JP4284746B2 - Flow rate calculation method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flow rate calculation method for measuring a flow rate of a fluid.
[0002]
[Prior art]
This type of conventional flow measurement device is provided with a pair of ultrasonic transducers on the upstream side and downstream side of the flow path of fluid, and from the upstream ultrasonic transducer to the downstream ultrasonic transducer, Alternatively, the propagation time of the ultrasonic wave from the downstream ultrasonic transducer to the upstream ultrasonic transducer is measured, and the flow rate of the fluid is calculated from the time difference to measure the flow rate (Japanese Patent Laid-Open No. 9-33308). Issue gazette).
[0003]
[Problems to be solved by the invention]
However, the conventional flow measuring device has the following problems.
[0004]
In other words, since the nth zero-cross point of the received ultrasonic waveform is used to measure the propagation time of the ultrasonic wave, noise such as an unnecessary vibration such as an ultrasonic vibrator is included in the ultrasonic wave transmission waveform or the received waveform. However, there is a problem that an error occurs in the measurement of the propagation time. That is, if this unnecessary vibration component is included in the received waveform, the nth zero-cross point for measuring the propagation time will include a jitter component that fluctuates in time, and becomes unstable accordingly. As a result, the measurement accuracy decreases.
[0005]
For example, even if the ultrasonic transducer is driven with a pulse waveform or a sine waveform having a single frequency, the transmitted ultrasonic waveform or the received ultrasonic waveform has a piezoelectric body constituting the ultrasonic transducer, Or, a so-called unnecessary vibration component other than the drive frequency due to the package or the fixing method will be included, and the zero-cross point will fluctuate with time and will include a jitter component, resulting in an error. was there.
[0006]
[Means for Solving the Problems]
In order to solve the above-described problems, the flow rate calculation method according to the present invention drives one of ultrasonic transducers disposed so as to face the upstream side and the downstream side of the flow path through which the fluid flows. A transmission step of transmitting ultrasonic waves, a reception step of receiving ultrasonic waves transmitted from the one ultrasonic transducer by the other ultrasonic transducer, and the one ultrasonic transducer transmitting ultrasonic waves A flow rate calculation method comprising: a measurement step for measuring a time from reception to reception by the other ultrasonic transducer; and a calculation step for calculating a flow rate of the fluid flowing through the flow path from the time measured in the measurement step. Thus, when the ultrasonic transducer is driven in the transmission step, a main frequency and a sub frequency that is an unnecessary vibration frequency of the ultrasonic transducer are synthesized. Thus, the ultrasonic vibrator is driven at the main frequency, and is composed of various frequencies including so-called unnecessary vibration components other than the driving frequency caused by the piezoelectric body, the package, or the fixing method constituting the ultrasonic vibrator. By driving the sub-frequency component at the same time, changing the amplitude of the sub-frequency component relative to the main frequency component to positive or negative, or changing its phase every time it is driven, for example, advance or delay, When the propagation time measurement at the zero cross point is performed, the time fluctuation of the zero cross point due to the unnecessary vibration component, that is, the jitter component is averaged. As a result, the flow rate can be measured with high accuracy without error.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, ultrasonic transducers are disposed opposite to an upstream side and a downstream side of a flow path through which a fluid flows, the ultrasonic transducers are driven at a driving frequency synthesized with a plurality of frequencies, and the vibrations The flow rate is calculated from the time difference during which the ultrasonic wave propagates between the children. With this configuration, it is possible to include various unnecessary vibration components generated when driving at a single frequency in addition to the main frequency component in the frequency at which the ultrasonic transducer is driven. The component can be increased or decreased, the zero cross point is more stable, and the accuracy of flow measurement is improved.
[0008]
The drive frequency is composed of a main frequency and a sub frequency, the sub frequency is an unnecessary vibration frequency of the ultrasonic vibrator, and the amplitude of the sub frequency is variously changed with respect to the amplitude of the main frequency. It was. With this configuration, it is possible to change the amplitude of the sub-frequency component consisting of unnecessary vibration components with respect to the main frequency component that drives the ultrasonic transducer, and to suppress this unnecessary vibration component, and to stabilize the zero cross point. , The accuracy of flow measurement is improved.
