JP3324720B2 - Flow velocity measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow velocity measuring device for measuring the flow velocity of a fluid using ultrasonic waves radiated into the fluid by elastic vibration excited by the piezoelectric effect of a piezoelectric body.

[0002]

2. Description of the Related Art In recent years, flow velocity measuring devices utilizing ultrasonic waves have been attracting attention for use in water meters, gas meters, and the like. This flow velocity measuring device constitutes an ultrasonic transmitter using a piezoelectric material such as a piezoelectric ceramic which is an electric / mechanical energy conversion element, thereby emitting an ultrasonic wave into a fluid and passing a predetermined propagation path in the fluid. The velocity of the fluid is measured by measuring the velocity of the propagating ultrasonic wave. Such a flow velocity measuring device is described in, for example, "Flow Measurement Handbook", Nikkan Kogyo Shimbun, Kawada et al.

[0003] The measurement principle of a flow velocity measuring device using ultrasonic waves will be described below with reference to FIG. In FIG. 10, T1 and T2 are ultrasonic transducers, each of which is constituted by a piezoelectric vibrator such as a piezoelectric ceramic. And
By applying an AC voltage near the resonance frequency to the piezoelectric vibrator, ultrasonic waves can be radiated to the outside, and conversely,
The transmitted ultrasonic waves can be converted into a voltage.

By applying an electric signal, the transducer T
When an ultrasonic pulse (burst) is radiated from 1, the ultrasonic pulse propagates along the propagation path L1 having a distance of L and reaches the transducer T2 after a time t. The arriving ultrasonic pulse is converted into an electric signal by the transducer T2. This electric signal is amplified and applied to the transducer T1, and the transducer T1 emits an ultrasonic pulse again. Such a repetition is called a sing-around, and the time required to go around the system is a sing-around period, and the reciprocal thereof is a sing-around.
Around frequency.

In FIG. 10, the flow velocity of the fluid is V, the velocity of the ultrasonic wave is C, and the angle between the direction in which the fluid flows and the propagation direction of the ultrasonic pulse is θ. If the sing-around period is t1 and the sing-around frequency is f1 when the transmitter / receiver T1 is a transmitter and the transmitter / receiver T2 is a receiver, the following equation (Equation 1) holds.

[0006]

F1 = 1 / t1 = (C + Vcos θ) / L Conversely, when the transducer T2 is a transmitter and the transducer T1 is a receiver, the sing-around period is t2, and the sing-around is If the frequency is f2, the following equation (Equation 2) holds.

[0007]

F2 = 1 / t2 = (C−Vcos θ) / L Therefore, the difference Δf between the two frequencies is given by the following expression (Expression 3), and the flow velocity V of the fluid can be obtained from the frequency difference Δf. it can.

[0008]

Δf = f1−f2 = 2Vcosθ / L According to this measurement principle, the velocity V of the fluid can be measured without being affected by the fluctuation of the velocity C of the ultrasonic wave due to temperature or the like.

In the flow velocity measuring apparatus utilizing the above-described sing-around, conventionally, an ultrasonic wave radiated into a fluid has a fixed frequency, generally a frequency of around 200 kHz. In addition, in terms of the structure of the ultrasonic transducer, a piezoelectric vibrator that vibrates a circular or square-type piezoelectric vibrator in the thickness vibration mode, or a piezoelectric vibrator by laminating many piezoelectric ceramic thin plates to realize low-voltage driving Those that constituted children were common.

[0010]

However, the conventional apparatus using one fixed frequency as the ultrasonic wave radiated into the fluid has the following problems.

That is, if the frequency is lowered (for example, 30 kHz to 100 kHz), the conversion efficiency of the piezoelectric vibrator from electric energy to mechanical vibration energy, and consequently,
Although the radiation efficiency of the ultrasonic wave can be increased, the rise of the received signal is instead delayed. That is, the time required for the converted voltage waveform to reach the comparison voltage becomes longer. Therefore, the flow velocity measuring device has an advantage of low power consumption, but has a disadvantage in that the measurement accuracy of the propagation time and, consequently, the measurement accuracy of the flow velocity are deteriorated, and particularly in the case of a low flow velocity, the measurement error increases. It is also disadvantageous in that the piezoelectric vibrator becomes large.

