one of several registers,     an angular range is assigned to each of the registers,     in each case the save is made into the register which has the angular range within which the arctangent for the corrected signal lies, and     the Fourier coefficients are determined by reference to the values saved into the registers.    
 
         [0032]     If the values stored in the registers are deleted after the Fourier coefficients have been determined, and a new determination of the Fourier coefficients is only then undertaken after the resisters have been adequately filled, this results in a particularly robust determination of the correction values.  
         [0033]     In the ideal case, the registers are only adequately filled when values have been saved into all the registers in accordance with the method described above. However, it is also possible to regard the registers as adequately filled at the point when values have been saved into a first group of the registers in accordance with the method described above, and in this case a second group of the registers is filled with values which are determined by reference to the values saved in accordance with the method described above.  
         [0034]     The evaluation of the values saved into the registers is particularly simple if certain registers are assigned to each Fourier coefficient and the Fourier coefficient concerned is determined by reference exclusively to the values which are saved in the registers assigned to the Fourier coefficient concerned. For appropriate assignment of the registers to the Fourier coefficients, it is then even possible to determine the Fourier coefficients solely by the formation of sums and differences of the values saved in the assigned registers.  
         [0035]     The correction of the measuring signals is particularly optimal if 
    for the purpose of determining the corrected signals pre-corrected signals are first determined,     the pre-corrected signals are determined from the measuring signals, with reference to the offset correction values, to the at least on amplitude correction values and/or to the at least one phase correction value, and     the corrected signals are then determined by reference to the pre-corrected signals and to the at least one correction value for the one or more higher-frequency wave in the measuring signals.    
 
         [0039]     Various approaches are possible for determining the corrected signals from the pre-corrected signals. It is thus possible, for example, to first determine a preliminary arctangent from the pre-corrected signals, and then to determine the corrected signals by applying the preliminary arctangent as the argument in a Fourier series expansion.  
         [0040]     It is thus possible to determine the corrected signals by reference to the pre-corrected signals, by the formation of functions of the form  
               x   ∝     =       x   c     -     a   ⁢       ∑     q   =   2     ∞     ⁢     [         c   q     ⁢     cos   ⁡     (     q   ⁢           ⁢     φ   c       )         +       d   q     ⁢     sin   ⁡     (     q   ⁢           ⁢     φ   c       )           ]                   (   6   )             and                           y   ∝     =       y   c     -     a   ⁢       ∑     q   =   2     ∞     ⁢     [         c   q     ⁢     cos   ⁡     (       q   ⁢           ⁢     φ   c       -     q   ⁢           ⁢     π   /   2         )         +       d   q     ⁢     sin   ⁡     (       q   ⁢           ⁢     φ   c       -     q   ⁢           ⁢     π   /   2         )           ]                   (   7   )             
 
 where x cc  and y cc  are the corrected signals, x c  and y c  the pre-corrected signals, a the signal amplitude, c q  and d q  are weighting factors determined by reference to the Fourier coefficients and φ c  the preliminary arctangent. 
 
         [0041]     The approach just described can be simplified by replacing the expression cos (qφ c −qπ/2) 
 
by cos (qφ c ) for q=0, 4, 8,  (8) 
 
by sin (qφ c ) for q=1, 5, 9,  (9) 
 
by −cos (qφ c ) for q=2, 6, 10, . . . and  (10) 
 
by −sin (qφ c ) for q=3, 7, 11,  (11) 
 
and the expression sin (qφ c −qπ/2) 
 
by sin (qφ c ) for q=0, 4, 8,  (12) 
 
by −cos (qφ c ) for q=1, 5, 9,  (13) 
 
by −sin (qφ c ) for q=2, 6, 10, . . . and  (14) 
 
by cos (qφ c ) for q=3, 7, 11,  (15) 
 
         [0042]     It is possible to effect further simplification by replacing the expression cos (qφ c ) by the expression  
               ∑     r   =   0       int   ⁡     (     q   /   2     )         ⁢         (     -   1     )     r     ⁢     (         q             2   ⁢           ⁢   r           )     ⁢       (     cos   ⁢           ⁢     φ   c       )       q   -     2   ⁢           ⁢   r         ⁢       (     sin   ⁢           ⁢     φ   c       )       2   ⁢           ⁢   r                 (   16   )             
 
 and the expression sin (qφ c ) by the expression  
               ∑     r   =   0       int   ⁡     [       (     q   -   1     )     /   2     ]         ⁢         (     -   1     )     r     ⁢     (         q               2   ⁢   r     +   1           )     ⁢       (     cos   ⁢           ⁢     φ   c       )       q   -     2   ⁢   r     -   1       ⁢       (     sin   ⁢           ⁢     φ   c       )         2   ⁢           ⁢   r     +   1                 (   17   )             
 