[0009]
The drive frequency is composed of a main frequency and a sub frequency, and the phase of the sub frequency is alternately inverted by 180 degrees with respect to the main frequency for driving. With this configuration, the phase of the sub-frequency component consisting of unnecessary vibration components can be changed and included with respect to the main frequency component that drives the ultrasonic transducer, this unnecessary vibration component can be suppressed, and the zero-cross point is stable. , The accuracy of flow measurement is improved. In the case of measuring a plurality of times, the phase is inverted every time driving is performed. By averaging the measurement results, the zero cross point is further stabilized and the measurement accuracy is improved.
[0010]
In addition, the drive frequency is configured by a main frequency and a sub frequency, and the phase of the sub frequency is sequentially changed with respect to the main frequency during a plurality of measurements and is driven. With this configuration, when measuring multiple times, the phase is changed sequentially for each drive so that the phase rotates once over the total number of measurements, so fluctuations in the zero cross point due to unwanted vibration are averaged and become more stable. Measurement accuracy is improved.
[0011]
Further, the sub-frequency is set to a resonance frequency caused by the package of the ultrasonic transducer. With this configuration, the variation of the zero cross point due to the resonance frequency caused by the package is suppressed, and the measurement accuracy is improved.
[0012]
The sub-vibration frequency is set to a resonance frequency caused by the piezoelectric body of the ultrasonic vibrator. With this configuration, the fluctuation of the zero cross point due to the resonance frequency caused by the piezoelectric body of the ultrasonic transducer is suppressed, and the measurement accuracy is improved.
[0013]
Further, the secondary vibration frequency is set to a resonance frequency caused by the measurement system of the ultrasonic flowmeter. With this configuration, the fluctuation of the zero cross point due to the resonance frequency caused by the measurement system of the ultrasonic flowmeter is suppressed, and the measurement accuracy is improved.
[0014]
In addition, a time difference in which the ultrasonic wave propagates is measured using a sing-around method. With this configuration, the ultrasonic flowmeter using the sing-around method for measuring a plurality of times works particularly effectively, and the zero cross point is averaged by a plurality of times of measurement, and the measurement accuracy is greatly improved.
[0015]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0016]
Example 1
1A and 1B are cross-sectional views of a flow rate measuring apparatus according to Embodiment 1 of the present invention. FIG. 1A shows a cross-sectional view of the flow path 1, and a pair of ultrasonic transducers 3, 4 are provided facing the upstream side and the downstream side of the portion 2 through which the fluid flows. The direction 6 in which the fluid flows (indicated by an arrow 5) and the direction 7 in which the ultrasonic wave propagates between the vibrators 3 and 4 intersect at an intersection angle An. FIG. 1B shows a side view of the flow path 1, W indicates the width of the flow path 1, and H indicates the height of the flow path 1. FIG. 2 shows a transmission waveform 8 and a reception waveform 9 of the ultrasonic transducer. Tt and Tr in FIG. 2 indicate a transmission start time and a reception time, respectively. In the normal case, the reception time Tr is often buried in the noise level and cannot be accurately specified. Therefore, the zero-cross point indicated by the nth Tr ′ is defined as the reception time, and the reception time and the transmission time are The difference, Tr′−Tt, is the ultrasonic wave propagation time. For example, the distance between the ultrasonic transducers 3 and 4 is L, the ultrasonic wave propagation velocity is Vs, the fluid flow velocity is Vf, and the ultrasonic wave from the upstream ultrasonic transducer 3 to the downstream ultrasonic transducer 4 is supersonic. When the propagation time of the sound wave is Tud and the propagation time of the ultrasonic wave from the downstream ultrasonic transducer 4 to the upstream ultrasonic transducer 3 is Tdu, the following equation is established.
