Measurement of flow rate of fluid

Abstract

The invention relates to measurement of a flow rate of a fluid. Transmitting and receiving a downstream first ultrasonic signal and a countercurrent second ultrasonic signal on a measurement path inclined relative to the flowing direction of the fluid, determining a first flight time of the first ultrasonic signal and a second flight time of the second ultrasonic signal, and determining the flow velocity according to the flight time difference, wherein the ultrasonic signal has a carrier frequency of a plurality of cycles, the amplitude of which is modulated in accordance with the envelope curve, the respective reception time for determining the time of flight is determined in accordance with selected oscillations of the ultrasonic signal, the oscillations being selected on the basis of the course of the envelope curve, in particular the maximum value of the envelope curve. A first difference between the first time-of-flight and a previously determined first time-of-flight and / or a second difference between the second time-of-flight and a previously determined second time-of-flight is calculated, and if the first and / or the second difference is an integer multiple of the carrier frequency period within a tolerance range, the carrier frequency period is determined. The first and / or second time of flight is corrected by an integer multiple of the carrier frequency period.

Description

[0001] The invention relates to a method and an ultrasonic measuring device for measuring the flow velocity of a fluid according to the preambles of claims 1 and 8 .

[0002] Fluid velocity in pipes and channels can be determined using ultrasonic measurement technology using the differential time-of-flight method. Ultrasonic signals are emitted and received by a pair of ultrasonic transducers, which are positioned opposite each other on the pipe wall at either end of a measuring path inclined relative to the fluid flow. The flow velocity is determined based on the difference in the time of flight of the ultrasonic wave along the measuring path, both upstream and downstream. The ultrasonic transducers operate alternately as transmitters and receivers. The ultrasonic signal transmitted through the fluid is accelerated in the direction of flow and decelerated in the opposite direction. The resulting time-of-flight difference is used to calculate the average fluid velocity using geometric variables. The cross-sectional area also determines the working volume flow rate, which is often the actual measured variable of interest for fluids measured by volume. For even higher measurement accuracy, additional measuring paths with ultrasonic transducers can also be provided.

[0003] The challenge in determining time of flight lies in temporally locating the ultrasonic signal. Traditionally, pulse packets or chirped signals are used as ultrasonic signals—multiple oscillations of the ultrasonic carrier frequency, whose amplitudes are modulated according to, for example, a bell-shaped envelope. The envelope's maximum value is not precise enough, but it can be used to orient a specific oscillation of the ultrasonic signal. The reception time is then determined, for example, based on the zero crossing of this selected oscillation. A problem with this approach is that the envelope's maximum value is neither precise nor stable enough for even relatively rough temporal alignment of a specific oscillation of the ultrasonic signal. This is particularly true if conditions change during operation of the ultrasonic measuring device (e.g., due to temperature fluctuations in the fluid, aging of the ultrasonic transducer, scattering objects in the fluid (e.g., bubbles or solids), or switching to a different, uncalibrated fluid). Such drift can cause the envelope to shift, potentially selecting a different oscillation. This measurement error is known as a phase jump, as the reception time virtually jumps to a different oscillation.

[0004] US 2007 / 0191990 A1 describes a flow measurement with bubble detection so that, if necessary, a warning can be issued or the output variable can be frozen until the bubble disappears. One criterion for the presence of a bubble is a change in amplitude, another criterion is the difference in flight time between time t and time t-1.

[0005] US 6 950 768 B2 discloses an ultrasonic measuring device that uses various diagnostics to identify and correct time-of-flight errors.

[0006] US 6 941 821 B2 relates to an ultrasonic flow measurement device which dynamically adjusts the number of measurement repetitions.

[0007] It is therefore an object of the present invention to further improve the measurement of flow velocity in the time-of-flight method using ultrasound.

[0008] This object is achieved by a method and an ultrasonic measuring device for measuring the flow velocity of a fluid according to claims 1 and 8. According to the ultrasonic differential time-of-flight method already briefly described in the introduction, a first ultrasonic signal downstream and a second ultrasonic signal upstream are emitted and received on an inclined measurement path (i.e., a measurement path having at least one component in the direction of flow or in a direction opposite to the direction of flow) to determine a first time of flight and a second time of flight. The flow velocity is determined based on the time difference between the two flight times (i.e., downstream and upstream). The ultrasonic signals each have a carrier frequency of multiple cycles, wherein the amplitude is modulated according to an envelope curve. Based on the direction of the envelope curve in the received ultrasonic signal, in particular characteristics such as the maximum value, the center of gravity, etc. of the envelope curve, the oscillation of the ultrasonic signal or a part of the oscillation is selected to determine the reception time point required for determining the flight time.