         [0043]     It is even possible to avoid the determination of trigonometric function values, if the expression cos (qφ c ) is finally replaced by the expression x c /a and the expression sin (qφ c ) by the expression y c /a.  
         [0044]     An alternative possibility consists in determining the corrected signals by reference to the pre-corrected signals, by forming functions of the form  
               x   cc     =       x   c     -       ∑     q   =   2     ∞     ⁢       b   q     ⁢     x   c   q                   (   18   )             and                           y   cc     =       y   c     -       ∑     q   =   2     ∞     ⁢       b   q     ⁢     y   c     ⁢   q                 (   19   )             
 
 where x cc  and y cc  are the corrected signals and x c  and y c  the pre-corrected signals, and b q  a weighting factor.
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     Further advantages and details are given by the following description of an exemplary embodiment, in conjunction with the drawings. These show, as schematic diagrams:  
         [0046]      FIG. 1 a  block diagram of a determination circuit for a position signal,  
         [0047]      FIG. 2 a  first form of embodiment of a first extract from  FIG. 1 ,  
         [0048]      FIG. 3 a  first form of embodiment of a second extract from von  FIG. 1 ,  
         [0049]      FIG. 4 a  simplification of the approach in  FIG. 3 ,  
         [0050]      FIG. 5 a  simplification of the approach in  FIG. 4 ,  
         [0051]      FIG. 6 a  simplification of the approach in  FIG. 5 ,  
         [0052]      FIG. 7 a  second form of embodiment of the first extract from  FIG. 1 ,  
         [0053]      FIG. 8  another extract from the determination circuit in von  FIG. 1 ,  
         [0054]      FIG. 9  an assignment of angular ranges to registers,  
         [0055]      FIG. 10 a  logical combination,  
         [0056]      FIG. 11 a  first approach for determining the correction values, and  
         [0057]      FIG. 12  an alternative approach for determining the correction values. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0058]     As shown in  FIG. 1 , a determination circuit, by means of which a position signal φ cc  is to be determined, has two sensors  1 ,  2  and a measuring scale  3 . The measuring scale  3  is moveable relative to the sensors  1 ,  2 . As shown in  FIG. 1 , it can for example be rotated about an axis of rotation  4 . This is indicated in  FIG. 1  by an arrow A. The measuring scale  3  has numerous (e.g. 1000 to 5000) equally-spaced scale divisions  5 . The sensors  1 ,  2  scan the measuring scale  3  and thereby supply corresponding measuring signals x, y.  
         [0059]     In the ideal case, the sensors  1 ,  2  have exactly equal sensitivities, and are ideally positioned. For a uniform movement of the measuring scale  3  relative to the sensors  1 ,  2 , the latter are therefore in a position to supply measuring signals x, y which satisfy the following conditions:  
         [0060]     They are periodic.  
         [0061]     They have an equal amplitude.  
         [0062]     They are exactly sinusoidal.  
         [0063]     They have a phase offset relative to each other of exactly 90°.  
         [0064]     They have a basic frequency fG which corresponds to the relative movement of the measuring scale  3 .  
         [0065]     One period of the measuring signals x, y then corresponds to a relative movement of one scale division  5  by the measuring scale  3 .  
         [0066]     In the ideal case therefore, the following applies within one scale division  5 : 
 
x=a cos (φ)  (20) 
 
y=a sin (φ)  (21) 
 
 where a is the amplitude of the measuring signals x, y. Correspondingly, the following applies for the position signal φ of the measuring scale  3  within a scale division  5 : 
 
φ=arctan ( y/x ) when x&gt;0  (22) 
 
φ=arctan ( y/x )+π when x&lt;0  (23) 
 
φ=(π/2) sign ( y ) when x=0  (24) 
 
