Method for operating a measuring device for determining a fluid variable, and measuring device

Abstract

The present application relates to a method for operating a measuring device (1), the measuring device comprising a vibration transducer (5), the vibration transducer being driven by a test excitation signal (13) to excite a wave (7) in a fluid, the wave being guided back along a propagation path (8) to the vibration transducer or to at least one additional vibration transducer (6), thereby exciting the vibration transducer or the additional vibration transducer to vibrate, obtaining an output signal (14) associated with the vibration, and determining the frequency (18) of the output signal that lies within an analysis interval (16), the analysis interval starting as soon as the driving of the vibration transducer excitation wave ends and / or as soon as a maximum value (42) of the vibration amplitude is reached or exceeded, and / or an error message or notification (19) is output to a user and / or a device external to the measuring device when an excitation condition is met, and / or the measuring device is in a fault state. The present application also relates to a measuring device for determining a variable of a fluid.

Description

Technical Field

[0001] The present invention relates to a method for operating a measuring device for determining a fluid variable related to a fluid and / or a fluid flow rate of a fluid, the measuring device comprising: a measuring container for holding a fluid and / or a fluid flowing through the measuring container; and a vibration transducer arranged on the measuring container. The present invention also relates to a measuring device for determining a fluid variable related to a fluid and / or a fluid flow rate of a fluid. Background Art

[0002] Ultrasonic measuring instruments are one possible way to detect flow rate or other measured variables related to fluids. These instruments use at least one ultrasonic transducer to couple ultrasonic waves into the fluid flowing through a measuring tube. The waves are then directed to a second ultrasonic transducer, either along a straight path or after several reflections off walls or special reflective elements. The flow rate through the measuring tube can be determined from the difference in propagation time between the ultrasonic transducers when the transmitter and receiver are interchanged. For other measurement tasks, such as fluid identification based on the velocity of sound in the fluid, even just one ultrasonic transducer (to which the waves are directed) may be sufficient.

[0003] In order to be able to easily integrate such a measuring device into a fluid circuit, it is preferred that the device has a space-saving design and implements low energy consumption, for example enabling the battery to be operated over longer time intervals. When a signal with a specific frequency is used here to excite the ultrasound, for a given vibration transducer size and a given excitation power, the amplitude of the received signal generally depends on this frequency, wherein particularly large signal amplitudes of the received signal can be achieved at a specific resonant frequency of the system. In order to obtain a high signal quality and therefore also a high measurement accuracy, it is therefore preferred to be able to operate the vibration transducer at the resonant frequency of the measuring system or at least close to the resonant frequency of the measuring system. Due to component tolerances, the relevant resonant frequency (in particular its response to changes in the measured parameter, for example to changes in temperature) can vary between measuring devices (which are in principle of identical construction) or can be changed as part of an aging process for the measuring device.

[0004] To automatically adjust the operating point of the operating frequency, document EP2725353B1 proposes varying the driving frequency of an ultrasonic sensor at intervals and determining the strength of the signal received by the ultrasonic receiver for each interval. The driving frequency is varied until the received signal strength reaches a maximum. Because this requires performing a relatively large number of measurements until the operating point can be determined, adjusting the operating point consumes a relatively large amount of energy, and during these intervals, flow velocity measurements cannot be performed, or at least cannot be optimally measured. Summary of the Invention

[0005] The object of the present invention is therefore to specify a method for operating a measuring device in which the excitation signal used in the measurement, in particular the frequency of said excitation signal, can be determined with little expenditure of energy and / or time and / or changes in the resonance frequency can be easily identified.

[0006] According to the invention, this object is achieved by a method of the type described in the introduction, wherein a control device of the measuring device drives a vibration transducer by means of a test excitation signal in order to excite a wave in the fluid, the wave being guided along a propagation path back to the vibration transducer or to at least one additional vibration transducer of the measuring device, which is arranged on the measuring container, so as to excite the vibration transducer or the additional vibration transducer to vibrate, wherein the control device acquires an output signal associated with the vibration and determines the frequency of that section of the output signal which lies within an analysis interval, wherein the analysis interval begins as soon as the driving of the vibration transducer with the excitation wave ends and / or as soon as a maximum value of the vibration amplitude is reached or exceeded, wherein the determination of the fluid variable is performed in accordance with the determined frequency, and / or in the event of fulfillment of an excitation condition (the fulfillment of which depends on the determined frequency), an error message or notification is output to a user and / or a device external to the measuring device, and / or the measuring device is in a fault state, wherein in particular no determination of the fluid variable is performed.

[0007] When wave is excited by very broadband excitation signal, the vibration of receiving vibration transducer will be excited to have the main frequency that equals the resonant frequency of system.Described main frequency can be determined as the maximum value of power density spectrum for example, or is determined using frequency counter.But, in order to realize the stable transmission of vibration from vibration transducer to additional vibration transducer, or the stable transmission that returns to identical vibration transducer along propagation path, should use relatively narrowband excitation signal, for example, determine the periodic signal of the main frequency of excitation, this periodic signal is modulated by amplitude envelope (envelope).But, the result of doing so is that the corresponding spectrum distribution of excitation signal is imposed on output signal, therefore when analyzing whole output signal, the main frequency of excitation signal or the intermediate frequency between this frequency and actual resonant frequency are usually determined as obvious resonant frequency.