[0009] The present invention is based on the following basic idea: to identify erroneous measurements caused by phase jumps due to sudden changes in the first flight time or the second flight time. A phase jump means that another oscillation is selected, for example because the envelope is deformed due to drift effects, in particular because it is widened due to dispersion. The flight time difference then has an error that is an integer multiple of the carrier frequency period. To detect this, a first difference between the first flight time and a previously determined (i.e., earlier) first flight time, and / or a second difference between the second flight time and a previously determined (i.e., earlier) second flight time is calculated. A check is performed to see whether the first difference and / or the second difference is an integer multiple of the carrier frequency period, with a certain tolerance allowed. If this is the case, the first flight time and / or the second flight time are corrected to an integer multiple of the carrier frequency period. Therefore, due to the coincidence with an integer multiple of the carrier frequency period, a phase jump is assumed and corrected, or it is offset by calculation. The term "difference" is somewhat overused here and should not be confused with different fields: the time-of-flight difference (from which the flow velocity is determined) is the difference between two different flight times, downstream and upstream. In contrast, the first difference and the second difference are the time differences or discrete derivatives within the first or second flight time.

[0010] The advantage of the present invention is that it largely eliminates the adverse effects of phase jumps on flow velocity measurements. The correction takes into account phase jumps in both time directions, i.e., phase jumps in the two ultrasonic signals in the downstream and upstream directions, and also considers multi-valued phase jumps in the carrier frequency over more than one cycle. This makes the measured values ​​more reliable, stable, and accurate, and allows for accurate measurements even after drift or under difficult measurement conditions (such as the formation of bubbles or particles in the fluid).

[0011] Preferably, the measurement is repeated in a sampling period of time resolution, and the previously determined first time of flight and / or the previously determined second time of flight are taken from a previous sampling period, in particular the immediately preceding sampling period. Thus, a time granularity is given to which the previously determined time of flight can be related, in particular by using the time of flight of the immediately preceding sampling period as the previously determined time of flight to calculate the first difference or the second difference. The measurement is thus carried out quasi-continuously with a discrete time resolution corresponding to the sampling period. Sampling should not be confused with a further finer digitization of the ultrasonic signal (for digital evaluation of the ultrasonic signal). In a sampling period in the sense referred to herein, the value of the flow velocity is determined by evaluating the corresponding (possibly previously digitized) ultrasonic signal. Therefore, the sampling period should preferably be fine enough so that the flow remains quasi-constant on the time scale of the sampling period. Otherwise, actual changes in the flow velocity or speed of sound could be confused with phase jumps.

[0012] Preferably, the tolerance is at most 20%, at most 15%, at most 10%, or at most 5% of the period. In practice, phase jumps do not lead to measurement errors that are precisely integer multiples of the carrier frequency period, which is why a certain tolerance is permitted. Several possible values ​​for the tolerance are mentioned, with intermediate values ​​or even smaller tolerances of less than 5% also conceivable. Depending on the tolerance, more Type I or Type II errors may occur, i.e., changes in actual measurement effects may be mistakenly identified as phase jumps, or unidentified measured phase jumps may be mistakenly interpreted as measurement effects.

[0013] Preferably, the integer multiple is one or two times the carrier frequency period. In particular, only phase jumps of one, i.e., exactly one, carrier frequency period are corrected. Larger deviations usually have other causes than phase jumps, in which case at least two times the carrier frequency period may still be a reasonable criterion.

[0014] Preferably, if the first flight time and / or the second flight time are corrected with the same integer multiple of the carrier frequency period over a plurality of sampling periods, in particular two sampling periods to ten sampling periods, the correction is reset. This is a credibility criterion for the phase jump, with which possible erroneous corrections that are not caused by phase jumps can be canceled. If the same correction seems to be required over a certain number of sampling periods, there is a risk that the assessment is artificially locked to an erroneous oscillation simply by correction. The value of this number is a parameter that can be selected within a range of two to ten and higher. The credibility criterion can be associated with one of the downstream and upstream flight times, or particularly preferably with both flight times.

[0015] Preferably, the calibration is reset if the signal quality of the first ultrasonic signal and / or the second ultrasonic signal is higher than a minimum signal quality during the calibration period, in particular within two sampling periods to ten sampling periods. This is another credibility criterion, which can be used instead of or cumulatively with the check of the calibration history of the previous period. Similarly, there is a parameter that determines the length of the time interval considered, which can be between two and ten or even more sampling periods. In order to comprehensively evaluate the signal quality within the time interval considered, statistical measures such as the mean or median can be used. High signal quality indicates that no calibration is necessary because it is more likely that there is a real measurement effect or an actual change in the flow velocity or sound speed. Therefore, in the case of high signal quality, if the above conditions are met, a possible calibration is reset or canceled.

