Method for ultrasonic Doppler fluid flow measurement with improved spatial resolution

0001The invention relates to a method for ultrasonic flow measurement, as specified in the preamble of claim 1.

0002It is known to use ultrasound, but not using the Doppler principle, to carry out flow measurements in the interior of pipelines. Such a method is among others described in DE-OS 30 20 282 for Fig. 1, where at an angle to the speed of the medium flowing inside the pipeline, ultrasound is emitted from a transmitter transducer to a receiver transducer in such a way that the ultrasound radiation also as far as possible the area of ​​the axis of the pipeline ( in an oblique direction). The associated receive converter is in a corresponding position in or positioned on the tube so that it can receive the ultrasound radiation emitted at an oblique angle. By using the receive transducer as a transmit transducer in a second operating phase and then using the previous transmit transducer as a receive transducer, an ultrasound measurement is carried out along the measurement path both in the forwards and forwards direction. By means of known difference measurement, as with other corresponding flow meters, the measured value is determined, which is an average over the entire measuring section. The accuracy of the flow measurement using this method is relatively high. However, the electronic effort for this is also considerable.

0003It is an object of the present invention to provide a method by means of which an overall also accurate detection or measurement of the flow rate in a pipeline can be achieved, but with less electronic effort.

0004This object is achieved with the aid of the method according to claim 1. Further refinements and developments of the invention emerge from the subclaims.

0005The invention is based on the finding that the flow occurring within a pipeline or the flow profile of a cross section of this pipeline is difficult and inaccurate to record with the methods known hitherto. However, this would be necessary for such flow measurements, on the accuracy of which considerable economic values, such as consumption measurements and their cost calculation, depend.

0006The solution principle according to the invention is based on operating at least one of the two transducers used (transmitting and / or receiving transducer) with a time-varying ultrasound direction. This includes that the direction of transmission and the direction of reception of the ultrasound radiation intersect in the interior of the tube, in contrast to the principle of DE-OS 30 20 282, FIG. 1, where the direction of transmission and reception 12 coincide and no intersection point has to be defined. However, if transmission and reception take place in intersecting directions, the intersection is a measuring point or, according to the cross-section of the ultrasound radiation and the cross-section of the receiving radiation lobe, the point of a differential flow volume for which the flow measurement according to the Doppler principle is currently the only one he follows. To speak here of ultrasound transmission beam of the emitting ultrasound transducer is conceptually unproblematic. For the reception, for which the reception sound lobe of the received ultrasound sound transducer, which is relevant for the transmission frequency f, has to be taken into account, the term "virtual reception beam" is to be used in the following.

0007In the present invention, therefore, the temporally changing angle of the transmitted beam and the changing angle of the virtual received beam are used. This change in direction over time can be effected mechanically. However, it is significantly more advantageous to do this by electrical means, for which purpose the corresponding embodiments of the invention are described below. Since the invention allows individual locations of the flow in the interior of the tube to be detected, the entire flow profile in the interior of the tube can be determined by appropriate spatial scanning - but often the scanning along a diameter of the tube cross section is sufficient. The various measured values ​​of the entire flow profile can be averaged, with the result that a considerably more precise statement about the flow volume transported per unit of time is achieved. Various weighting factors can be derived from the flow profile, which can now be exactly determined with the invention, for individual locations, which are introduced into the averaging and considerably increase the accuracy of the mean. It is also possible with the aid of the invention to determine a measurement location from the recorded flow profile which gives a representative value, ie which is quantitatively equal to the mean value of the flow and is then only subsequently determined instead of this mean value.

0008With the help of the invention, the actual position of the measuring location in the interior of the pipe can be reconstructed.

0009The flow velocities are relatively low for measuring points near the pipe wall. With the aid of the invention or with the aid of the averaging made possible by the invention, the measuring accuracy of the known "clamp-on" method can even be improved, in which measurements are made only on a single measuring path defined in the pipe interior for the forwards and forwards direction , such as in Fig. 1 of DE-OS 30 20 282. The clamp-on method takes its name from the fact that the transducers are attached to the pipe from the outside, possibly also subsequently.

