DE19639213A1 - Shearing-Speckle pattern interferometry for oscillating object - Google Patents

Abstract

The method involves detecting derivatives of the shaping of a surface of an oscillating object in at least one selected direction, whereby the object surface is irradiated with coherent light and a diffuse reflected light is recorded in an optics system comprising a Shearing element. At least one origin Shearing pattern and a load Shearing pattern are produced from the image of the object surface. Both Shearing patterns are subtracted from each other, and a reference Shearing pattern, recorded in the respectively preceding video clock, is used instead of the origin Shearing pattern. The object is put into oscillations and a laser beam is synchronized to it in such way, that the irradiation of the object results respectively only during preselected oscillation conditions.

Description

The invention relates to methods and devices for the examination of vibrating Objects using shearing speckle interferometry.

Methods and devices of this type which have become known to date (DE 44 14 287 A1, DE 44 46 887 A1) deal almost exclusively with the examination of stationary arranged objects. For this purpose, a so-called zero shea program is first carried out when there is no load Object generated with an optoelectronic image sensor, especially on CCD Base, recorded and then in a memory, e.g. B. a frame grabber chert. The object is then loaded, and then it becomes a second, so-called Bela Stungshearogramm recorded and also saved. By subtraction or Adding both shea programs can result in pixel by pixel on one screen being represented. If a mathematical evaluation is also required, the Shearograms additionally with several, precisely defined phase shifts of e.g. B. 0 °, + 120 ° and -120 ° added, the phase shift z. B. by means of a Device like a Michelson interferometer.

In addition, it is already known (DE 44 46 887 A1), objects excited to vibrate to investigate during the vibration process by z. B. closures one  Imaging optics are controlled so that their half, single or double frequency with the excitation frequency of the vibrating object coincides with the closures e.g. B. as acoustic or electro-optical modulators. This makes it in Principle possible to see pure in-plane and out-of-plane strains in real time do.

When examining vibrating objects, however, there are considerable problems. On the one hand, these consist in the fact that the previously usual difference formation from an earlier one stored, unloaded state of the object and one due to the vibration resulting randomly loaded state of the object to very fuzzy real-time Leads pictures. This is mainly due to the fact that every CCD cell during the usual video clocks of 25 msec duration is exposed in different ways, so that a large number of superimposed picture elements are created, which together form a mixed picture with low contrast. There is also the major disadvantage that no defined phase shifts possible during the vibrations and therefore the evaluation methods usually used cannot be carried out. An arithmetic So far, it has not been possible to determine the strains.

In view of this prior art, the invention is based on the object described at the outset to improve methods and devices explained in that one hand Real-time observations made with good contrast and on the other hand those of the Shearograms obtained from vibrating objects can be evaluated mathematically can.

The features of claims 1, 4 and 6 serve to achieve this object.

Further advantageous features emerge from the subclaims.

The invention is described below in conjunction with the accompanying drawing Exemplary embodiments explained in more detail. Show it:

Figure 1 shows a known device for performing digital shearography.

2 shows in a diagram the variations of the conventional real-time strip function [1 - J _o (Ω)] against Ω, with Ω = δ ×.

FIG. 3 is a graph × the changes in the real-time function in strip application of the invention with renewed reference picture J _o (Ω) against Ω with Ω = δ;

Fig. 4 real-time representations of digital time averaging shearograms for a circumferentially clamped rectangular steel plate with four recesses on the back when excited with different frequencies, (a) showing the shape and position of the four recesses and (b) to (f ) show the digital time averaging shearograms corresponding to the vibration frequencies of 4317 Hz, 4411 Hz, 4486 Hz, 4875 Hz and 4960 Hz;

Fig. 5 digital time averaging Shearogramme a mutually clamped, thin, circular aluminum plate at harmonic excitation at the frequencies (left to right) 1270 Hz, 4300 Hz and 4860 Hz;

Figure 6 schematically illustrates the structure of an apparatus for digital shearography with stroboscopic illumination using an acousto-optic modulator (AOM) for vibration measurements.

Fig. 7 phase distributions of Shearogrammen, which at application of the same excitation method, the deformation gradients for the thin circular aluminum plate of FIG 5 and (again from left to right) show the same frequencies.

