FI98959B - Method and arrangement for determining the height of a surface when measuring the planarity of the surface - Google Patents

Description

98959

Method and apparatus for determining surface elevation information in surface planarity measurement

The invention relates to a method for determining surface elevation information in a surface planarity measurement, in which a pattern is projected or otherwise formed on a surface to be inspected, radiation reflected from a surface is detected to form a 2-dimensional line pattern type image signal comprising several image rows, altitude information.

The invention also relates to an apparatus for determining surface elevation information in a surface planarity measurement, the apparatus comprising means for generating a pattern on a surface, means for generating a 2-dimensional multi-line line pattern to generate an image signal from surface reflective radiation, means for displaying image signal carrier phase information and intermediate phase information.

The invention can be applied in particular to the flatness monitoring of plate-like and strip-like products in the metal industry. The method can be used to detect: · products with excessive surface bumps, dents or other similar shape defects which may cause. 25 throws, for example, worn or otherwise in poor condition *: rolling rollers.

• *. The shape of the detector means, such as a camera, is. · The left signal is a line pattern. A line pattern refers to a line pattern of the interferometry type comprising dark lines and light lines between them. The dark lines in the line pattern seen by the camera correspond to the in- • · | * minimum intensity point. The horizontal lines of the line pattern seen by the camera correspond to the maximum point of the intensity measured by the camera. The lines in the line pattern are in a way a height curve, and in this case, for example, the transition from the dark line of the line pattern to the next dark line means something like an increase of 3 mm on the surface to be inspected.

The line pattern may be a Moire pattern, such as a line pattern formed by the projection-5 Moire method, or the line pattern may be generated by an interferometer. Thus, the term interferometer-type line pattern also includes line patterns other than line patterns generated by an interference measuring device. Such line patterns include, for example, line patterns formed by the projection Moire method and line patterns formed by the shadow Moire method. For example, a line pattern formed by the projection-Moire method is obtained by projecting a lattice on a surface to be inspected with light from a light source and recording the actual line pattern, i.e. a 2-dimensional intensity signal, i.e. an image signal, through the other lattice.

In a line pattern containing a carrier formed by the projection-Moire method, the surface height information, i.e. the z-coordinate, is encoded in the phase of the carrier because the shape of the surface modulates the carrier. The height information of the surface is thus obtained by detecting the phase and converting the phase information into height information. The shape of the surface affects the deflection of the lines and the distance between the lines.

'1 * Several different methods have been used to interpret Moire patterns. 25 methods, such as temporal phase shifting and spatial carrier phase shift (SCPS) methods.

»·. ·. The known solutions involve several problems. One known method is disclosed in U.S. Pat. No. 4,212,073. Currently known calculation methods are not readily applicable to automatic and real-time analysis of a disturbed line pattern. Interference with the line pattern is caused by, for example, text, color markings, or other surface markings on the surface of a steel plate, for example. has been done. Such markings impair the reliability of the planar measurements if they are interpreted as a surface elevation. The problem with the SCPS method is the change in the carrier frequency caused by the curved surfaces, which results in a phase error signal.

It is an object of the present invention to provide a new type of method and apparatus which avoids the problems associated with known solutions.

This object is achieved by a method according to the invention, characterized in that a phase-locked loop is used for phase detection, in which a phase comparison is performed between the input signal generated by the image signal and the reference signal produced by the feedback controlled control 15 is used as a control for generating a new reference signal to be input for phase comparison with the input signal, and for determining phase information in the method in addition to the instantaneous phase difference signal in a phase locked loop, determining absolute cumulative phase information by determining the phase difference of each pixel from the calculation start point.

Said object is achieved by a device φ · .V according to the invention, which is characterized in that the means for detecting the phase information comprise a phase signal that reads the image signal; 25 locked loops comprising a phase comparator to which '··. the video signal is connected as one input signal, phase * »·. ·. a low-pass filter means coupled to the phase comparator, the phase-locked loop further comprising a control signal generating control signal and integrating an absolute cumulative phase, the line comprising a feedback control, the reference signal produced by the control element being ··· connected as a second input signal to the phase comparator.