         [0067]     In a real situation however, the sensors  1 ,  2  are not exactly positioned and they also have sensitivities which are—at least slightly—different. In the real situation therefore, for a uniform relative movement of the measuring scale  3  the measuring signals x, y have amplitudes which are only broadly the same, are only broadly sinusoidal in shape and only broadly have a phase offset of 90° relative to each other. On the other hand, the basic frequency fG of the measuring signals x, y is retained.  
         [0068]     The following formulation can therefore be made for the measuring signals x, y as a function of the actual position φ of the measuring scale  3  within a scale division  5 : 
 
 x=a c (φ+Δ)+ x   0   (25) 
 
 y =(1 +m )  a s (φ)+ y   0   (26) 
 
 where c and s are periodic functions of the form 
 
 c (φ)=cos (φ)+Σ q=2   28    [c   q  cos ( q φ)+ d   q  sin ( q φ)]  (27) 
 
 and 
 
 s (φ)=sin (φ)+Σ q=2   ∞   [c   q  cos ( qφ−q π/2)+ d   q  sin ( qφ−qπ/ 2)]  (28) 
 
         [0069]     The functions c and s are phase-shifted relative to one another by 90° or π/2, as applicable. Hence s(φ)=c(φ−π/2) applies.  
         [0070]     In the above formulae, x 0  and y 0  represent offset errors, m an amplitude error and Δ a phase error. c q  and d q  are tracking signal distortions due to harmonics of the basic frequency fG, that is distortions caused by higher-frequency waves in the measuring signals x, y. The following applies as a general rule 
 
| x   0   /a|, |y   0   /a|, |m|, | 2 Δ/π|, |c   q   |, |d   q |&lt;&lt;1.  (29) 
 
         [0071]     These signal errors must be determined and compensated for.  
         [0072]     The method in accordance with the invention is executed iteratively. It is now assumed below that values have already been determined for the signal errors x 0 , y 0 , m, Δ, c q , d q . At the start of the method, however, the value can be set to predetermined starting values, e.g. to x 0 =y 0 =m=Δ=c q =d q =0.  
         [0073]     The measuring signals x, y detected by the sensors  1 ,  2  are initially fed to a first correction block  6 , as shown in  FIG. 1 . Also fed to the correction block  6  are the correction values x 0 , y 0 , m and Δ for the offset, amplitude and phase errors. The first correction block  6  determines from these—see  FIG. 2 —pre-corrected signals x c , y c  in accordance with the ratios 
 
 y   c =( y−y   0 )/(1 +m )  (30) 
 
 x   c =( x−x   0   +y   c  sin Δ)/cos (Δ)  (31) 
 
         [0074]     For the pre-corrected signals x c , y c , the following approximations apply 
 
 x   c   ≈a  cos (φ)+ a Σ q=2   ∞   [c   q  cos ( q φ)+ d   q  sin ( q φ)]  (32) 
 
 y   c   ≈a  sin (φ)+ a Σ q=2   ∞   [c   q  cos ( qφ−q π/2)+ d   q  sin ( qφ−q π/2)]  (33) 
 
         [0075]     Using the pre-corrected signals x c , y c  and the correction values c q , d q  for the higher-frequency waves in the measuring signals x, y, it is then possible in a second correction block  7  to determine corrected signals x cc , y cc , in doing which the tracking signal distortions are also largely compensated.  
         [0076]     There are several possibilities for determining the corrected signals x cc , y cc .  
         [0077]     For example, using the pre-corrected signals x c , y c  it is possible—see  FIG. 3 —to determine first a preliminary arctangent φ c  from the ratios 
 
φ c =arctan ( y   c   /x   c ) when x c &gt;0  (34) 
 
φ c =arctan ( y   c   /x   c )+π when x c &lt;0  (35) 
 
φ c =(π/2) sign ( y   c ) when x c =0  (36) 
 
 and thence to determine the corrected signals x cc , y cc  by utilizing the preliminary arctangent φ c  as the argument in a Fourier series expansion. The corrected signals x cc , y cc  are then formed in this case, for example, by forming functions of the form 
 
 x   cc   =x   c   −a Σ q=2   ∞   [c   q  cos ( q φ c )+ d   q  sin ( q φ c )]  (37) 
 
 y   cc   =y   c   −a Σ q=2   ∞   [c   q  cos ( q φ c   −q π/2)+ d   q  sin ( q φ c   −q π/2)]  (38) 
 
         [0078]     For the corrected signals x cc , y cc  determined in this way, it is then true to a very good approximation that 
 
x cc =a cos (φ  (39) 
 
y cc =a sin (φ  (40) 
 
         [0079]     By analogy with the formulae 1 to 3 it is thus possible, using the measuring signals x, y and the correction values x 0 , y 0 , m, Δ, c q , d q , to determine with great accuracy an arctangent φ cc , and hence also the position φ cc  of the measuring scale  3  within a scale division  5 . That is to say, using the corrected signals x cc , y cc  it is possible to determine the position signal φ cc  for the measuring scale  3  relative to the sensors  1 ,  2 , by using the equations 
 