[0008] In the context of the present invention, it has been found that this problem can be avoided or significantly reduced by considering only the analysis interval, which begins once the driving of the vibration transducer excitation wave is completed or once the amplitude of the vibration reaches or exceeds the maximum value. This is because during the driving of the vibration transducer or the incident wave to excite the vibration, that is, before the said time, a forced vibration of the vibration transducer or the additional vibration transducer occurs, the frequency of which is determined by the frequency of the external excitation. On the other hand, once the driving of the vibration transducer excitation wave ends, in particular when it is possible to additionally delay the expected transmission time of the wave back to the vibration transducer or to the additional vibration transducer, or in the event of reaching the maximum value of the vibration amplitude, it can be assumed that, at least for most of the analysis interval, the vibration transducer or the additional vibration transducer is in free vibration, and therefore the vibration spectrum is dominated by the resonant frequency of the system or the vibration transducer or the additional vibration transducer. By considering only the output signal within the analysis interval, the influence of the spectrum of the test excitation signal on the determined output signal spectrum can be at least roughly eliminated, so that the frequency of the output signal within the analysis interval, in particular the resonant frequency of the system or the vibration transducer or the additional vibration transducer, can be determined with high accuracy by a single measurement. Thus, the excitation frequency can in particular be adjusted to a certain resonance frequency of the system, or in particular of the vibration transducer.

[0009] In particular, the determined frequency can be the dominant frequency of the output signal in the analysis interval. Various options for determining this dominant frequency will be described in more detail below. As part of the method according to the invention, in particular, the excitation frequency of the measuring excitation signal is determined as a function of the determined frequency. The excitation frequency can be identical to the determined frequency, can be offset from the determined frequency by a fixed offset, can be determined using a lookup table, etc. In particular, the frequency can be the frequency of a periodic signal, such as a sine wave or a square wave. The periodic signal can be amplitude modulated by an envelope, such as a square wave function of a defined length, a cosine squared window, or a Blackman-Harris window, etc.

[0010] As will be described in greater detail below, the determined frequency is preferably used to determine an excitation signal for the vibration transducer, which is used to determine the fluid variable. Additionally or alternatively, a parameter or correction factor that depends on the determined frequency can be taken into account when determining the fluid variable, in particular when processing the measurement data. For example, the parameter or correction factor can be determined from a lookup table, which is obtained, for example, empirically. For example, the correction factor can be used to correct for effects due to aging of the measuring device or environmental conditions (e.g., temperature), which also affect the resonant frequency or the main frequency in the system.

[0011] Additionally or alternatively, when determining the fluid variable based on a specific frequency, a notification or error message can be issued under certain conditions based on the specific frequency. For example, when the specific frequency is outside a target range or deviates from a target value by more than a limit value, the triggering condition for issuing a notification or error message can be met. For example, the resonant frequency of the measuring device, which can be determined as the specific frequency, can change due to aging or damage of the vibration transducer or an additional vibration transducer. Slight changes in the resonant frequency can be offset by adjusting the measurement excitation signal. However, if the deviation is too large, or the magnitude of the deviation indicates that measurement accuracy may be affected (e.g., due to aging or damage of the measuring device), this can be notified to the user, or a suitable notification or error message can be issued to a device external to the measuring device, particularly wirelessly, for example, to recommend repairing the measuring device or replacing certain components. The notification can be issued by a notification device installed on the measuring device, such as a loudspeaker, display, etc. However, a suitable signal can also be sent for the purpose of issuing the notification via a radio link, for example, to a central device or monitoring system of the supplier or manufacturer of the measuring device, or to the user's mobile phone. Additionally or alternatively, the measuring device can be switched to a fault state when a triggering condition is met. In the fault state, no measurement values ​​can be acquired, or measurement data acquired in the fault state is marked as erroneous. This prevents the acquisition of corrupted measurement data or measurement data that does not have the guaranteed accuracy when the determined frequency indicates a significant change in the characteristics of the measuring device.

[0012] When the frequency is determined at a plurality of time-spaced instants as described above, it may also be advantageous to store and / or analyze variations in the determined frequency for diagnostic purposes. For example, a characteristic change in the frequency near the end of the life cycle of the vibration transducer may be identified, or a step change in the frequency may be identified, which may indicate damage to the vibration transducer, such as a crack, etc.

[0013] In the following explanations and examples, emphasis is placed on a measuring device having a vibration transducer for coupling waves directly into a fluid and receiving waves directly from the fluid. However, the teachings of the present invention can also be applied to measuring devices in which the coupling of waves into the fluid and / or out of the fluid is performed indirectly through the wall of a measuring vessel (e.g., a pipe wall). For example, a vibration transducer and / or an additional vibration transducer can be configured to first excite a guided wave in the wall of the measuring vessel, which then couples the wave into the fluid. For example, Lamb waves can be coupled into the wall of the measuring vessel as guided waves.