[0017]
Tud = L / [Vs + Vf * COS (An)]
Tdu = L / [Vs-Vf * COS (An)]
From this, [Vs + Vf * COS (An)] = L / Tud
[Vs−Vf * COS (An)] = L / Tdu
Therefore, 2 * Vf * COS (An) = [(L / Tud)-(L / Tdu)]
Also, Vf = [(L / Tud)-(L / Tdu)] / [2 * COS (An)]
The fluid flow velocity Vf obtained by the above calculation is multiplied by the cross-sectional area (W * H) of the flow path 1 to obtain a flow rate. Therefore, the more accurately the propagation time from the upstream side to the downstream side or from the downstream side to the upstream side can be measured, the more accurate the flow rate measurement.
[0018]
FIG. 3 shows the result of observing the ultrasonic reception waveform 9 with a spectrum analyzer. In FIG. 3, the horizontal axis indicates the frequency, and the vertical axis indicates the intensity of the frequency component. The largest peak 10 indicates the main frequency component of the received waveform 9, and small peaks such as 11, 12, and 13 of the side lobes indicate a plurality of unnecessary vibration components. Thus, the received waveform 9 includes various unnecessary vibration components. Unnecessary vibration components 11, 12, and 13 included in the received waveform 9 greatly affect the zero-cross point Tr ′ for measuring the propagation time, and the zero-cross point Tr ′ vibrates with these unnecessary vibration components. That is, it means that a jitter component is included. Therefore, the propagation time can be measured more stably as the unnecessary vibration components are smaller and the jitter components are smaller.
[0019]
Note that these unnecessary vibration components change depending on which vibration mode is used as the main frequency of the piezoelectric vibrator constituting the ultrasonic vibrator. For example, when longitudinal vibration is used as the main frequency, the lateral vibration mode, the thickness shear mode, or the contour mode corresponds to the lower frequency side. Further, various unnecessary vibration components are generated depending on the package such as which part of the piezoelectric vibrator is bonded or fixed. Moreover, an unnecessary vibration component is also generated by a measuring system such as a sing-around method.
[0020]
FIG. 4 shows a transmission waveform for suppressing unnecessary vibration. 8 indicates a main frequency, and 14 indicates a sub-frequency. In FIG. 4, Tm represents the period of the main frequency 8, Td represents the start time difference between the main frequency 8 and the subfrequency 14, and Amp represents the amplitude ratio of the subfrequency component to the amplitude of the main frequency component. The main frequency was a burst wave consisting of three rectangular waves, and the sub-frequency was a burst wave consisting of two rectangular waves.
[0021]
In addition, the stability evaluation of the flow rate measurement was performed by repeating the flow rate measurement 100 times and judging from the variation of the measured value. When Td is changed in the range of (0.75 to 1.50) * Tm and the amplitude Amp is changed in the range of (0.1 to 0.4), the transmission waveform has a single frequency only. Compared with the case of a single frequency, the stability of the flow rate measurement was improved by about 5 to 20%. That is, the flow measurement result when the transmission waveform is composed of only a single frequency of the main frequency is Qmeas ± dQ, but when the transmission waveform composed of the main frequency and the sub-frequency is used, Qmeas ± dQ * It means that it was improved to (0.95-0.80).
[0022]
In this way, the ultrasonic transducer is not driven only at the main frequency, but by including sub-frequency components composed of various frequencies, the amplitude or phase of this sub-frequency component is changed, so that the received waveform is changed. The unnecessary vibration component contained can be reduced, the jitter component in time measurement can be reduced, and the accuracy of time measurement can be improved.
[0023]
(Example 2)
The frequency for driving the ultrasonic vibrator is configured by the main frequency 8 shown in FIG. 4 and the frequency constituting the unnecessary vibration component shown in FIG. That is, the received waveform of the ultrasonic wave was analyzed with a spectrum analyzer as shown in FIG. 3, and the frequencies of unnecessary vibration components such as 11, 12, and 13 were obtained and used as sub-frequency. Thus, the sub-frequency could be easily determined by performing spectrum analysis on the received waveform of the ultrasonic wave. In this way, the sub-frequency component can be set easily, and the jitter at the zero cross point of the received waveform can be efficiently suppressed.
[0024]
(Example 3)
FIG. 5 shows the phase relationship between the main frequency and the sub frequency used for even-number propagation time measurement.
