CA2279257A1

CA2279257A1 - An ultrasonic flow velocity measuring method

Info

Publication number: CA2279257A1
Application number: CA002279257A
Authority: CA
Inventors: Hak Soo Chang
Original assignee: Changmin Tech. Co. Ltd.; Hak Soo Chang
Current assignee: Changmin Tech Co Ltd
Priority date: 1998-09-03
Filing date: 1999-07-30
Publication date: 2000-03-03
Also published as: KR100298474B1; CN1155831C; DE19942138B4; KR20000018659A; DE19942138A1; RU2193208C2; CN1248703A

Abstract

An ultrasonic flow velocity measuring method comprises an ultrasonic transit time difference flow velocity measuring method, without transmitting/receiving an ultrasonic pulse and measuring the ultrasonic transit time if a flow velocity is measured in a river, an open sluice way channel and a pipe of a larger inner diameter based on the ultrasonic transit time difference method, including steps of amplitude-modulating a continuous ultrasonic sinewave f C into a frequency f M, transiting/receiving the amplitude-modulated signal and using the amplitude-modulation wave in measuring the ultrasonic transit time; and a phase difference flow velocity measuring method irrelevant to the change of the sound velocity, if the flow velocity is measured based on the phase difference method, including steps of amplitude-modulating an ultrasonic wave into a predetermined frequency f M, if a phase difference .DELTA.~C transited in the directions similar and contrary to the flow velocity is equal to n.pi. + a.pi., obtaining n.pi. using the signal f M
and measuring a part a.pi. in that a.pi. < .pi., obtaining the phase difference between the signals f C and obtaining the total phase differences .DELTA.~C, precisely.

