JPS597217A - Ultrasonic flowmeter - Google Patents

Abstract

PURPOSE:To improve the accuracy in measurement by determining the autocorrelation function of a prescribed modulation signal which modulates a carrier signal from said modulation signal and the reception signal received in the other of ultrasonic wave transmitter and receivers and obtaining the velocity and rate of flow of fluid. CONSTITUTION:The output S1 of an oscillator 4 which generates a carrier signal of a specified frequency is supplied to a modulator 5. A carrier signal S1 is subjected to modulation S3 in accordance with a modulation signal S2 and is supplied to a transmission circuit 6. The signal S3 is amplified there and is supplied with a switch 7 to one ultrasonic wave transmitter and receiver 1a, from which an ultrasonic wave is transmitted. On the other hand, the received ultrasonic wave is converted to the corresponding reception signal by an ultrasonic wave transmitter and receiver 1b, and is then supplied with the switch 7 to a reception circuit 8. The reception signal is amplified and supplied to a demodulator 9. The signal S2 and demodulation signal S5 are multiplied with a calculator 10, and the average value of the multiplied results is determined, whereby the autocorrelation function of the signal S2 is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention detects the flow velocity and flow rate of fluid flowing through the pipe from the time of propagation of ultrasonic waves transmitted and received between ultrasonic transducers disposed diagonally opposite to the pipe. Regarding the desired ultrasonic flow rate needle, in particular, the transmission signal that drives the ultrasonic transducer is fiAI-transmitted with a predetermined modulation signal, and the autocorrelation of this modulation signal is calculated based on the reception signal obtained accordingly and the Km signal. The present invention relates to an ultrasonic flowmeter in which the flow rate and flow rate of the fluid are determined using the autocorrelation function.

Fig. 1 is a diagram showing an example of the basic configuration of an ultrasonic flowmeter.As shown in this figure, ultrasonic pulses emitted by one ultrasonic transducer 1 & are propagated in the fluid 3 in the pipe line 2. Then, it is received by the other ultrasonic transducer 1b disposed diagonally opposite the ultrasonic transducer 1a. Here, if the diameter of the conduit 2 is the diameter of the conduit 2, the angle of incidence of the ultrasonic pulse is θ, the sound velocity in the fluid 3 is 01, and the flow velocity of the fluid 3 is ■, then the ultrasonic transducer 1& emits an ultrasonic pulse. The time 110T1 required for the ultrasonic transducer 1b to receive this ultrasonic pulse is given by: The time T2 required for reception at 1a is given by: Then, by eliminating the sound speed C from these equations (1) and (2) and finding the flow velocity V of the fluid 3, we get the following equation. Here, the values normally used in such an ultrasonic flowmeter are D=100tnms θ=22
By substituting ° into equation (8), and assuming that the sound velocity 0 of the fluid 3 measured by this ultrasonic flowmeter is 1A30 m/8 and the flow velocity V is 1 cm/s, the difference in propagation time of the ultrasonic pulse is T * - T
2G = about 8 ps (B X I O""I" seconds)
becomes. In this way, in an ultrasonic flowmeter, the propagation time difference TI''/)' of ultrasonic pulses is a very small value, so accurate measurement of the propagation time of ultrasonic pulses is essential.

By the way, the conventional sing-around method (or PLL method) is used to measure the propagation time of such ultrasonic pulses.
The pulse propagation time difference method and the pulse propagation time difference method are known. Both the sing-around method and the pulse propagation time difference method calculate T1 and T2 individually from the propagation time of the ultrasonic pulse, but they have the following drawbacks.

(υ Since the ultrasonic waves used for measurement are in the form of pulses, if there are air bubbles or dust in the propagation path of the ultrasonic pulses, the ultrasonic pulses will be attenuated and a difference will occur in the measurement of propagation time.

