Abstract:
A method and circuit arrangement for processing signals which are produced during disturbance-free examination of objects such as pipes or sheet metal, by reflecting ultrasonic waves at defective spots on the structure of the object. According to the method, a complete wave front is emitted on at least one section of the object which is to be examined by means of a plurality of independent transmitting elements, a wave reflected by the structure of the object is received by means of a plurality of receiving elements which are independent of each other, the signals received by the receiving elements are digitalized, and the digitalized signals are stored in a storage element according to amplitude and propagation time. In order to detect defective points on the structure of the object in a faster manner with improved signal/noise ratio, the defective points are detected by a phase-locked addition of the stored amplitude values of the propagation time.

Description:
This application is a filing under 35 USC 371 of PCT/EP2004/008048 filed Jul. 19, 2004. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a method for processing signals and to a circuit arrangement. 
   A method for processing signals which correspond to reflected waves which are transmitted or interrupted by a structure in order to be able to gather information and analyze the structure of this material is described in EP 0 825 453 B1. With the method described, the fact that every point of an object to be examined leads to a reflected wave which is then stored at positions of the storage unit which are distributed in the form of parabolic curves, the characteristics of which depend on the distance of the point relative to the probe and on the radiation pattern of each element. In the known method, a probe is used which is configured linearly and composed of a plurality of transmitter/receiver elements of small dimensions. The same probe is then used for transmission and reception. First, a wave front is transmitted into the object to be examined and those waves which are reflected by the structure are received. 
   The information which is emitted by the sensor elements is then digitalized and stored and, more particularly, is stored in a storage element which has a line for each sensor element. 
   Subsequently, the structure of the object is reconstructed and/or analyzed with reference to the information that was stored in the storage elements. For each point of the object the position of the storage element is computed which contains the signals which are located by the sensor elements and correspond to the waves reflected or transmitted from this point. This position is computed with aid of an addressing rule whose parameters depend on the position of this point in relation to the sensor elements. Furthermore, the lines of the field storage are read for each point at the respective positions which were previously computed for this point, a mathematical operation being used for the values read for this point in order to obtain a result which is representative for the range of the wave. For the computation, all cells of the storage field are read in parallel for this point at the positions which are designated for this point in the respective allocated addressed storage units. 
   Since the computation of each reading principle for reading out the storage would take too long to effect in real time, this computation is done in advance and the results stored in specific “address storage units” which are allocated to each line of the “field storage unit”. 
   Hence, it follows that the described method is only suitable for identifying specific, i.e. previously defined structures. After performing the mathematical operation, the contents of the field storage unit are stored in image storage unit, location and propagation time information being taken into consideration. In the method, computed B images are evaluated, as is common, for example, in medical diagnostics. However, for automatic material examination, B images should not be referred to since long evaluation times are required for the evaluation. 
   Furthermore, it should be noted that there are limits in defect detection with vertical acoustic irradiation since only one defective position can be detected. Furthermore, uncertainties occur in the defect evaluation, since fixed geometric relations between the probe and the test piece are a prerequisite, since location information is required for the defect determination. Therefore, the known method is very sensitive to conventional misalignments of the probe. 
   SUMMARY OF THE INVENTION 
   Based on this, the object of the present invention is to further develop a method and a circuit arrangement of the known type in such a way that defect locations of the structure of the object can be detected at high speed and with an improved signal/noise ratio. Furthermore, the method should be insensitive to possible misalignments of the probe. 
   It is thereby provided that a defect location is determined by a phase-locked addition of the stored amplitude values received at equal propagation times. Use is hereby made of the fact that, when an object is radiated by means of a probe configured as a Phase Array Transducer, i.e. when emitting a complete wave front, echoes are received in each of the receiver elements of the probe switched simultaneously to receive, the amplitude values of the received echoes being received at the same propagation times. This offers the possibility of adding the amplitude values of the received signals along a propagation time, with the advantage that the amplitude peaks identifying the defect location in the structure are amplified and that the additional signals received by the individual receiver elements almost cancel one another. 
   Consequently, the method according to the invention is distinguished, on the one hand, by a very high speed of defect identification, since the sequential method common in the prior art is avoided due to the emission of a complete wave front and, on the other hand, by an improved signal/noise ratio being obtained in comparison to the prior art. The method is also robust against e.g. a misalignment of the probe, since location information is not involved in the method. 