φ cc =arctan ( y   cc   /x   cc ) when x cc &gt;0  (41) 
 
φ cc =arctan ( y   cc   /x   cc )+π when x cc &lt;0  (42) 
 
φ cc =(π/2) sign ( y   cc ) when x cc =0  (43) 
 
         [0080]     It should be remarked at this point that for the purpose of determining the complete position of the measuring scale  3  it is also necessary to know which scale graduation  5  has just been sensed by the sensors  1 ,  2  (the so-called coarse position). However, it is generally known how to determine this coarse position, and this is not a subject of the present invention. Rather, within the context of the present invention it is taken as given.  
         [0081]     Formulae 37 and 38 are mathematically correct, but require a large computational effort because sine and cosine values must be determined both for qφ c  and for (qφ c −qπ/2). For this reason, in accordance with the generally familiar addition theorems for sine and cosine the following substitutions—see  FIG. 4 —are made:  
         [0082]     The expression cos (qφ c qπ/2) is replaced 
 
by cos (qφ c ) for q=0, 4, 8,  (44) 
 
by sin (qφ c ) for q=1, 5, 9,  (45) 
 
by −cos (qφ c ) for q= 2 ,  6 ,  10 , . . . and  (46) 
 
by −sin (qφ c ) for q=3, 7, 11,  (47) 
 
         [0083]     In addition, the expression sin (qφ c −qπ/2) is replaced 
 
by sin (qφ c ) for q=0, 4, 8,  (48) 
 
by −cos (qφ c ) for q=1, 5, 9,  (49) 
 
−sin (qφ c ) for q=2, 6, 10, . . . and  (50) 
 
by cos (qφ c ) for q=3, 7, 11,  (51) 
 
         [0084]     After this it only remains necessary to determine the sine and cosine values of qφ c .  
         [0085]     Formula 37, and the modified formula 38 which is arrived at by modification in accordance with the formulae 44 to 51, can however be yet further simplified. Because it is possible, as shown in  FIG. 5 , to replace the expression cos (qφ c ) in these formulae by the expression  
               ∑     r   =   0       int   ⁡     (     q   /   2     )         ⁢         (     -   1     )     r     ⁢     (         q             2   ⁢           ⁢   r           )     ⁢       (     cos   ⁢           ⁢     φ   c       )       q   -     2   ⁢           ⁢   r         ⁢       (     sin   ⁢           ⁢     φ   c       )       2   ⁢           ⁢   r                 (   52   )             
 
         [0086]     Furthermore, the expression sin (qφ c ) can be replaced by the expression  
               ∑     r   =   0       int   ⁡     [       (     q   -   1     )     /   2     ]         ⁢         (     -   1     )     r     ⁢     (         q               2   ⁢           ⁢   r     +   1           )     ⁢       (     cos   ⁢           ⁢     φ   c       )       q   -     2   ⁢           ⁢   r     -   1       ⁢       (     sin   ⁢           ⁢     φ   c       )         2   ⁢           ⁢   r     +   1                 (   53   )             
 
         [0087]     After this it only remains necessary to determine the sine and cosine of φ c .  
         [0088]     However, even the determination of these trigonometric functions can be avoided. Because it is possible—see  FIG. 6 —to replace the expression cos (φ c ) by the expression x c /a and the expression sin (φ c ) by the expression y c /a.  
         [0089]     In many case, the measuring signals x, y arise from a mapping of the signals 
 
 x   cos   =a  cos (φ+Δ)+ x   0   (54) 
 
 y   cos =(1 +m )  a  sin (φ)+ y   0   (55) 
 
 by means of a (common) non-linear characteristic curve f. The following then applies 
 
x=f(x cos )  (56) 
 
y=f(y cos )  (57) 
 
         [0090]     In this case, the correction values d q  vanish, that is they have a value of zero. In this case it is therefore possible—see  FIG. 7 —to determine the corrected signals x cc , y cc  using the pre-corrected signals x c , y c  by forming functions of the form 
 
 x   cc   =x   c −Σ q=2   ∞   b   q   x   c   q   (58) 
 
 y   cc=   y   c −Σ q=2   ∞   b   q   y   c   q   (59) 
 
         [0091]     Here, the coefficients bq are determined by the ratio 
 
b q =a −1 Σ q′=q   Q h q,q′ c q   (60) 
 
 where h q,q′  are matrix coefficients. Here, the matrix coefficients h q,q′  can be determined as follows: 
 
