METHOD AND MEASURING DEVICE FOR DETERMINING A MEASURED QUANTITY RELATING TO A FLOW

Abstract

A method determines a measured quantity relating to the flow of a fluid through a measuring tube, in two propagation directions, and a receive signal is captured. A transit time difference is determined depending on the position of the main maximum of a cross-correlation of the receive signals. Whereupon the measured quantity is determined depending on the transit time difference, and the transmitting ultrasonic transducer is controlled in each case with an excitation signal. The excitation signal has a fixed carrier frequency. The excitation signal has a phase shift and/or an envelope with a plurality of temporally spaced maxima, and/or, if a trigger condition is fulfilled, the fulfilment of which depends on the height of the main maximum and/or of at least one secondary maximum of the cross-correlation. The determination of the measured quantity is modified compared with a normal operating mode and/or a message is output.

Claims

1. A method for determining a measured quantity relating to a flow of a fluid through a measuring tube by means of a measuring device, which comprises the steps of: emitting, for two propagation directions, an ultrasonic signal in each case by a transmitting ultrasonic transducer of the measuring device and being transmitted via the fluid to a receiving ultrasonic transducer of the measuring device, wherein a receive signal is captured via the receiving ultrasonic transducer for a respective propagation direction; depending on a position of a main maximum of a cross-correlation of receive signals for the two propagation directions or a cross-correlation of processing signals which depend in each case on one of the receive signals or on a partial signal of the receive signal, a transit time difference between transit times of a respective ultrasonic signal for the respective propagation direction from the transmitting ultrasonic transducer to the receiving ultrasonic transducer is determined; determining the measured quantity in dependence on the transit time difference, wherein the transmitting ultrasonic transducer being controlled in each case with an excitation signal, the excitation signal having a fixed carrier frequency, the excitation signal having a phase shift and/or an envelope with a plurality of temporally spaced maxima; and determining if a trigger condition is fulfilled, a fulfilment of the trigger condition depending on a height of the main maximum and/or of at least one secondary maximum of the cross-correlation, the determining of the measured quantity is modified compared with a normal operating mode and/or a message is output to a user of the measuring device and/or to a further device outside the measuring device if the trigger condition exists.

2. The method according to claim 1, wherein if the trigger condition is fulfilled, modifying a determination of the measured quantity compared with the normal operating mode in such a way that the receive signals are rejected and either a previously determined measured quantity is used as a current measured quantity or a determination of the receive signals is repeated in order to provide new receive signals, wherein the determination of the new receive signals is performed either unchanged or with at least one modified determination parameter compared with the determination of the receive signals, and the measured quantity is determined on a basis of the new receive signals, and/or the transit time difference is determined depending on a position of one of the at least one secondary maxima of the cross-correlation and/or a determination rule is modified in order to determine the measured quantity from the transit time difference.

3. The method according to claim 1, wherein if the trigger condition is fulfilled and/or a spectral condition depending on a spectral composition of at least one of the receive signals is fulfilled, a second determination of the receive signals is carried out following a first determination of the receive signals, wherein the fixed carrier frequency of the excitation signal and/or a size of the phase shift is modified compared with the first determination.

4. The method according to claim 3, wherein a fulfilment of the spectral condition depends on a frequency range in which frequencies have a maximum amplitude in a respective one of the receive signals or in a selected time segment of the receive signal.

5. The method according to claim 1, which further comprises determining the receive signals multiple times in temporal succession for the propagation directions and the cross-correlation, wherein a time characteristic of the height of the main maximum and/or at least one of the secondary maxima of the cross-correlation and/or a time characteristic of a processing result which depends on heights of the main maximum and at least one of the secondary maxima is recorded, wherein the fulfilment of the trigger condition depends on the time characteristic.

6. The method according to claim 1, which further comprises emitting a test ultrasonic signal multiple times by the transmitting ultrasonic transducer and the test ultrasonic signal is transmitted via the fluid to the receiving ultrasonic transducer for at least one of the propagation directions before a transmission of the ultrasonic signal, wherein a respective test receive signal is captured via the receiving ultrasonic transducer, wherein the transmitting ultrasonic transducer is controlled to emit the test ultrasonic signal in each case with a test excitation signal, wherein test excitation signals differ from one another in terms of their carrier frequency and/or a size of their phase shift, wherein the test excitation signal for which a maximum phase shift and/or a maximum separation of local maxima of an envelope of the test receive signal occur in a resulting test receive signal is chosen as the excitation signal for determining the transit time difference.