On the other hand, the frequency of the ultrasonic wave is high (for example, 10
In the case of 0 kHz to 500 kHz), the rising of the received signal is fast, but the conversion efficiency, that is, the radiation efficiency of the ultrasonic wave is low. Therefore, the flow velocity measuring device has an advantage that measurement accuracy is high and measurement is possible down to a low flow velocity, but it is disadvantageous in that the power consumption of the piezoelectric vibrator, that is, the power consumption of the drive circuit increases.

As described above, in the conventional apparatus using one fixed frequency ultrasonic wave, it is possible to measure the flow velocity with high accuracy even when the flow velocity is low by increasing the measurement precision while keeping the power consumption low. It was difficult to do.

Further, as described above, in an ultrasonic transducer using a piezoelectric vibrator for vibrating a circular or square piezoelectric vibrator in a thickness vibration mode, the thickness of the piezoelectric ceramic is about 1 cm. As a result, there is a problem that a high drive voltage is required and power consumption is increased. On the other hand, when a piezoelectric vibrator is formed by laminating a number of piezoelectric ceramic thin plates for low voltage driving, the cost is high and there is a problem in reliability.

Accordingly, the present invention solves the above-mentioned conventional problems, and provides a flow velocity measuring apparatus which consumes a small amount of power, has a high measuring accuracy, and can measure the flow velocity with high accuracy even at a low flow velocity. The purpose is to do. In addition, it is another object of the present invention to provide a highly reliable flow velocity measuring device that is structurally inexpensive while realizing low power consumption.

[0016]

Means for Solving the Problems] Features of the flow rate measuring device according to the present invention uses two vibration modes of the piezoelectric vibrators constituting the ultrasonic transmitter, the according to the magnitude of the flow velocity of the measurement object By switching the vibration mode of the piezoelectric vibrator, when the predicted flow velocity is equal to or higher than a predetermined value, the frequency of the ultrasonic wave radiated into the fluid is switched to a lower frequency, and when the predicted flow velocity is lower than the predetermined value, the above frequency is changed. Is to switch to the higher one. Preferably, as the two vibration modes of the piezoelectric vibrator, a primary vibration and a higher-order (for example, tertiary) vibration of the same vibration are used. Alternatively, different vibration modes with different resonance frequencies may be used.

[0017]

[0018]

According to the above feature, when the predicted flow velocity is equal to or higher than the predetermined value, the frequency of the ultrasonic wave radiated into the fluid is switched to a lower frequency, thereby converting the electric / mechanical vibration energy conversion efficiency of the piezoelectric vibrator. As a result, the radiation efficiency of the ultrasonic waves can be increased, and the power consumption can be reduced. on the other hand,
When the predicted flow velocity is less than the predetermined value, by switching the frequency of the ultrasonic wave radiated into the fluid to a higher one, the power consumption increases, but the rise of the received signal becomes faster.
That is, the time required for the converted voltage waveform to reach the comparison voltage for detection is shortened, and the measurement accuracy of the propagation time and, consequently, the measurement accuracy of the flow velocity are improved. As a result, it is possible to accurately measure even a small flow velocity.

[0019]

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings. (Embodiment 1) FIG. 1 is a perspective view schematically showing a piezoelectric vibrator 4 used in an ultrasonic transducer of a fluid measuring device according to an embodiment of the present invention. In the figure, reference numerals 1 and 2 denote piezoelectric ceramics which are polarized in the thickness direction and have electrodes formed on upper and lower surfaces, respectively, and are bonded together to form a piezoelectric vibrator 4. Reference numeral 3 denotes a driving power supply, which generates an AC voltage for driving the piezoelectric vibrator 4. When an AC voltage having a frequency near the resonance frequency is applied to the piezoelectric ceramics 1 and 2 constituting the piezoelectric vibrator 4 by the driving power supply 3, the piezoelectric vibrator 4 oscillates in thickness and emits ultrasonic waves to the outside. I do.