(II) Since an ultrasonic vibrator that generates ultrasonic waves does not vibrate immediately even when a voltage is applied, the ultrasonic pulse obtained by such an ultrasonic vibrator is
As shown in the figure, the envelope a becomes gentle. For this reason, the propagation time is measured by setting a comparison level l when measuring the propagation time and measuring the time when the ultrasonic pulse exceeds this comparison level l. If the image width of the pulse (specific wave) that first exceeds the comparison level l changes, a measurement error will occur.

(1) Furthermore, if the comparison level l is set low in order to eliminate measurement errors due to envelope changes, etc., not only will it be more susceptible to the effects of noise, etc., but the acoustic energy of the ultrasonic pulse with level h will also be wasted. Become.

(■) In order to eliminate measurement errors due to envelope changes using ultrasonic pulses with a sharp rise, it is necessary to instantaneously apply a high voltage to the ultrasonic transducer. It is necessary to devise ways to Furthermore, in this case, the circuit becomes complicated and the reliability of the circuit elements decreases because a high voltage is applied to the circuit elements, and since high voltage is used, it is not preferable from the point of view of explosion protection.

On the other hand, a phase difference method using continuous waves is also known as a measurement method different from the above measurement method, but such a phase difference method has the following drawbacks.

(V) Since the propagation time is measured using a continuous wave, a carrier wave is required to obtain the propagation time difference T1-T2 shown in equation (11) above, and measurement errors occur due to frequency vibration of this carrier wave. .

((Source) Furthermore, the propagation times T1 and T2 of the ultrasonic pulses as shown in equation (3) above cannot be determined individually.

The present invention solves the above points and provides an ultrasonic flowmeter that can eliminate the drawbacks shown in (II) to (1) above and can determine the propagation times T1 and T2 of ultrasonic pulses with high accuracy. The carrier signal is P &
One side of the front tube ultrasonic transducer is driven with a faintly modulated signal, and the resulting ultrasonic wave is received by the other side of the Ninghong ultrasonic transducer to obtain a received signal, and this received signal and the above-mentioned It is characterized in that the autocorrelation function of the modulation signal is determined from the modulation signal, and the flow rate and flow rate of the fluid are determined based on this autocorrelation function.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a block diagram showing a first embodiment of the ultrasonic flowmeter according to the present invention. In this figure, 4 is an oscillator that generates a carrier signal of a constant frequency, and the output of this oscillator 4 (!R transmission signal 81) modulates the carrier signal S1 according to KlliIl (!R transmission signal 81). , FM conversion), and the output of this modulator 5 (drive signal 83) is supplied to the transmitting circuit 6. The transmitting circuit 6 is for amplifying the drive signal -963, and supplies the rotational pressure obtained by this amplification operation to one of the ultrasonic transducers 1a via the switch 7, and transmits and receives the same ultrasonic waves. Ultrasonic waves are transmitted from the transducer 1a. On the other hand, the ultrasonic wave received by the ultrasonic transducer 1b is converted into a corresponding reception signal by the ultrasonic transducer 1b, and then supplied to the receiving circuit 8 via the switch 7. The receiving circuit 8 is for amplifying the received signal, and the output of this receiving circuit 8 (Iha signal s4) is supplied to the abdominal tone adjuster 9. The demodulator 9 demodulates the signal g84,
The output of this demodulator 9 (demodulated signal s5) is supplied to an arithmetic unit 10.

The calculation gain 0 is to multiply the modulation signal S2 and the demodulation signal S5 and calculate the average value of the multiplication results to obtain the autocorrelation function of the modulation 4Aqsz. ) is output from the output terminal 11. That is, here, if the ^IJ notation signal S2 is shifted from Yu (1), the rear trunk ID number S5 is changed to m (t - τ), so the performance,
The autocorrelation function ψ (ri) obtained by the device 10 is as follows.Here, the modulation signal S2 is an M-sequence whose clock period (parameter indicating a pseudo-random number) is t, and the signal period indicating repetition is N. If it is a signal, the arithmetic unit 10
The autocorrelation interval aVr(r> obtained at
1-1)・t, when in the /7J range, where k is an integer.