0010A very particularly important embodiment of the present invention is that in which transducers according to DE-OS 30 20 282 mentioned above are used to carry out the method according to the invention. With regard to the structure, mode of operation, mode of operation and other special features of these converters of the above-mentioned DE-OS, reference is made here to the content of the description of this document, which is hereby also part of the present application.

0011In FIG. 1 of the above-mentioned DE-OS and also the present application, 1 designates a pipeline in which the medium to be measured (not shown here) flows and ultrasound through the interior thereof - in this known embodiment - with the fixed angle is transmitted from one converter 2 to the other converter 3 and to form the difference from converter 3 to converter 2. The transducer 2 with a plate-shaped body K made of piezoelectric ceramic is built into the wall of the tube 1 in such a way that the tube inner wall has as little or no discontinuity in its shape as possible. In particular, the inside 4 of the body K of the transducer 2 can have a curvature corresponding to the inner tube wall, or an additional attachment piece with a corresponding curvature can also be provided on a flat body K.

0012The converters 2 and 3 preferably have the same structure. For example, a customary full-surface electrode coating is arranged on the respective inner side 4 of the body K, which for example can also be provided with a protective coating. The electrode coating 5 is provided with a corresponding connecting line.

0013Arranged on the outer surface 6 of the transducers 2, 3 is the interdigital structure described in more detail in DE-OS, which consists of two interdigitated comb structures with their electrode fingers, the fingers of each comb structure being connected to a busbar. The fingers 7 shown in section in FIG. 1 belong to one comb structure and the fingers 8 belong to the other, interdigital comb structure.

0014The arrows 11 denote the permanent polarization of the piezoelectric ceramic material of the transducers 2, 3. This polarization can also be directed in the opposite direction to the arrows 11 shown. It should be pointed out that for the sake of simplicity, converters 2 and 3 of the same construction are provided; these converters can also have different structures within the framework of the principle of DE-OS (and the present invention).

0015The converter 2 or 3 (currently) used as a transmitter is fed with an alternating voltage of a frequency f. The two comb structures of a respective interdigital structure with 180 ° phase-shifted alternating voltages U are expediently provided with respect to the electrode 5 which is advantageously connected as a ground₁ and U₂ to feed the same frequency f. An ultrasound signal is picked up by the transducer 3 which is then used (at the moment) as a reception transducer and which has a Doppler frequency shift in accordance with the velocity component of the flow in the tube falling into the radiation 12. If the functions of the transducers 2 and 3 are interchanged, the oppositely directed transit time or phase shift is ascertained, and from the formation of the difference and the angle α the - for such an embodiment average flow velocity - for a diameter of the flow cross section is determined.

0016As already described in detail in the above-mentioned DE-OS, the advantage of such an interdigital transducer is that it can be used to radiate ultrasound radiation at an angle α obliquely to the transducer surface, so that no flow obstacle is associated with the installation of such a transducer.

0017It should be pointed out again here that the interdigital structure of such a converter of the above-mentioned DE-OS can be realized jointly both by corresponding finger electrode structure 7, 8, by alternating polarization (FIG. 2 of DE-OS 30 20 282.5) and by using both principles can.

0018The condition d = λ / cos α results in α = arccos c fd with c = speed of sound of the medium and f = frequency of the exciting voltage U.

00192 and 3 show preferred embodiments of the invention, namely:<ul id="ul0001" list-style="none"><li>2 shows an embodiment with a converter of any embodiment according to DE-OS 30 20 282 and a conventional second converter,</li><li>3 shows an embodiment with two converters of DE-OS 30 20 282 and</li><li>4 shows a circuit example of the electronics.</li></ul>

0020With 21 a transducer according to Fig.1 or DE-OS 30 20 282 is designated, which is installed in the tube wall 1 as in Fig.1. A conventional transducer is designated by 22, which is, for example, a piezo-ceramic disk provided with electrodes covering the entire surface on both sides. In the invention, as shown in FIG. 2, this converter 22 can be installed in the tube wall 1 in one plane with the same. The transducer 22 is used continuously as a receiving transducer and it is sufficient for the invention if this transducer 22 has a (receiving) sound lobe directed only in the diameter direction 23 due to its construction and installation .