. ·· *. In Moire images, phase information appears as a two-dimensional signal produced by 35 cameras. The invention takes advantage of, firstly, the finding that the frequency of the line patterns in successive rows cannot differ significantly from each other, and secondly, that the phase difference of the line patterns in successive rows cannot be limited in a limited time. • · 9 ·· · »♦ ·« • »98959 4 be higher than specified in the application. Thus, unexpected changes in the phase can be filtered out because they are not caused by surface bumps, but by, for example, text or color tone changes.

The invention achieves several advantages. Using the developed calculation method, a fully automatic measuring device can be implemented, which operates with a considerably worse signal-to-noise ratio, i.e. for more disturbed image signals. The calculation method can be implemented with a single commercial signal processor. The Algo-15 rhythm automatically adapts to the geometric and photometric changes in the measurement environment. Good interference tolerance and adsorbivity enable accurate phase detection of the measurement signal and thus also an accurate measurement result. Since the algorithm operates with a noisy signal, the optical and mechanical requirements of the measuring device se-20 and the measuring environment can be mitigated. The invention eliminates the so-called unwrapping problem, which means that ‘·}, · that absolute cumulative phase information cannot be computed ·: · in which case the method malfunctions. Calculation of the level of requirements for optical and mechanical components · '. and only one signal processor can be used- I *. considerably less expensive than the '···' construction. The required cos and sin values can be locked, so that the calculation of pixel phase information does not require atan, acos, or asin functions, which would be computationally slow and cumbersome operations.

/: The invention will be explained in more detail below with reference to the accompanying drawings, in which Fig. 1 shows a measuring arrangement. Fig. 2 shows a block diagram of a phase-locked loop used in the detection of phase information. Fig. 3 shows a 2-way traversal of image rows Fig. 4 shows Moire line patterns, Fig. 5a shows an intensity-5 signal measured from a surface with all intensities of image rows combined in series, Fig. 5b shows an absolute step Fig. 6 shows a block diagram of an image processing unit and a corresponding method.

Figure 1 thus shows a measuring arrangement. Figure 1 shows the surface 1 to be inspected, the planarity of which is to be measured. In Fig. 1, an apparatus for measuring the flatness of surface 1, in particular for determining the height information z of surface 1, comprises means 2-5 for generating a sinusoidal pattern on surface 1. The apparatus also comprises means 6-9 for generating a 2- to 2-dimensional line pattern image signal from surface 1. . In the case of the projection-Moire technique, the means 2-5 comprise an illuminating means 2, a condensing lens 3 3, a first grating 4 and an optic 5, and then means 6-9 in turn comprise an optic 6, a second grating 7, a field lens 8 and a detector 9 practically ka- .V Meran 9 like matrix camera. The projection Moire technique «. * · 'Is characterized in that the surface 1 is illuminated through the first lattice 4 25 and the surface is imaged through the second lattice 7, whereby a projection Moire pattern comprising a carrier such as« φ · is obtained; * · Line pattern in Figure 4. ·: * made on the surface l through the lattice 4. the pattern formed by the illumination is also a line pattern, but its frequency is much higher than the actual line pattern generated on the second lattice 7 measured by the camera 30 of Fig. 4: * / vio.

* · * In Figure 1, the direction of travel of a surface such as a steel plate ·: * is away from the viewer and the lines of the line patterns are the same.

'**; In this case, in the Moire pattern in Fig. 4, a transverse sinusoidal signal is formed. Figure 4 Moire rubber-m • M »

• V

6 98959, i.e. in the image taken by the camera 9, the dark lines correspond to the low points of the intensity signal, i.e. the image signal, of Fig. 5a. In the Moire pattern of Fig. 4, i.e. in the image taken by the camera 9, the light lines correspond to the high points of the intensity signal, i.e. the image signal, of Fig. 5a.