When the time difference τ is outside the above range, ψ(τ)=−1 (6).

Therefore, if the &IAI signal S2 is selected in this manner, the autocorrelation function V(τ)l= of the modulated signal S2 exhibits the characteristics shown in FIG. That is, here, the clock period tf bait period N is (N-1)t, (T, (ato...)
- If the value is selected to satisfy the equation (7), one ultrasonic propagation time T1 can be determined from the value of the autocorrelation function F(τ) obtained by measurement and the equation (5).

In this example, if the ultrasonic propagation time T1 is within the range shown by equation (7) above, as is clear from equation (5), the autocorrelation function F (T,) at this time is the variable Nto-1. Since it changes linearly with respect to .degree., once the autocorrelation function W(T1) is found, the ultrasonic propagation time T can be easily found from this autocorrelation function F(T,).

Furthermore, the autocorrelation function ψ(r) has a time difference τ of O~(N-1
) When in the range of to, its value is constant at -+,
It has a large gain when it is in the range of (N-1)t0 to Nto. In this way, the autocorrelation function S'(T,)Gj(
N-1) td7) Since it has an offset, the accuracy of time measurement can be improved, and the ultrasonic propagation time T can be determined with high accuracy. Furthermore, by switching the switch 7, the ultrasonic propagation time T2 in the opposite direction can be obtained in the same manner as the ultrasonic propagation time T1 described above.

Next, the integration period T in the ultrasonic flowmeter according to the present invention will be discussed using specific numerical values.

First, in this invention, the autocorrelation function of the modulated signal S2 is determined by setting the integration period T to infinity as shown in equation (4) above, but the actual integration is performed over a finite interval ( (finite period) TI: Therefore, the autocorrelation function Sfl(τ) in this case has an error with respect to the true value F(τ).

The relationship between this error and the integration period T0 will be described below.

First, the expected value E(F'(τ) of the autocorrelation function r'(τ)
] is given by the formula, and the variance Var(V
r'(τ)] is (@"...(9) 3N2.

It is given by the formula. Therefore, here 1. 7XlO
'+tp<τ)-E(ψ'(τ)) l , S7 V'a
If r (baka(-1-), then to/
To < 3.5 X I Cr0E was obtained, and the above (9
) formula gives us %/To<JIXIO'″8. Therefore, the integration period T. is t.

"0>8 X I O"""'"
``°1 and oak, especially when the signal period N is N=127,
TO>105, To −・・・(11
) However, To is the ultrasonic propagation time. Therefore, if the pipe diameter of the pipe line 2 (see FIG. 1) is D=25 mm, the integral period 1 is T,>1. s
(s v: a) ・・・・・・(12
), becomes. As is clear from this equation (confirmation), the integration period To is a sufficiently small and practical value.

Next, using specific numerical values, we will consider the difference between the resolution required for the ultrasonic flowmeter according to the present invention and the resolution required for a conventional ultrasonic flowmeter. First, we will discuss the resolution that is insufficient for a conventional ultrasonic flow meter. Ultrasonic propagation time T in a conventional ultrasonic flow meter. is obtained from equations (1) and (2) above. In addition, in the above formula (8), T2
If −TI=δT, sin 2θ J T = −・V −T,・T.

1゛“2θ・V-T2 ・・・・・・(14
) Ding-Tm 〇 is obtained. Therefore, the ultrasound propagation time Tx, T2
The resolution δT/T 0 required for interviewing is as follows. Here, if the incident angle θ of the ultrasonic wave is 22°, the sound velocity in the fluid 3 is 011450 m/8, and the flow i [V of the fluid 3 is 1 crn, then the resolution δT/T 0 shown in equation (15) is
XI0-cI-0,-(16)c.

