EP1412737A1

EP1412737A1 - Defect identification in bodies consisting of brittle material

Info

Publication number: EP1412737A1
Application number: EP02754343A
Authority: EP
Inventors: Stefan Maetschke; Thomas Völkel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2001-07-10
Filing date: 2002-07-10
Publication date: 2004-04-28
Also published as: US20030167845A1; WO2003006983A1; DE10133510C1

Abstract

The invention relates to a non-linear method, which permits a simple, rapid and precise defect identification in a body consisting of brittle materials. According to said method, the body is subjected (1a, 2b) to acoustic vibrations of different intensities in at least two temporally staggered operations (A, B), a vibration response of the body is recorded (2a, 2b) in the time range, the vibration response is normed (3a, 3b), the start of the vibration is determined (4a, 4b) and a section of the vibration response is determined (5a, 5b), the start of said section forming the previously determined start of the vibration and said section having a specific length. A correlation coefficient of the sections of the respective vibration responses of the two or more operations constitutes (6) a characteristic value for the defect identification.

Description

description

Fault detection in bodies made of brittle materials

The invention relates to a method for the detection of

Defects in bodies made of brittle materials, e.g. B. made of glass or ceramic.

In addition to visual, vibroacoustic methods are also used to detect defects in bodies made of brittle materials, in particular for crack detection in roof tiles. To do this, a roof tile is vibrated using a mechanism, the vibration is recorded and then evaluated. In principle, the evaluation can be carried out using two different methods. With the non-linear

Methods described in the article Johnson, P., The new wave in Acoustic Testing, Materials World, September 1999, a brick is excited at least twice with different strength or different frequencies. If the brick is defective, a non-linear shift of the resonance frequencies depending on the excitation can be observed, while the resonance frequencies of the perfect brick show no shift. This effect is not taken into account in the linear method (e.g. by measuring the RMS value of the vibration signal).

On the other hand, it is advantageous with the linear method that a simple stop of the tile is sufficient. It is disadvantageous that linear methods generally have to be specially adapted for each type of brick (shape, color) (characteristic, threshold values) in order to correctly recognize defective ones

Ensure bricks. If the above-mentioned non-linear method is used for the brick test, the brick must be hit several times. For each attack, the vibration of the brick must be recorded, the Fast Fourier Transformation (FFT) calculated, the resonance frequencies identified and the shift in the resonance frequencies determined. In particular the calculation of the FFT and the Identification of the resonance frequencies is complex and involves inaccuracies, which is reflected in the quality of the crack detection.

The invention has for its object to provide a non-linear method which allows simple, fast and accurate error detection in a body made of brittle materials.

This object is achieved by a method for the detection of defects in a body made of brittle materials, in which method the body in at least two temporally staggered processes

Is vibrated with different strength,

A vibration response of the body is recorded in the time domain,

• the vibration response is standardized,

• the beginning of the oscillation is determined and • a section of the oscillation response is determined, the beginning of which forms the previously determined beginning of the oscillation and which has a specific length, a correlation coefficient of the sections of the respective oscillation responses of the at least two processes being formed as a characteristic value for the error detection becomes.

The method according to the invention is based on the non-linear method mentioned above, but is characterized by greater simplicity, greater accuracy and faster calculation. The body made of brittle material is vibrated in (at least) two temporally staggered processes, hereinafter also referred to as stop processes, with different strengths and the vibration responses of the body are recorded. Then the two vibration responses are standardized (e.g. with mean = 0, standard deviation = 1) and the beginning (the starting point) of the actual vibration is determined. outgoing From the starting point found, a short part (typically 500 to 2000 measured values) of the signal - the recording of the vibration response - is cut out and the correlation coefficient between the section from the first striking process and the section from the second

Velocity determined. The method according to the invention works directly in the time domain on a short section of the vibration response and determines the correlation between the vibration responses of two or more stroke processes. The inaccuracies in determining the

Shifting of the resonance frequencies and the calculation of the FFT in the above-mentioned non-linear method are thus avoided. Since only part of the vibration response is required for the evaluation, the two stops can be carried out very quickly in succession and the analysis time can be shortened compared to previously known methods.