   To identify defect locations on an outer surface of the object (outer field), in particular during the non-destructive examination of pipes by means of ultrasound, point-wave signals which proceed from reflections at outer defects of the object are evaluated. An addition of the amplitude values stored in the storage unit does thereby not take place along one and the same propagation time, but in a direction which extends at a right angle, or essentially at a right angle, to the interference patterns of the received amplitude values of the point-waves of the outer defects AF. 
   Due to the phase-locked addition of the amplitude values along a propagation time, defect locations can generally be identified, whereby, however, information about whether or not it is an outer defect or an inner defect cannot be derived from the sum signal. As previously stated, outer defects can be identified by adding amplitude values at a right angle or essentially at a right angle to interference patterns of the point-waves emanating from the outer defect. Finally, a comparison of the signal detected during the phase-locked addition of stored amplitude values along a propagation time with the signal detected during the addition of the amplitude values of the interference patterns takes place due to a coincidence circuit, in which an outer defect is present when both signals indicate a defect location. 
   To set a beam angle α adapted to the measuring conditions, e.g. type of defect (longitudinal defect, inclined defect), property of the material and the shape of the object to be examined, the individual transmitter elements of the Phased Array Transducers can be controlled in a time-delayed manner (phasing). When examining e.g. a pipe, an ultrasonic irradiation can take place in dependence on a defect location (inclined defect, longitudinal defect), optionally in the longitudinal direction or in the peripheral direction. 
   Furthermore, it is provided that, to determine the location of the defect, a propagation time dependent amplitude correction of the sum signal determined during the addition can be carried out, the received sum signal being compared with a reference value. 
   To further reduce the data, it is provided that the received signals are filtered immediately after the digitalization, preferably by means of a wavelet filter. 
   The object is solved by a circuit arrangement in which a summer is provided for the phase-locked addition of the amplitude values stored in the storage module. 
   Furthermore, the invention relates to a method for the non-destructive examination of a contour, such as an edge, bend or curvature of an object. 
   According to the prior art, contours of the aforementioned type in the form of edges are examined by ultrasound with an essentially planar wave front. An interference pattern corresponding essentially to the contour of the object to be examined is thereby received, but the interference pattern is unsuitable for evaluating defect locations in the structure of the object. 
   Based on this, the object of the invention is to provide a method for the non-destructive examination of a contour of an object by means of ultrasonic waves which enables an improved identification of defect points in the structure of the object in the area of the contour. 
   To achieve the object, it is proposed that the contour of a surface of the section of the object to be examined is detected and stored, that the independent transmitter elements are controlled in a time-delayed manner in such a way that the emanating wave front extends parallel or approximately parallel to the surface contour and that the waves reflected by the object are received in a time-delayed manner and produce an essentially planar interference pattern. 
   In a preferred procedure, it is provided that the contour of the object is identified by transmission a wave front which is planar relative to a transmission plane to the contour to be examined, that the waves reflected by the contour of the object are received and digitalized by means of the plurality of receiver elements that are independent of one another and the digitalized signals are stored in the storage element at least according to their propagation time and that the contour of the object is computed from the defined distance A of the probe to the object and the different propagation times of the received signals. 
   The method is distinguished by a time-delayed actuation of the independent transmitter elements which transmit a wave front in such a way that it extends parallel or essentially parallel to a surface of the structure of the object to be examined. 
   When the wave reflected by the contour of the object is received, a time-delayed reception by means of the controllable receiver elements also occurs, as a result of which signals are received which generate an interference pattern extending parallel or essentially parallel to the surface of the contour of the object. Defect locations of the structure of the object can then be easily identified in this interference pattern. 
   Due to fluctuations in production, irregularities can occur in the contour of the object to be examined, so that, according to a further preferred procedure, it is provided that the contour of the object is preferably identified on-line, i.e. during a measurement process. For this purpose, it is provided that a planar wave front is transmitted onto the contour to be examined, that the waves reflected by the contour of the object are received and digitalized by means of the plurality of receiver elements which independent of one another and the digitalized signals are stored at least according to propagation time in a storage element and that the contour of the object is computed from the defined distance of the probe to the object and the varying propagation times of the received signals. 