On the other hand, in the ultrasonic flowmeter according to the present invention, the ultrasonic propagation time T0 is
17). Therefore, when calculating the ultrasonic propagation times T1 and T2 from this trunk polymorphism and the above formulas (to) and (2), the required table resolution δT/t is as follows. That is, this ultrasonic flow 1. In total, (
The required resolution can be lowered by 89 cycles. For example, if the signal period N'IN=127, the resolution at this time is δT/log, δT - Hori 7 X I O-'”-'-
(19) becomes 0.

In this way, in this ultrasonic flowmeter, if the time base (clock frequency JuJ) t, t knee is broken down to the required accuracy, it is relatively easy to obtain the required g! Ultrasonic propagation period T1 with precision
, T2 can be obtained.

Note that in this embodiment, the propagation period T1 is (N-1)
The clock period t and the signal period N are set so that t, 0(T, <, N-to, that is, this propagation time T matches the period of the modulation signal S2. ,
The clock period and the signal period N may be set so that the propagation time T1 is approximately an arbitrary integral multiple of Nt.

FIG. 5 shows the ultrasonic flow 1 according to the present invention. FIG. 3 is a block diagram showing a second example of the total. In this figure, 12 is a modulator that modulates the carrier signal S1 obtained by the oscillator 4. The IMI device 12 performs IFM modulation on the carrier signal 81 based on the modulation signal 82m supplied via the terminal 13, and the drive signal 83a obtained thereby (this drive signal 5
31L is a signal in which a signal with a frequency fhigh and a signal with a frequency 'LOW are alternately combined) into the intermodulation circuit 1.
4. On the other hand, the ultrasonic wave received by the ultrasonic wave transmitter/receiver 1b is converted into a corresponding reception signal here, and then sequentially passed through the switch 7 and the reception circuit 8 to the cross-modulation 101.
14. The cross-modulation circuit 14 mixes the driving signal (M number S3a) and the signal 84a supplied via the receiving circuit 8, and supplies the beat signal s6 obtained by the mixing to the counter 15. Here, if the drive signal S3a is as shown in FIG. 6(A), and the signal +384a is as shown in FIG. 6(B), the frequency of this beat signal S6 is the frequency of the drive signal 83a. Since it is zero when the frequency of the signal S4a and the frequency of the signal S4a are the same, and fhigh - fLow when they are different, the counter 1
The heat signal s6 supplied to 5 is as shown in FIG. 6(c). That is, the counter 15 is supplied with a pulse train beat signal s6 proportional to the phase difference (ultrasonic propagation time) T between the drive signal S3a and the signal S4a and the frequency difference fhighfLOW of the drive signal S3. Therefore, the count result of the counter 15 is proportional to L-ψ"(T1), and the count result of the counter 15 is supplied to the arithmetic unit 10a.

The arithmetic unit 10a determines the ultrasonic propagation time T from the counting results, and also calculates the flow velocity (2) from this ultrasonic propagation time T1 and the ultrasonic propagation time T2 obtained by switching the switch 7.

(7) j5km, two frequencies fltigh, f
Even when low is set to IN, the autocorrelation function F(τ) can be equivalently determined from the argument result of the counter 15.

Furthermore, in this case, similar measurement results can be obtained by using a frequency comparator instead of the intermodulation circuit 14 and integrating the output of this frequency comparator.

FIG. 7 is a block diagram showing a third embodiment of the ultrasonic flowmeter according to the present invention. In this figure, parts corresponding to those in FIG. 3 are given the same reference numerals. In this figure, 16 is a switch for circuit calibration, and the common contact 16a and the contact 16b of this switch 16 are connected at predetermined time intervals or at arbitrary timing. As a result, the fpA signal S2 is supplied to the arithmetic unit 10 via the switch 16, and the autocorrelation function number W(o) at this time is determined in the arithmetic unit 10.