The beginning of the oscillation is determined in a simple manner, starting from the beginning of the recording of the oscillation response, determining the point in time at which the value of the oscillation response exceeds a certain threshold value (for example twice the standard deviation) for the first time. With exact triggering, the calculation of the starting times can be dispensed with and the start of the oscillation can be automatically based on the

Time is determined in which the body is vibrated. As a rule, however, the trigger mechanism is less precise than the computational method.

The detection of an error in the body advantageously takes place in that the feature value for the error detection is compared with a feature value of an error-free body previously determined in the same way, a feature value which is lower in comparison to the feature value of the error-free body indicating an error in the body. This characteristic value is lower for a defective body (e.g. r = 0.36) than for a faultless body (r = 0.67), since here the correlation between the first and second stroke process is lower.

Because the determination of the starting point is associated with a certain error, it is proposed that others

Excerpts of the vibration response are determined, the beginning of which is shifted relative to the beginning of the previously determined excerpt and which have a certain length, an average value being formed from correlation coefficients of the further excerpts of the respective vibration responses of the at least two processes as a feature value for error detection. For example, four further correlation coefficients are calculated, which result from the virtual shift of the starting points by one or two measured values to the left and to the right. The final characteristic value, which is used to identify defective bodies, is then calculated from the mean of the five correlation coefficients.

The proposed method can also be carried out if the body is vibrated at different frequencies instead of with different strengths in at least two temporally staggered processes. The body's vibration response is typically recorded acoustically or with acceleration sensors. Bodies made of brittle materials are bodies made of glass, made of ceramic materials, and the like. Ä. Typical errors that are recognized by the process are cracks and irregularities in the structure of the material. The method for crack detection in bricks is particularly suitable.

An advantage of the proposed method is that the two stops can take place very shortly after one another, since only a short section of the vibration response is required for the evaluation. There is no need to wait for the signal to decay completely. The invention is described and explained in more detail below on the basis of the exemplary embodiment illustrated in the figures.

Show it:

1 shows a flowchart of the method for detecting defects in a body made of brittle materials with two staggered stop processes,

2 typical vibration responses of a body when the body is struck twice,

3 shows an example of a standardized vibration response with the threshold value shown,

4 shows a section of the vibration response of a faultless body after a hard impact,

5 shows a section of the vibration response of the faultless body after a soft stroke,

6 shows a section of the vibration response of a faulty body after a hard stop,

7 shows a section of the vibration response of a faulty body after a soft stop and

8 shows a graphical representation of the values of mean correlation coefficients for fault-free and faulty bodies determined using the described method.

1 shows, by way of example, a flow chart of the method for detecting defects in a body made of brittle materials with two time-delayed stop processes A, B. A vertical time axis 10 is shown, on which a first time t1 indicates the start of a first process A and a second time t2 the start of a second process B. The process A comprises five steps, which are identified in their chronological order by the reference numerals la to 5a, the process B accordingly comprises five steps lb to 5b at different times. Both processes A, B merge into a common path, the steps of which are identified by reference numerals 6 and 7.

The sequence of the proposed method is described and explained in the following using FIG. 1 as an example. The method is suitable for the detection of defects on bodies made of brittle materials, in the example it is to be used for the detection of cracks in roof tiles. The

Roof tiles thus serve as a typical example of a body made of brittle materials, but the statements made about the roof tiles can certainly be transferred to other brittle bodies, such as glass or other ceramic materials. The first process A of the method begins at time t1 with the first