[0014] Determining the fluid variable is preferably at least frequency-dependent, i.e., a measurement excitation signal is determined in accordance with the determined frequency, and the control device outputs this measurement excitation signal to the vibration transducer or an additional vibration transducer as part of determining the fluid variable. In particular, the excitation frequency of the measurement excitation signal can be determined in accordance with the determined frequency and, for example, set to this frequency. Thus, for example, excitation at the resonant frequency of the vibration transducer can be achieved. This allows for obtaining a sufficient signal amplitude with relatively low energy input, thereby ensuring low energy consumption of the measuring device.

[0015] The control device can determine digital measurement data from the output signal, which describe the instantaneous changes in the output signal, wherein the frequency and / or time at which the vibration amplitude maximum is reached is determined from the digital measurement data. In this case, the output signal is preferably converted by an analog-to-digital converter whose conversion rate is greater than the main frequency of the test excitation signal or the measurement excitation signal, or greater than the determined frequency or the resonant frequency to be determined, for example at least 3, 5 or 10 times greater. The digital processing of the measurement data makes it possible, for example, to determine the envelope of the output signal and thus the maximum amplitude particularly easily. In addition, the power density spectrum of the analysis interval or a subinterval of the analysis interval can be determined without difficulty, for example by means of a Fourier transform, in particular a fast Fourier transform, so that the main frequency (as a maximum of the power density spectrum) can be easily determined. The acquisition of the digital measurement data can, for example, begin at the end of the wave excitation or be offset relative to it by a specific time interval.

[0016] The frequency can be determined by determining the maximum value of the power density spectrum of the output signal in the analysis interval, or by analyzing the time intervals between the output signal and crossings above and / or below a specific limit value within the analysis interval. As described above, the analysis can be performed on digital measurement data. In this case, the power density spectrum can be generated by a Fourier transform. The intervals between zero crossings or other crossings above or below a limit value can also be easily determined in the digital measurement data. However, the time intervals between crossings above and / or below a specific limit value, in particular the time intervals between zero crossings, can also be determined without prior analog-to-digital conversion, for example by means of a frequency counter, period measurement, or (usually) a time-to-digital converter.

[0017] The control device can determine the envelope or instantaneous signal amplitude of the output signal, wherein the start and / or end of the analysis interval depends on the envelope or instantaneous signal amplitude. In particular, the maximum or minimum value of the output signal or digital measurement data between each two crossings above and / or below a limit value (thus, in particular between each two zero crossings) can be determined as the instantaneous signal amplitude. The envelope can be determined, for example, by low-pass filtering, squared, or otherwise rectified the output signal or by correspondingly processing the digital measurement data.

[0018] In particular, the analysis interval can begin at a maximum value of the vibration amplitude or at a specific time after this maximum value, wherein the maximum value can be determined from the signal amplitude envelope or the instantaneous signal amplitude. The end of the analysis interval can be selected such that the instantaneous signal amplitude or envelope at the end of the analysis interval falls below a specific limit value. As already explained, this selection of the analysis interval start can ensure that the spectrum of the test excitation signal has no or only a minimal effect on the determined frequency. This selection of the analysis interval end can advantageously exclude from the frequency determination time intervals in which the output signal or digital measurement data is dominated by noise.

[0019] Since the method for frequency determination according to the invention only considers the analysis interval, rather than the entire output signal, any influence of the spectrum of the test excitation signal on the frequency determination can be significantly reduced even at this stage, as previously described. However, if the dominant frequency of the test excitation signal is very close to the frequency to be determined, this can negatively impact the accuracy of the frequency determination. Furthermore, it should be avoided that the frequency to be determined lies at a minimum in the power density spectrum, as this can make it more difficult to identify the resonant frequency.

[0020] Therefore, in the method according to the present invention, a measuring device can be used in which, for a given test excitation signal amplitude, the amplitude of the output signal has a local maximum at at least one resonant frequency, wherein the test excitation signal is a periodic signal amplitude-modulated by an envelope, the resonant frequency and the excitation frequency of the periodic signal differ by at least 1% or at least 3% of the resonant frequency, and / or the excitation frequency and the envelope are selected such that the minimum of the power density spectrum of the test excitation signal is offset from the resonant frequency by at least 1% or at least 3% of the resonant frequency. Preferably, the resonant frequency and the excitation frequency of the periodic signal can differ by 5% or more of the resonant frequency, and / or the minimum of the power density spectrum of the test excitation signal can be offset from the resonant frequency by 5% or more of the resonant frequency.

[0021] When using a measuring device in which the amplitude of the output signal has a local maximum at at least one additional resonant frequency or a main frequency (when white noise is used as the excitation signal), and the test excitation signal is a periodic signal amplitude-modulated by an envelope, the envelope can preferably be selected so that the spectrum of the test excitation signal has a local minimum at the additional resonant frequency or the main frequency. For example, for a rectangular or pulse-shaped envelope, the pulse width can be selected in this way. When the spectrum of the test excitation signal has a minimum at a frequency, this frequency is also suppressed in the output signal. This prevents additional, undesired resonant or main frequencies from causing errors in the detected frequency or resonant frequency.