FIG. 2 is a sectional view of an ultrasonic transducer 8 using the piezoelectric vibrator 4. In the figure, reference numeral 5 denotes a matching layer for efficiently radiating ultrasonic waves from the end face of the piezoelectric vibrator 4, and also serves to narrow the directivity of radiated sound pressure. Reference numeral 6 denotes a cushioning material that prevents the sound wave from radiating from surfaces other than the acoustic radiation surface of the piezoelectric vibrator 4. The piezoelectric vibrator 4, cushioning material 6, and matching layer 5 are provided in a case 7.
It is stored in. The ultrasonic transducer 8 configured as described above can be used with the case 7 securely fixed.
Ultrasonic pulse (burst) stably from the sound emitting surface only
Can be emitted (in the direction of the arrow). Also, conversely,
The arriving ultrasonic pulse is efficiently input to the piezoelectric vibrator 4 by the matching layer 5 and is converted into a voltage signal. FIG. 3 shows the frequency characteristics of admittance as viewed from the drive terminal of the ultrasonic transducer. In the figure, a frequency fr1 is a resonance frequency of the first-order thickness vibration, and a frequency fr2 is a resonance frequency of the third-order thickness vibration.

FIG. 4 is a block diagram of a flow velocity measuring apparatus according to the present embodiment using the above-described ultrasonic transducer. In this figure, T1 and T2 are the ultrasonic transducers shown in FIG. Also, in the following description, similarly to FIG.
V is the flow velocity of the fluid, C is the velocity of the ultrasonic wave, and θ is the angle formed by the direction in which the fluid flows and the propagation direction of the ultrasonic pulse.

In FIG. 4, when the measurement of the flow velocity of the fluid is started by the start circuit 11, the switching circuit 10
Switching is performed so that 1 is a transmitter and the transmitter / receiver T2 is a receiver. Further, the start circuit 11 notifies the time measurement circuit 19 of the start of measurement. And the trigger circuit 18
Triggers the drive circuit 12 to drive the transducer T1, and an ultrasonic pulse is emitted into the fluid. The ultrasonic pulse propagates along the propagation path L1 having a distance L, reaches the transducer T2, and is converted into a voltage signal. The output signal of the transducer T2 is amplified by the amplifier 13 and compared with the set level by the comparison circuit 14, whereby it is recognized that the ultrasonic pulse has reached the transducer T2. Then, after the elapse of a predetermined delay time by the delay circuit 17, the trigger circuit 18 again applies a voltage pulse to the transducer T1 to emit an ultrasonic pulse. This delay time corresponds to the time until the previous ultrasonic wave disappears from the measurement system. This operation is repeated the number of times set by the repetition setting circuit 15, and the elapsed time is measured by the time measuring circuit 19. The repetition control circuit 16 is responsible for terminating the repetition operation. By dividing the elapsed time at this time by the number of repetitions and subtracting the delay time, an average sing-around period t1 is obtained, and an average sing-around frequency f1 is obtained from its reciprocal. This calculation is executed by the flow speed calculation circuit 20, and the calculation result is temporarily stored.

Next, the switching circuit 10 is switched so that the transmitter / receiver T2 is used as a transmitter and the transmitter / receiver T1 is used as a receiver. Then, the same repetitive operation as described above is performed, and the average sing-around period t2 and the frequency f2 are calculated by the flow velocity calculating circuit 20. Then, the flow velocity V is calculated from the frequency difference Δf between the average sing-around frequencies f1 and f2 based on the above equation (Equation 3). The flow velocity calculation circuit 20 performs this calculation.

