DE102014112368A1 - Measuring device for characterizing a test specimen by means of ultrasound - Google Patents

Abstract

A measuring device for characterizing a test specimen by means of ultrasound has at least one measuring head (1) with first and second piezo elements (3, 4). Transverse oscillations can be generated or detected in the device under test with the first piezo elements (3), while longitudinal vibrations can be generated or detected with the second piezo elements (4). The first and second piezoelectric elements (3) are arranged side by side in a regular pattern on the measuring head (1) and have the same external shape, so that the measuring head (1) can be equipped with either type of piezoelectric elements.

Description

FIELD OF THE INVENTION

The invention relates to a measuring device for characterizing a test specimen by means of ultrasound. The measuring device has at least one measuring head with at least one piezoelectric element. Each piezoelectric element has a stop body for application to the test specimen and a piezoelectric actuator for generating and / or detecting vibrations in the stop body.

BACKGROUND

The characterization of specimens by means of ultrasound makes it possible to non-invasively record various properties of the specimen. In particular, this technique may e.g. be used to determine the internal structure of the specimen or certain material properties thereof (e.g., sonic velocity or elastic modulus). The technique can be used in particular for quality testing of concrete.

A device suitable for ultrasonic characterization usually has at least one measuring head, in which at least one piezoelectric element is arranged. The piezoelectric element has a piezoelectric actuator for generating and / or detecting a vibration as well as a stop body, which can be applied to the specimen.

The piezo element can be designed to generate a longitudinal oscillation or a transverse oscillation (heavy wave) in the test object.

EP 1 394 538 describes a device with a measuring head with a relatively complex piezoelectric element, which is capable of selectively generating both longitudinal and transverse vibrations in the test piece.

PRESENTATION OF THE INVENTION

It has as its object to provide a device of the type mentioned, with which in the simplest possible way both transversal and longitudinal vibrations generated in the test specimen and / or can be measured.

This object is fulfilled by the measuring device according to claim 1. Accordingly, the measuring head has different first and second piezo elements. The piezo elements differ from each other at least in that the first piezo elements are designed to generate and / or detect transversal oscillations and the second piezo elements to generate and / or detect longitudinal oscillations in the test specimen. This makes it possible to generate both modes of vibration, but the individual piezo elements can be kept simple in their construction.

The measuring device can be designed to operate the piezo elements in each case in resonance. In this case, the resonance frequency for the transverse oscillation of the first piezoelectric elements advantageously differs from that for the longitudinal oscillation of the second piezoelectric elements by at most 20%, in particular by at most 10%. It can thus be achieved that the measurements with the transverse and longitudinal oscillations take place in the same spectral range in each case. In addition, the electronics for exciting and / or measuring the two vibration modes can each be optimized for the same frequency range.

The term "a piezo element is operated in resonance", is to be understood that the piezoelectric element is driven by a periodic signal whose frequency deviates at most by 10%, in particular at most by 5%, from a resonant frequency of the piezoelectric element.

By "resonance frequency" or "natural frequency" is advantageously the lowest (fundamental) resonant frequency of the piezoelectric element for each type of vibration to understand, i. the piezoelectric elements are preferably operated in their lowest resonance.

In a further embodiment, the piezoelectric actuators each extend between a first and a second end. In this case, the second end is connected to the respective stop body. The distance between the first and second ends defines the length of the piezoelectric actuator. In this case, the length of the piezoelectric actuators of the first piezoelectric elements is advantageously smaller than the length of the piezoelectric actuators of the second piezoelectric elements. Had the two types of actuators the same lengths, the fundamental transverse natural frequency of the piezoelectric actuators of the first piezoelectric elements would be significantly lower than the fundamental longitudinal natural frequency of the piezoelectric actuators of the second piezoelectric elements, which is disadvantageous for the reasons already mentioned. Due to the shorter design of the actuators of the first piezoelectric elements, this frequency difference can be counteracted.