Fig. 8 shows the dynamic study of a turbine blade, wherein (a) the investigated turbine blade, (b) gradient the phase distribution of Shearogramms for the deformation ∂w / ∂x and the frequency f = 1572 Hz, (c) the unwrapped phase distribution and (d ) shows the 3D diagram of the field of the deformation gradients;

9 shows stripe patterns in each case, with (a) deformation field (w), which is obtained by integrating the unfolded phase image from FIG. 8c, and (b) the 3D deformation field, ie the vibration state;

Fig. 10 (a) is a strip pattern that the Biegedehungsfeld ε _xx, which was shown Shearogramms 8b by differentiating the phase image of the in Fig obtained, and (b) the 3D diagram of the bending strain field.

FIG. 11 shows a block diagram of the control device according to FIG. 6; and

Fig. 12 shows the front panel of a housing of the control device according to Fig. 6.

Vibrations in general and resonances in particular are undesirable phenomena in Components and structures because they are accompanied by increased tensions and energy losses; she should therefore be avoided or largely reduced.

Holography and speckle pattern interferometry (SPI) are two typical experi mental analysis methods for full-area and non-contact deformation measurement. In recent years, they have been used for vibration analysis of components and structures e.g. B. applied as a qualitative, surface-optical modal analysis. The limits of this Techniques include sensitivity to rigid body movements and strong ones Streak increase due to the deformation that the interpretation of the stripe pattern complicate. The further disadvantage of the methods stems from the small measuring range, so that only small deformations can be measured.

The solution to the problem that is insensitive to rigid body displacement and allows a larger measuring range, shearography (also speckle pattern shearing Interferometry or SPSI called). Instead of the deformation measurement using Hologram interferometry and SPI directly measure the derivative of the shearography Whole-field deformation caused by the "self-calibrating" optical system. With the application of image processing, wet developing and optical recon structure of the shearograms on films is eliminated, the shearographic measurement in in industry and in the laboratory more economical and faster.

In conventional real-time observation technology relates to vibration measurement With the double exposure method, the reference image is usually on the stationary one ren ground state; the running images are in from the original reference image  Subtracted in real time and the result can be in video clock with less stripe contrast to be observed.

Real-time observation using a digital shea program of a vibrating object

Digital shearography (DSPSI or TV shearography) is a further development of photographic shearography. There is no difference in the optical theory between the two methods, but DSPSI is a computer-aided process, which leads to an accelerated test sequence because the shea program can be observed in real time (with video clock). Fig. 1 shows the experimental arrangement of the digital shearo graph. The test object is illuminated by an expanded laser beam. The light reflected from the object surface interferes in the image plane of the CCD shearing camera, in front of the lens of which there is a Michelson interferometer, so that laterally overheard images of the object in the image plane of the CCD camera are generated by the mirror 1 being off the vertical position is rotated by a small angle. The light rays of the two images interfere and form an interferogram or speckle pattern.

For vibration measurement using the conventional real-time subtraction method, the CCD camera first records the intensity distribution I _r (x, y) of the speckle pattern corresponding to the stationary basic state of the object and stores it in the reference image memory according to Eq. (1):

I _r (x, y) = 2 I _o {1 + γcos [Φ (x, y)]} (1)

in which I _{o is} the mean intensity of two light waves that have been exchanged, γ is the modulation of the interference term and Φ (x, y) is any relative phase angle between two images that have been exchanged. If the object is excited, the intensity distribution of the speckle pattern changes slightly due to the difference in the relative phase angle distribution according to Eq. (2):

I (x, y) = 2 I _o {1 + γcos [Φ (x, y) + Δ (x, y, t)]} (2)

in the Δ the relative phase change due to the light path difference due to the oscillation object is. The digital subtraction between the current and the reference image results in a visible stripe pattern, a so-called digital shea program. A commercial one Image processing card allows the running image from the reference image in real time subtract and watch the digital shea program on the monitor in video clock.

If the object vibrates in a steady state at a frequency that is higher than the video clock, the speckle pattern changes accordingly and its intensity is integrated by the CCD chip of the camera during the time period T _f for image recording. While the object is swinging, the image is recorded in the current image memory as a time averaging correlogram ( Fig. 2).