         [0092]     For the sake of simplicity and with no loss of generality, the assumption is initially made in what follows that the correction values x 0 , y 0 , m and Δ are zero.  
         [0093]     We now assume further that the non-linear function f can be expanded as a Taylor series and the Taylor coefficients of the function f correspond to the coefficients b q  and that |bq|&lt;&lt;1. Then the measuring signal x resulting from a position φ is given by 
 
x=Σ q=0   ∞ b q a q [cos (φ)] q   (61) 
 
 Using the ratio  
               cos   ⁡     (   β   )       =       ∑     r   =   0       int   ⁡     (     q   /   2     )         ⁢         (     -   1     )     r     ⁢     (         q             2   ⁢           ⁢   r           )     ⁢       (     cos   ⁢           ⁢   β     )       q   -     2   ⁢           ⁢   r         ⁢       (     sin   ⁢           ⁢   β     )       2   ⁢           ⁢   r                   (   62   )             
 
 which applies for any angle β, and the ratio (cos β) 2 +(sin β) 2 =1 which is also generally valid, it is however possible to determine coefficients g q,r  such that 
 
[cos (φ)] q =Σ r=0   q g q,r  cos (rφ)  (63) 
 
         [0094]     The coefficients g q,r  are independent of β or φ, as applicable. The first coefficients g q,r  turn out as g 0,0 =1, g 1,0 =0, g 1,1 =1, g 2,0 =½, g 2,1 =0, g 2,2 =−½, g 3,0 =0, g 3,1 =¾g 3,2 =0, g 3,3 =¼. This allows equation 61 to be rewritten as 
 
x=Σ q=0   ∞ b q a q Σ r=0   q g q,r  cos (rφ)=Σ q=0   ∞ c q  cos (qφ)  (64) 
 
where 
 
c q =Σ q′=q   ∞ b q′ a q′ g q′,q   (65) 
 
         [0095]     In practice, it is only necessary to consider a finite number of the coefficients bq. The others can to a good approximation be assumed to be zero. As a result, the system of equations in equation 65 is reduced to a finite system of equations, which for known correction values cq can be solved for the coefficients bq. The trigger produces a system of equations in the form of equation 60. The matrix coefficients hq,q′ can thus be determined by a comparison of coefficients. In this way one obtains, for example, h0,0=1, h0,1=0, h1,1=1, h0,2=−1, h1,2=0, h2,2=2, h0,3=0, h1,3=−3, h2,3=0, h3,3=4.  
         [0096]     For the purpose of compensating for the errors in the measuring signals x, y, arising from the non-linear function f, one can simply subject the pre-corrected signals x c , y c  to an inverse mapping. For small errors, that is to say for |c q |&lt;&lt;1, this inverse mapping is given approximately by 
 
 x   cc   =x   c −Σ q=2   ∞   b   q   x   c   q   (66) 
 
 y   cc   =y   c −Σ q=2   ∞   b   q   y   c   q   (67) 
 
         [0097]     The above assumes throughout that the correction values x 0 , y 0 , m, Δ, c q , d q  are known, and thus it is possible to effect the compensation. However, the correction values x 0 , y 0 , m, Δ, c q , d q  must also be determined. For this purpose, we proceed according to  FIG. 1 , as follows:  
         [0098]     For each position φ cc  which is determined, the sum of the squares of the corrected signals x cc , y cc , or the square root of this sum, is also determined as applicable. That is, from the corrected signals x cc , y cc  is derived a supplementary signal r cc   2  or r cc  as applicable, in the form 
 
 r   cc   2   =x   cc   2   +y   cc   2  and  r   cc =√{square root over (x cc   2 +y cc   2 )}  (68) 
 