   By the on-line resetting or adjustment to the changing contour of the object, an always uniform evaluation of defect locations is possible. 
   The readjustment is made possible by comparing the received interference pattern extending parallel or essentially parallel to the contour of the surface of the object with a desired pattern, a wave front which is parallel or essentially parallel relative to the plane of the transmitter element being emitted when the received interference pattern deviates from the desired pattern in order to compute the actual contour of the object to be examined from the reflected waves. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details, advantages and features of the invention can be found not only in the claims, the features found therein—alone and/or in combination—but also in the following description of a preferred embodiment that can be found in the drawings, in which: 
       FIG. 1  shows a schematic representation of a wave front emanating from a probe (Phase Array Transducer) through a medium such as water onto an object to be examined, e.g. a steel plate, 
       FIG. 2  shows a representation of amplitude values of the amplitudes received from the individual receiver elements EL 1 -ELN of a probe with varying propagation times (typical B-scan), 
       FIG. 3  shows a representation of selected ray paths of the ultrasonic wave front according to  FIG. 1 , 
       FIG. 4  shows a representation of the amplitude curve over the propagation time of an individual receiver element ELX according to  FIG. 2 , 
       FIG. 5  shows a representation of the amplitude curve of the sums of amplitude values having the same propagation times of various receiver elements for the evaluation of a defect location, 
       FIG. 6  shows a test assembly in principle for the non-destructive examination of a pipe, 
       FIG. 7  shows a schematic block diagram of a circuit arrangement for processing the received signals, 
       FIG. 8  shows a basic representation of the ray paths of a defect location situated on the outer surface, 
       FIG. 9  shows a basic representation of the ray paths of a defect location situated on the inner surface, 
       FIG. 10   a  shows a schematic representation of a wave front emanating from a probe for determining the contour of an object to be examined, 
       FIG. 10   b  shows a schematic representation of a received interference pattern (B-scan) of the contour of the object to be examined, 
       FIG. 11   a  shows a schematic representation of a wave front emanating from a probe which extends parallel or essentially parallel to the surface of the contour of the object to be examined, and 
       FIG. 11   b  shows a schematic representation of an essentially planar interference pattern for the simple evaluation of defect locations. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows, in a purely diagrammatic manner, an arrangement  10  for the non-destructive examination of an object  12  which is shown as a steel plate in the illustrated embodiment, by means of an ultrasonic wave front  16  emanating from a probe  14 , the ultrasonic wave front being coupled into the object  12  through a liquid medium  18  such as water. Defect locations  20  in the structure of the object  12  are to be detected by the examination. 
   The probe  14  comprises a plurality of transmitter/receiver elements EL 1  to ELN which are each preferably switched simultaneously or phase-displaced as transmitter elements and preferably simultaneously as receiver elements. 
   With reference to a grey-scale image,  FIG. 2  shows a conventional B-scan of an outer defect location of a planar object which was taken with a Phased Array Transducer at a beam angle of α=18°. The wave front transmitted onto the object  12  was generated by simultaneous switching on of all transmitter elements. 
   The individual receiver elements EL 1 -ELN are plotted over the horizontal axis, the signals received from the individual receiver elements being shown according to their propagation time which is plotted over a vertical axis, in succession in the vertical direction. 
   The intensity of the illustrated signals is defined by their brightness. It can be seen in the B-scan of  FIG. 2  that different structures of the object generate different signal patterns. Thus, the surface of the object  12  generates surface echoes OE which are shown as diagonally extending, continuous lines in  FIG. 2 . The defect echoes FE 1 -FE 12  allocated to the individual receiver elements EL 1 -ELN are shown as horizontal lines. Thus, for example, the defect echoes FE 1  to FE 4  of a receiver element ELX and the defect echoes FE 5  to FE 8  or the defect echoes FE 9  to FE  12  of additional receiver elements ELX 1 , ELX 2 , respectively, are shown. In the upper left area of the B-scan, interference echoes IE can be identified which are produced due to interferences between wave fronts of the individual transmitter elements of the Phase Array Transducer. 
   Furthermore, the B-scan shown in  FIG. 2  shows that defect echoes of several receiver elements, e.g. the defect echoes FE 1 , FE 5  and FE 3 , FE 6 , FE 9  as well as FE 4 , FE 7  and FE 10 , are each received at equal propagation times. The same propagation times of, for example, the defect echoes FE 2  and FE 5  as well as FE 3  and FE 6  are to be described with reference to  FIG. 3 . 