In the case of the sine wave, the amplitude A (t) versus time can be as follows to be written:

A (t) = A sin ωt = (ue _x + ve _y + we _z ) sin ωt (4)

where A and ω are the amplitude vector and the angular velocity; u, v, w represent the components of the amplitude vector in the x, y and z direction and e _x , e _y and e _z the unit vectors also in the x, y and z direction. The relative phase difference Δ (x, y, t ) is linked to the gradient of the deformation due to the shearing principle of shearography. If the shear direction is in the x direction, Δ (x, y, t) is given as follows:

Δ (x, y, t) = δx [(k _s A / x)] sin ωt
= δx (u / xk _s e _x + v / xk _s e _y + w / xk _s e _z ) sin ωt (5)

Here δx is the shear amount in the x direction and k _{s is} the sensitivity vector. k _s is the bisector between the direction of illumination and observation and its size | k _s | corresponds to (4π / λ) cos (Θ / 2); Θ is the lighting angle ( Fig. 1). By turning the lighting in the direction perpendicular to the object surface, it coincides with the z direction, whereby k _s e _x = k _s e _y = 0 and k _s e _z = / k _s / = (4πλ (for Θ = 0 ° and λ = wavelength of the laser), so that the relative phase difference Δ (x, y, t) is only associated with the out-of-plane tendency to deform:

Eq. (3) are written as follows:

where J _{o is} the Bessel function of the first kind of zero order and Ω = δx (4π / λ) (δw / δx). In the conventional real-time subtraction method, the current images corresponding to I (x, y) _{ave are} repeatedly subtracted in video clock intervals from the reference images I _r (x, y) recorded in the stationary object state and appear in real time on the screen as Eq. (8) shown:

Eq. (8) shows that the real-time subtraction stripe pattern of a vibrating object is modulated by a stripe system with [1 - J _o (Ω)] ( FIG. 2), with poor streak contrast.

The real-time subtraction technique with repeatedly renewed reference image according to the invention shows the stripe pattern of a vibrating object that is modulated by a stripe system with J _o ² (Ω) and not with [1 - J _o (Ω)] as in the previous case , which makes the strip contrast of the "vibration show program" much better. During the measurement, the current image is continuously subtracted from the immediately preceding image and not from the reference image of the steady state. The phase shift is carried out during the video sequence in every further image, namely by means of the electrically driven and PC-controlled piezo mirror 2 in the Michelson interferometer ( FIG. 1). If the current image is the Nth image, the reference image is the (N-1 )th image and not the first image as in the conventional method. Therefore the reference picture is constantly renewed.

If the test object vibrates in a steady state at a frequency higher than the video clock, then the recorded image as the (N-1) th image is a time averaging shea program during the (N-1) th image period T _f (see Eq. (7)) and determined as follows:

I _N-1 (x, y) _ave = 2 I₀ [1 + γcos Φ (x, y) J₀ (Ω)] (9)

After the introduction of a 90 ° phase shift (as with this experimental setup, but any other value is also possible) the Nth image is recorded:

I _N (x, y) _ave = 2 I₀ {1 + γcos [Φ (x, y) + 90 °] J₀ (Ω)}
= 2 I₀ [1-γ sin Φ (x, y) J₀ (Ω)] (10)

The current picture, e.g. B. the Nth picture, is constantly digital from the previous picture for example subtracted from the (N-1) th image. Squaring the digital subtrak result (or the absolute value of the subtraction) appears on the monitor as visible pattern:

and

I _s ² = 8 I² I₀γ² cos² [Φ (x, y) - 45 °] J₀² (Ω) (12)

If the frequency of the object vibration changes, the stripe pattern changes accordingly and can be observed in quasi real time. Eq. (12) describes a real-time subtraction stripe pattern of a vibrating object according to the invention, which is modulated by a stripe system J _o 2 (Ω), the stripe contrast of which is excellent. Furthermore, the time difference between the registration of the current and reference images is greatly shortened, as a result of which low-frequency interference due to thermal air and ambient vibrations are suppressed. These advantages over conventional real-time subtraction technology, in which the strip system is only modulated with [1-J _o (Ω)], are used for qualitative analysis such as NDT and surface-optical "modal analysis".