The response characteristics shown in FIG. 5 are obtained when the ultrasonic transducer shown in FIG. 2 is used. FIG. 5A shows the case where the frequency fr1 in the first mode of the thickness vibration is used.
FIG. 4B shows the frequency fr2 in the third mode of the thickness vibration.
Respectively shows the received waveforms of the ultrasonic transducer when using. Recognition of the received waveform is considered that an ultrasonic pulse has arrived when the received waveform level exceeds a predetermined level (hereinafter, referred to as a trigger level). Therefore, it is considered that the ultrasonic pulse has arrived when the time td1 has elapsed from the received wave in FIG. 10A and when the time td2 has elapsed since the received wave in FIG. Accordingly, the accuracy of the time measurement that can recognize the received waveform is higher when the third mode of the thickness vibration, which is a high frequency, is used than when the first mode is used, and the measurement accuracy of the flow velocity is also higher. . However, when the frequency of the ultrasonic wave is increased, the power consumption of a transmitter that generates a voltage pulse, an amplifier circuit, and the like increases.

As can be seen from equation (3), when the flow velocity V of the fluid is small, the difference in the average sing-around time becomes small, and high time measurement accuracy is required. Focusing on this point, in the device according to the present invention, when the flow velocity V of the fluid is small, the tertiary mode of the thickness vibration, which is a high frequency, is used to increase the measurement accuracy. The power consumption is reduced by using the third mode of thickness vibration. In this embodiment, the switching of the vibration mode is performed by switching the oscillation frequency of the driving power supply 3 between fr1 (frequency near) and fr2 (frequency near) by various known methods. (Embodiment 2) FIG. 6 is a sectional view of a piezoelectric vibrator used in an ultrasonic transducer of a flow velocity measuring apparatus according to a second embodiment of the present invention. In the figure, reference numerals 21 and 22 denote piezoelectric ceramics which are polarized in the thickness direction and have electrodes formed on upper and lower surfaces, respectively, and are bonded together with a metal plate 23 therebetween to constitute a piezoelectric vibrator. V1 and V2 are drive power supplies, which generate an AC voltage for driving the piezoelectric vibrator.
When an AC voltage near the resonance frequency is applied to the piezoelectric ceramics 21 and 22, the piezoelectric vibrator generates thickness vibration and emits ultrasonic waves.

FIG. 7 is a frequency characteristic diagram of admittance as viewed from the drive terminal of the ultrasonic transducer formed using the piezoelectric vibrator shown in FIG. In the figure, the frequency fr3
Is the resonance frequency when the polarities of the output voltages of the driving power supplies 1 and 2 are reversed, and the frequency fr4 is
Are the resonance frequencies when the polarities of the output voltages are the same.

As described above, in the present embodiment, two driving power supplies are used, and the polarity of one of them is adjusted to the polarity of the other or switched so as to change the ultrasonic wave output from the ultrasonic transducer. Switching frequency. Then, as in the first embodiment, when the flow velocity V of the fluid is small, the measurement accuracy is increased by using high frequency ultrasonic waves, and when the flow velocity V of the fluid is large, low frequency ultrasonic waves are used. To reduce power consumption. The block diagram of the flow velocity measuring device is the same as that of FIG. 4 described in the description of the first embodiment, and its response characteristics are also the same as those shown in FIG. (Embodiment 3) Next, a description will be given of an embodiment of a highly reliable fluid measuring device which is low in power consumption and low in manufacturing cost. FIG. 8 is a perspective view of a piezoelectric vibrator of the ultrasonic transducer used in the apparatus of the present embodiment. In the figure, reference numeral 30 denotes a piezoelectric ceramic element having electrodes formed on an inner peripheral surface and an outer peripheral surface of a cylindrical shape and polarized in a radial direction. When a drive AC voltage, which is the output of a drive power supply, is applied to the piezoelectric element 30, the piezoelectric element 30 vibrates in a longitudinal (axial) vibration mode and emits ultrasonic waves in the direction indicated by the arrow.

