Method for operating an ultrasonic flow meter and ultrasonic flow meter

Abstract

A method (1) for operating an ultrasonic flow meter (2) for measuring the flow rate through a measuring tube (4) through which a medium (3) flows is presented and described, wherein the ultrasonic flow meter (2) comprises at least one transmitting ultrasonic transducer (5) for transmitting ultrasonic signals (6) and at least one receiving ultrasonic transducer (7) for receiving ultrasonic signals (6) and a control and evaluation unit (8), wherein the control and evaluation unit (8) controls the transmitting ultrasonic transducer (5) such that it emits the ultrasonic signal (6), the receiving ultrasonic transducer (7) receives the emitted ultrasonic signal (6), and the control and evaluation unit (8) determines at least an indirect value for the flow rate of the medium (3) through the measuring tube (4) by evaluating emitted and received ultrasonic signals (6). Consistent measurement quality is achieved by the control and evaluation unit (8) adjusting an ultrasonic signal frequency (f) of the emitted ultrasonic signal (6) in such a way that a change in the opening angle (phi) of the sound lobe (10) of the ultrasonic signal (6) caused by a change in the value of the speed of sound (v_sos) of the medium (3) is at least partially compensated.

Description

[0001] The invention relates to a method for operating an ultrasonic flow meter for measuring the flow rate through a measuring tube through which a medium flows, wherein the ultrasonic flow meter comprises at least one transmitting ultrasonic transducer for transmitting ultrasonic signals and at least one receiving ultrasonic transducer for receiving ultrasonic signals and a control and evaluation unit, wherein the ultrasonic transducers are arranged such that they realize an ultrasonic measuring path in the medium, and wherein the control and evaluation unit controls the transmitting ultrasonic transducer such that it emits an ultrasonic signal, the receiving ultrasonic transducer receives the emitted ultrasonic signal, and the control and evaluation unit determines at least an indirect value for the flow rate of the medium through the measuring tube by evaluating emitted and received ultrasonic signals.Furthermore, the invention also relates to such an ultrasonic flow meter.

[0002] Flow measurement using ultrasonic waves has been known for decades. Regardless of the specific measurement method used (for example, transit-time measurement, transit-time difference measurement (with and against the flow direction), frequency measurement / Doppler effect), flow measurement is always based on the transmission of ultrasonic waves in the medium flowing through the measuring tube, the flow velocity of which is to be measured.

[0003] In the vast majority of ultrasonic flowmeters, the characteristic dimensions of the measuring tube (in the case of the usual round measuring tubes, the measuring tube diameter) are significantly larger than the wavelength of the ultrasonic signal used for the measurement, so that the ultrasonic waves propagating in the medium can be considered free-space waves. It is known that the shape of the emitted ultrasonic signals can be influenced by various measures, for example, by a specific design of the ultrasonic transducer geometry. In this context, the opening angle of the ultrasonic lobe of the emitted ultrasonic signal is often discussed. If the maximum sound pressure is present in a main emission direction of the transmitting ultrasonic transducer, then it is determined at what angle, deviating from the main emission direction, only half the sound pressure is present, for example.This angle is then interpreted as the opening angle of the sound beam, although sound pressure can still be detected at larger opening angles. In any case, the opening angle of the ultrasound signal's sound beam is a suitable measure of the ultrasound signal's focus.

[0004] It is known from the prior art that the opening angle of the sound lobe depends on various physical parameters, see for example DE 27 03 439 A1, DE 20 2013 104 569 U1 or DE 10 2021 115 546 A1.

[0005] A small beam angle of the ultrasonic signal is advantageous for achieving a good signal-to-noise ratio. In ultrasonic flowmeters with multiple measurement paths and several pairs of ultrasonic transducers, small beam angles can prevent or at least reduce mutual interference between the transducers. If installation situations involve components protruding into the measurement space traversed by the measurement path (other sensors, agitators, etc.), small beam angles also prevent or reduce disruptive reflections. Conversely, the beam angle must be sufficiently large to ensure that the ultrasonic signal reliably covers a sufficiently large reception area.

[0006] The object of the present invention is to further develop the method for operating the ultrasonic flow meter and the corresponding ultrasonic flow meter in such a way that a consistent measurement quality is ensured even in highly variable measurement situations.