Step la, the second process B correspondingly at time t2 with the first step lb. In the following, both processes A, B are described together, since they have equivalent steps, albeit at different times. In step la, lb, the roof tile is vibrated using a suitable mechanism. He is z. B. struck by a stop mechanism with a certain strength. The vibrations that the brick then executes, that is to say its vibration response, are recorded in the second step 2a, 2b via acoustic sensors or via acceleration sensors. In the following third step 3a, 3b the vibration response is normalized (e.g. with mean = 0, standard deviation = 1), in the fourth step 4a, 4b the start (the starting point) of the actual vibration is determined. Starting from the starting point found, in the fifth step 5a, 5b a short part (typically a sequence with five hundred to two thousand measured values) of the Signals - ie the recording of the vibration response - cut out. After the fifth step 5a, 5b has been completed, a section of the vibration response of the first process A or the second process B is thus available in the exemplary embodiment. The two processes A, B differ not only in terms of their starting time but also in terms of the strength with which the striking mechanism vibrates the brick. In the first process A, the brick is chipped hard in the first step la, in the subsequent first step 1b of the second process B the same brick is chipped comparatively softly. The time difference between the times t1 and t2 is chosen so that the beginning of the first vibration response can be recorded before the second vibration excitation takes place. It is not necessary to wait at the start of the second process B until the first vibration has completely subsided. The two present sections of the standardized vibration responses are now correlated with one another in the sixth step 6 of the method. The calculated result of the correlation is a correlation coefficient, which is a measure of the agreement between the two vibration responses. The correlation coefficient serves as a characteristic value for error detection. Flawless, intact roof tiles have a higher correlation coefficient value than faulty roof tiles, e.g. B. roof tiles with a crack. If you know the typical range of values in which the correlation coefficient that can be determined by the proposed method is for faultless bricks, a further determined correlation coefficient can be compared with this typical range of values and a statement can be made as to whether the brick tested with the method is error-free (value of Correlation coefficients within the typical value range) or incorrect (value of the correlation coefficient below the typical

Value range). This check for errors is carried out in the seventh step 7 of the method. The Steps 3a, 3b to 7 are processed in a computer to which the recorded vibration responses are fed as signals. The bricks assessed as defective with the aid of the described method are subjected to an additional visual inspection in further steps, which are not shown graphically here, or are immediately sorted out as unusable.

2 shows typical vibration responses S2 of the brick when the brick is struck twice. The diagram D2 shown in FIG. 2 shows the amplitude of the signal of the vibration responses S2 against the vertical axis 11 and the time against the horizontal axis 12. The times t1, t2 already shown in the flowchart in FIG. 1 are on the horizontal axis 12 with the same

Reference numerals. At the first time t1, the brick is set into vibration, the amplitude of which has almost completely decayed at the second point in time t2 when the brick is set into vibration for the second time. Since the brick is hit harder the first time than the second time, the maximum amplitude of the vibration response S2 is visibly larger the first time.

FIG. 3 shows an example of a normalized vibration response S3 with the threshold value 15 shown in a diagram D3. The normalized amplitude of the vibration response S3 is plotted against the vertical axis 13, the time against the horizontal axis 14. In the fourth step 4a, 4b of the method, the start of the vibration response S3 is determined, for example, by determining the point in time at which the normalized value of the vibration response S3 exceeds the previously defined threshold value 15 for the first time. In the exemplary embodiment, the threshold value 15 is fixed at the value of twice the standard deviation. FIG. 4 and FIG. 5 show sections 24, 25 from vibration responses of an error-free brick, FIG. 4 for a hard stop, FIG. 5 for a subsequent soft stop. FIG. 6 and FIG. 7 show corresponding sections 26, 27 from vibration responses of a defective brick, again for a hard stop (FIG. 6) and for a soft stop (FIG. 7). The normalized amplitude of the vibration responses is plotted against the vertical axis 16 and the time against the horizontal axis 17 in FIGS. 4 to 7. The excerpts 24 to 27 of vibration responses graphically reproduced in FIG. 4 to FIG. 7 are the result of the fifth step 5a, 5b of the method. In the process, the excerpts are available as data sets, e.g. B. in the form of tables. These data sets are correlated with one another in sixth step 6. It can be seen with the naked eye that the

Cutouts 24, 25 of the vibration responses of the faultless brick correlate more strongly than the corresponding cutouts 26, 27 of the faulty brick. This is also the result of the calculation of the respective correlation coefficients with the data quantities on which the graphical representations are based. In the example, the correlation coefficient of the vibration responses of the faultless brick has the value 0.67, the correlation coefficient of the vibration responses of the faulty brick is only 0.36, which is significantly lower.