As an investigation for NDT is shown in a vibrating plate with notches below of different geometry that the resonance when the excitation frequency is continuously increased frequencies make the different notches visible at higher values than the base plate itself. The other application of this technique is the qualitative "modal analyze "for easy finding of resonance frequencies. The stripe pattern of the time-averaged digital shearogram shows the derivation of the amplitude vector on each Point of the object surface; in it is for the strip with the greatest brightness Ω = 0. Taking into account that Ω = δx (4π (λ) (∂w / ∂x) and the shear distance δx (4π / λ) ≉ 0, the brightest stripes are those for which the derivative of the amplitude vek tors (δw / δx) is zero. Therefore, the places where the deformation gradients are zero are, d. H. with strips of zero interference order, easily in a digital time averaging Detect shearogram. The zero order stripe is an important parameter for the further analysis and evaluation of digital shearograms.

Numerical evaluation of the digital shearogram of a vibrating object

The numerical analysis of a digital shea program sets the knowledge of the phase ver division ahead. Although time averaging technology allows this, the phase step Method the application of optical phase modulation synchronized with the object vibration in frequency and phase ahead, which is the control and automation complicated.  

With K. Creath's phase shift technology, the phase distribution of interferograms can be obtained. However, this technique is only suitable for evaluating stripe patterns of quasi-static deformations because it requires the registration of three or four interferograms of the same deformation state. With the use of stroboscopic lighting using an acousto-optic modulator (AOM) according to the invention, the interferogram can be frozen at any point in the oscillation period if the object is illuminated with short light pulses which are synchronized with the object oscillation ( FIG. 6). . With the phase shifting technique, the phase distribution of the shearogram for vibrating objects can be obtained.

The relative phase distribution Φ _α and Φ _β (in the vibration state α and β) can only be determined within the limits of 2π, because the determined relative phase difference Δ is developed in the range 0 to 2π and results in a modulo-2π-stripe pattern as phase distribution. The black dots correspond to zero phase values and the white dots correspond to 2π phase values. An undeveloped phase distribution can be determined by means of an unfolding algorithm, so that automatic and numerical evaluations cannot be carried out as long as the undeveloped phase distribution is present.

Fig. 8 shows the dynamic study 8a shows a turbine blade and in Fig., The test object. The phase distribution of the shearogram shows the deformation gradients ten δw / δx at an excitation frequency f = 1572 Hz ( Fig. 8b). FIG. 8c shows the demodu profiled phase distribution.

Since the phase distribution for each pixel point in the demodulated phase values of the Shearograms is numerically determined, the deformation field w (x, y, t) as Schwin be determined by integration. The key to this lies in the exact Knowledge of the size for δw / δx and zero in the determination of the interference fringe Order, d. H. for the digits δw / δx = 0. I.a. have to consider the boundary conditions of the analyzing object for integration, but using the interference fringes These zeros can easily be determined using the time averaging method will. Furthermore, the stripe pattern, including the full-field bending and shearing strains as a second derivative of out-of-plane deformation in plates due to differences  The unfolded phase distribution can be obtained for δw / δx.

About the two practical techniques for qualitative and quantitative vibration The real-time sub according to the invention thus belongs to analysis using digital shearography traction method with the continuously renewed reference picture, so that the time difference to register the current and reference image is greatly reduced compared to conventional technology. This not only eliminates rigid body displacements - due to the "self-referencing" optical system of shearography - but also suppresses low-frequency interference such as air and ambient vibrations.

By using light pulses with an acousto-optical modulator and under Use of the phase shifting technique allows the phase distribution of the shearogram for one vibrating object can be obtained. The resulting relative phase difference, that describes the deformation derivatives is determined numerically. Although only a one-dimensional dynamic phenomenon is investigated, level and spatial dynamic problems are also analyzed.