[0016] The signal quality is preferably calculated based on the Fourier transform of the first and / or second ultrasonic signals by evaluating the spectral components of the carrier frequency relative to the other spectral components. This is a method for comparing the useful signal with the noise to assess the signal-to-noise quality. For example, a spectral band of fixed width surrounding the carrier frequency is determined, and the integral of the Fourier transform within this spectral band is compared with the integral of the remaining spectrum outside of this spectral band. This value can then be compared to a minimum signal quality, with normalization being performed if necessary. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other features and advantages of the present invention will be described in more detail below based on exemplary embodiments and with reference to the accompanying drawings. In the accompanying drawings:

[0018] Figure 1 The basic structure of an ultrasonic measuring device based on time of flight is shown, with the measuring path of the ultrasonic measuring device and the labeled geometric variables for determining the flow rate;

[0019] Figure 2 A schematic diagram of an ultrasonic signal is shown for explaining the selection of oscillations to determine a reception time point;

[0020] Figure 3 Shows something like Figure 2 Schematic diagram of a graph where other oscillations are incorrectly selected due to dispersion effects;

[0021] Figure 4 An exemplary flow chart for correcting the time of flight in the event of a phase jump and checking the plausibility of the correction is shown;

[0022] Figure 5 shows exemplary measurement data of a first downstream flight time and a second upstream flight time as a function of time without the correction according to the present invention;

[0023] Figure 6 Shown according to Figure 5 a flight time difference between a first flight time and a second flight time calculated from the measurement data;

[0024] Figure 7 Shows something like Figure 5 Exemplary measurement data of , now with the correction according to the invention;

[0025] Figure 8 Shown according to Figure 7 a flight time difference between a first flight time and a second flight time calculated from the measurement data;

[0026] Figure 9 Shows the Figure 7 A graphic representation of the corrections performed; and

[0027] Figure 10 A diagram showing the signal quality of the measured data is shown.

[0028] Figure 1 The basic structure of an ultrasonic measuring device 10 based on time of flight is shown. Two ultrasonic transducers 12, 14 are arranged at an angle α measured relative to the vertical on the wall of a pipe 16 in which a fluid 18 flows in the direction of the arrow 20. The ultrasonic transducers 12, 14 operate alternately as transmitters and receivers under the control of a control and evaluation unit 22. The ultrasonic signal transmitted by the fluid 18 on the measuring path 24 is accelerated in the direction of flow and decelerated in the direction opposite to the flow direction. Via circuit elements (not shown), such as amplifiers and A / D converters, the corresponding received signals are transmitted to the control and evaluation unit 22 and digitally evaluated. For this purpose, according to v=L / (2cosα)(1 / t v -1 / t r ), convert the obtained flight time difference into the required flow rate, or according to Q = v 1 / 4D 2 π is converted to working volume flow, where Figure 1 The geometric relations shown in are described by the following variables:

[0029] v: flow velocity of the fluid in the pipe,

[0030] L: length of the measuring path between the two ultrasonic transducers,

[0031] α: The angle of transmission and reception of the ultrasonic transducer,

[0032] Q: volume flow rate,

[0033] D: diameter of the pipe,

[0034] t v : The time of flight of the ultrasonic wave downstream,

[0035] t r : Flight time of ultrasonic waves in countercurrent.

[0036] The calculation of the above variables should be understood as exemplary, and there are other possibilities for measuring the difference in flight time and thereby determining the flow velocity or volume flow. In particular, the design of the ultrasonic transducers 12, 14 and the geometry of the measuring path (including the possibility of multiple measuring paths) can be varied in a manner known per se. The control and evaluation unit 22 can be integrated into the ultrasonic measuring device 10, provided as an external device, or in the form of a mixture of the two. There is provided at least one preferably digital computing module, such as a microprocessor or CPU (Central Processing Unit), FPGA (Field Programmable Gate Array), DSP (Digital Signal Processor), ASIC (Application-Specific Integrated Circuit), etc. The external computing unit can be a computer of any structural type, including a laptop, smartphone, tablet, dedicated controller, or a local network, edge device or cloud.