Advantageously, the measuring device has more second piezo elements (ie piezo elements for generating and / or detecting longitudinal oscillations) as first piezo elements (ie piezo element for generating and / or detecting transversal Vibrations), since at about the same dimensioning and control of the piezoelectric elements with each first piezoelectric element usually more power coupled into the device under test (or decoupled from the device under test) can be as with every second piezoelectric element.

In a further advantageous embodiment, the piezoelectric elements are arranged on the measuring head such that each first piezoelectric element has at least one second piezoelectric element as the nearest neighbor and vice versa. By this mixing of the piezoelectric elements, it becomes possible to generate or detect the longitudinal and transverse vibrations in the approximately same spatial region of the test object.

In a further embodiment, the measuring head has a mechanical holder for each piezoelectric element, in which the piezoelectric element is held. In this case, the holders for the first and the second piezo elements are substantially identical, i. they each have the same shape, such that each holder can accommodate both a first and a second piezoelectric element. This makes it possible to equip each holder optionally with a first or a second piezoelectric element. In this way it becomes possible to use the same measuring head, e.g. to equip only with first piezo elements, only with second piezo elements or with a mixture of the two types of piezo elements, so that different measuring heads can be manufactured by the manufacturer with only a few components.

For the same reason, the first and the second piezoelectric elements each have the same external shape, with the possible exception of the shape of the stopper body. The latter can differ - since they do not interact with the holders of the measuring head, they can be optimized in their shape for the transmission of the respective vibration mode.

In a further advantageous embodiment, the piezoelectric elements are arranged in a rotationally symmetrical pattern on the measuring head in such a way that each piezoelectric element can be converted into itself or into another piezoelectric element by rotation through 360 ° / n. In this case, n> 2, whereby the measuring head is particularly suitable for measuring cylindrical specimens (for example boron cores).

The symmetry of the pattern is advantageous in such a way that, with a rotation of 360 ° / n, each piezo element can either be converted into itself or into a piezo element of the respective other type (i.e., a first piezo element into a second piezo element and vice versa). In this case, n should be straight, so that an alternating arrangement of first and second piezo elements is possible. In a preferred embodiment, n = 6.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments, advantages and applications of the invention will become apparent from the dependent claims and from the following description with reference to FIGS. Showing:

1 a view of a measuring head,

2 the measuring head of 1 from the measuring side,

3 a section through the measuring head of 1 .

4 a view of a first piezoelectric element for transverse vibrations,

5 a section through the first piezoelectric element,

6 a view of a second piezoelectric element for longitudinal vibrations,

7 a section through the second piezoelectric element and

8th a block diagram of the components of the measuring device.

WAYS FOR CARRYING OUT THE INVENTION

1 to 3 shows a measuring head 1 for an ultrasonic measuring device. The measuring head has an approximately cylindrical housing 2 in which first and second piezo elements 3 . 4 are arranged. The first piezo elements 3 serve to generate or detect transversal vibrations in the test specimen, while the second piezo elements serve to generate longitudinal vibrations in the specimen or to detect such oscillations.

The structure of the first piezo elements 3 for generating or detecting transverse vibrations is in 4 and 5 shown. Each piezo element 3 has a substantially cylindrical sleeve 6 , which preferably consists of plastic. Measuring side closes to the sleeve 6 a stopper body 7 which serves to be applied to the DUT. The stopper body 7 has a tip 8th made of metal, advantageously steel, which is in a cap 9 is embedded. The cap 9 preferably also consists of metal, in particular nickel-plated brass or aluminum.

In the sleeve 6 and coaxial with this is a piezoelectric actuator 10 intended. This is rod-shaped and has a first, facing away from the measurement side end 11 and a second, the measuring side facing the end 12 , The second end 12 is with the stopper body 7 connected. To that of the piezoelectric actuator 10 generated transverse movements well to the stop body 7 to transfer, it sits in the embodiment shown in a recess 13 the stopper body 7 and is there eg glued.