[0022] For ultrasonic transducers, the resonant frequency can be around 1 MHz, for example, it can be 1.05 MHz. Thus, the offset can be, for example, at least 10 kHz, at least 30 kHz, or 50 kHz or more. Preferably, the resonant frequency can be a frequency that is determined as the determined frequency in the method according to the invention. For a given test excitation signal amplitude, the amplitude of the output signal can in particular have an overall maximum at the resonant frequency. However, the measurement path or the vibration transducer or the additional vibration transducer can also have multiple resonant frequencies, wherein, for example, the excitation will be carried out at a resonant frequency that is not the overall maximum.

[0023] For example, a sufficient offset of the minimum of the power density spectrum of the test excitation signal from the resonant frequency can be achieved by making the main maximum of the power density spectrum of the test excitation signal relatively wide, for example having a width of at least 10% or at least 20% or at least 30% of the resonant frequency.

[0024] A maximum of 25, or a maximum of 15, or a maximum of 10 periods of the periodic signal can lie within the envelope. For example, 6 or 12 periods of the periodic signal can lie within the envelope. A larger width of the main maximum of the power density spectrum of the test excitation signal can be achieved by using a relatively short envelope. In particular, the amplitude of the test excitation signal can be zero outside the specified length of the envelope. A square wave function can be used as the envelope, so that, for example, a specified number of periods of the periodic signal are output without additional amplitude modulation, while no signal is output before or after this number of periods. The periodic signal can be, for example, a sine wave.

[0025] The measurement excitation frequency of the measurement excitation signal can be read from a lookup table based on the determined frequency. By determining a suitable lookup table, systematic errors in determining the frequency (e.g., the resonant frequency) can be eliminated. Similarly, when necessary, using a suitable lookup table can ensure that the excitation occurs at a specific position relative to the resonant frequency. Alternatively, the determined frequency can be used directly as the measurement excitation frequency, and / or the determined frequency can be scaled, and / or an offset can be added to determine the measurement excitation frequency.

[0026] In addition to the method according to the invention, the present invention also relates to a measuring device for determining a fluid variable associated with a fluid and / or a fluid flow rate of the fluid, the measuring device comprising a measuring container for holding the fluid, a vibration transducer arranged on the measuring container, and a control device, wherein the measuring device is configured to carry out the method according to the invention. In particular, the vibration transducer can be arranged and disposed on the measuring container such that, when excited by a test excitation signal from the control device, a wave excited in the fluid is guided along a propagation path back to the vibration transducer or to at least one additional vibration transducer of the measuring device, which is arranged on the measuring container, thereby exciting the vibration transducer or the additional vibration transducer to vibrate. The control device can be configured in particular to control the vibration transducer according to the method according to the invention, to obtain an output signal of the vibration transducer or the additional vibration transducer, and to process the output signal according to the method according to the invention.

[0027] The features described with respect to the method according to the invention can be used to develop a measuring device so that it has the advantages associated with the method, and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further advantages and details of the invention are shown in the following exemplary embodiments and in the accompanying drawings, in which:

[0029] Figure 1 Schematically shows an exemplary embodiment of a measuring device according to the invention;

[0030] Figure 2 A flow chart showing an example embodiment of a method according to the present invention;

[0031] Figure 3 schematically illustrates power density spectra of different test stimulus signals that can be used in an exemplary embodiment of the method according to the invention; and

[0032] Figure 4-7 The diagram schematically shows measurement and processing data obtained in an exemplary embodiment of the method according to the invention. DETAILED DESCRIPTION

[0033] Figure 1 A measuring device 1 is shown for determining a fluid variable, in particular a flow rate of a fluid through a measuring container 3 carrying the fluid. A vibration transducer 5 and an additional vibration transducer 6 are arranged on a side wall 2 of the measuring container 3. A control device 4 is capable of supplying the vibration transducers 5, 6 with relevant excitation signals in order to excite the transducers to vibrate and thus in each case excite a wave 7 for the vibration transducer 5, e.g. Figure 1As shown in . The wave 7 excited by the vibration transducer 5 is guided along the propagation path 8 to the additional vibration transducer 6, and vice versa. The special vibration transducers 5, 6 at which the wave 7 is incident are excited so as to vibrate. Therefore, the special vibration transducers 5, 6 provide an output signal, which is related to the vibration and is acquired by the control device 4. The wave 7 is guided along the propagation path 8 by means of ultrasonic mirrors 9, 10. Alternatively, it is also possible, for example, to use vibration transducers 5, 6, which transmit the wave 7 obliquely into the measuring container 3, so that the ultrasonic mirrors 9, 10 can be omitted. In another embodiment, the vibration transducers 5, 6 can first excite the side wall 2 to vibrate, which in turn excites the wave 7, instead of directly exciting a suitable wave in the fluid.