Description

ULTRASONIC FLOW VELOCITY MEASURING METHOD
Background of the Invention The invention is related to providing a method of measuring a flow velocity using an ultrasonic wave for calculating a flow rate of fluid in a larger river or open sluice way channel and a flow rate of liquid and gas in a pipe of a larger inner diameter.
Prior Arts A core portion of a recent well-known ultrasonic flow rate measuring system for the larger open sluice way channel and the pipe of a larger inner diameter is designed to measure a flow velocity of liquid and gas, so the system is normally called "a flowmeter'.
Most of the flow rate measuring systems are supposed to measure a flow velocity based on an ultrasonic transit time difference flow velocity measuring method.
As shown in Fig. l, the ultrasonic transit time difference flow velocity measuring system is as follows: ultrasonic transducers 1 and 2 for transmitting/
receiving an ultrasonic wave are mounted at an angle a to face against each other.
A switch circuit 3 functicms to switch the transducers 1 and 2 in turns to the inputs of transmitting and receiving circuits, for example, an ultrasonic pulse oscillator 4 and an ultrasonic receiving signal amplifier 5. A pulse shaping circuit 6 receives an amplified signal and shapes it into a pulse signal of a shorter period. A time interval measuring apparatus 7 rr~easures transit times tl and t~ in an interval distance L
from the transmitting time till the i°eceiving time. An arithmetic logic Lmit 8 computes a flow velocity based on expression ( 1 ).
That is to say, the transit time tl, which the ultrasonic pulse is transmitted ,.., from the transducer 1 to the transducer ? (as shown in Fig. 1 ), is measured. On the contrary, the transit time t~, which the ultrasonic pulse is transmitted from the ,~ transducer 2 to the transducer 1, is measured. These times measured are made as follows:
t 1 C -~- vcosa ' t = C - L cos~~
It seems as if the transit time difference (0t = t2 - tl) could be presented as follows:
Llt= ~Lcoacr h C 1 Wherein, C is a sound velocity of liquid or gas, L is an interval between transducers 1 and 2 and V is an average flow velocity in the interval L.
The flow velocity V from the expression ( 1 ) is deduced as follows:
- alt C' ('?) 2 L cos,x It may be called "A Transit Time Difference Flow Velocity Measuring Method", because the flow velocity V is proportional to the transit time difference fit. It seems that the transit time difference flow velocity measuring method is related to the sound velocity, because there is an item C~ of the square of the sound velocity in the expression (2). It appears as if the item C~ of the sound velocity must be measured. The ,square of the sound velocity is represented as follows:
C?- L.a t 1 . t .o The sound velocity item C:' is substituted into the expression (2) to make the final flow velocity measuring expression as follows:
L , t.> t 1 - L ~' t~' t ~ (;>>
2 L co sc~ t i ~ t ~ ? d t I ~ t ., Then, the flow velocity is obtainable by measuring only the ultrasonic transit times t~ and tl and computing the expression (3), because LJ/2d = const.
,.... Typical prior arts are disclosed in U.S.A. Patent x,531,124 granted at July 2, 1996, Japanese Patent No.2,676,321 granted at July 25, 1998, Manual of Ultrasonic flow Measuring and Apparatus thereof and Ultrasonic Flowmeter related to Model UF-2000C manufactured by Ultraflux Co.
The transit time difference flow velocity measuring method has a great advantage in that the flow velocity measuring is simply performed as illustrated in the expression (3), even though the sound velocity is seriously changed in fluid.
That is, although the expression (3) seems like being related to the square of the sound velocity according to a deliberative method of the flow velocity measuring expression, it is not principally related to the flow velocity.
For example, the difference between the reciprocal numbers with respect to the transit times tl and t2 is obtained as follows:
1 1 2 Tlcosc~
tl _ t, - L
The items of the sound velocity C are offset to each other. Therefore, the flow velocity V is as follows:
v- L r 1 _ 1 )- L2 C t?_ tt ) cosc~ ' tl t~ 2d tl ~ tz Wherein, d = Lcosa.
As a result, the expression obtained is the same as the one (3).
It has a great advantage in that the transit time difference flow velocity measuring method has no relation with the change in the great range of the sound velocity C in flLlld. But, tile transit time difference flow velocity measuring method is limited in its using. For example, when the transit distance L is very small and/or the flow velocity V is very low, it is very difficult to measure the glow velocity, precisely. If L = O.OSm, 'V = 0.1 m/s, ec = ~~t~ and C ~ 1500m/s, 0t ~ 3.14 I
0 ~S.
If it is intended to measure a very little time difference within the en-or range of I%, the time difference absolute measuring el-1-or should not exceed the range of 3 ~ 10 1 1 S. Mleasuring the time difference based on such like a method needs a ,.... relative complex time intel-val measuring apparatus. Also, an apparatus for catching moments of transmittin';/receiving the ultrasonic pulses must be very stable and -, ,.,, precise. As mentioned below, the transit time difference flow velocity measuring method causes many problems, when the gas flow velocity is measured in the pipe, or the horizontal flow velocity is measured in the channel or river.
In addition to the transit time difference flow velocity measuring method, an ultrasonic phase difference flow velocity measuring method is also well-known.