[0025]
The sub-frequency was determined as shown in Example 2. 15 is a drive waveform consisting of three rectangular waves consisting of main frequency components, 16 is a two-wave rectangular wave consisting of sub-frequency components consisting of unwanted vibration components, 17 is the same frequency as 16, and 16 2 also shows two rectangular waves whose phases are delayed by 180 degrees. For example, ultrasonic waves are transmitted from the ultrasonic transducer 3 on the upstream side, received by the ultrasonic transducer 4 on the downstream side, the ultrasonic propagation time at that time is measured, and conversely, the ultrasonic transducer on the downstream side The ultrasonic waves are transmitted from 4 and received by the ultrasonic transducer 3 on the upstream side, and the ultrasonic propagation time and the time difference at that time are measured. In normal cases, such measurement is performed even times or a plurality of times, and the accuracy is improved by averaging the measured propagation times.
[0026]
In the case of measuring even number of times, for example, for the ultrasonic propagation time measurement of the odd number of times, a driving waveform consisting of 15 and 16 is used, the ultrasonic transducer is driven, the ultrasonic propagation time is measured, In ultrasonic propagation time measurement, an ultrasonic transducer is driven using a drive waveform consisting of 15 and 17, ultrasonic propagation time is measured, and these measurement results are further averaged to further increase the ultrasonic propagation time. It is possible to reduce jitter at the nth zero-cross point in ultrasonic propagation time measurement. For example, if a positive jitter occurs in the odd-number measurement, a negative jitter occurs in the even-number measurement. Therefore, by performing the averaging process, these positive and negative jitter components can be eliminated, and the measurement accuracy is further improved.
[0027]
(Example 4)
FIG. 6 shows the phase relationship between the main frequency and the sub-frequency used for multiple times of propagation time measurement.
[0028]
The sub-frequency was determined as shown in Example 2. Reference numeral 18 denotes a driving waveform consisting of three rectangular waves consisting of main frequency components, 19 denotes two rectangular waves consisting of sub-frequency components composed of unnecessary vibration components, 20 denotes the same frequency as 19 and 19 2 shows two rectangular waves whose phases are delayed by φ degrees. When the number of times of ultrasonic propagation time is determined in advance, for example, when measuring 10 times, the ultrasonic propagation time is measured using a drive waveform consisting of 18 and 19 for the first measurement. In the second measurement, the ultrasonic wave propagation time is measured using a drive waveform composed of 18 and a waveform 19 whose phase (φ) is delayed by 36 degrees.
[0029]
Thus, in the k-th measurement, the ultrasonic propagation time is measured using a drive waveform including 18 and the waveform 19 whose phase (φ) is delayed by 36 * (k−1) degrees. When the measurement is performed 10 times in this way, the drive waveform during the 10 measurements includes the waveform 20 whose phase is sequentially delayed from 0 degree to 324 degrees. Therefore, by measuring the ultrasonic wave propagation time using these drive waveforms and averaging these results, the jitter at the zero cross point for measuring the ultrasonic wave propagation time can be eliminated by these phase delays. Measurement accuracy will be further improved.
[0030]
(Example 5)
The frequency for driving the ultrasonic vibrator is constituted by the main frequency 8 shown in FIG. 4 and the sub frequency component 14 is constituted by an unnecessary vibration component caused by the package of the ultrasonic vibrator. FIG. 7 shows an overview of the ultrasonic transducer, and FIG. 8 shows a sectional view thereof. Reference numeral 21 denotes an acoustic matching layer made of a resin material bonded to the upper surface of the metal package 22, and 23 denotes a metal terminal block. A piezoelectric ceramic 24 is bonded to the inner surface of the metal package 22. Reference numerals 25 and 26 denote electrodes formed on the upper and lower surfaces of the piezoelectric ceramic 24, and the electrode 26 is electrically connected to the terminal 27 of the metal terminal block 23 through the metal package 22.