   The structure according to  FIG. 1  is shown in  FIG. 3 , signal paths a to i being shown purely diagrammatically. The thickness of the steel plate is designated by s and the ray path within the object  12  by t. 
   It appears that the propagation time of the defect echo FE 2  from the sections d+2t+e and the propagation time of the defect echo FE 5  from the sections or propagation times e+2t+d each comprise an identical propagation time. The propagation times of the defect echo FE 3  and defect echo FE 6  are also identical, it being noted that they are received in different receiver elements. 
   The signals received from a receiver element ELX can also be shown in their amplitude over time. A corresponding illustration (A-scan) is shown in  FIG. 4 . In this case, the individual echoes OE, FE 1  to FE 4  are shown as amplitude swings at different propagation times. According to the method of the invention, the amplitude values of the signals received at the same propagation times, e.g. FE 2  and FE 5 , FE 3 , FE 6 , FE 9 , or FE 4 , FE 7  and FE 10 , are added along their propagation time, the signal shown in  FIG. 5  being obtained over the time which very clearly represents information about the field position  20  in the object  12  by an amplitude increase in the range of an order of magnitude. 
   Since the defect echoes received from various elements ELX, ELX 1  or ELX 2  have the same propagation time, the amplitude values are added, so that the sum signal increases. Since the noise signals contained in the A-scan of  FIG. 4  cancel one another stochastically, the signal/noise ratio is improved. 
     FIG. 6  shows a test assembly  22  for the non-destructive examination of a pipe wall  24  by means of probes  26 ,  28  which are arranged along the periphery of the pipe and have curved surfaces  30 ,  32  adapted to the surface of the pipe  24 , to which the transmitter/receiver elements EL 1  to ELN, N being e.g. 128, of each element transmitter/receiver element EL 1 -ELN can be switched both as a transmitter and as a receiver (sic). The spatial extent of an element is adapted to the ultrasonic frequency f used, which is in the range of 0.2 MHz≦f≦20 MHz, preferably f=6 MHz. 
   The probes  26 ,  28 , which are also designated as Phased Array Transducers, are each connected via data lines  34 ,  36  with a first signal processing unit  38 ,  40  which controls the transmitter/receiver element EL of the probes  26 ,  28  and amplifies and digitalizes the received signals. The signal processing units  38 ,  40  are each connected to a signal evaluation unit  46 ,  48  via a data connection  42 ,  44 , which can e.g. be configured as FSL (Fast Serial Link). Furthermore, the circuit arrangement comprises an interface  50  for connection to an external personal computer  52  and a microprocessor unit  54  for processing and evaluating (sorting, marking) the information from the units  46  and  48 . 
     FIG. 7  shows a detailed block diagram of the components of the circuit arrangement  22  according to  FIG. 6 . The signal processing unit  38  comprises a pulse unit PE for controlling the transmitter/receiver elements EL 1 -EL 128 , which comprise 128 elements in the present example. The transmitter elements are thereby controlled according to a definite time pattern (phasing). The receiver elements are subsequently switched to simultaneous reception and the received analog signals are then conducted via a multiplexer MUX to corresponding A/D converters AD, in the present example, to each of 32 A/D converters. The received analog signals are digitalized by the A/D converters AD and stored in a storage element SP as a so-called A-image or A-scan. The e.g. 32 A-scans stored in the storage element SP according to  FIG. 4  are then added during their propagation time in a summing element SUM, according to the invention by a phase-locked addition of the amplitude values, to form the A-scan shown in  FIG. 5  in their propagation time, whereby the advantage results that the amplitude peaks add up, as a result of which the defect location is clearly accentuated and noise signals mutually cancelled. 
   Thus, the signal shown in  FIG. 5  is available at the output of the summing element SUM as an A-scan which is forwarded via an interface I of the further signal evaluation device  46  which also has an interface I. The B-scan is then forwarded to one or more evaluation modules F 1  F 2  in the signal evaluation device  46  for further signal processing. The individual components of the circuit units  38 ,  46  are controlled via a digital signal processor DSP which is connected with the personal computer  52  and with the microprocessor unit  54  via a bus coupler BK and the interface  50 . The results of the signal processing are displayed on a screen. 
