the use of correlation analysis of the image in phase optoelectronic devices

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  • The possibility of using phase-correlation analysis of an image in optoelectronic devices to measure smallangular and linear displacements of objects is considered. It is shown that the measurement accuracy isthereby increased.Key words: optoelectronic system, correlation analysis of the image, accuracy of displacementmeasurement.

    In geodesy and measurement technique, various phase optoelectronic devices are employed at the present time tomeasure the angular and linear displacements of objects, their nonaxiality, to balance the position of individual items ofinstruments, etc. [13]. These phase devices differ from other similar devices in having a simpler construction, stability ofthe characteristics, speed of response, reliability, a wide dynamic range of the reproduced signals and high efficiency of opto-electronic signal conversion. Sometimes they possess higher sensitivity and higher noise immunity. Compared with ampli-tude optoelectronic devices they are less sensitive to changes in the radiation flux incident in the image analysis plane.

    The drawbacks of phase optoelectronic devices are the low resolving power, due to technical errors in the rasters ofthe image analyzers, the low noise immunity from the effect of background and interference, incident in the angular field ofthe device, and also from internal noise. The technological errors of image analyzers include deviations of the raster line peri-ods from their nominal value, the different slope of the lines, and, for rotating analyzers, the eccentricity of their rotation axeswith respect to the center of the raster. In addition, the high resolving power of phase devices may sometimes be limited bythe errors of standard industrial phase-measuring instruments (phasemeters), which are difficult to match with the accuracycharacteristics of image analyzers.

    In existing phase optoelectronic devices, the phase difference between the useful signal and a standard voltage is mea-sured. This is proportional to the displacement of the image of the object or a mark in the analyzer raster plane. One can alsouse as a raster certain multielement radiation receivers with signal sampling from their elements in accordance with a certainrule, as a result of which a virtual displacement of the elements (pixels) of the receiver is produced [2]. Only receivers withan arbitrary selection of the signal with elements, for example of CMOS type, are suitable for phase analysis [3].

    The effect of technical errors of the image analyzer raster on the accuracy of existing phase devices is explained bythe fact that, when measuring the informative parameter of the useful signal, in this case the phase difference, only limitedinformation on the signal is employed, corresponding to characteristic instants of time during the measurements, and the vol-ume of information which other signal points carry is not taken into account. Such characteristic points are, for example,points where the signal passes through zero or its extremal values, points where the signal intersects a chosen threshold level,etc. If, in a phase device for measuring phase difference, only one characteristic point is used, as often happens in practice,the result of the measurements will obviously depend to a considerable extent both on the spread in the geometrical param-eters of the raster and on the signal distortions, due to the actions on it of internal noise, external interference and the nonuni-form background, and sometimes also vibrations.

    Measurement Techniques, Vol. 48, No. 5, 2005


    V. P. Soldatov UDC 621.397.3:621.317.77:621.035

    Translated from Izmeritelnaya Tekhnika, No. 5, pp. 3739, May, 2005. Original article submitted December 20,2004.

    0543-1972/05/4805-0471 2005 Springer Science+Business Media, Inc. 471

  • To reduce the effect of the factors considered above on the accuracy of measurements, one can use the phase-corre-lation method of analyzing the image, in which information from all points (values) of the signal over a complete measure-ment period is taken into account. The scheme of the phase-correlation angle gauge, proposed in the Department ofOptoelectronic Instruments at the MIIGAiK is shown in Fig.1 [4].

    A bunch of rays from the target is focused by an objective lens 1 in the plane of the raster of a drum image analyzer 3,which rotates with constant angular velocity, on parts diametrically opposite it. It then passes through a beam-splitting unit 2.Modulated radiation fluxes 1 and 2 are incident on photoreceivers 4, in the circuits of which electric signals U1(t) and U2(t)are produced. After amplification by amplifiers 5, they are incident on the inputs of corresponding similar voltmeters 6, whichmeasure the effective values of the voltages, and at the inputs of the correlator 7. The outputs of the correlator 7 and of bothvoltmeters 6 are connected to a computer unit 8, for example, a microprocessor, which calculates the displacement x of the imageof the target in the plane of the analysis, on which the phase difference of the signals U1(t) and U2(t) depends.