[0034] The measuring device 1 shown can be used, for example, to determine the flow velocity of the fluid 3 and, therefore, the flow rate of the fluid 3 by analyzing the difference in the propagation time of the wave 7 from the vibration transducer 5 to the additional vibration transducer 6 and vice versa. The process described below can also be used for measuring devices for determining other fluid properties, for example, for determining the fluid type by measuring the speed of sound. In this case, for example, the wave 7 can also be guided back from the vibration transducer 5 along a propagation path (not shown) to this transducer, without the need for the additional vibration transducer 6. Figure 1 In the example shown, this can be achieved, for example, by omitting the ultrasonic mirror 9 , so that the waves 7 are reflected back by the opposite side wall 2 to the vibration transducer 5 .

[0035] In order to achieve high measurement quality with low energy consumption, it is preferred that the main frequency of the measurement excitation signal for driving the vibration transducers 5, 6 (as part of determining the fluid variable) is equal to the resonant frequency of the system or the vibration transducers 5, 6, resulting in a particularly large amplitude of the output signal for a given excitation power. Since this frequency generally depends on operating parameters (such as temperature) and can also vary between different measuring devices due to component tolerances or due to aging of the measuring device, it is preferred to determine the corresponding frequency in a specific operating situation or to determine the corresponding frequency regularly during operation of the measuring device 1. Figure 2 The corresponding method implemented by the control device 4 is described in more detail.

[0036] In step S1, a periodic signal 11, such as a sine wave, is first generated at a determined frequency. The frequency of the periodic signal is selected so that it has a certain offset from the assumed resonant frequency, although the offset is not too large. For example, the frequency of the periodic signal may differ from the assumed resonant frequency by between 1% and 10%.

[0037] In step S2, an envelope 12 is generated, which is used in step S3 to amplitude modulate the periodic signal 11 in order to provide a test excitation signal 13. In the simplest case, the envelope 12 can be a square wave function, so that the test excitation signal 13 can correspond to a specific number of periods of the periodic signal 11, for example. However, other envelopes 12, which can also be referred to as window functions, are also possible, such as a cosine squared function, a trapezoidal envelope, etc. It is preferred to use a relatively short envelope 12. This results in a broad frequency spectrum and, therefore, robust excitation of the resonant frequency, even when the periodic signal is significantly detuned from the resonant frequency.

[0038] In step S4, the control device 4 outputs the test excitation signal 13 to the vibration transducer 5 to excite the transducer to vibrate. As a result, the wave 7 is excited in the fluid and guided along the propagation path 8 to the additional vibration transducer 6 as described above, thereby exciting the vibration of the additional vibration transducer 6.

[0039] In step S5, the control device 4 acquires the output signal 14, which is provided by the vibration of the additional vibration transducer 6. In particular, in step S6, an analog-to-digital conversion of the output signal 14 can be performed in order to provide digital measurement data 15. Optionally, this step can also be omitted, but the other steps for signal processing can be performed in analog form.

[0040] In step S7, an analysis interval 16 is determined for the output signal 14 or for the digital measurement data 15, specifically, for which the aforementioned resonant frequency is to be determined. To prevent the spectral composition of the test excitation signal 13 from causing errors in the frequency determination, analysis interval 16 is selected so that it does not begin until after the excitation of the wave 7 in step S4 has been completed. This can be easily achieved, for example, by setting a suitable time marker in the control device 4. Alternatively or in addition, for the same purpose, analysis interval 16 can be selected so that it begins only when or after a maximum amplitude of the vibration or output signal 14 or digital measurement data 15 is reached. For example, the maximum amplitude can be determined by calculating an envelope. Preferably, the end of analysis interval 16 is selected so that at this point in time the amplitude or envelope of the output signal 14 or digital measurement data 15 falls below a predetermined limit value. This allows for the exclusion of time intervals in which the output signal is primarily characterized by noise.

[0041] In step S8, the analysis interval 16 or a subinterval thereof is subjected to a Fourier transformation, in particular by means of a fast Fourier transformation, in order to obtain a power density spectrum of the output signal 14 in the analysis interval 16. Then, in step S9, the frequency 18 can be determined, in particular, as a global or local maximum of the power density spectrum 17.

[0042] As mentioned above, the determined frequency 18 is a good measure of the resonant frequency of the measuring system or the vibration transducers 5, 6. A sharp or sudden change in the determined frequency 18 can indicate that the vibration transducers 5, 6 or other components of the measuring device 1 are seriously aged or damaged, so that, for example, the measuring device 1 needs to be repaired or maintained. Therefore, a check can be performed in step S10 to confirm whether an excitation condition is met, for example, the excitation condition can be when the frequency 18 is outside a determined target range, or when the deviation from the target value exceeds a limit value. Alternatively or in addition, the instantaneous changes of the frequency 18 determined for multiple time intervals can also be analyzed by means of the excitation condition. For example, this can be used to identify step changes or similar changes in the frequency 18, which can indicate, for example, cracks or other damage to the vibration transducers 5, 6.

[0043] When the triggering condition is met in step S10, a notification 19 or error message can be output in step S11. For this purpose, the notification device 11 of the measuring device, such as a display, speaker, etc., can be used. It is particularly preferred, additionally or alternatively, that the notification or error message be sent wirelessly to a system that can store, manage, and visualize all meter data from the system operator or the water supply network. An example of such a system is the IZAR portal from Diehl Metering. Alternatively, suitable notifications can also be sent, for example wirelessly, to the user's mobile communication device.