The actuator 10 has two piezoelectric bodies for generating or measuring transverse waves 14a . 14b , which abut each other, are mechanically connected to each other and from the first end 11 and second end 12 extend. These two bodies are as indicated by arrows 5 indicated, perpendicular to a from the first to the second end 11 . 12 extending connecting line polarized. In the present embodiment, the polarization directions of the two bodies are parallel (not anti-parallel) to each other.

Next are three electrodes 15a . 15b . 15c provided, of which the middle electrode 15b between the two bodies 14a . 14b is arranged while the outer electrodes 15a . 15c each opposite of the electrode 15b on the outside of the body 14a . 14b are arranged. The two outer electrodes 15a . 15c are at the same potential. If between the outer electrodes 15a . 15c and the inner electrode 15b a voltage is applied, the electric field is in the first body 14a eg parallel to the direction of polarization and in the second body 14b antiparallel, or vice versa. Thus, the one body extends in the longitudinal direction (ie in the direction of the connecting line between the two ends 11 . 12 ) together, while the other expands, creating in the area of the end 12 a transverse vibration can be generated.

Alternatively to the arrangement according to 5 can the two bodies 14a . 14b also be polarized anti-parallel to each other, in which case the electrodes are to be designed so that the electric fields in the two bodies 14a . 14b are rectified. In this case, the middle electrode can also be omitted.

As well as out 5 seen, is the sleeve 6 with a potting compound 17 poured, with which the vibration of the actuator can be damped. To generate vibrations to the electrodes 15a - 15c to apply or to measure oscillations voltages from the electrodes 15a - 15c to tap is a cable 18 , eg a coaxial cable.

In 4 be on the outside of the sleeve 6 tangential stops 20a . 20b . 20c as well as an axial stop 21 shown. These serve to the piezoelectric element in a holder of the measuring head described below 1 to fix in rotation and longitudinal direction.

The structure of the second piezo elements 4 for generating or detecting longitudinal vibrations is in 4 and 5 shown. The structure of the second piezo elements 4 corresponds largely to that of the first piezoelectric elements 3 , so that in the following mainly only the differences between the two types of piezo elements will be discussed. In particular, the first and second piezo elements have 3 . 4 each same outer shape, with exceptions in the area of the stop body 7 , Specifically, in the illustrated embodiment, at least the outer shape of the sleeves 6 and the attacks 20a to 20c and 21 identical. This makes it (as already mentioned) possible to mount in the same holder optionally a first or second piezoelectric element.

How out 7 can be seen, also has the second piezoelectric element has a piezoelectric actuator 10 which is rod-shaped and extends from a first end 11 to a second end 12 extends. The second end 12 is in turn with the stopper body 7 connected.

The actuator 10 of the second piezoelectric element 4 has a piezoelectric body for generating or measuring longitudinal waves 14 up, moving from the first end 11 and second end 12 extends. The body 14 is how in 7 indicated by an arrow, perpendicular to one from the first to the second end 11 . 12 extending connecting line polarized.

Next are two electrodes 15d and 15e provided, which are located on opposite longitudinal sides of the body 14 and with which an electric field perpendicular to the longitudinal direction and parallel or anti-parallel to the polarization direction of the body 14 can be generated. When creating such a field, the body tightens 14 in the longitudinal direction (ie in the direction of the connecting line between the two ends 11 . 12 ) together, or it elongates, depending on the sign of the applied voltage, so that longitudinal vibrations at the ends 11 and 12 can be generated.

How out 7 can be seen, is the actuator 10 laterally (ie radially) from a metallic shield 20 surrounded, which suppresses electrical crosstalk. (Also the first piezo elements 3 could be equipped with appropriate shielding, but in practice it that the shield at the second piezo elements 4 more important from a metrological point of view.)

THE SHIELD 20 IS IN PLEASURE 6 HELD.