         [0099]     In what follows, only the approach for a supplementary signal r cc  is considered. The approach for the supplementary signal r cc   2  is completely analogous.  
         [0100]     The supplementary signal r cc  and the position φ cc  are fed into a Fourier block  8 —see  FIGS. 1 and 8 . As shown in  FIG. 8 , the Fourier block  8  has a number of registers  9 . The supplementary signal r cc  which is instantaneously being fed in is saved into one of these registers  9 .  
         [0101]     As shown in  FIG. 9 , an angular range, α 1  to αn, is assigned to each of the registers  9 , where n is preferably a power of 2. The Fourier block  9  then has a selector  10 . The position signal φ cc  is fed to the selector  10 . By reference to the position signal φ cc , the selector  10  activates that register  9  for which the position signal φ cc  lies within its assigned angular range α 1  to αn, in order to save the supplementary signal r cc  concerned into this register  9 .  
         [0102]     In addition, a flag  11  is assigned to each register  9 . As well as saving away the supplementary signal r cc  into one of the registers  9 , the selector  10  at the same time also sets the flag  11  which is assigned to the register  9  concerned.  
         [0103]     The flags  11  are linked to a trigger element  12 . By reference to the flags  11 , the trigger element  12  determines whether a trigger condition is satisfied. If the trigger condition is not satisfied, the trigger element  12  does not activate a computational block  13 . On the other hand, if the trigger condition is satisfied, it activates the computational block  13 . So a determination of the Fourier coefficients E i , F i  is only undertaken if the trigger condition is satisfied.  
         [0104]     If the trigger condition is satisfied, the computational block  13  determines the Fourier coefficients E i , F i  for the supplementary signal r cc  by reference to the totality of the values saved in the registers  9 . It thus determines the Fourier coefficients E i , F i  in such a way that the following applies 
 
 r   cc   =E   0 +Σ i=1   ∞   [E   i  cos ( i φ)+ F   i  sin ( i φ)]  (69) 
 
         [0105]     After the determination of the Fourier coefficients E i , F i , the computational block  13  resets the flags  11  again. Furthermore, it also clears the values saved in the registers  9 . A re-determination of the Fourier coefficients E i , F i  will thus not take place again until the trigger condition is again satisfied.  
         [0106]     In the simplest case, the trigger condition is only satisfied when values have been saved into all the registers  9  in accordance with the method described above. In this case, it is only necessary to check whether all the flags  11  have been set.  
         [0107]     However, it is also possible for the trigger condition to be satisfied when values have been saved into only a first group of the registers  9  in accordance with the method described above. For example, it may be assumed that there is an adequate filling of the registers  9  if it is true for each register  9  that its assigned flag is set and/or the flags  11  assigned to both the immediately neighboring registers  9  are set. This can be determined—for each register  9  individually—by means of a logical combination, an example of which is shown in  FIG. 10 . In particular, in this case the remaining registers  9  can be filled with values which are determined by reference to the values already saved. For example, into each register  9 , in which a value has not yet been saved in accordance with the above method, could be saved the mean of the two values which have been saved into the two registers  9  which are immediately neighboring in terms of angle.  
         [0108]     The computational block  13  thus determines—see  FIGS. 11 and 12 —the Fourier coefficients E i (i=0, 1, . . . ) and F i  (i=1, 2, . . . ) in a manner known per se. In principle, the Fourier coefficients E i , F i  are thus determined in the computational block  13  in accordance with the usual approach. For example, they can be determined in accordance with the formulae 
 
 E   0 =(1 /n )Σ m=0   n-1   r   cc ( m )  (70) 
 
 E   i =[(1/(2 n )]Σ m=0   n-1   r   cc ( m ) cos (2 πim/n )  (71) 
 
 F   i =[(1/(2 n )]Σ m=0   n-1   r   cc ( m ) sin (2 πim/n )  (72) 
 
         [0109]     Preferably, however, certain registers  9  are assigned to each of the Fourier coefficients E i , F i . These registers  9  can, in particular, be those of the registers  9  for which the contribution, of the value saved in the register  9  concerned to the Fourier coefficients E i , F i  concerned, is particularly heavily weighted, i.e. the value of cos (2πim/n) or sin (2πim/n) lies close to one. The computational effort can then be significantly reduced without any essential change in the value determined for the Fourier coefficients E i , F i . It is thus possible to determine the Fourier coefficients E i , F i  concerned exclusively by reference to the values which are saved in the registers  9  assigned to the Fourier coefficients E i , F i . The registers  9  which are assigned to the Fourier coefficients E i , F i  concerned are here obviously determined individually for each Fourier coefficient E i , F i .  
         [0110]     The approach just outlined can even be extended to the point that the only registers  9  assigned to each Fourier coefficient E i , F i  are those for which the cosine or sine, as applicable, assumes the maximum absolute value. In this case it is possible to determine the Fourier coefficients E i , F i  exclusively by the formation of sums and differences of the values saved in the assigned registers  9 .  
         [0111]     As can be seen from  FIGS. 11 and 12 , the offset correction values x 0 , y 0  are determined from the Fourier coefficients E 1 , F 1  for the basic frequency component of the supplementary signal r cc . The amplitude correction value m and the phase correction value Δ are determined from the Fourier coefficients E 2 , F 2  for the first harmonic component in the supplementary signal r cc . Because, for small error variables x 0 , y 0 , m, Δ the following applies to a very good approximation 
 