[0007] The derived problem in the method described at the beginning for operating an ultrasonic flow meter is initially solved by the control and evaluation unit controlling the emitting ultrasonic transducer in such a way that the emitted ultrasonic signal is emitted with a defined ultrasonic signal frequency, so that at a defined value of the speed of sound of the medium flowing in the measuring tube, the ultrasonic signal is emitted with a defined opening angle of a sound lobe of the ultrasonic signal.

[0008] These defined conditions and settings of the ultrasonic flowmeter often correspond to the conditions and specifications underlying the ultrasonic flowmeter's design for a particular medium, such as water. Many processes using ultrasonic flowmeters involve relatively stable process conditions.

[0009] The present invention is based on observations made in connection with gaseous media whose composition is subject to changes, which can have a significant impact on the ultrasound signal, in particular on the opening angle of the sound lobe of the emitted ultrasound signal and thus on the measurement. Especially when working with light gases (for example, hydrogen), significant differences in the propagation of sound waves can occur when the gas (for example, to natural gas) or its composition changes. This is because the speed of sound in a gaseous medium is inversely proportional to the square root of the molar mass of its molecules. For example, nitrogen at 0 °C and at standard pressure has a speed of sound of approximately 337 m / s, while hydrogen under the same conditions has a speed of sound of approximately 1261 m / s.

[0010] While changes in the speed of sound have practically no influence on the determination of the medium's flow velocity and thus on the flow rate measurement in conventional transit-time difference measurements, it has been recognized that significant changes in the speed of sound have a considerable impact on the opening angle of the sound beam of the ultrasound signal excited at the defined ultrasound signal frequency. Such a change in the opening angle of the sound beam can severely impair the measurement quality and is therefore undesirable.

[0011] In connection with the invention, it has also been recognized that the opening angle of the sound beam of the emitted ultrasound signal can also be influenced by changing the ultrasound signal frequency. According to the invention, it is therefore provided that the control and evaluation unit receives a current value for the speed of sound of the medium flowing in the measuring tube, and that when the received current value of the speed of sound changes compared to the defined value of the speed of sound of the medium, the control and evaluation unit changes the ultrasound signal frequency of the emitted ultrasound signal to a current value of the ultrasound signal frequency, so that a change in the opening angle of the sound beam of the ultrasound signal caused by the change in the value of the speed of sound of the medium is at least partially compensated.The described measure can counteract an unwanted change in the opening angle of the sound beam of the ultrasound signal, so that the desired measurement quality can be maintained.

[0012] In an advantageous embodiment of the method, the ultrasonic signal frequency of the emitted ultrasonic signal is modified such that the opening angle of the sound beam of the emitted ultrasonic signal remains within a tolerance range around a nominal opening angle. This takes into account that an exact setting of the opening angle of the sound beam of the ultrasonic signal is often not critical, and certain deviations are acceptable.

[0013] In a further development of the method, the ultrasound signal frequency is abruptly changed to the current value, specifically by jumping from one tolerance limit of the tolerance range to another. In this further development, not every change in the speed of sound in the medium results in a change in the emitted ultrasound signal frequency; rather, the excitation frequency is delayed until a tolerance range is exceeded. This is advantageous, for example, when the transmitting ultrasound transducer is to be operated at only a few different frequencies, perhaps because advantageous excitation is only possible at these specific frequencies (resonances, utilization of radial and axial excitation modes).

[0014] An alternative embodiment of the method involves continuously adjusting the ultrasound signal frequency to reflect the current, changed value. This is particularly suitable when the current value for the speed of sound in the medium is frequently updated or readily available, and fine-tuning the ultrasound signal frequency is not problematic.

[0015] One variant of the method is characterized by the fact that the ultrasonic signal frequency of the emitted ultrasonic signal is only changed when the change in the obtained current speed of sound relative to the known speed of sound in the medium exceeds a predetermined change threshold. Since the opening angle of the ultrasonic signal's sound beam changes depending on the speed of sound in the medium, the change in the speed of sound can also be used directly to adjust the ultrasonic signal frequency accordingly.

[0016] In an advantageous embodiment of the method, the current value for the speed of sound of the medium flowing in the measuring tube is calculated by the control and evaluation unit itself, based on a transit-time measurement of the emitted ultrasonic signal and a known propagation length of the emitted ultrasonic signal. This is particularly easy to achieve if the ultrasonic flowmeter is configured to perform transit-time difference measurements, in which signal transit-time measurements are taken with and against the flow direction, since the flow velocity of the medium automatically cancels out in the calculation when the transit times are appropriately summed.