[0044] If it is determined in step S10 that the triggering condition is not met, the measuring device 1 is configured to continue operating. Alternatively to the illustrated exemplary embodiment, in this context, steps S12 to S14 described below can also be performed when the triggering condition in step S10 is met. For example, the measuring device 1 can initially continue normal operation until repairs can be performed.

[0045] In step S12, a measurement excitation frequency 20 is determined based on the determined frequency 18. For example, a lookup table can be used for this purpose to correct for systematic errors as part of determining the frequency 18 and / or to specifically ensure that the measuring device is operated at a frequency different from the resonant frequency. In step S13, a measurement excitation signal 21 is then generated, the main frequency of which is equal to the measurement excitation frequency 20. This can be implemented, for example, by having the measurement excitation signal 21 as a periodic signal with the measurement excitation frequency 20 and amplitude modulated by an envelope.

[0046] In step S14, the vibration transducers 5 and / or 6 are driven using the measurement excitation signal 21 in order to determine the fluid variable. For example, the measurement excitation signal 21 can be used to determine the individual waves 7 as part of determining the transit time difference between the vibration transducers 5, 6, as described above, in order to determine the flow rate. Aside from using the measurement excitation signal 21 having the measurement excitation frequency 20 determined as described above, the processing here can correspond to the processing known from the prior art for determining transit time differences and will not be described in detail.

[0047] Alternatively or additionally, knowledge of the modified resonance frequency can also be used to determine a correction factor, in particular an empirically derived correction factor, for determining the fluid variable as part of the measurement. This can be achieved, for example, by means of a lookup table.

[0048] The drive data and signals provided and / or processed by the control device 4 are described in more detail below with reference to a number of examples. Figure 3 The power density spectra 22, 23, 24 for three different available test excitation signals 13 are shown. In all three cases, the periodic signal 11 of the test excitation signal 13 is a sine wave with a frequency of 1.1 MHz. In each case, a square wave function is used as envelope 12, the length of which is selected so that twenty-four vibration periods are output in the case of the power density spectrum 22, twelve vibration periods are output in the case of the power density spectrum 23, and six vibration periods of the sine wave are output in the case of the power density spectrum 24. Outputting fewer vibration periods results in a broadening of the maximum values ​​of the relevant power density spectra 22, 23, 24. In this respect, it is generally advantageous to use relatively few vibration periods. When a relatively large number of vibration periods is output, as in the example of the power density spectrum 22, a relatively large number of minima 25 can lead to a search for a region of resonant frequency, which can interfere with the determination of the resonant frequency, as explained in more detail below.

[0049] Figure 4 An example of an output signal 14 acquired by an additional vibration transducer 6, or digital measurement data 15 representing the output signal 14, is shown. In this example, a test excitation signal 13 having a power density spectrum 23, i.e., a main frequency of 1.1 MHz and 12 vibration periods, is used. As soon as the vibration of the additional vibration transducer 6 is forced to be excited, the amplitude of the output signal 14 initially increases. Since the characteristic frequency or resonant frequency of the system is to be determined, as described above, the analysis interval 16 is selected so that it does not begin until after a maximum value 42 of the amplitude of the output signal 14 has been reached.

[0050] Reference below Figure 5 The selected results of the analysis interval 16 are explained in more detail. Figure 5The instantaneous changes 26, 27, 28 of the corresponding main frequencies of the output signal 14 are shown for different main frequencies of the relevant test stimulus signal 13. Here, the instantaneous change 26 is related to the Figure 4 The output signal 14 shown in FIG corresponds to a short-time Fourier transform of the output signal 14, which is generated by the main frequency of the test excitation signal 13 of 1.1 MHz. For the transient 27, the main frequency of the test excitation signal 13 is 1.05 MHz, and for the transient 28, the main frequency is 1.0 MHz. The resonant frequency is slightly higher than 1.05 MHz. Figure 5 The relatively strong quasi-periodic oscillations of the instantaneous variations 26, 27, 28 clearly visible in the graph are mainly caused by the relatively short windows used for the associated short-time Fourier transform. For longer transform windows, a small change of the determined main frequency over a period of time will be obtained. However, despite this, Figure 5 In the example, a clear distinction can be made between the time interval before the maximum value 42 of the vibration amplitude is reached and therefore before the analysis interval 16, and the changes within the analysis interval 16. Before the start of the analysis interval 16, for each change 26, 28 in which the main frequency of the test excitation signal 13 differs significantly from the resonant frequency, the frequency is determined to be a main frequency between the main frequency 13 of the test excitation signal and the resonant frequency. As long as forced vibrations are detected at the vibration transducer 6, it is therefore almost impossible to reliably determine the resonant frequency. In the region of the amplitude maximum 42 of the output signal 14, i.e. at the start of the analysis interval or shortly before the start, the transient changes 26, 27, 28 of the main frequency converge towards the resonant frequency, so that in the analysis interval 16, the same main frequency (i.e., the resonant frequency) is determined to be approximately independent of the main frequency of the test excitation signal 13.