   To obtain a data reduction prior to storing the signals applied to the A/D converters AD 1 -AD 32 , it can be provided that they be filtered, preferably wavelet filtered, prior to storing. 
   The method described thus far generally serves to quickly and effectively identify defect locations in an object. Since the location information of the defect location is lost during summation of the amplitude values along the same propagation times, further steps are required to enable a detection of an outer defect. For this purpose, it is proposed that the point-wave signals reflected by an outer defect of the object when examined with an ultrasonic wave be evaluated. 
     FIG. 8  shows strictly schematically the ray paths when the pipe  24 , which has a defect location AF in an outer surface  56 , is ultrasonically irradiated.  FIG. 9  shows the ray paths when the pipe  24 , which has a defect location IF on an inner surface  58 , is ultrasonically irradiated. The point-wave signals emanating from the outer defect AF in  FIG. 8  can also be seen in the B-scan shown in  FIG. 2 , namely as defect echoes FE 1  to FE 4  and their interference pattern. To identify an outer defect, a “diagonal” addition can be performed, i.e. not during a specific propagation time but at a right angle or approximately at a right angle to the interference patterns of the received amplitude values, so that an amplitude peak is produced in this direction which contains information about an outer defect AF. 
   To reach a decision about whether there is an inner or outer defect, an evaluation by a coincidence method of the sum signal of the amplitude values of the same propagation times with the sum signal of the amplitude values in direction at a right angle or approximately at a right angle to the interference patterns of the amplitude values received from the point-wave signals is effected, e.g. in one of the evaluation modules F 1 , F 2 , an outer defect being present if both signals show a defect location. However, if a defect location is identified during the phase-locked addition of the amplitude values along a propagation time without, however, a defect location being detected during addition of the amplitude values at a right angle or approximately at a right angle to the interference patterns, then there is an inner defect. 
   The following should be noted as special advantages of the method according to the invention: the signals obtained for evaluating the defect locations exhibit a better signal/noise ratio in comparison to the prior art and can thus be evaluated better automatically. As a result of the planar ultrasonic irradiation of the object to be examined on the basis of the use of a probe with Phased Array Technology, i.e. by simultaneous transmission of a wave front and simultaneous reading of the signals, a considerable time advantage is obtained in comparison to methods with sequential technology known from the prior art. 
   A data reduction also takes place by the phase-locked addition, so that the subsequent signal processing is also accelerated and simplified. 
     FIG. 10   a  shows a schematic representation of a probe  60  which corresponds in its structure to the probe  14  described above. A wave front  64  extending parallel or essentially parallel to a plane  62  fixed by the transmitter elements is emitted by the probe  60 . The wave front  64  hits an object  66  having a contour, e.g. an edge, bend or curvature, which, in the present example, has a first surface  68  which extends at an angle α relative to a second surface  70 . 
   To determine the contour of the object  60  to be examined, the waves reflected by the surfaces  68 ,  70  are received, digitalized and stored in a storage element SP corresponding to the circuit arrangement of  FIG. 7 . The contour of the object  66  can then be computed on the basis of a defined distance A from a reference point of the object  66  and the determined and stored propagation times of the signals. A corresponding interference pattern (B-scan)  72 , which essentially corresponds to the contour of the object  66 , is shown in  FIG. 10   b.    
   Based on the computed contour of the object  66 , the transmitter elements EL 1 -ELN are then controlled during the actual measuring process in a time-delayed manner in such a way that a wave front  74  emanating from the probe  60  extends parallel or essentially parallel to the contour, i.e. the surfaces  68 ,  70  of the object  60  to be examined. The waves reflected by the object  66  are also received in a time-displaced manner and from the signals thus received an interference pattern  76 , shown in  FIG. 11   b , is generated, the wave front of the interference pattern lying in a plane. 
   Defect locations can be easily identified from the data of this interference pattern. 
   To enable an adaptation to a changing contour of the object, it is provided that the received interference pattern  76  be compared with a desired pattern and that, when the received interference pattern  76  deviates from the desired pattern, a renewed determination of the actual contour of the object  66  takes place by e.g. a planar wave front  64  being sent onto the object  66  proceeding from the probe  60 , even during a measuring process, to undertake a renewed contour measurement, on which the further measurement is then based.