[0017] In an alternative embodiment of the method, the current value for the speed of sound of the medium flowing in the measuring tube is specified, in particular by an external parameter input. The current value for the speed of sound can, for example, be determined by an external measurement process, wherein the determined value is distributed by a control computer to ultrasonic flow meters operating according to the invention.

[0018] In an advantageous embodiment of the method, the required change in the value of the ultrasonic signal frequency to compensate for the influence of the change in the speed of sound in the medium on the opening angle of the ultrasonic signal's sound lobe is calculated based on a physical-mathematical model of wave propagation. In particular, an equation describing the sound pressure as a function of a pole angle relative to a main radiation direction of the transmitting ultrasonic transducer is used. More generally, a wave equation for sound waves is solved, taking into account the geometric and physical boundary conditions (ultrasonic transducer, medium).The physical-mathematical model is used to analytically or numerically determine the dependence of the opening angle of the ultrasound signal's sound beam on the speed of sound in the medium and the ultrasound signal frequency of the excited ultrasound signal. From the relationship between opening angle, speed of sound, and ultrasound signal frequency, the required change in the ultrasound signal frequency is determined to at least partially compensate for the influence of the changing speed of sound on the opening angle of the ultrasound signal.

[0019] The derived problem is also solved with the previously described ultrasonic flowmeter, whereby the control and evaluation unit performs the previously described procedure during the operation of the ultrasonic flowmeter.

[0020] In detail, there are numerous possibilities for designing and further developing the inventive method for operating an ultrasonic flow meter and the corresponding ultrasonic flow meter. Reference is made, on the one hand, to the claims subordinate to the independent claims, and on the other hand, to the following description of exemplary embodiments in conjunction with the drawing. The drawing shows Fig. 1 schematically a method for operating an ultrasonic flow meter and an ultrasonic flow meter on which the method is operated, wherein an ultrasonic signal is emitted with a sound beam at an opening angle, Fig. 2a, Fig. 2b schematically a method for operating an ultrasonic flow meter and an ultrasonic flow meter to illustrate the influence of a changing speed of sound in the medium, Fig. 3. Schematic representation of the method for operating an ultrasonic flow meter and the corresponding ultrasonic flow meter with compensation for a change in the opening angle of the sound beam due to a changing speed of sound in the medium. Fig. 4 schematically the procedure for operating an ultrasonic flow meter with illustration of the pole angle dependence of the sound pressure in the far field of a cylindrical ultrasonic transducer, Fig. 5. Schematic representation of the procedure for operating an ultrasonic flow meter, showing the dependence of the opening angle of the sound beam of a cylindrical ultrasonic transducer on the speed of sound in the medium and on the ultrasonic signal frequency. Fig. 6 schematically the procedure for operating an ultrasonic flow meter according to Fig. 5 and a strategy for changing the ultrasound signal frequency in response to a changing speed of sound in the medium.

[0021] The figures schematically show various aspects of a method 1 for operating an ultrasonic flow meter 2 for measuring the flow through a measuring tube 4 through which a medium 3 flows.

[0022] Fig. Figure 1 shows the ultrasonic flowmeter 2 in operation, on which the method 1 is carried out. The ultrasonic flowmeter 2 comprises a transmitting ultrasonic transducer 5 for transmitting ultrasonic signals 6 and a receiving ultrasonic transducer 7 for receiving the ultrasonic signals 6. The ultrasonic flowmeter 2 also includes a control and evaluation unit 8. The ultrasonic transducers 5 and 7 are arranged such that they create an ultrasonic measurement path 9 in the medium 3. The control and evaluation unit 8 controls the transmitting ultrasonic transducer 5 so that it emits the ultrasonic signal 6. The receiving ultrasonic transducer 7 receives the emitted ultrasonic signal 6, and the control and evaluation unit 8 determines a value for the flow rate of the medium 3 through the measuring tube 4 by evaluating the emitted and received ultrasonic signals 6.

[0023] In the illustrated embodiment, the control and evaluation unit 8 implements a transit-time difference measurement; that is, the signal transit times of the ultrasonic signal 6 are measured in the direction of flow and against the direction of flow, and a flow velocity is determined from this, which allows a conclusion to be drawn about the volumetric flow rate of the medium 3. Fig. Figure 1 shows that the ultrasonic signal 6 is emitted in the direction of flow, i.e., from left to right, by the transmitting ultrasonic transducer 5. Due to the transit-time difference measurement, the ultrasonic transducers 5 and 7 can also act alternately as transmitting ultrasonic transducer 5 and receiving ultrasonic transducer 7. The double arrow between the control and evaluation unit 8 indicates that the control and evaluation unit 8 interacts with the other components of the ultrasonic flowmeter 2, in particular with its ultrasonic transducers 5 and 7, for the purpose of performing the flow measurement.