7. The method according to claim 6, which further comprises determining a size of the phase shift by performing a Fourier transform in each case for a plurality of windows of the test receive signal.

8. The method according to claim 1, wherein the fulfilment of the trigger condition depends on a difference and/or a quotient of the height of the main maximum and the height of one of the at least one secondary maxima.

9. The method according to claim 1, wherein the fulfilment of the trigger condition depends on the height of the main maximum and/or the at least one secondary maximum of the cross-correlation of the processing signals, wherein a respective processing signal of the processing signals is determined through normalization of the receive signal or of a respective intermediate signal determined from the receive signal and/or in that, the height of the main maximum and of the at least one secondary maximum of the cross-correlation is determined following a normalization of the cross-correlation.

10. The method according to claim 6, wherein the test excitation signal is amplitude-modulated by the envelope.

11. The method according to claim 8, wherein the height of one of the at least one secondary maxima is derived from a highest secondary maximum.

12. A measuring device for determining a measured quantity relating to a flow of a fluid through a measuring tube, the measuring device comprising: a measuring tube guiding the fluid; at least two ultrasonic transducers disposed in or on said measuring tube; and a controller configured to control said at least two ultrasonic transducers, to capture receive signals via said at least two ultrasonic transducers and to determine the measured quantity depending on receive signals, said controller is configured to carry out the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0049] FIG. 1 is a diagrammatic, sectional view of an exemplary embodiment of a measuring device according to the invention;

[0050] FIG. 2 is a block diagram showing a sequence of one exemplary embodiment of the method according to the invention;

[0051] FIG. 3 is an illustration showing excitation signals usable in the method according to the invention and resulting receive signals;

[0052] FIGS. 4 and 5 are graphs showing cross-correlations of receive signals in exemplary embodiments of the method according to the invention; and

[0053] FIG. 6 is a block diagram showing steps for determining a suitable excitation signal in one exemplary embodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0054] Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a measuring device 1 for determining a measured quantity relating to the flow of a fluid 3 through a measuring tube 4, in particular a flow rate or a flow volume. The fluid 3 flows here through the measuring tube 4 in the direction shown by an arrow 2. A control device 7 controls, on one hand, the ultrasonic transducer 5 as the transmitting ultrasonic transducer in order to transmit the ultrasonic signal 14 in the direction of flow, the ultrasonic signal 14 being received by the ultrasonic transducer 6 following a reflection on the tube wall, wherein the corresponding receive signal is captured by the control device 7 via the receiving ultrasonic transducer 6. Conversely, the control device 7 controls the ultrasonic transducer 6 as the transmitting transducer in order to transmit the ultrasonic signal 15 against the direction of flow to the now receiving ultrasonic transducer 5, via which a corresponding receive signal is captured by the control device 7.

[0055] On the basis of these two receive signals, a transit time difference can be determined between the transit time of the ultrasonic signal 14 from the ultrasonic transducer 5 to the ultrasonic transducer 6 and the transit time of the ultrasonic signal 15 from the ultrasonic transducer 6 to the ultrasonic transducer 5, wherein, as is well known per se, the transit time difference correlates with the flow rate of the fluid 3 so that, with a known measuring tube geometry, a volume flow can be determined for the fluid 3.

[0056] Details relating to a method implemented by the control device 7 to determine a measured quantity 24, for example the flow volume, are explained below with additional reference to FIG. 2.