[0031]
On the other hand, the electrode 26 is electrically connected to a terminal 30 hermetically fixed to the terminal block 23 by an insulating glass 29 via a lead wire 28. When a drive signal is input to the terminals 27 and 30 of the ultrasonic transducer having such a configuration, the piezoelectric ceramic 24 vibrates longitudinally or spreads and ultrasonic waves are emitted through the acoustic matching layer 21. The At this time, various resonance vibrations accompanying the metal package 22 are generated as unnecessary vibrations. That is, the cylindrical metal package 22 generates contour vibrations or flexural vibrations to generate unnecessary low-frequency vibrations. The low-frequency unnecessary vibration associated with this package is emitted as ultrasonic vibration from the transmission-side vibrator or is induced by the received ultrasonic vibrator in the reception-side vibrator.
[0032]
Since these unnecessary vibrations are reverberated for a long time without being attenuated because the frequency is low, they are measured as jitter at the nth zero-cross point in the ultrasonic time measurement, which is a cause of a decrease in accuracy. Therefore, the accuracy of ultrasonic propagation time measurement can be greatly improved by using the resonance frequency resulting from these metal packages as a sub-frequency component and a driving waveform together with the main frequency component.
[0033]
(Example 6)
FIG. 9 shows the impedance characteristics of the ultrasonic transducer. A portion 31 indicates a resonance portion of an ultrasonic transducer that emits as ultrasonic vibration, 32 indicates an anti-resonance portion, and 33 indicates another vibration mode portion. For example, when the vibration emitted as the ultrasonic vibration is a longitudinal vibration, the other vibration mode 33 is an unnecessary vibration component such as a bending vibration, a lateral vibration, or a contour vibration. Therefore, by using these unnecessary vibrations as sub-frequency components as a drive waveform together with the main frequency component, these unnecessary vibrations can be suppressed to a small level, and the accuracy of ultrasonic propagation time measurement is greatly improved.
[0034]
(Example 7)
The case of the sing-around measurement method will be described below with reference to FIG. In the case of the sing-around measurement method, for example, an ultrasonic wave is emitted from the upstream ultrasonic transducer 3 toward the downstream ultrasonic transducer 4 and, for example, about 200 μsec later, Ultrasound is received. When an ultrasonic wave is received, the delay circuit operates and, after about 150 μsec, a signal is transmitted when the ultrasonic wave is received from the downstream ultrasonic vibrator to the upstream ultrasonic vibrator. This signal transmission is counted by a counter circuit, and for example, the time to repeat 100 times is measured by the single-around method. When this method is used, even a 10 MHz clock can measure time with a time resolution equivalent to 1000 MHz.
[0035]
In such a sing-around measurement, the ultrasonic transducer on the receiving side receives an ultrasonic wave every about 350 μsec during the measurement. For this reason, the ultrasonic transducer on the receiving side receives forced vibration at about 1/350 μsec, that is, 2.86 kHz. This forced vibration causes a jitter at the zero cross point, resulting in a decrease in measurement accuracy. Therefore, the accuracy of ultrasonic propagation time measurement is greatly improved by using these forced vibrations as sub-frequency components, that is, using the resonance frequency caused by the measurement system as a sub-frequency as a drive waveform together with the main frequency component.
[0036]
(Example 8)
The present invention in which no jitter is included in the nth zero cross point for time measurement is particularly effective for jitter of low frequency components. That is, once a jitter component having a low frequency is generated, it continues for several tens of cycles until attenuation, and the accuracy of time measurement is significantly reduced during that period. Therefore, as described in the seventh embodiment, the present invention is particularly effective for a sing-around measurement method including a low frequency component of about 2.86 kHz. That is, it is very difficult to measure the ultrasonic propagation time with high accuracy using the sing-around method without using the drive waveform composed of a plurality of frequency components of the present invention.
[0037]
【The invention's effect】
As is apparent from the above description, the flow rate calculation method of the present invention provides the following effects.
[0038]
(1) Since a pair of ultrasonic transducers are installed facing each other on the upstream side and the downstream side of the flow path, and the ultrasonic transducer is driven at a drive frequency synthesized with a plurality of frequencies, Jitter at the zero-crossing point for measuring the propagation time can be suppressed, and the measurement accuracy is improved.