[0051] When only the main frequency is considered, care should be taken to ensure that the main frequency of the test excitation signal 13 is not too close to another resonant frequency. When this is the case, although the resonant frequency 34 in the power density spectrum still leads to a local maximum 43, as shown in FIG. Figure 6 , but it may be the case that the overall maximum 44 is at the additional resonant frequency 35. Figure 6By way of example, the following are shown: a power density spectrum 29 of the output signal 14, which results from an excitation at a main frequency of 800 kHz; a power density spectrum 30 of the output signal 14, which results from an excitation at a main frequency of 850 kHz; a power density spectrum 31 of the output signal 14, which results from an excitation at a main frequency of 900 kHz; a power density spectrum 32 of the output signal 14, which results from an excitation at a main frequency of 950 kHz; and a power density spectrum 33 of the output signal 14, which results from an excitation at a main frequency of 1.2 MHz. As can be seen, an overall maximum 43 is obtained only for the power density spectra 32 and 33 at the sought resonant frequency 34. The main frequency of excitation for the power spectra 29, 30, 31 is significantly closer to the additional resonant frequency 35, with the result that most of the excitation energy excites vibrations at this frequency. However, in Figure 6 It is also evident that even for these power density spectra 29, 30, 31 at least a local maximum 43 at the resonance frequency 34 results, so that even when using a test excitation signal 13 with a relatively low frequency, the resonance frequency 34 can be located by also taking into account the local maxima 43, 44 or by only taking into account the maxima 43, 44 in a special frequency range (e.g. above 1 MHz).

[0052] The described process (in which the frequency to be determined is determined in an analysis interval 16 in which the forced excitation of the additional vibration transducer 6 has already ended) substantially eliminates the influence of the spectrum of the test excitation signal 13 on the determination of the frequency 18, as previously described. However, it should be avoided that the frequency to be determined lies at a minimum in the power density spectrum of the test excitation signal 13. This will be discussed below with reference to Figure 7 Let me explain in detail. Figure 7 Four power density spectra 36, ​​37, 38, and 39 of the output signal 14 generated by different test excitation signals 13 are shown. In each of the obtained power density spectra 36 and 37, the dominant frequency or frequency of the periodic signal 11 of the test excitation signal 13 is 1.05 MHz, i.e., very close to the resonant frequency. In contrast, power density spectra 38 and 39 are obtained for an excitation frequency of the periodic signal 11 of 1.1 MHz. In the case of power spectra 36 and 39, the envelope is selected such that 24 cycles of the periodic signal are output. When obtaining power density spectra 37 and 38, only six cycles of the relevant periodic signal 11 are output in each case.

[0053] As expected, the power density spectra 36, ​​37, and 38 all have a maximum value 43 in the region of the resonant frequency 34, while the maximum value 40 of the power density spectrum 39 shifts sharply relative to the resonant frequency 34, and a local minimum 41 of the power density spectrum 39 can be seen close to the resonant frequency 34. This initially unexpected behavior is Figure 3 This is easily explained when looking at the power density spectrum 22 of the test excitation signal 13 shown in FIG, which is used to obtain the power density spectrum 39 or the associated output signal 14. This also has a minimum 25 at the location of the minimum 41, so that due to the power density spectrum 22, the test excitation signal 13 is able to oscillate at the resonant frequency 34 without substantially any input of energy.

[0054] In order to avoid this problem and the consequent possible incorrect identification of the sought frequency 18, it is preferred to obtain a relatively wide excitation or at least a relatively wide first maximum of the power spectrum of the test excitation signal 13 by using a short envelope 12, so that, for example, even a periodic signal 11 (e.g. a sine wave) only outputs 6 cycles or less.