[0057] An excitation signal 8, 42 having a fixed carrier frequency and amplitude is initially provided, whereupon an amplitude modulation 13 of the excitation signal 8 is performed with an envelope 10. Alternatively, it would also be possible to generate the excitation signal 8, 42 directly with a time-variable amplitude which is predefined by a corresponding envelope 10. The amplitude-modulated excitation signals 8, 42 are fed, typically in succession, to the ultrasonic transducers 5, 6, as a result of which the latter emit the ultrasonic signals 14, 15, the reception of which in the respective other of the ultrasonic transducers 5, 6 enables the provision of the respective receive signals 16, 17 to the control device 7. Optionally, a processing signal 18, 19 can first be determined from the respective receive signal 16, 17, for example by up-sampling, filtering and/or a scaling, in particular a normalisation. Alternatively or additionally, a partial signal can be selected from the respective receive signal in order to determine the processing signal. The computing requirement and therefore the energy requirement of the calculation can be reduced by restricting the cross-correlation to partial signals.

[0058] A cross-correlation 20 of the receive signals 16, 17 or the processing signals 18, 19 is then performed. The cross-correlation 20 of two signals x, y, which is denoted here as R.sub.xy, can be calculated as follows:

R.sub.xy(τ)=∫.sub.−∞.sup.∞x(t).Math.y(t+τ)dt.

[0059] With a time-discrete capture of the receive signals, the integral can also be expressed as the sum of the individual samples.

[0060] Optionally, the cross-correlation 20 can itself also be normalised, for example scaled depending on the amplitudes of the autocorrelations of the receive signals or intermediate signals, as already explained above, or by dividing the function R.sub.xy by the value of this function at τ=0.

[0061] Examples of the signals that are used and the resulting cross-correlations are discussed below with additional reference to FIGS. 3 to 5. It is initially assumed here that the envelope 10 is a rectangular function, so that the respectively used excitation signal 8, 42 is output on the respective ultrasonic transducers 5, 6 for a number of cycles with a fixed amplitude and the control is then abruptly ended.

[0062] In a first example which is shown in the top line in FIG. 3, a normal sinusoidal oscillation is used as the excitation signal 42. The x-axis 40 indicates the time characteristic in μs and the y-axis 41 indicates the amplitude. The receive signal 43, for example, is produced due to the transmission characteristics of a normal measuring distance with the use of this excitation signal 43.

[0063] If two receive signals 43 of this type are cross-correlated, the cross-correlation 20 shown in FIG. 4 is produced as a result. A sample number, and therefore ultimately a time, is plotted here on the x-axis, and the amplitude of the cross-correlation 20 is plotted in any given units on the y-axis. In order to highlight the discussed features more clearly, FIG. 4 and FIG. 5 show only a relatively small section of the respective cross-correlation 20 representing the highest maxima of the respective cross-correlation 20. FIG. 4 shows that the height 25 of the main maximum 28 differs from the height of the next-highest secondary maximum 29 by only a small differential amount 30. Even relatively slight interference in the measurement can thus result in the secondary maximum 29 rather than the main maximum 28 being identified as the main maximum. Since the time axis of the cross-correlation correlates with the transit time difference, this would result in a substantial measurement error for the transit time difference and therefore for the measured quantity also.

[0064] As will be explained later, corresponding interference can normally be identified by evaluating a trigger condition 27 and can elicit a corresponding response so that, in principle, a robust determination of the measured quantity using the excitation signal 42 is also possible. However, an approach for reducing the risk of a measurement error of this type from the outset will first be explained below.

[0065] As shown in FIG. 2 and in the bottom line of FIG. 3, an excitation signal 8 having a phase shift 9, in the example of 180°, can be used. Alternatively, other sizes of the phase shift, for example 90° or 270°, or values in between, can also be used. A phase shift through 180° is particularly simple to implement, since it can be produced e.g. by inverting the signal at the zero crossing. However, if the excitation signal is, for example, digitally generated and converted via an analogue-to-digital converter into an analogue control signal before or after the amplitude modulation by the envelope 10, phase shifts of any size can be implemented without additional hardware outlay.

[0066] The receive signal 16, 17 resulting from an excitation of this type is similarly shown in FIG. 3. This also has a phase shift in the area 34 and the amplitude of the signal is significantly reduced in this area 34. The cross-correlation 20 of two receive signals 16, 17 of this type is shown in FIG. 5. Compared with the cross-correlation shown in FIG. 4, it is evident that the height 25 of the main maximum 28 is significantly reduced through the use of the phase shift 9, but the height 26 of the secondary maxima 29 is even more significantly reduced in comparison, resulting, both relatively and absolutely, in an even greater differential amount 30 than would occur without the use of this phase shift 9. The main maximum 28 and therefore its position 21 can thus be identified significantly more robustly by using the phase shift 9 in the excitation signal 8.