Next is an elastic spacer 22 provided with which the actuator 10 coaxial with the sleeve 6 is fixed without hindering him in his ultrasonic vibrations. Preferably, the shield is 20 electrically with the stopper body 7 connected and grounded.

To generate vibrations to the electrodes 15d . 15e to apply or to measure oscillations voltages from the electrodes 15d . 15e turn off, is in turn a cable 18 , eg a coaxial cable.

Like a comparison of 5 and 7 shows are the piezoelectric actuators 10 the first piezo elements 3 shorter than those of the second piezo elements 4 while having approximately equal cross-sections (ie, cross-sectional shapes perpendicular to the longitudinal direction). As a result, as already mentioned, it can be achieved that the fundamental transverse natural frequency of the actuator 10 from 5 the fundamental longitudinal natural frequency of the actuator 10 from 7 can be adjusted.

As well as a comparison of 5 and 7 shows are the stopper body 7 the first and second piezo elements 3 . 4 slightly different structure. In particular, the cap 9 the stopper body 7 the second piezoelectric elements designed less stiff, so that the tip 8th longitudinal vibrations with the highest possible amplitude can generate.

In practice, it turns out that (depending on the used piezoelectric material and geometry of the actuators) the lengths should usually differ by at least a factor of 2. Specifically, the length of the actuator 10 of the first piezoelectric element 3 eg 10 mm, while that of the actuator 10 of the second piezoelectric element 4 28 mm, while the cross section perpendicular to the longitudinal direction for both actuators, for example, 4 × 4 mm2. Of course, other sizes can be used, depending on the material used, the desired geometry and the desired natural frequency.

The fundamental transverse natural frequency of the actuator 10 the first piezo elements 3 and the fundamental longitudinal natural frequency of the actuator 10 the second piezo elements 4 are preferably not too high, otherwise the depth of penetration of the waves in the specimen can be greatly affected. Advantageously, these frequencies are at about 50 kHz. Accordingly, the measuring device is preferably designed to excite the piezoelectric elements in each case with a frequency of at most 100 kHz, in particular a frequency between 30 and 70 kHz.

As can be seen, only transversal oscillations can be generated or detected with the first piezoelectric elements, while only longitudinal oscillations can be generated or detected with the second piezoelectric elements. This is to be understood that the piezoelectric elements are designed such that the signal of the other (unwanted) vibration mode has a power of at most 10% of that of the desired mode of vibration.

As in 1 to 3 are shown in the measuring head 1 a plurality of first and a plurality of second piezo elements 3 . 4 arranged. For this, the measuring side is in the measuring head 1 a number of mechanical holders corresponding to the number of piezoelements 30 provided, each of which each has a piezoelectric element 3 . 4 receives. Because the first and second piezo elements 3 . 4 (except for the stopper body) have the same outer shape, the holder can 30 be constructed the same for both types of piezo elements. As mentioned above, this allows the manufacturer to provide different measuring heads for transversal, longitudinal and combined measurements with only a few components. Every holder 30 forms a recording room 31 , the space for receiving a piezoelectric element 3 . 4 offers. The recording room 31 is laterally of guide sleeve 32 limited. In the recording room 31 is a spring 33 provided, the measuring side of the axial stop 21 the piezo element is supported and back against a crown 34 supported. The crown 34 is with the rear end of the guide sleeve 32 screwed.

The feather 33 pushes the piezo element 3 . 4 towards the measuring side. In the maximum relaxed position of the springs 33 as they are in 3 is shown, the stop body lie 7 all piezo elements in a common plane. If the surface of the test specimen is uneven, the piezo elements become 3 . 4 against the respective force of the springs 33 individually so deep in the brackets 30 pushed in that every stop body 7 remains in contact with the surface of the specimen.

How out 2 it can be seen, in the measuring head of the illustrated embodiment, six outer piezoelectric elements uniformly distributed on a circle and arranged concentrically around a central piezoelectric element. The pattern of the piezoelectric elements is chosen such that, by rotation about 360 ° / n about the central axis of the measuring head, each of the outer piezoelectric elements is in each case transferred to another of the outer piezoelements, where n = 6, during the middle piezoelectric element is converted into this rotation in itself.