Here, if this modulation signal S2 is, for example, the aforementioned M-sequence signal, the autocorrelation function F (0) is f (0) =
1, so using this llf (o) = 1, we can calculate n
The number of autocorrelation functions constituting the device 10 is 39. It is possible to calibrate circuits, integration circuits, etc.

FIG. 8 is a block diagram showing a fourth embodiment of the ultrasonic flowmeter according to the present invention, and in this figure, parts corresponding to those in FIGS. As shown in this figure, in this ultrasonic flowmeter, switch 18
By switching /, the intermodulation circuit 13 to the arithmetic unit 10a can be calibrated.

Further, in the first to fourth embodiments described above, an M sequence signal is used as the fly signal, but this may be used, for example, as an average remainder sequence (L sequence), a twin prime number sequence, a Berger sequence,
A periodic pseudo-random signal such as a series or a sine wave signal may be used. In this case as well, since the autocorrelation functions of all these modulated signals have periodic sharp peaks, it is possible to apply an offset to the time measurement as in the case described above.

As explained above, the ultrasonic flow rate of 6t according to the present invention is
One of the ultrasonic transducers is driven by a modulated signal obtained by modulating a carrier signal with a predetermined modulation signal, and the ultrasonic wave thus obtained is received by the other of the ultrasonic transducers. The autocorrelation function of this modulated signal is determined from the received signal obtained thereby and the modulation signal described above, and the flow rate of the fluid is determined based on this autocorrelation function, so that the following effects can be obtained. can.

(1) Since the signal in the circuit including the ultrasonic transducer is a continuous wave, it is easy to apply AGO (auto gain control) and is not affected by attenuation in the fluid.

(n) (i+) If FM modulation or the like is used as the modulation, detection errors caused by a delay in the rise of the ultrasonic transducer, etc. can be eliminated.

(P/) Since the signal in the circuit is a continuous wave, large acoustic power can be obtained even if the drive amount ratio of the ultrasonic transducer is low, and the transmitter circuit can also be simplified, improving the reliability of the circuit. It can be closed. Furthermore, in this case, since the voltage is low, it is advantageous in terms of explosion protection.

(V) Even when the frequency of the carrier No. 111 that drives the ultrasonic transducer changes, the side value does not change, so the excavator that generates the transmission signal can be converted to f1ij. In this case, f! I! Signal clock frequency 3<sl
Since tu affects measurement accuracy, it is necessary to stabilize the clock period t, but in this case, a crystal oscillator or the like may be used.

(The VO ultrasound propagation times T1 and T2 can be determined individually.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of the basic configuration of a general ultrasonic flowmeter, Fig. 2 is a waveform diagram for explaining Fig. 1, and Fig. 3IA is a first embodiment of the ultrasonic flowmeter according to the present invention. 4 is a characteristic diagram for explaining FIG. 5, FIG. 5 is a block diagram showing a second embodiment of the ultrasonic flowmeter according to the present invention, and FIG. 6 is a characteristic diagram for explaining FIG. 5. Waveform diagram for
The figure is a block diagram showing a third embodiment of the ultrasonic flow rate needle according to the present invention, and FIG. FIG. la, lb...Ultrasonic transducer, 2...Tube Nir, 3 fluid, 5.5a...Modulator, 9
... Demodulation eH110, 10a ... Arithmetic unit, 1
4...Conversion training circuit, 15...Counter.

Claims (1)

[Claims] In an ultrasonic flowmeter that determines the flow rate and flow rate of fluid flowing in a conduit from the propagation time of ultrasonic waves transmitted and received between ultrasonic transducers disposed diagonally opposite to the conduit, a carrier signal is set to a predetermined value. Driving one of the ultrasonic transducers with a modulated signal modulated by the modulated signal, and receiving the resulting ultrasonic waves at the other of the ultrasonic transducers to obtain a reception signal,
An ultrasonic flow needle characterized in that an autocorrelation function of this modulation I6 is determined from the received signal and the modulation signal, and the flow velocity and flow rate of the fluid are determined based on this autocorrelation function.