Fig. 6 shows the principle of digital shearography invention with stroboscopic lighting using a controlled by a control device acousto-optical modulator (AOM). The stroboscopic lighting is achieved by triggering the Bragg cell. The light pulses, which are synchronized with the object vibration, are derived by the function generator for generating the vibration and passed on by the control unit, at which the trigger position and width are set ( FIG. 6 below). The control unit first tunes the trigger position for generating the pulses with a fixed phase α in each oscillation period. The intensities associated with each flash of light are summed up by the CCD camera and result in a shearing speckle interferogram. It can be seen that the interferogram is frozen for a certain state of deformation. Therefore, it can be further processed with the phase shifting technique in order to obtain the phase distribution of the shearogram for an oscillation of the object, for which purpose the piezoelectrically controlled mirror 2 in the Michelson interferator ( FIG. 6) is used. The D / A converter provides the necessary voltage steps for the phase shift from 0, π / 2, π to 3π / 2 through the mirror 2 . This gives 4 images that determine the relative phase distribution Φ _{α of} the interferogram from the intensity data:

Φ _α = arc tan [(I₄-I₂) / (I₁-I₃)] (13)

The synchronizer then adjusts the further trigger position in order to generate light flashes for a specific phase β of a deformation state in each oscillation period. Another 4 images with intensity data are recorded by shifting the phase by the same amounts as for the first data set. The relative phase distribution Φ _{β of} the changed interferogram is calculated like Φ _α . From this, the relative phase difference Δ can be calculated simply by subtracting with Φ _β from Φ _α , and the phase distribution of the shea program shows the following relative phase distribution

FIG. 7 shows the phase distributions of a shea program for a vibrating circular plate according to FIG. 5, excited with the same frequencies 1270 Hz, 4300 Hz and 4860 Hz (from left to right). Since the relative phase difference Δ of the shearography is linked to the deformation gradients ∂w / ∂x (in the x-shear direction) or ∂w / ∂y (in the y-shear direction) for vertical illumination and observation, the phase distribution of the shearogram describes full-field information the deformation gradients ∂w / ∂x or ∂w / ∂y numerically.

Further details of the real-time observation according to the invention and the stroboscopic illumination according to the invention in connection with the phase shift method result from FIGS. 6 and 11, 12 as follows:

The light sources that are diffusely reflected by a vibrating or moving object are stored after their superposition as a speckle interferogram, which you can choose from the superimposition of a multitude of single interferograms emerged, each represents an instantaneous state of the moving object. The superposition the individual intensities result from the fact that the speed of the object  motion is small compared to the speed of light. In differential steps the intermediate states of the object are thus recorded.

According to the invention, the real-time difference technique with a constantly renewed reference image means that the stripe pattern of a vibrating object which is modulated by a stripe system and is described with J _o 2 (Ω) instead of [1 - J _o (Ω)], in which J _o the Bessel function of the first kind of zero order and Ω = δx (4π / λ) (∂w / ∂x) mean can be observed in real time. The contrast of the stripes is much better than with the real-time difference method with a fixed reference image. Furthermore, the time interval between taking the current and the reference images is much shorter. Therefore, low-frequency disturbances due to heat streaks and low-frequency vibrations are largely suppressed.

With the introduction of stroboscopic lighting that provides acousto-optical Using the modulator (AOM), the interferogram, namely the speckle pattern, can be used in every point of the vibration process can be frozen by holding the object with short stroboscopic flashes of light that are synchronized with the vibration excitation, is illuminated; it can be evaluated using the phase shifting technique Obtain phase distribution of the shearogram for a vibrating object.

The relative phase distributions Φ _α (in the state of deformation 1) and Φ _β (in the state of deformation 2) can be determined within the limits of 2π, so that the calculated relative phase difference Δ = Φ _α - Φ _{β is} in the range between 0 and 2π and the Phase distribution is a modulo 2π stripe pattern. The black digits correspond to zero phase values and the white digits correspond to 2π phase values. A demodulated phase distribution can be determined using a phase demodulation algorithm, which is a prerequisite for automatic and numerical evaluation.

When an RF voltage is applied to the AOM, it is activated for the laser lighting. In the device according to FIG. 6, the control unit generates the control signal for the piezoelectric vibration exciter at the object under examination from an external signal source (computer-controlled function generator) and synchronously with the control signal that can be set in phase position and duration for opening the ultrasound-vibrating grating in the AOM.

The connection of an external signal source as an output variable has the advantage that per Manual or computer adjustment through the resonance points of the examination object can drive.

As a basic requirement, the control unit must always have the exposure time back to the same point on the excitation curve, even if the frequency of arousal changes. So there must be a phase-locked connection between the pathogen sig nal of the test object and the opening signal of the AOM.