In the illustrated embodiment are in the measuring head 1 three first (transverse) and four second (longitudinal) piezoelectric elements 3 respectively. 4 arranged. However, these numbers can be adapted to the respective requirements.

How farther 1 and 3 can be seen on and in the measuring head shown 1 also connections 40 . 41 as well as electronics on printed circuit boards 42 . 43 be arranged.

The measuring head shown 1 can basically serve as transmitter, receiver or transmitter and receiver for ultrasonic signals. If the measuring device is operated in reflection, then all measurements can be carried out with only one measuring head 1 be performed. If the measuring device is operated in transmission, then two measuring heads can be used, one of which serves as transmitter and the other as receiver of the ultrasonic signals, wherein the test object is arranged between the two measuring heads.

8th shows the most important components of the measuring device when operating in transmission. In particular, the device has a controller 44 which controls the components. The control 44 controls via a driver 45 the piezo elements 3a . 4a on, with which the ultrasonic signals are to be generated (in 8th For the sake of simplicity, only three of these piezo elements are shown). The control 44 can eg via an electronic switch 46 select whether the first or the second piezo elements 3a respectively. 4a be driven, ie whether transverse or longitudinal waves are to be generated.

The emitting piezo elements 3a . 4a lie on a first side of the specimen 47 and are arranged in a first measuring head. All piezo elements of each type are in each case operated parallel to one another, so that the oscillation amplitude of each piezo element is maximum.

On the opposite side of the specimen 47 are receiving piezo elements 3b . 4b provided in a second measuring head (of which in 8th again only three are shown).

The signals of all piezo elements 3b . 4b each type are each added electronically, so that a maximum signal level is achieved.

The control 44 can eg via an electronic switch 48 Select whether the transverse or longitudinal vibrations should be measured. The signals are through a filter / amplifier 49 led and then evaluated.

The electronic components of the circuit according to 8th can eg on the circuit boards 42 . 43 be arranged one of the measuring heads, or they can be at least partially realized in an external control.

The measuring device is advantageously configured, in operation, alternately periodic signal trains to the first and the second transmitting piezoelectric elements 3a . 4a to create so alternately transversal and longitudinal vibrations in the test specimen. The by the examinee 47 transmitted signals can then alternately through the first and second receiving piezoelectric elements 3b . 4b be caught. In this way, a good signal separation between the two measurement channels is achieved.

While preferred embodiments of the invention are described in the present application, it should be clearly understood that the invention is not limited to these and may be practiced otherwise within the scope of the following claims.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1394538 [0005]

Claims (18)