[0037] Figure 2A schematic diagram of an ultrasonic signal for time-of-flight measurement is shown. In each case, a linear frequency-modulated signal or pulse packet is transmitted and received, which comprises a plurality of oscillations of the carrier frequency of the ultrasonic wave, and the amplitude of these oscillations is modulated throughout the oscillation process. For simplicity, the oscillations are shown only as sawtooth waves 26 with the same amplitude as each other; in reality, these oscillations would be embedded in an envelope curve 28 generated by amplitude modulation. An oscillation 30 is selected based on the maximum value of the envelope curve 28, and the reception time 32 is determined, for example, based on the zero crossing of the oscillation. In alternative embodiments, at least one additional adjacent oscillation can also be added to determine the reception time 32.

[0038] Figure 3 Shows something like Figure 2 Schematic diagram of a flow diagram in which the envelope curve 28 widens due to dispersion effects, for example due to temperature fluctuations, aging, or interfering objects (e.g., bubbles or particles in the flow). As a result, the maximum of the envelope curve 28 shifts, as indicated by arrow 34. This in turn means that a different oscillation 30 is now used to determine the reception time 32. The problematic result is a phase jump in the time-of-flight measurement, i.e., the flight time is extended by one carrier frequency period despite the same flow velocity.

[0039] Figure 4 An exemplary flow chart for identifying phase jumps and correcting the flight time accordingly is shown. Not all steps have to be performed, in particular the last two plausibility criteria are optional. In step S1, the flight time downstream and the flight time upstream are determined. In particular, as for Figure 1 As explained, ultrasonic signals are emitted and received in two directions along the measuring path 24. Figure 2 As explained, the corresponding reception time is determined, so that the flight time is obtained in conjunction with the known transmission time and the calibration of the internal signal delay.

[0040] Figure 5 Exemplary measurement data for a first time of flight downstream (below) and a second time of flight upstream (above) are shown as a function of time. The time of flight is measured in sampling periods, but the sampling periods are preferably tight enough to form a discrete but quasi-continuous variation for practical applications. Figure 5 Many phase jumps can be seen in the right part of Figure 5 No correction according to the invention is performed. Figure 6 The corresponding flight time difference is shown, i.e. Figure 5 In the right part, many phase jumps are propagated, unless by chance the same phase jump occurs simultaneously in both flight times, causing the error to cancel out at some point in time.

[0041] Back to Figure 4 , determine a first difference between the current first flight time and the previous first flight time and / or a second difference between the current second flight time and the previous second flight time, in preparation for finding a phase jump. Preferably, at each time point t, the difference with the previous time point t-1 is determined. This is a discrete approximation to the time derivative of the measured flight time, other approximation methods (for example, using other previous time points) are also conceivable. It is then checked whether the first difference and / or the second difference is equal to an integer multiple of the carrier frequency period. If this condition holds, it is evaluated as identifying a phase jump, because this is more likely than a sudden change in flow velocity that happens to be an integer multiple of the carrier frequency period. An implicit prerequisite here is that the sampling period for measuring the flight time is short enough, i.e. the sampling period is tight enough compared to the expected change in flow velocity or sound speed. Preferably, the comparison is performed within a certain tolerance. That is, if the first and / or second difference values ​​are within ±x% of the carrier frequency period (where x = 5, 10, 15, 20, or similar values), the first and / or second difference values ​​are still considered to be equal to an integer multiple of the carrier frequency period. The integer multiple is preferably one, since a single phase jump is most likely. In a preferred embodiment, two is also permitted, and even higher integer multiples are conceivable, but rarely occur in practice. In general, regarding the correction in step S3, it should also be noted that the measured time of flight is plausibly checked to detect phase errors, i.e., sudden changes at integer multiples of the carrier frequency period. This is in stark contrast to conventional methods, which attempt to determine the reception time with particularly high precision through additional knowledge of ambient conditions (e.g., the velocity of sound of the fluid or the temperature of the fluid) or through special evaluations, or to more precisely detect the envelope curve 28 and, therefore, its temporal position, by, for example, using ultrasonic transducers 12, 14 with particularly high bandwidths. On the other hand, the present invention employs different methods, which does not exclude the use of such measures to supplement the present invention.

[0042] Figure 7 Shows something like Figure 5 Example measurement data, now with the correction according to the invention. As can be seen in the right part, a large number of phase jumps can be corrected. But this is only part of the positive effect. If both flight times have phase jumps, then as long as the phase jumps are the same, it is just as useful as correcting the phase jump. This is in Figure 8 , where the flight time difference between the first flight time and the second flight time is calculated based on Figure 7The same phase jump disappears in the time-of-flight difference, so overall there are no relevant phase jumps here except for two remaining short outliers. Figure 9 It also supplements the Figure 7 Graphical representation of the corrections performed.