FIG. 9 is a perspective view of an ultrasonic transducer 32 constituted by using the piezoelectric element 10 shown in FIG. In the figure, reference numeral 31 denotes a matching material (matching layer) for efficiently radiating sound waves from one end face of the piezoelectric element 30, and also serves to reduce the directivity of radiated sound pressure. Although the matching layer 31 has a disk shape here, the directivity of radiated sound pressure can be improved by making the acoustic radiation surface concave as shown in the cross-sectional view shown in FIG. In addition, by covering the piezoelectric element 30 with a cushioning material as in the case of the ultrasonic wave transducer shown in FIG. 2, it is possible to prevent the emission of ultrasonic waves from surfaces other than the acoustic radiation surface.

In the piezoelectric element of this embodiment, since the drive voltage is applied between the electrodes formed on the inner and outer circumferences of the cylindrical piezoelectric element, the impedance seen from the drive terminals can be reduced. Therefore, an ultrasonic pulse can be emitted more efficiently at a lower driving voltage than a disk or square plate type piezoelectric vibrator used in a conventional flow velocity measuring device that vibrates in a thickness vibration mode. Further, since the structure is simpler than that in which a piezoelectric vibrator is formed by laminating a large number of piezoelectric ceramic thin plates, a low-cost and highly reliable piezoelectric element can be realized.

[0031]

According to the present invention, by using two vibration modes (for example, primary and higher order) of the piezoelectric vibrator and switching the vibration mode (that is, frequency) according to the magnitude of the flow velocity, the power consumption is reduced. It is possible to provide a flow velocity measuring device capable of measuring flow velocity with high accuracy even when the flow velocity is small and the measurement precision is high and the flow velocity is low.

[0032]

[Brief description of the drawings]

FIG. 1 is a perspective view of a piezoelectric vibrator used in a fluid measuring device according to a first embodiment of the present invention.

FIG. 2 is a sectional view of an ultrasonic transducer using the piezoelectric vibrator shown in FIG. 1;

FIG. 3 is a diagram showing admittance versus frequency characteristics of the ultrasonic transducer shown in FIG. 2;

FIG. 4 is a block diagram of a fluid measuring device.

FIG. 5 is a diagram showing a received waveform of an ultrasonic transducer.

FIG. 6 is a sectional view of a piezoelectric vibrator used in a fluid measuring device according to a second embodiment.

FIG. 7 is a diagram showing frequency characteristics of admittance of the ultrasonic transducer shown in FIG. 6;

FIG. 8 is a perspective view of a piezoelectric element used in a fluid measuring device according to a third embodiment.

FIG. 9 is a perspective view of an ultrasonic transducer using the piezoelectric element shown in FIG. 8;

FIG. 10 is a conceptual diagram for explaining the measurement principle of the ultrasonic flow velocity measuring device.

[Explanation of symbols]

 1,2; 21,22 Piezoelectric ceramic 3 Drive power supply 4; 30 Piezoelectric vibrator 5; 31 Matching layer 6 Buffer material 7 Case 8; 32 Transmitter / receiver 23 Metal plate

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01F 1/66

Claims (3)

(57) [Claims] An apparatus for measuring the flow velocity of the fluid by driving an ultrasonic transducer to emit an ultrasonic wave into a fluid and measuring a time required for the ultrasonic wave to propagate through a predetermined propagation path. When the predicted flow velocity is equal to or higher than a predetermined value by switching the vibration mode according to the predicted flow velocity using two vibration modes of the piezoelectric vibrator constituting the ultrasonic transducer. The present invention relates to a flow velocity measuring device, wherein the frequency of an ultrasonic wave radiated into a fluid is switched to a lower frequency, and the frequency is switched to a higher frequency when an estimated flow velocity is less than a predetermined value. 2. The first vibration and the higher vibration of the same vibration are used as two vibration modes of the piezoelectric vibrator.
The flow velocity measuring device according to the above. 3. The flow velocity measuring device according to claim 1, wherein different vibration modes having different resonance frequencies are used as the two vibration modes of the piezoelectric vibrator.