In addition, the tent point of the lighting on the excitation curve should be in degrees be adjustable (0-360 °). The opening time of the AOM grille should also be in degrees steps must be set (1-90 °).

To generate the excitation signal, 360 input pulses (f _in ) are counted onto a binary counter according to FIG. 11. The parallel counter outputs then serve as address lines for a memory E-PROM. The values of a sine function are stored digitally in the E-PROM, which are read out one after the other by counting up the counter values. Then the digital values are sent to the outside via a D / A conversion and an amplifier to control the piezo amplifier to excite the test object.

While each input pulse (f _in ) generates a value of the exciter sine curve, a delay in the circuit is started at the same time as the value 00, which after an adjustable number of input pulses (ie to a specific value of the exciter sine curve) the RF modulator causes the AOM to open the grating.

This can be 1/360 at every. Part of the sine curve (i.e., 1 °). After expiration of again adjustable 1-90 input pulses (≅ 1-90 ° of the sine curve) the vibrating grille of the AOM is brought to rest. After 360 entrance pulses (i.e. a complete sine period) are all counters and delays register is deleted and the process is repeated.  

For easier operation, there is a monitor output on the HF part of the device, which can independently measure and, if necessary, display the exit gate. An external entrance allows the AOM to be used even without the control unit functioning.

Due to the internal structure, the input frequency must be exactly 360 times greater than that desired excitation frequency. An optional frequency display of the excitation frequency allows easier adjustment of the resonance points of the test specimen.

The control unit is installed in a standard electronics housing measuring 500 × 140 × 320 mm³ ( Fig. 12).

There are two function boards internally

• - gate generation
• - digital sine wave generator

as well as a power supply unit and the purchased RF modulator for the acousto-optical modulator (AOM) in the housing. A frequency display of the excitation frequency (f _out ) is optionally installed and can be read on the front panel. It would also be conceivable to install an adjustable rectangular generator for the internal generation of the input pulses (f _in ) in the housing.

The controls such as are located on the front panel

• - Reset button
• - 3-digit thumbwheel switch for 0-360 ° deceleration
• - Thumb wheel switch 2-digit for deceleration 1-99 °
• - Rotary potentiometer for the output amplitude of f _out
• - power switch

and the BNC connection sockets for

• - The input pulses f _in
• - the excitation frequency f _out
• - Monitor output for the gate signal (TTL level)
• - RF output for connecting the AOM
• - External control input for RF modulator (0-5 V).

The mains connection is on the back of the device.

Claims (9)

1. A method for shearographically determining derivatives of the deformation of a surface of a vibrating object in at least one selected direction, in which the object surface is irradiated with coherent light and the light diffusely reflected by this is imaged in an image plane by means of an optical system having a shearing element and at least one zero shea program and one load shea program are recorded from the object surface and both shea programs are subtracted from one another, characterized in that a reference shea program recorded in the respective previous video cycle is used instead of the zero shea program. 2. The method according to claim 1, characterized in that the phase of the loading Shearogram compared to the reference shearogram by a preselected one Value, for example by 90 °, is shifted. 3. The method according to claim 1 or 2, characterized in that the result of Subtraction, in particular the absolute value of the subtraction result, squared and then made visible on the screen. 4. Method for shearographically determining derivatives of the deformation of a Surface of a vibrating object in at least one selected direction, in which the surface of the object is irradiated with coherent light and diffusely reflected by it Light by means of an optical system having a shearing element in one Mapped image plane and at least one zero shea program and of the object surface a stress shea program is recorded, characterized in that the object vibrated and the laser beam is controlled synchronously so that the Exposure of the object takes place only during preselected vibration conditions. 5. The method according to claim 1, characterized in that control signals for control the vibrations of the object and the synchronous control of the switch-on times and durations of the laser beam are generated.   6. Device for the shearographic determination of derivatives of the deformation Surface of a vibratable object, characterized in that it to carry out the method according to one or more of claims 1 to 5 is set up. 7. The device according to claim 6, characterized in that it additionally means for Has phase shift. 8. The device according to claim 6 or 7, characterized in that it has means for optionally has permanent activation of the laser beam. 9. The device according to claim 7, characterized in that the means an acousto- optical modulator (AOM) included.