In a projection-Moire pattern containing a carrier, as in Fig. 4, the 3D information, i.e., the z-coordinate of the surface, is encoded in the carrier phase φ, since the shape of the surface modulates the carrier.

With particular reference to Figures 1 and 6, the device comprises an image processing unit 11. The image processing unit 11 in turn comprises means 13 for detecting the phase information φ of the image signal carrier and means 14 for converting the phase information φ into a surface height information z. In addition, an interpreting unit 15 is detected, which determines whether or not there is a flatness error.

Referring to Fig. 2, the means 13 for detecting phase information comprise a phase locked loop 13 (PLL, Phase Lock-20 ed Loop) reading a 2-dimensional image signal one line at a time, comprising a phase comparator 13a to which the image signal is connected as one input signal, the phase locked loop 13 comprising a phase locked loop 13 a low-pass filter means 13b, a phase-locked loop, further comprising a reference signal generating and ab-. A control element 13c integrating 25 cellular cumulative phases, comprising a feedback control 13d. from the pass filter means 13b. The ver- ·· produced by the control member 13c. The input signal is connected as a second input signal to the phase comparator 13a.

In the preferred embodiment, the low-pass filter means 13b comprises a 1-dimensional filter element FH to be filtered in the image row direction and a second 1-dimensional filter element FV to be filtered in the transverse direction of the image row. Imaging ···

horizontal filter element FH to be filtered in the vin direction

»··. * 35 ensures that there are no large phase changes in the adhesive ·· ···» • · 98959 7 within one row of images. The filter element FV, which is filtered in the transverse direction of the image row, i.e. the vertical filter element, ensures that a single image row cannot move too much, i.e. slide, compared to the previous image rows.

In the image processing unit 11 before the phase locked loop 13 (PLL), the device 1, i.e. in practice the image processing unit 11 of the device, in a preferred embodiment comprises pre-filter means 11a, the center frequency of which substantially corresponds to the center frequency of the image signal. Thus, in the method, prior to the detection of the 10 steps, pre-filtering is performed, by which frequencies deviating from the center frequency of the carrier are filtered out of the image signal. In this case, the device and the method operate adaptively, which improves the advantages of the method. Prior to the phase-locked loop 13, the device in a further preferred embodiment further comprises means 11b for adjusting the gain and level of the image signal. Thus, in addition to pre-filtering, the method performs gain and signal level correction. It can also be seen from Figure 6 that the first block in the image processing block is the calculation area delimitation block 20 block lie.

With regard to the method, it is stated that it is thus a method for determining the height information z of the surface 1 in the measurement of the planarity of the surface. In the method, a high-frequency sinusoidal shape is formed on the surface 1 to be inspected. 25 line pattern, radiation reflected from surface 1 is detected: a lower frequency, line pattern comprising several rows of images. a fault type 2-dimensional image signal (Figures 4 and 5a). form. In the method, a list of 2-dimensional image signals is expressed by the carrier phase information ilma, and the phase information 30 is determined by the surface height information z.

According to the invention, the phase itt detection uses a phase-locked loop 13, PLL, in which: a phase comparison is performed between the input signal generated by the 2-dimensional image signal and the reference signal produced by the feedback controlled control element 13c. The instantaneous phase difference signal obtained from the phase * · »e ψ 98959 8 comparison is low-pass filtered to attenuate excessive phase squares. The filtered signal 0vh is used as a control, i.e. a correction factor, for the control element 13c, with which a new corrected reference signal 5 is generated, which is fed to the phase comparison with the input signal. In order to determine the phase information to be used for calculating the height information z in the method in addition to the instantaneous phase difference signal #vh in the phase locked loop 13, the PLL determines the absolute cumulative phase path 10 to 4> abs by determining the phase difference of each pixel from the calculation start point S. The starting point S is observed in Figures 3, 4 and 5a. Figure 5b shows the absolute cumulative phase information φabs.