    We will obtain the relation between the cross-correlation function K() of the signals U1(t) and U2(t) and the phasedifference between them. The function K() is described by the expression


    where is the correlation interval and t is the time.Since U1(t) and U2(T) are sinusoidal signals, between which there is a phase shift , we can write


    where = 2mn is the angular frequency of the signals, m is the number of periods of the raster of the image analyzer, n isthe rotation frequency of the analyzer, and U1max and U2max are the amplitudes of the signals in the first and second chan-nels of the angle gauge, respectively.

    The signals U1(t) and U2(t) can be considered as individual samples of the corresponding stationary random func-tions. From (1), taking (2) into account, and fact that the random functions U1(t) and U2(t) are stationary, we obtain


    where T is the period of variation of the functions.


    U tU t dtT

    ( ) sin sin ( ) ,max max = +1 1 20

    U t U tU t U t

    1 1

    2 2

    ( ) sin ;( ) sin( ) ,




    = +

    K U t U t dt( ) ( ) ( ) , = +

    1 2









    7 8

    Fig. 1. Sketch of the phase-correlation angle gauge: 1) objective; 2) beam-splitting unit; 3) imageanalyzer; 4) photoreceiver; 5) amplifiers; 6) voltmeters; 7) correlator; 8) computer unit.

  • After integrating in (3), we obtain

    K() = 0.5U1maxU2maxcos. (4)

    Expression (4) shows that the argument of the cross-correlation function K() is the phase shift = between therandom functions U1(t) and U2(t).

    Consequently, from (4) we obtain an expression which describes the change in the phase shift of the signals U1(t)and U2(t) as a function of the change in the cross-correlation function K() of these signals in the form


    We will convert this formula as follows. Assuming the random functions U1(t) and U2(t) are stationary, and theirmathematical expectations m1(t) and m2(t) are equal to zero, taking (2) into account we can write the following expressionsfor the variances D1(t) and D2(t) of these functions:


    Substituting the values of U1max and U2max from (6) into (5), we obtain

    Since the expression is the normalized cross-correlation function Kn() of the signals U1(t) and U2(t), we

    can write the following simpler relation for the required phase difference:

    = arccosKn(). (7)

    We will replace in this formula by the shift x of the position of the target in the plane of the drum image analyz-er, corresponding to the conversion function of this analyzer. As follows from [1], the conversion function has the form

    = marcsin(x /R),

    where R is the radius of the drum image analyzer.Taking this formula into account, after simple reduction we obtain from (7) the following formula describing the

    relation between the shift x of the image of the target in the analysis plane and the cross-correlation function of the signalsU1(t) and U2(t):

    x = Rsin[m1arccosKn()].

    Hence the value of the mismatch angle , measured by the instrument, is expressed in terms of Kn() in the form

    = arctan{(R /)sin[m1arccosKn()]}, (8)

    where is the equivalent focal length of the objective of the angle gauge.


    D D

    ( )1 2



    ( ).


    D D1 2

    D tT

    U t m t dt U

    D tT

    U t m t dt U



    1 1 12


    2 2 22


    1 0 5

    1 0 5

    ( ) [ ( ) ( )] . ;

    ( ) [ ( ) ( )] . .



    = =

    = =


    arccos( )


    max max


    1 2

    KU U


  • When the angle gauge is operating, its computer unit 8 calculates the required angle from formula (8) in accor-dance with the values of the correlation function K(), obtained by the correlator, the values of D1(t) and D2(t), determinedby the voltmeters, and the constant parameters R, m, and .

    As follows from (8), the values of the angles theoretically do not depend on the period 2a of the image analyzerraster, while the resolving power p of the angle gauge increases as the number of periods m of the raster increases, i.e., asthe periods decrease. Consequently, the technical deviation of the period 2a of the raster from the nominal value also doesnot give rise to any corresponding error when measuring the angle. It also follows from (8) that the errors in the mea-sured angles are also independent of the instability of the amplitudes of the signals U1(t) and U2(t) in both channels of theinstrument, since the n


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