For example, there are Dutch Patent Laid-Open Publication No. DE 19722140 at November 12, 1997 and Tapanese Patent Laid-Open Publication No. Hei 10-104039 published at April 24, 1998, which are entitled "A multi-channel flow rate measuring system".
Figs. 2A and 2B show a typical configuration of a phase difference flow velocity measuring system. Ultrasonic transducers 1, 1' and 2, 2' are positioned to face against each other. A sinewave oscillator 9 generates a sinewave having a frequency f. A phase shifter 10 adjusts the phase of received ultrasonic signals. An amplifier 11 amplifies the received signals from the phase shifter 10 and the transducer 1'. A phase difference discriminator 1 ? measures the phase difference between the received phase signals. When the sinewave oscillator 9 is operated, the transducers 2 and 2' transmit ultrasonic waves at the same phase. At that time, the phase signals which the receiving transducers 1 and 1' receive are as follows:
W =2~rf~ tl+ ~o ~ ~P?=2~rf t,+ ~o Wherein, __ L __ L
t 1 C - hcosc~ ~ t 2 C -~ ZTcos cr cp0 is an initial phase that the ultrasonic wave is firstly transmitted.
Therefore, the phase difference Ocp between the received signals is as follows:
.~~p = ~p 1- ~~ ~, = 2 ,~f~l t= ~,~f 2L ~ ,osc~ c .-1 Herein, the flow velocity is as follows:
-~~ W) 4 ~cf Z.cosa ,,~ The phase difference method has features in that the ultrasonic waves can be continuously transmitted and the phase difference Ocp is proportional to the frequency f unlike the tr<~nsit time difference method. Therefore, even if L
and V
are very small, when the ultrasonic frequency f is selected at a higher one, the phase difference becomes largf:r, so that the phase difference measuring is conveniently and precisely done.
Also, if L is relatively larger, the damping factor is very small over the ultrasonic pulse, becaus~° the ultrasonic continuous waves are transmitted/
received. Further, even though the amplitude of the received signal significantly pulsates, the received signal can be sufficiently amplified, because the receiving moment is not measured. And an automatic gain control circuit can be used in the method. It means that there is not any problem in measuring the phase difference at all. Only, the phase dii:ference method is preferably used under the condition that the sound velocity C is not almost changed or in case that any other means measures the sound velocity C. For example, in order to measure the gas flow rate, the sound velocity of gas can be easily calculated under the condition that a pressure gauge and a thermometer are mounted in the pipe.
As mentioned above, the great advantage of the ultrasonic transit time difference method can bf: utilized even under the situation that the sound velocity in tZuid is significantly changed. But, if the interval L between the transducers becomes larger, the following problems occur due to the transmitting/receiving of the ultrasonic pulse.
First, the ultrasonic pulse has a larger damping factor over the sinewave because of its sufficient harmonic wave components or overtones. If the ultrasonic transit distance L becomes larger, it is difficult to receive the transmitted ultrasonic wave and the received pulse becomes a bell form due to the serious damping problem. For all that, it cannot help increasing the ultrasonic wave intensity that ".. can be auxiliary adjusted. If the intensity becomes higher, the cavity phenomenon occurs in a river, so that the ultrasonic wave is not transmitted. Especially, as the pulse frequency becomes. lower in order to reduce the damping factor, the ultrasonic intensity also becomes lower, which causes the cavity phenomenon.
Second, the ultrasonic pulse is not damped only by the distance L in the procedure of being transmitted, but the amplitude of the ultrasonic wave seriously pulsates, by which the ultrasonic wave is diffused and reflected because of various sizes of eddy currents, the concentration change of floating particles, the temperature change of water, etc. in the open sluice way channel. It sometime happens that the ultrasonic wave is not received.
When the flow velocity in gas is measured, the damping factor of the ultrasonic pulse is larger than that in liquid. The serious damping and pulsation of the ultrasonic pulse cause many errors, when it is subjected to catch the moment that the ultrasonic pulse reaches. Thus, the flow velocity measuring error is increased.
Due to these reasons, the ultrasonic transit distance L is limited in that the ultrasonic pulse is transrnitted/received and the flow velocity is measured based on a time difference method. Thus, it has big trouble in measuring the flow velocity in the open larger sluice w;~y channel or river and the larger pipe.
If the phase difference method is used for measuring the flow velocity, its damping factor is decreased two or three times over that of the ultrasonic pulse, because the ultrasonic continuous waves (sinewaves) are transmitted/received.
Also, the phase difference mEahod is not relevant to the amplitude pulsation of the received signals, because it is not related to catching the moment that the ultrasonic pulse reaches, but the phase difference between two sinewaves is measured.
Nevertheless, the phase difference method is limited to its use. If the phase difference .~cp between two sinewaves is equal to nn + ~3, a general phase difference measuring apparatus cannot detect n(l, ?, 3, ~ ~ ). If the ultrasonic transit distance L or the flow velocity V is lamer, ~lcp becomes greater than n. For example, if it is ,.... intended to measure the glow rate of gas in the pipe having an inner diameter ~ of 300mm, the cross-sectional average flow velocity V of gas is generally 10 ~
30m/s.