[0043] In optional steps S4 and S5, further credibility criteria are checked. Depending on the embodiment, these steps can be performed together, separately or not at all. Further credibility criteria are intended to prevent the evaluation from being locked to a wrong oscillation due to the correction. In step S4, it is checked whether the two most recent flight times have each corrected the same integer multiple of the carrier frequency period. This can be done for the first flight time and / or the second flight time and, for example, can be traced back to two, five, ten or a similar number of sampling periods. Step S4 is based on the following heuristic principle: a longer period of similar correction no longer indicates the presence of a phase jump, but rather a real measurement effect, and the correction is therefore reset or canceled.

[0044] In step S5, the signal quality is checked over a number of sampling periods as another possible confidence criterion, which may also be two, five, ten or a similar number of sampling periods. Preferably, the signal quality is summarized using a statistical measure, such as the mean or median over the considered sampling periods. Figure 10 A graphical representation of the signal quality of the measurement data considered so far is shown. In this example, the signal quality is calculated by first performing a Fourier transform. During the Fourier transform, a spectral band is determined around the carrier frequency, and the signal components in this spectral band are compared with those in the remaining spectrum, where the signal components are determined, for example, by integration. Preferably, a normalization process is then performed so that the signal quality lies in the range of 0-100%. However, signal quality, in particular the signal-to-noise ratio, can also be evaluated in different ways, for which various signal processing methods are known.

[0045] A higher signal quality, for example, predefined by a threshold value such as 90%, 95%, 98%, 99%, or 99.5%, indicates that a correction is inappropriate because it may be a real measurement effect. Therefore, if the signal quality is high during the sampling period under consideration, the correction is reset. The confidence criteria of steps S4 and S5 can be combined. In one example in this regard, if the same correction is performed during ten sampling periods and the average signal quality is determined to be greater than 99%, the correction is reset.

Claims

1. A method for measuring the flow rate of a fluid (18), wherein: A first ultrasonic signal in the downstream direction and a second ultrasonic signal in the upstream direction are emitted and received again on a measuring path (24) inclined relative to the flow direction of the fluid (18), a first flight time of the first ultrasonic signal and a second flight time of the second ultrasonic signal are determined, and the flow velocity is determined based on the flight time difference between the first flight time and the second flight time, wherein the ultrasonic signal has a carrier frequency (26) of multiple cycles whose amplitude is modulated according to an envelope curve (28), and corresponding reception time points (32) of the ultrasonic signal are determined based on selected oscillations (30) of the ultrasonic signal to determine the flight time, wherein the oscillations are selected based on the course of the envelope curve (28), in particular the maximum value of the envelope curve (28), It is characterized by: A first difference between the first flight time and a previously determined first flight time and / or a second difference between the second flight time and a previously determined second flight time is calculated, and if the first difference and / or the second difference are integer multiples of the period of the carrier frequency (26) within a tolerance range, the first flight time and / or the second flight time are corrected to an integer multiple of the period of the carrier frequency (26).

2. The method according to claim 1 , wherein the measurement is repeated in a sampling period of the time resolution, and the previously determined first time of flight and / or the previously determined second time of flight are taken from a previous sampling period, in particular the immediately preceding sampling period.

3. The method according to claim 1 or 2, wherein: The tolerance is at most 20%, at most 15%, at most 10% or at most 5% of the period of the carrier frequency (26).

4. A method according to any one of the preceding claims, wherein The integer multiple is one or two times the period of the carrier frequency (26).

5. A method according to any one of the preceding claims, wherein If the first flight time and / or the second flight time are corrected by the same integer multiple of the period of the carrier frequency (26) within a plurality of sampling periods, in particular two sampling periods to ten sampling periods, the correction is reset.

6. A method according to any one of the preceding claims, wherein If the signal quality of the first ultrasonic signal and / or the second ultrasonic signal is higher than a minimum signal quality during the calibration, in particular within two sampling periods to ten sampling periods, the calibration is reset.

7. The method according to claim 6, wherein: The signal quality is calculated from a Fourier transform of the first ultrasonic signal and / or the second ultrasonic signal by evaluating spectral components of the carrier frequency relative to other spectral components.

8. An ultrasonic measuring device (10) for measuring the flow rate of a fluid (18), the ultrasonic measuring device comprising: At least two ultrasonic transducers (12, 14) arranged opposite one another, between which a measuring path (24) is formed which extends obliquely with respect to the flow of the fluid (18); and a control and evaluation unit (22) which is designed to measure the flow velocity using the method according to one of the preceding claims.

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