[0067] A similarly substantial lowering of the height of the secondary maxima 29 can also be achieved without the use of a phase shift 9 if, as shown in FIG. 2, an envelope 10 having a plurality of temporally spaced maxima 11, 12 is used. Both approaches can obviously also be combined, as shown in FIG. 2.

[0068] Again with reference to FIG. 2, following the determination of the cross-correlation 20, the position 21 of the main maximum 28 can be obtained therefrom, for example by locating the global maximum. This position 21 corresponds to the time shift between the receive signals 16, 17 with which the latter are as similar as possible to one another and therefore to the transit time difference 22 of the ultrasonic signals 14, 15. Depending on how the time window is chosen during which measurement data are acquired for the receive signals 16, 17, the position 21 of the main maximum 28 can also differ by a fixed offset from the transit time difference.

[0069] The measured quantity 24, i.e., for example, a flow rate or flow volume over time, can therefore be determined from the transit time difference 22 by means of a determination rule 23. In the simplest case, the determination rule 23 can involve a multiplication by a predefined constant, but it is also possible to take account of non-linearities, for example due to a change in the flow profile depending on the flow rate, for example by using a look-up table or a defined mathematical relationship as the determination rule 23 or as part thereof.

[0070] Particularly if an excitation signal without a phase shift 9 is used as the excitation signal 42, and a simple envelope, for example a rectangular envelope, is used, but also in the other cases discussed above, it is appropriate to check on the basis of the captured receive signals 16, 17 whether a robust determination of the measured quantity 24 is likely to be possible or whether, for example, maintenance of the measuring device or a new determination of measurement data is required due to interference or a manipulation attempt.

[0071] For this purpose, as shown in FIG. 2, the heights 25, 26 of the main and secondary maximum 28, 29 can be determined from the cross-correlation 20, whereupon a trigger condition 27 depending on these quantities or at least one of these quantities can be evaluated. The effects of different interferences and influences on the heights 25, 26 of the main maximum 28 and the secondary maxima 29 have already been explained in detail in the general part, so that only individual points will be singled out here by way of example. Time characteristics, for example, of the heights 25 or 26 can be recorded via a multiplicity of measurements, and ageing processes can be identified on the basis of the change over time in the heights 25, 26 and thus, for example, a message 33 can be output to a user or to a further device in order to indicate a maintenance requirement. A message 33 of this type can also be output, for example, if interference is detected repeatedly or with a certain frequency or said interference cannot be compensated by adjusting 31 the determination of the measured quantity 24 compared with the normal operating mode.

[0072] A multiplicity of options which have already been discussed in the general part are available for adjusting 31 the determination of the measured quantity 24. Only some examples of adjustments 31 will therefore be mentioned below. It is thus possible, for example, to reject the previously captured receive signals 16, 17 if the trigger condition 27 is fulfilled and to carry out a new determination, i.e. to repeat the sequence shown in FIG. 2 from the start. It is possible here, in particular, for the carrier frequency and/or the size of the phase shift 9 of the excitation signal 8 to be modified. The procedure explained later with reference to FIG. 6, for example, can be used for this purpose.

[0073] However, instead of a new determination, it would also be possible, for example, initially to continue to use the measured quantity determined in a preceding iteration and to determine the measured quantity again only at a later time at which the trigger condition, for example, is no longer fulfilled.

[0074] As similarly already described in the general part, the evaluation of the trigger condition 27 can, however, also serve to identify measurement situations with a high flow through the measuring tube. In this case, it can be advantageous to retain the receive signals 16, 17 and to modify only the subsequent further processing of the resulting cross-correlation 20. The determination rule 23, for example, can be modified by taking account of non-linear effects at high flow rates or turbulences in the flow. In some cases, however, it can also be appropriate to use the position 32 of the secondary maximum 29 instead of the position 21 of the main maximum 28 to determine the measured quantity 24 or the transit time 22. This can be the case, for example, if the heights 25, 26 of the main and secondary maximum 28, 29 are very similar and, at the same time, the position 21 of the main maximum 28 indicates a relatively low flow, whereas, for example, the height 25 of the main maximum 28 indicates more of a weak correlation due to the distortion of one of the receive signals 16, 17 due to a high flow.