In Figs. 5a and 5b, the horizontal axis 15 of the coordinate system shows the number of pixels P calculated from the beginning of the image signal. Figure 4 has 512 image rows and 512 pixels in each image row. In Figures 5a and 5b, a full 360 degree wave, i.e. a distance of 2π, corresponds to 45 pixels, for example, and then one image line would be slightly less than 12 full waves, i.e. the phase value would be about 24π and multiplied by the number of image lines 512 gives the absolute maximum phase value in Figure 5b.

The phase information is determined as the difference between the current phase 9 * · * of the control element 13c and the phase • 25 of the theoretical constant frequency carrier.

**: By feedback, the control element 13c is controlled so that the phase difference between the control element and the input signal is at least li * *: ·. at least 90 ° or a multiple thereof. In this case, the method is • computationally fast, because such a setting minimizes the computation time of trigonometric functions.

... In the preferred embodiment with reference to Fig. 3, the ab- • · · * * cellular phase information $ abs is stored by reading the 2-dimensional image signal alternately in opposite directions so that after the false data of the previous image row is calculated, the phase information from the next image row is calculated. opposite- ·· ···· • · 98959 9 in the direction of the fireplace. In Fig. 3, the notation R1 shows the first line of the image, which is read from left to right. In Figure 3, the notation R2 shows a second row of images read from right to left. In the preferred embodiment, the method then operates in such a way that when the calculation direction of the phase information changes, the latest absolute cumulative phase information from the previous display line is used as the initial value in the next display line, whereby the absolute phase information is stored. Alternating reading of the image rows in different directions also eliminates the problems associated with calculating the order of line patterns. Most preferably, the camera image is read (Figure 4) so that the counting starts from the top left and ends at the bottom right. Therefore, it should be noted from Figs. 5a and 5b that the beginning of the signals 15 in Fig. 4 corresponds to the left edge of the first or top row of images, and the ends of the signals in Fig. 4 correspond to the right edge of the last or bottom row of images.

In the preferred embodiment, the absolute cumulative phase information 0abs is determined by the control element 13c producing the reference signal 20, which integrates the absolute cumulative phase $ abs, whereby the device solution in the phase-locked loop 13, PLL is simple.

In a preferred embodiment, the method is such that the instantaneous phase difference obtained from the phase comparison. The filtering of the 25 signals by the low-pass filtering means 13b * «uses the desired locking limit, which is exceeded by a phase change. attenuated. In this case,% · can be effectively prevented. for example, interpreting text or hue changes as a surface bump.

30 In order to improve the reliability of the method, the method calibrates optical, mechanical or other similar distortions at one or more points in the image area by performing a zero measurement with a calibration means 50, *. on the basis of which the absolute cumulative 35 phase data is corrected. Such a calibration is easily • »» · 98959 10 because it does not depend on the measurement environment.

The phase of the control element 13c is £ (φρρ + φνh), where φρρ is the previously known phase difference between the two calculation points 5 (often often a pixel) and 0vh is the phase correction term or control signal given to the control element 13c by the loop filter or low-pass filter means 13b. Since the control element 13c integrates the absolute phase between Οπ and oo, the phase of the control element is obtained for each calculation-10 point. The difference between the current or instantaneous phase £ (φρρ + φνh) of the control element 13c and the instantaneous phase £ (φρρ) required by the constant frequency carrier is the result of the calculation point, from which the difference of the z-coordinate of the surface to the calculation point can be determined by means 14. The interpreting unit 15 determines whether there is a flatness error or not.

Although the invention has been described above with reference to the examples according to the accompanying drawings, it is clear that the invention is not limited thereto, but can be modified in many ways within the scope of the inventive idea set forth in the appended claims.