E 0 =a  (73) 
 
 E   1   =x   0 +( a /2) c   2 −( a /2)d 2   (74) 
 
 F   1   =y   0 +( a /2) c   2 +( a /2)d 2   (75) 
 
 E   2 =−( a /2) m   (76) 
 
 F   2 =−( a /2)Δ  (77) 
 
         [0112]     Under the realistic assumption that the correction values c 2 , d 2  vanish or are negligibly small relative to the offsets x 0 , y 0 , these equations thus give uniquely the base correction values (i.e. the offset, amplitude and phase correction values) x 0 , y 0 , m and Δ.  
         [0113]     On the other hand, for higher-frequency waves in the measuring signals x, y, the assignment of the Fourier coefficients E i , F i  to the correction values c q , d q  are ambiguous. Because for n=0, 1, 2, . . . it is approximately true that 
 
 E   3+4n =( a /2)( c   2+4n   +d   2+4n   +c   4+4n   +d   4+4n )  (78) 
 
 F   3+4n =( a /2)(− c   2+4n   +d   2+4n   −c   4+4n   +d   4+4n )  (79) 
 
 E   4+4n   =a ( c   3+4n   +c   5+4n )  (80) 
 
 F   4+4n   =a ( d   3+4n   +d   5+4n )  (81) 
 
 E   5+4n =( a /2)( c   4+4n   −d   4+4n   +c   6+4n   −d   6+4n )  (82) 
 
 F   5+4n =( a /2)( c   4+4n   +d   4+4n   +c   6+4n   +d   6+4n )  (83) 
 
         [0114]     These ambiguities can be resolved in various ways.  
         [0115]     Namely, the above system of equations has been derived using first partial derivatives. It is therefore possible, for example, to take into account also higher order derivatives, and thus to arrive at further ratios between the Fourier coefficients E i , F i  on the one hand, and the correction values c q , d q  on the other hand. The ambiguity could possibly be eliminated in this way. However, this approach requires a very high computational effort. Also, the resulting system of equations is generally no longer analytically soluble, but only numerically.  
         [0116]     In practice, however, one can often make simplifying assumptions, on the basis of which the assignment of the Fourier coefficients E i , F i  to the correction values c q , d q  becomes unambiguous.  
         [0117]     A first possible assumption is that in the measuring signals x, y the higher-frequency waves which arise are essentially only those with a frequency which is an odd number multiple of the basic frequency fG. The result of this assumption, which is most cases is perfectly applicable, is that only equations 80 and 81 must be solved. So, it is only necessary to determine the correction values c q , d q  for the at least one higher-frequency waves, in the measuring signals x, y, the frequency of which is an odd integral multiple of the basic frequency fG of the corrected signals x cc , y cc .  
         [0118]     It can also be assumed without major error that the only relevant higher-frequency waves in the measuring signal x,y are those with a frequency of three, and possibly also five times the basic frequency fG of the corrected signals x cc , y cc . It is therefore sufficient to solve equations 80 and 81 for n=0, and thus to determine the correction values c 3 , d 3 , c 5 , d 5 . For this purpose there are two alternative possibilities, which are shown in  FIGS. 11 and 12 .  
         [0119]     On the one hand it can be assumed—see  FIG. 11 —that the correction values c 5 , d 5  have a predetermined ratio to the correction values c 3 , d 3 . For example, it is assumed that the correction values c 3  and c 5  are in the ratio 3:1, that is the correction value c 3  is always three times as large as the correction value c 5 . Other (even negative) ratios are however also conceivable. With this assumption, the correction values c 3  and c 5  can be determined uniquely from equation 80. For the correction values d 3  and d 5 , either the same assumption can be made or a different one.  
         [0120]     Alternatively it can also be assumed—so to speak as a special case of this approach—that the harmonic wave with a frequency five times that of the basic frequency fG of the corrected signals x cc , y cc  vanishes, that is the correction values c 5  and d 5  have a value of zero. In this case it is only necessary to determine the correction values c 3 , d 3  for those higher-frequency waves in the measuring signals x, y with a frequency which is three times the basic frequency fG. Then in this case, for example, c 3 =E 4 /E 0 . This approach is shown in  FIG. 12 .  
         [0121]     Depending on the situation, it can indeed be logical in an individual case to assume that both the correction values d 3  and d 5  and also the correction value c 5  vanish, i.e. have a value of zero.  
         [0122]     Using the Fourier coefficients E i , F i  it is then possible to adjust the correction values x 0 , y 0 , m, Δ, c q , d q . For example, in the case where correction values c 3 , d 3  are determined only for the third harmonic, the following adjustment rules can be executed: 
 
 a:=a+αE   0   (84) 
 
 x   0   :=x   0   +αE   1   (85) 
 
 y   0   :=y   0   +αF   1   (86) 
 
 m:=m− 2 αE   2   /E   0   (87) 
 