[0075] The quality of the receive signals 16, 17 and therefore the robustness of the determination of the measured quantity 24 can further be monitored by evaluating a spectral condition 37 which, for the sake of clarity, is shown for only one of the receive signals 16. The spectral composition 35 of the respective receive signal 16, 17 can first be determined for this purpose and the frequency range 36 in which the frequencies of the receive signal 16, 17 have the maximum amplitude can then be determined. The spectral condition 37 can be fulfilled, for example, if the frequency of the excitation signal 8, 42 lies outside this frequency range 36, since it can indicate, for example, a detuned resonant frequency of one of the ultrasonic transducers due to ambient conditions, damage, or for other reasons.

[0076] In particular, if the spectral condition 37 is fulfilled, the carrier frequency and/or the size of the phase shift 9 of the excitation signal 8, 42 that is used can be adjusted during the next determination of the receive signals 16, 17 or during a repetition of the determination of the receive signals 16, 17.

[0077] FIG. 6 shows steps S1-S6 which can be used in order to define a suitable excitation signal 8. These steps can be carried out, for example, during a calibration of the measuring device 1 following manufacture or following installation at the location of use or also if, for example, the trigger condition 27 or the spectral condition 37 is fulfilled, in order to determine a suitable carrier frequency or a suitable size of the phase shift 9. Here, in step S1, a plurality of different test excitation signals 44 are first provided or generated which differ from one another in terms of their carrier frequency 45 and/or the size 46 of the phase shift 9.

[0078] In step S2, the respective test excitation signal 44 is multiplied or amplitude-modulated by the envelope 10 and output to one of the ultrasonic transducers 5, 6.

[0079] In step S3, assigned test receive signals 47 are received via the respective other ultrasonic transducer. In step S4, a Fourier transform is then performed in each case for a plurality of windows of these test receive signals 47, wherein not only the amplitude for a respective frequency, but also a phase is determined. The size 48 of the phase shift of the respective test receive signal 47 can be determined by comparing the phases of the dominant frequency in adjacent windows.

[0080] In step S6, the test excitation signal 44 for which the size 48 of the phase shift of the resulting test receive signal 47 was greatest is then selected as the excitation signal 8.

[0081] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention.

REFERENCE NUMBER LIST

[0082] 1 Measuring device

[0083] 2 Arrow

[0084] 3 Fluid

[0085] 4 Measuring tube

[0086] 5 Ultrasonic transducer

[0087] 6 Ultrasonic transducer

[0088] 7 Control device

[0089] 8 Excitation signal

[0090] 9 Phase shift

[0091] 10 Envelope

[0092] 11 Maximum

[0093] 12 Maximum

[0094] 13 Amplitude modulation

[0095] 14 Ultrasonic signal

[0096] 15 Ultrasonic signal

[0097] 16 Receive signal

[0098] 17 Receive signal

[0099] 18 Processing signal

[0100] 19 Processing signal

[0101] 20 Cross-correlation

[0102] 21 Position

[0103] 22 Transit time difference

[0104] 23 Determination rule

[0105] 24 Measured quantity

[0106] 25 Height

[0107] 26 Height

[0108] 27 Trigger condition

[0109] 28 Main maximum

[0110] 29 Secondary maximum

[0111] 30 Differential amount

[0112] 31 Adjustment

[0113] 32 Position

[0114] 33 Message

[0115] 34 Area

[0116] 35 Spectral composition

[0117] 36 Frequency range

[0118] 37 Spectral condition

[0119] 38 X-axis

[0120] 39 Y-axis

[0121] 40 X-axis

[0122] 41 Y-axis

[0123] 42 Excitation signal

[0124] 43 Receive signal

[0125] 44 Test excitation signal

[0126] 45 Carrier frequency

[0127] 46 Size

[0128] 47 Test receive signal

[0129] 48 Size

[0130] S1-S6 Step