[0039]
(2) Since the drive frequency is composed of a main frequency and a sub frequency, and the sub frequency is set as an unnecessary vibration frequency of the ultrasonic transducer, it is possible to easily perform sub analysis by analyzing the spectrum of the received ultrasonic wave. The frequency can be determined.
[0040]
(3) During even-number measurement, the phase of the sub-frequency is alternately inverted by 180 degrees with respect to the main frequency, and the measurement result is averaged to improve measurement accuracy.
[0041]
(4) The measurement accuracy can be improved by sequentially changing the phase of the sub-frequency with respect to the main frequency during a plurality of measurements and averaging the measurement results a plurality of times.
[0042]
(5) Since the sub-vibration frequency is set to the resonance frequency caused by the package of the ultrasonic transducer, the jitter component of the resonance frequency caused by the package can be suppressed, and the measurement accuracy can be improved.
[0043]
(6) Since the sub-vibration frequency is set to the resonance frequency caused by the piezoelectric body of the ultrasonic vibrator, the jitter component of the resonance frequency caused by the piezoelectric body can be suppressed, and the measurement accuracy can be improved. it can.
[0044]
(7) Since the sub-vibration frequency is set to the resonance frequency caused by the measurement system of the ultrasonic flowmeter, the jitter component of the resonance frequency caused by the measurement system can be suppressed, and the measurement accuracy can be improved. it can.
[0045]
(8) Since low-frequency jitter can be removed, it is particularly effective for flow rate measurement using the sing-around method.
[Brief description of the drawings]
1A is a plan sectional view of a flow rate measuring apparatus according to a first embodiment of the present invention, FIG. 1B is a side view of the apparatus, and FIG. 2B is a diagram showing transmission waveforms and reception waveforms in the apparatus. Fig. 4 is a diagram showing main and sub frequency components constituting the drive waveform in the apparatus. Fig. 5 is a diagram for explaining the drive waveform in the flow rate measuring apparatus according to the third embodiment of the invention. FIG. 7 is a perspective view showing the appearance of an ultrasonic transducer in a flow rate measuring apparatus according to a fifth embodiment of the present invention. FIG. FIG. 9 is a cross-sectional view of an ultrasonic transducer in the flow rate measuring device of Example 5 of the present invention. FIG. 9 is an impedance characteristic diagram of the ultrasonic transducer in the flow rate measuring device of Example 6 of the present invention.
DESCRIPTION OF SYMBOLS 1 Flow path 3 Upstream ultrasonic transducer 4 Downstream ultrasonic transducer 8 Drive waveform 9 Reception waveform 10 Main frequency 11, 12, 13 Unnecessary vibration waveform 22 Metal package 24 Piezoelectric ceramic

Claims (5)

A transmission step of transmitting an ultrasonic wave by driving one of the ultrasonic transducers arranged to face the upstream side and the downstream side of the flow path through which the fluid flows;
A receiving step of receiving the ultrasonic wave transmitted from the one ultrasonic transducer with the other ultrasonic transducer;
A measurement step of measuring a time from when the one ultrasonic transducer transmits an ultrasonic wave until it is received by the other ultrasonic transducer;
A flow rate calculation method comprising a calculation step of calculating the flow rate of the fluid flowing through the flow path from the time measured in the measurement step,
A flow rate calculation method for synthesizing a main frequency and a sub frequency which is an unnecessary vibration frequency when the ultrasonic transducer is driven in the transmission step. The flow rate calculation method according to claim 1, wherein the sub-frequency is a resonance frequency caused by the package of the ultrasonic transducer. The flow rate calculation method according to claim 1, wherein the sub-frequency is a resonance frequency caused by the piezoelectric body of the ultrasonic vibrator. The flow rate calculation method according to claim 1 , wherein the sub-frequency is a resonance frequency caused by the measurement system of the ultrasonic flowmeter. The flow rate calculation method according to any one of claims 1 to 4, wherein a time difference in which ultrasonic waves propagate is measured using a sing-around method .

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