Because the determination of the starting point of the vibration in the vibration responses has a certain error, four further correlation coefficients are calculated in the exemplary embodiment, which result from the virtual displacement of the starting points by one or two measured values to the left and to the right. In the example, the measurement signal of the vibration response is sampled at discrete points in time, ie there are measured values that are equidistant in time. The final feature value 20, 22, which is used to identify defective bricks, is calculated in this case from the mean of the five calculated correlation coefficients. These characteristic values 20, 22 are plotted graphically in FIG. The averaged correlation coefficient of the respective feature value 20, 22 is plotted against the vertical axis 18, and the number of the tested brick is plotted against the horizontal axis 19. In total, the feature values 20, 22 of sixty different bricks are shown. The characteristic values 20 of thirty-three defective bricks can be found in the area designated by reference numeral 21, whereas the characteristic values 22 of twenty-seven defect-free bricks are found in the area designated by reference numeral 23. It can be clearly seen that the averaged correlation coefficient of the vibration responses of the flawless bricks has significantly higher values than the corresponding correlation coefficient of the faulty bricks. The feature values 20, 22 determined using the described method can therefore be used to distinguish between faultless and faulty bricks.

In summary, the invention thus relates to a nonlinear method which allows simple, fast and accurate error detection in a body made of brittle materials. In the method, the body is set into vibration with different strength in at least two time-shifted processes A, B la, 2b, if a vibration response of the body is recorded in the time domain 2a, 2b, if the vibration response is normalized 3a, 3b, the start of the Vibration determines 4a, 4b and a section of the vibration response is determined 5a, 5b, the beginning of which forms the previously determined start of the vibration and which has a specific length, a correlation coefficient of the sections of the respective vibration responses of the at least two processes being formed as a feature value for the error detection turns 6.

Claims

claims

1. A method for detecting defects in a body made of brittle materials, in which method the body in at least two staggered processes (A, B)

Is vibrated with different strength (la, lb),

• a vibration response of the body is recorded in the time domain (2a, 2b), • the vibration response is standardized (3a, 3b),

• the start of the oscillation is determined (4a, 4b) and

A section of the oscillation response is determined (5a, 5b), the beginning of which forms the previously determined start of the oscillation and which has a specific length, a correlation coefficient of the sections of the respective oscillation responses of the at least two processes being formed as a characteristic value for error detection (6 ).

2. The method according to claim 1, so that the start of the oscillation is determined by starting from the beginning of the recording of the oscillation response, determining a point in time at which a value of the oscillation response exceeds a specific threshold value (15) for the first time.

3. The method according to claim 1 or 2, d a d u r c h g e k e n e z e i c h n e t that the feature value for the error detection is compared with a feature value of an error-free body previously determined in the same way, wherein a feature value lower than the feature value of the error-free body indicates an error of the body.

4. The method according to any one of the preceding claims, characterized in that further sections of the vibration response are determined, the beginning of each of which compared to the beginning of the previous certain section is shifted and have a certain length, a mean value being formed from correlation coefficients of the further sections of the respective vibration responses of the at least two processes as a feature value for error detection.

5. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the vibration response of the body is recorded acoustically.

6. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the vibration response of the body is recorded with acceleration sensors.

7. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the method is used to detect defects in a body made of glass.

8. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the method is used to detect defects in a body made of ceramic materials.

9. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the start of the vibration is automatically determined based on the time at which the body is set in vibration.

10. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the body stagger in at least two times

Processes with different frequency is vibrated.