[0055] Reference Number Table

[0056] 1. Measuring device

[0057] 2 Sidewalls

[0058] 3 Measuring container

[0059] 4 Control device

[0060] 5. Vibration transducer

[0061] 6 Vibration transducer

[0062] 7 waves

[0063] 8 Transmission channels

[0064] 9 Ultrasound mirror

[0065] 10 Ultrasound mirror

[0066] 11 Signal

[0067] 12 Envelope

[0068] 13 Test stimulus signal

[0069] 14 Output signal

[0070] 15 Measurement Data

[0071] 16 Analysis Interval

[0072] 17 Power Density Spectrum

[0073] 18 Frequency

[0074] 19 Notice

[0075] 20 Measuring the excitation frequency

[0076] 21 Measuring stimulus signal

[0077] 22 Power Density Spectrum

[0078] 23 Power Density Spectrum

[0079] 24 Power Density Spectrum

[0080] 25 minimum

[0081] 26 changes

[0082] 27 changes

[0083] 28 changes

[0084] 29 Power Density Spectrum

[0085] 30 Power Density Spectrum

[0086] 31 Power Density Spectrum

[0087] 32 Power Density Spectrum

[0088] 33 Power Density Spectrum

[0089] 34 Resonant frequency

[0090] 35 Resonant frequency

[0091] 36 Power Density Spectrum

[0092] 37 Power Density Spectrum

[0093] 38 Power Density Spectrum

[0094] 39 Power Density Spectrum

[0095] 40 Maximum

[0096] 41 minimum

[0097] 42 Maximum

[0098] 43 Maximum

[0099] 44 Maximum

[0100] Steps S1–S14

Claims

1. Method for operating a measuring device (1) for determining a fluid variable related to a fluid and / or a fluid flow of a fluid, the measuring device comprising: A measuring container (3) holding a fluid and / or a fluid flowing through the measuring container (3); and a vibration transducer (5) arranged on the measuring container (3), wherein a control device (4) of the measuring device (1) drives the vibration transducer (5) by means of a test excitation signal (13) so as to excite a wave (7) in the fluid, the wave (7) being guided back along a propagation path (8) to the vibration transducer (5) or to at least one additional vibration transducer (6) of the measuring device (1) arranged on the measuring container (3), thereby exciting the vibration transducer (5) or the additional vibration transducer (6) to vibrate, wherein the control device (4) obtains a signal related to the vibration transducer (5). the vibration-related output signal (14) and determining the frequency (18) of the output signal (14) that is located in the analysis interval (16), wherein the analysis interval (16) begins as soon as the driving of the excitation wave (7) of the vibration transducer (5) ends and / or as soon as a maximum value (42) of the vibration amplitude is reached or exceeded, the determination of the fluid variable is performed according to the determined frequency (18), and / or an error message or notification (19) is output to a user and / or a device external to the measuring device when an excitation condition is met, the satisfaction of the excitation condition being dependent on the determined frequency (18) and / or the measuring device (1) being in a fault state, in which the determination of the fluid variable is not performed; Characterized in that a measuring device (1) is used, wherein for a given amplitude of a test excitation signal (13), the amplitude of the output signal (14) has a local maximum (43, 44) for at least one resonant frequency (34, 35), wherein the test excitation signal (13) is a periodic signal (11) amplitude modulated by an envelope (12), the resonant frequency (34, 35) and the excitation frequency of the periodic signal (11) differ by at least 1% of the resonant frequency (34, 35), and the excitation frequency and the envelope (12) are selected such that a minimum (25) of the power density spectrum (22, 23, 24) of the test excitation signal (13) is offset relative to the resonant frequency (34, 35) by at least 1% of the resonant frequency.

2. The method according to claim 1, wherein: The determination of the fluid variable is dependent on the determined frequency, wherein a measurement excitation signal (21) is determined as a function of the determined frequency (18), and the control device (4) outputs the measurement excitation signal (21) to the vibration transducer or the additional vibration transducer (5, 6) as part of the determination of the fluid variable.

3. The method according to claim 1 or 2, characterized in that: The control device (4) determines digital measurement data (15) based on the output signal (14), which describe the instantaneous change of the output signal (14), wherein the frequency (18) and / or the time at which the maximum value (42) of the vibration amplitude is reached is determined based on the digital measurement data (15).

4. The method according to claim 1 or 2, characterized in that: The frequency (18) is determined by determining the maximum value (43, 44) of the power density spectrum (17, 29-32, 36-39) of the output signal (14) in the analysis interval (16), or by analyzing the time interval between the crossing points of the output signal (14) above and / or below a determined limit value within the analysis interval (16).

5. The method according to claim 1 or 2, characterized in that: The control device (4) determines the envelope or the instantaneous signal amplitude of the output signal (14), wherein the start and / or end of the analysis interval (16) depends on the envelope or the instantaneous signal amplitude.

6. The method according to claim 1, characterized in that: A maximum of 25 periods of the periodic signal (11) are within the envelope (12).

7. The method according to claim 1 or 2, characterized in that: A measuring device (1) is used, wherein the amplitude of the output signal (14) will have a local maximum for at least one additional resonant frequency or the main frequency when white noise is used as the excitation signal, the test excitation signal (13) being a periodic signal (11) amplitude modulated by an envelope (12), the envelope (12) being selected so that the spectrum of the test excitation signal has a local minimum at the additional resonant frequency or the main frequency.

8. The method according to claim 1 or 2, characterized in that: The measurement excitation frequency (20) of the measurement excitation signal (21) is read from the lookup table according to the determined frequency (18).

9. The method according to claim 1, wherein: The resonant frequency (34, 35) and the excitation frequency of the periodic signal (11) differ by at least 3% of the resonant frequency (34, 35).

10. The method according to claim 1, wherein: The excitation frequency and the envelope (12) are selected such that the minimum (25) of the power density spectrum (22, 23, 24) of the test excitation signal (13) is shifted relative to the resonant frequency (34, 35) by at least 3% of the resonant frequency.

11. The method according to claim 6, characterized in that: A maximum of 15 periods of the periodic signal (11) are within the envelope (12).

12. The method according to claim 6, characterized in that: A maximum of 10 periods of the periodic signal (11) are within the envelope (12).

13. A measuring device for determining a fluid variable related to a fluid and / or a fluid flow rate of a fluid, the measuring device comprising a measuring container (3) for holding the fluid, a vibration transducer (5) arranged on the measuring container, an additional vibration transducer (6) and a control device (4), characterized in that The measuring device (1) is configured to carry out a method according to any one of the preceding claims.

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