[0024] The ultrasonic flow meter according to Fig. 1. The transmitting ultrasonic transducer 5 is controlled by the control and evaluation unit 8 such that the ultrasonic transducer 5 emits ultrasonic signals 6 with a defined ultrasonic signal frequency f_det. If the medium 3 has a defined value of the sound velocity v_sos,det of the medium 3 flowing in the measuring tube 4, then the ultrasonic signal 6 is emitted with a defined opening angle of a sound lobe 10 of the ultrasonic signal 6. The aforementioned defined values ​​are the values ​​on which the factory calibration of the ultrasonic flowmeter 2 is based.

[0025] In connection with the flow measurement of gaseous media 3, it has been recognized that gases of very different masses, whose gas molecules have very different molar masses, have a significant influence on the speed of sound in the medium 3 and thus also on the opening angle phi of the sound beam 10 with which the ultrasonic signals 6 are emitted into the medium 3; this is described in more detail in Fig. 2 shown.

[0026] Fig. Figure 2a shows the conditions for flow measurement when the defined values ​​for the ultrasonic signal frequency f_det and for the speed of sound v_sos,det are present in the medium 3 flowing through the measuring tube 4. Under these conditions, the ultrasonic signal 6 is emitted with an ultrasonic beam 10 with the defined opening angle phi_det. The opening angle phi of the sound beam 10 depends on the speed of sound v_sos in the medium 3, which here, by assumption, corresponds to the defined value v_sos,det (speed of sound), and the ultrasonic signal frequency f, which here, by assumption, corresponds to the defined ultrasonic signal frequency f_det. Therefore, the following relationship applies: phi_det = phi(v_sos,det; f_det).

[0027] In Fig. Figure 2b shows that the medium 3 has changed insofar as it exhibits a current sound velocity v_sos,akt, which differs from the defined value v_sos,det of the sound velocity of the medium 3; the sound velocity v_sos,akt is greater than the defined value v_sos,det. While maintaining the defined ultrasonic signal frequency at the value f_det, the opening angle phi(v_sos,akt; f_det) of the sound beam 10, with which the ultrasonic signal 6 is emitted into the medium 3, increases compared to the opening angle phi_det. This can have undesirable effects on the measurement quality, as the signal-to-noise ratio decreases and mutual interference occurs from several adjacent pairs of ultrasonic transducers (not shown) that are arranged close to each other and define multiple measurement paths.

[0028] Fig. Figure 3 shows the method 1 and the ultrasonic flowmeter 2, in which a countermeasure is taken to compensate for the increased opening angle phi caused by the changed speed of sound v_sos,akt. The control and evaluation unit 8 receives the current value v_sos,akt for the speed of sound of the medium 3 flowing in the measuring tube 4, and the control and evaluation unit 8 changes the ultrasonic signal frequency f of the emitted ultrasonic signal 6 to a current value f_akt of the ultrasonic signal frequency when the received current value v_sos,akt of the speed of sound changes compared to the defined value v_sos, det of the speed of sound of the medium 3, so that a change in the opening angle phi of the sound lobe 10 of the ultrasonic signal 6 caused by the change in the value of the speed of sound of the medium 3 is at least partially compensated.The opening angle phi changes as a result of the measure to the again reduced opening angle phi(v_sos,akt; f_akt).

[0029] In the described method 1, the required change in the value of the ultrasound signal frequency f to the current value f_actual of the ultrasound signal frequency f to compensate for the influence of the change in the speed of sound v_sos in the medium 3 on the opening angle phi of the sound lobe 10 of the ultrasound signal 6 is calculated based on a physical-mathematical model of wave propagation. The basis for the method is the law applicable to ideal gases, according to which the speed of sound in the gaseous medium 3 is inversely proportional to the square root of the molar mass of the gas (in addition to the dependence on temperature): v_sos=γRTM

[0030] Furthermore, an equation to describe the far field of the sound pressure p of a cylindrical ultrasonic transducer 5 as a function of a pole angle phi to a main radiation direction phi_0 of the transmitting ultrasonic transducer 5 is used as a physical-mathematical model of wave propagation: p(r,phi,t)=j2ρcUarka[2J1(ka sin(phi))ka sin(phi)]ej(wl−kr)

[0031] The angle phi here has the meaning of the angle phi given in the figures and corresponds to the pole angle measured from the main radiation direction phi_0 of the transmitting ultrasonic transducer 5. The equation has the usual form of an acoustic wave equation in spherical coordinates, starting from a cylindrical shape of the ultrasonic transducer 5.