Measuring device for characterizing a test specimen by means of ultrasound comprising at least one measuring head ( 1 ) with at least one piezo element ( 3 . 4 ), which has a stop body ( 7 ) for application to the test specimen and a piezoelectric actuator ( 10 ) for generating and / or detecting vibrations in the stop body ( 7 ), characterized in that the measuring head ( 1 ) a plurality of different first and second piezoelectric elements ( 3 . 4 ), wherein the plurality of first piezoelectric elements ( 3 ) for generating and / or detecting transversal oscillations and the several second piezo elements ( 4 ) are designed for generating and / or detecting longitudinal vibrations in the test piece. Measuring device according to claim 1, wherein the measuring device is adapted to the piezo elements ( 3 . 4 ) in each case to operate in resonance, and wherein the resonant frequency of the transverse vibration of the first piezoelectric elements of the resonant frequency of the longitudinal vibration of the second piezoelectric elements ( 3 . 4 ) differs by a maximum of 20%, in particular by a maximum of 10%. Measuring device according to one of the preceding claims, wherein the piezoelectric actuators ( 10 ) between a first and a second end ( 11 . 12 ), the second end ( 12 ) with the respective stop body ( 7 ), wherein a distance between the first and the second end ( 11 . 12 ) a length of the actuator ( 10 ), and wherein the length of the piezoelectric actuators ( 10 ) of the first piezo elements ( 3 ) is less than the length of the piezoelectric actuators ( 10 ) of the second piezo elements ( 4 ). Measuring device according to claim 3, wherein the piezoelectric actuators ( 10 ) Electrodes ( 15a - 15e ) in order to generate electric fields perpendicular to connecting lines between the first and second ends ( 11 . 12 ) and wherein the piezoelectric actuators ( 10 ) are polarized parallel and / or antiparallel to the respective fields. Measuring device according to claim 4, wherein each actuator ( 10 ) of the first piezo elements ( 3 ) two are juxtaposed between the first and second ends ( 11 . 12 ) extending piezoelectric body ( 14a . 14b ), wherein the electrodes ( 15a . 15b . 15c ) and the bodies ( 14a . 14b ) are polarized such that when a voltage is applied to the electrodes ( 15a . 15b . 15d ) the first body ( 14a ) parallel to the electric field and the second body ( 14b ) is polarized antiparallel to the electric field, or vice versa. Measuring device according to one of claims 3 to 5, wherein the actuators ( 10 ) of the first and the second piezo elements ( 3 . 4 ) have the same cross sections. Measuring device according to one of the preceding claims, wherein the measuring device is designed to the piezo elements ( 3 . 4 ) each with a frequency of at most 100 kHz, in particular a frequency between 30 and 70 kHz, excite. Measuring device according to one of the preceding claims, wherein the measuring device more second piezoelectric elements ( 4 ) has as first piezo elements ( 3 ). Measuring device according to claim 1, wherein the stop bodies ( 7 ) of all piezo elements ( 3 . 4 ) are arranged in a common plane. Measuring device according to one of the preceding claims, wherein the piezo elements ( 3 . 4 ) on the measuring head ( 1 ) are arranged such that each first piezoelectric element ( 3 . 4 ) at least one second piezoelectric element ( 4 ) as the next neighbor and vice versa. Measuring device according to one of the preceding claims, wherein the measuring device is designed to alternately periodic signals to the first and the second piezo elements ( 3 . 4 ) so as to produce alternately transverse and longitudinal oscillations in the test specimen. Measuring device according to one of the preceding claims, wherein the measuring head ( 1 ) for each piezo element ( 3 . 4 ) a mechanical holder ( 30 ), wherein the holders ( 30 ) for the first and the second piezo elements ( 3 . 4 ) are identical so that each holder ( 30 ) both a first and a second piezoelectric element ( 3 . 4 ). Measuring device according to one of the preceding claims, wherein the first and the second piezo elements ( 3 . 4 ) in each case the same external shape, with the possible exception of the shape of the stop body ( 7 ), exhibit. Measuring device according to one of the preceding claims, wherein the first and the second piezo elements ( 3 . 4 ) Stop body ( 7 ) have different shape. Measuring device according to one of the preceding claims, wherein the piezo elements ( 3 . 4 ) in a rotationally symmetrical pattern on the measuring head ( 1 ) are arranged such that by rotation by 360 ° / n each piezoelectric element ( 3 . 4 ) in itself or in another piezo element ( 3 . 4 ), where n> 2, in particular n = 6. Measuring device according to claim 15, wherein the pattern is such that with a rotation of 360 ° / n each piezoelectric element ( 3 . 4 ) either in itself or in a piezoelectric element ( 3 . 4 ) of the other type, where n is even. Measuring device according to one of the preceding claims, wherein at least every second piezo element ( 4 ) a metallic shield ( 20 ), which the piezoelectric actuator ( 10 ) of the respective piezoelectric element ( 4 ) surrounds at least laterally. Measuring device according to one of the preceding claims, wherein with the first piezo elements ( 3 ) only transversal oscillations can be generated or detected and with the second piezo elements ( 4 ) each only longitudinal vibrations.

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