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Claims (10)

1. A method for determining the height data in measuring the flatness of a surface, in which method a figure 5 is projected or otherwise formed on the surface (1) to be examined is detected by the surface reflected radiation for generating a plurality of image rows comprising, 2-dimensional line image type signal, phase information for a carrier path is detected from the image signal, and the phase information determines the height data (z) of the surface, characterized in that a phase-read loop (13, PLL) is used in the phase detection in which the phase comparison is performed. between an input signal generated by the image signal and a control signal generated by feedback (13c), a instantaneous phase difference signal obtained from the phase comparison low-pass filter for attenuation of large phase changes, the filtered signal being used as control for control means (13). which generates a new comparison signal input into the phase comparison with the input signal, and In order to determine the phase information, the method is determined in addition to the instantaneous phase difference signal in the phase. Iästä loop (13, PLL) absolute cumulative phase information \ (tfabs) by determining the phase difference of each pixel in relation to the starting point (S) of the calculation. A method according to claim 1, characterized in that the control means is controlled by feedback so that the phase difference of the control means and the input signal is always at least about n x 90 °, where n = 1, 2, 3 ... 3. A method according to claim 1, characterized in -30 characterized in that the absolute phase information (· abs) is maintained by reading the 2-dimensional image signal in turns in opposite directions, so that there is one. The phase information of the current image row has been counted, the phase formation from the following image row is counted in the opposite direction. The method according to claim 3, characterized in that when the counting direction of the phase information is changed, the latest absolute cumulative phase information ($$ abs) from the previous image row is used as the initial value of the following image line. Process according to any of the preceding claims, characterized in that the absolute cumulative phase information ($ abs) is determined by the comparator signal generating control means (13c) which integrates the absolute cumulative phase ($ abs). Method according to Claim 1 or 5, characterized in that the phase information is determined as the difference between the current-phase phase (13c) and the constant frequency theoretical phase path. The method according to claim 1, characterized in that the desired phase difference signal obtained from the filter comparison obtained from the filter comparison uses the desired reading limit, whereby the phase changes exceeding the limit are attenuated. 20 Θ. Method according to claim 1, characterized in that, before the phase detection, the pre-filtration process is carried out by which the mean frequencies of the carrier path are filtered from the image signal, ·· · ”and that in addition to the pre-filtering a correction of the gain and signal level is performed. The method according to claim 1, characterized in - * I; characterized in that, in the process, optical, mechanical, or other corresponding distortions are calibrated at one or more points in the image area by measuring a zero-m level with a calibrator (50) on the basis of which the absolute cumulative phase information ($ (abs) is measured). 10. Device for determining height data in measuring the flatness of a surface, which device comprises [35 means (2-5) for generating a figure on the surface, means (6-9) ·· m · 98959 for generating a 2-dimensional, multiple image rows comprising line image type image signal from surface reflected radiation, means (13) for detecting phase information (φ) for the image path carrier and means (14) for converting the phase information ion (φ) to the surface height data (z), characterized by the fact that the means (13) for detecting the phase information (φ) comprise a phase-read loop (13, PLL) which reads the image signal and includes a phase comparator re (13a) to which the image signal is coupled as an input signal, wherein the phase-read loop (13, PLL) further comprises a low-pass filter means (13b) coupled to the phase comparator (13b), the phase-read loop (13, PLL) further comprising a control means (13c) which generates a comparison signal and integrates an absolute cumulative phase and comprises a feedback control (13d) from the low pass filter means, the comparison signal generated by the control means (13c) being coupled as a second input to the phase comparator (13a). 11. Device according to claim 10, characterized in that the low-pass filter means (13b) comprises a 1-dimensional filter which is filtered in the direction of the frame. . device (FH) and one in the transverse direction of the image line, filtering the other 1-dimensional filter means (FVj. ••• J 12. Device according to claim 10, characterized in that: before the phase-read loop (13, PLL) ··· ..... the device comprises a pre-filtering means (11a) whose jj *: average frequency substantially corresponds to the average ΓΓ: frequency of the image signal, and before the phase-read loop (13, PLL) comprises the device additional means (lib) for controlling the amplification and level of the image signal. • · ·. --- ---- T ------ »· ♦ ·" "'1“ Τ7-ΠΓ'. '. • ♦ · ···. '' ···· · ~ · - · - - ------ ··· i: ··· ·· • ·· 9 9 m