[0032] The dependence of the sound pressure p on the pole angle phi, which is of interest, is given solely by the term in square brackets. J1 is the first-order Bessel function, k is the wavenumber, and a is the radius of the cylindrical ultrasonic transducer 5.

[0033] Fig. Figure 4 shows the expression in parentheses as a function of the argument x = k*a*sin(phi). The sound lobe is defined by a sound pressure drop of -6 dB compared to the sound pressure in the main radiation direction phi_0. This attenuation corresponds to a pressure drop of half for a power square root quantity such as the sound pressure p. The value 0.5 for the expression in parentheses is reached for x = 2.212.

[0034] Since k = 2*pi*f / v_sos and a = D / 2, where D is the diameter of the ultrasound transducer, the following applies: phi=arcsin[Kd*v_sosD*f]

[0035] Here, Kd = x / pi = 0.70.

[0036] Based on this relationship, Fig. 5 the dependence of the opening angle phi of the sound lobe 10 (defined by the drop in sound pressure by -6 dB) on the speed of sound v_sos in the medium 3, which in turn depends on the molar mass M of the gas molecules, for an ultrasonic transducer 5 with a diameter of D = 10 mm.

[0037] The three curves correspond to different specifications for the ultrasound signal frequency f, namely for the values ​​f = 300 kHz, f = 454 kHz, and f = 706 kHz; thus, parameter lines in the ultrasound signal frequency f are shown. It is clearly visible how changing the ultrasound signal frequency f can influence the opening angle phi of the ultrasound beam 10 of the ultrasound signal 6. Therefore, it is possible to counteract a change in the opening angle phi of the sound beam 10 of the ultrasound signal 6 caused by a change in the speed of sound v_sos of the medium 3 by a corresponding change in the ultrasound signal frequency f of the ultrasound signal 6.

[0038] Fig.Figure 6 shows a realized concept for changing the ultrasound signal frequency f to compensate for a change in the opening angle phi of the sound lobe 10 of the ultrasound signal 6. Here, the ultrasound signal frequency f of the emitted ultrasound signal 6 is changed so that the opening angle phi of the sound lobe 10 of the emitted ultrasound signal 6 remains within a tolerance range delta_phi around a nominal opening angle phi_nom of 5°.

[0039] The ultrasound signal frequency f is abruptly changed to the altered current value f_actual of the ultrasound signal frequency, whereby the change to the altered value f_actual of the ultrasound signal frequency jumps from one tolerance limit phi_tol1 (6°) of the tolerance range delta_phi to the other tolerance limit phi_tol2 (4°) of the tolerance range delta_phi. This means that not every change in the speed of sound v_sos of the medium also leads to a change in the frequency of the excited ultrasound signal 6.

[0040] In alternative methods 1, which are not shown here, the ultrasound signal frequency f is continuously changed to the changed current value f_act of the ultrasound signal frequency f.

[0041] In the method 1 shown and the ultrasonic flow meter 2 shown, the current value v_sos,akt for the speed of sound v_sos of the medium 3 flowing in the measuring tube 4 is calculated by the control and evaluation unit 8 itself on the basis of a time-of-flight measurement of the emitted ultrasonic signal 6 and a known propagation length of the emitted ultrasonic signal 6. Reference sign 1 Procedure 2 Ultrasonic flow meters 3 Medium 4 measuring tube 5 transmitting ultrasonic transducers 6 Ultrasound signal 7 receiving ultrasound transducer 8 Control and evaluation unit 9 Ultrasound measurement path 10 Sound beam of the ultrasound signal v_sos speed of sound of the medium v_sos, det defined value of the speed of sound in the medium v_sos,akt current value of the speed of sound in the medium f Ultrasound signal frequency f_det defined ultrasound signal frequency f_akt current ultrasound signal frequency phi Opening angle of the sound lobe phi_det defined opening angle of the sound lobe phi_nom nominal opening angle of the sound lobe phi_0 Main radiation direction p sound pressure

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