Δ:=Δ−2 αF   2 /E 0   (88) 
 
 c   3   :=c   3   +αE   4   /E   0   (89) 
 
 d   3   :=d   3   +αF   4   /E   0   (90) 
 
         [0123]     Here, the factor α is a positive number which is less than one. It is preferably the same for all the adjusted values a, x 0 , y 0 , m, Δ, c 3 , d 3 . However, it can also be defined separately for each individual value which is adjusted, a, x 0 , y 0 , m, Δ, c 3 , d 3 .  
         [0124]     The above is a description of the fact that, and how, the correction values x 0 , y 0 , m, Δ, c 3 , d 3  have been determined using a supplementary signal r cc . Here, the supplementary signal r cc  (or r cc   2  as applicable) corresponded respectively to the sum of the squares of the corrected signals x cc , y cc , or the square root of this sum.  
         [0125]     By means of the approach in accordance with the invention it is thus also possible to correct higher-frequency waves in the measuring signals x, y in a simple manner. This is indicated in  FIG. 1  by dashed lines. In this case, the equations 
 
 x   cc   =x   0   +a  cos (φ cc +Δ)+ a Σ q=2   ∞   [c   q  cos ( q φ cc   +q Δ)+ d   q  sin ( q φ cc   +q Δ)]  (91) 
 
 y   cc   =y   0   +a (1 +m ) sin (φ cc )+ a (1 +m )Σ q=2   ∞   [c   q  cos ( q φ cc   −q π/2)+ d   q  sin ( q φ cc   +q π/2)]  (92) 
 
 must be equated to the corresponding Fourier expansions 
 
 x   cc   =XR   0 +Σ q=1   ∞   [XR   q  cos ( q φ cc )+ XI   q  sin ( q φ cc )  (93) 
 
 y   cc   =YR   0 +Σ q=1   ∞   [YR   q  cos ( q φ cc )+ YI   q  sin ( q φ cc )  (94) 
 
         [0126]     In this case, the assignment of the Fourier coefficients XR q , XI q , YR q , YI q  to the correction values c q , d q  can be made simply and uniquely. However, the principle of the approach, that is in particular the manner in which the Fourier coefficients XR q , XI q , YR q , YI q  are determined, the adjustment of the correction values x 0 , y 0 , m, Δ, c q  and d q  by reference to the Fourier coefficients XR q , XI q , YR q , YI q  which have been determined, and the determination of the corrected signals x cc , y cc  by reference to the measuring signals x, y and the correction values x 0 , y 0 , m, Δ, c q , d q , is just as previously described for the supplementary signal r cc .  
         [0127]     In particular cases, there may be small differences between the correction values c q , d q  determined by evaluation of the equations 91 and 93 on the one hand and 92 and 94 on the other. For this reason it is preferable, as shown in  FIG. 13 , to determine the Fourier coefficients XR q , XI q , YR q , YI q  for both corrected signals x cc , y cc . In this case, the correction values c q , d q  for the higher-frequency waves in the measuring signals x, y will be adjusted using the Fourier coefficients XR q , XI q , YR q , YI q  for both corrected signals x cc , y cc . In particular, mean values can be formed.  
         [0128]     Unlike the sums of the squares of the corrected signals x cc , y cc , the corrected signals x cc , y cc  themselves show a marked fluctuation at the basic frequency fG. It can be logical therefore to begin by using the arctangent φ cc  and the amplitude a to determine expected signals x′, y′ according to the equations 
 
x′=a cos φ cc  and  (95) 
 
y′=a sin φ cc   (96) 
 
 and to subtract these expected signals x′, y′ from the corresponding measuring signals x, y. That is to say, in this case supplementary signals δx, δy are formed, corresponding to the difference between the measuring signals x, y and the expected signals x′, y′. The correction values x 0 , y 0 , m, Δ, c q , d q  are in this case adjusted using the Fourier coefficients of the supplementary signals δx, δy. 
 
         [0129]     By means of the approach in accordance with the invention it is thus also possible to correct higher-frequency waves in the measuring signals x, y in a simple manner.