0021The transducer 21 can be operated so that it α with its ultrasound at variable angles_n sends out. With 24 and 25 are two examples with the angles α₁, α₂ designated. As shown, these different emission directions 24 and 25 lead to two different measuring locations 124 and 125, for which the indicated differential measuring volumes 124, 125 result according to the sound lobes. They lie in the area of ​​the respective intersection of transmission direction 24, 25 and fixed reception direction 23. The radiation angle α₁, α₂ are by operating the converter 21 with AC voltages of the frequencies f₁ and f₂ causes. By appropriate selection of the frequency f, with appropriate positioning of the transducers 21 and 22 to one another, all locations along a diameter of the interior of the tube wall 1 (but at least those of the half-diameter indicated by r) can be determined, so that a reliable image can be ascertained over the entire flow profile is.

0022The relationship between (punctual) flow velocity v and Doppler frequency f is as follows in the arrangement according to FIG. 2:<maths id="math0001" num=""><img file="EP0138017A1_D0001.tif" /></maths>

0023By inserting the above relationship for cos one obtains<maths id="math0002" num=""><img file="EP0138017A1_D0002.tif" /></maths>

0024This means that the measured Doppler frequency Δ_f only depends on the flow velocity v and the periodicity constant d of the interdigital transducer. It is therefore not necessary in the method according to the invention. to relate the measured Doppler frequency to the transmission frequency. There is even no dependence on the speed of sound c of the flowing medium, which disadvantageously has to be taken into account with all its changes in conventional ultrasonic Doppler flowmeters.

0025Instead of the structure according to FIG. 2, two converters according to the principle of DE-OS 30 20.282 can advantageously be used for the invention, as shown in FIG. It is easy to see from FIG. 3 what diverse possibilities result for the detection of the interior of the tube wall 1. 224 and 225 denote the differential measurement volumes, which are once for the frequency f₁ and for the frequency f₂ the operating alternating voltage of the transmitting converter 21 (or 31). It should be pointed out that in the embodiment of FIG. 3, the transmit and receive functions of the converters 21 and 31 can also be interchanged. Such a converter 21, 31 has a frequency-dependent reception direction as shown.

0026As can be seen from the illustration in FIG. 3 with knowledge of the inventive concept, measures must be taken that the (transmitting or receiving) sound lobes 24, 25 of the transducer 21, namely for a frequency f and a frequency f₂ actually cut with each other with the respective sound lobe 24 ', 25' of the second transducer 31 for the respective frequency. One way to achieve this is to install at least one of the transducers 21, 31 somewhat obliquely in the inner tube wall 1. However, a major advantage of the converter of DE-OS 30 20 282 would be at least partially revealed. On the other hand, it is advantageous to use the two converters 21 and 31 with different periodicity constants d₁ and d₂to form, which on the one hand results in different angles α different α 'at the same frequency f for sending and receiving and thus inevitably, as shown intersection points 224, 225, etc.

0027The following should then be set analogously to equation (II):<maths id="math0003" num=""><img file="EP0138017A1_D0003.tif" /></maths>

0028Since the oblique-beam interdigital transducers of DE-OS 3020282 generally have two symmetrical sound lobes, there are two measurement points per se for a frequency f, as is only shown in broken lines and is designated by 224. However, as described in the aforementioned DE-OS, the transducers can also be designed so that they only radiate unidirectionally or received, with which such an ambiguity could also be used as a simultaneous measurement in two measurement volumes. With suitably chosen angles, the resultant averaging of the values ​​from the two measurement volumes could improve the accuracy of the measurement. To obtain more comprehensive information about the flow profile inside a tube 1 and to increase redundancy, more than two transducers 21, 22; 21, 31 can be used in combination.

00294 shows an example of an electronic circuit to be used for the invention with the transmitter converter 21 and the receiver converter 22. The transmitter converter 21 is fed by an voltage-controlled oscillator (VCO) 46 with an AC voltage with a controllably variable frequency. The signal of the receive converter 22 is amplified in the amplifier 42 and fed to the mixer 43. The mixer 43 also receives an electrical signal with the frequency at which the transmitter converter 21 is currently being fed. The Doppler frequency signal goes from the mixer 43 via the line 44 to the circuit 45, which contains both the control for the oscillator 46 and the evaluation electronics for converting the Doppler frequency signal into a flow rate value which can be read on the display 47.