Then, assumed that the :>ound velocity C is 400m/s, the ultrasonic frequency f is selected at 400KHz in order to be beyond the frequency band of noises and an angle a is 45~, the changing width of the phase difference Ocp is as follows: .
4~p=9.42---2«. 26rad= (2~r+0.998%r)--(8%~+0.9950 That is, Ocp ) n.
If L = l Om, V = 3m/s, f = 200KHz and C = 1500m/s in a relatively smaller open channel, the phase difference ~cp is as follows:
~J~~ 16.746rad = 5 ~r+ 0 . 33 ~r> ~r Thus, the phase Ciifference method cannot be used in measuring the flow velocity in the relatively smaller open channel. In other words, the transit time difference method has an advantage in being used under the situation that the sound velocity is changed in a larger range. But, it has disadvantages in that if the flow velocity measuring interval L is larger, the ultrasonic pulse becomes unstable, because the ultrasonic pulse is greatly damped due to its own property during the transmitting/receiving.
The phase difference method has advantages in that the damping factor is relatively smaller and the received signal is easily processed, because the ultrasonic sinewave is transmitted/received. But, if the phase difference exceeds n radians by which the interval L and the flow velocity V is larger or the sound velocity is lower, it is not possible to measure the glow velocity based on the phase difference method. Also, the phase difference method has a disadvantage in that the sound velocity should be separately measured.
An object of the invention is to provide an ultrasonic flow velocity measuring method for measuring a tZow velocity based on an ultrasonic tow velocity transit time difference method a phase difference method, smoothly, if a flow velocity measuring interval L is relatively larger, for example if a horizontal average flow velocity is measured in an open sluice way channel or river.
... The other object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on an ultrasonic flow velocity transit time difff:rence method and a phase difference method, smoothly, if a flow velocity measuring interval L is relatively larger, for example if a gas flow velocity is measured in a pipe of a relatively larger inner diameter.
Another object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on an ultrasonic flow velocity transit time difference method and a phase difference method, smoothly, if a gas or liquid flow vf:locity is measured in a pipe of a relatively larger inner diameter.
Still another object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on an ultrasonic flow velocity transit time difference method and a phase difference method, smoothly, if a flow velocity is relatively larger and the sound velocity is relatively lower.
SUMMARY OF THE INVENTION
According to the invention, an ultrasonic flow velocity measuring method based on a transit time difference method for measuring a flow velocity without transmitting/receiving an ultrasonic pulse comprises steps of: amplitude-modulating a continuous ultrasonic sinewave carrier into a lower frequency and transmitting the amplitude-modulated signals, whenever the ultrasonic transit time is measured;
demodulating the received signals; detecting or discriminating the amplitude-modulated signal and measuring the time interval between the moments that the transmitted wave is amplitude-modulated and the received amplitude-modulated signal is demodulated.
An ultrasonic flow velocity measuring method based on a phase difference method not depending ~.ipon a sound velocity, comprises steps of: amplitude-modulating an ultrasonic wave into a lower frequency, if a phase difference between the ultrasonic w~~.ves transmitted in a direction contrary to the flow velocity ,,.". exceeds n radians beyond the measuring range of a general phase difference discriminator and becomes Wren + (3, and transmitting/receiving the amplitude-modulated signal; measuring the phase differences between the amplitude-modulated signals and bcaween the carried ultrasonic waves and obtaining m;
and enabling the very accurat~° measurement of the phase difference between the carried ultrasonic waves.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention now will be described in detail with reference to the accompanying drawings, in which:
Fig. 1 is a schematic block diagram illustrating an ultrasonic transit time difference flow velocity measuring system according to a prior art;
Figs. 2A and 2B are schematic block diagrams illustrating an ultrasonic phase difference flow velocity measuring system according to a prior art;
Fig. 3 is a timing chart illustrating the processing of an ultrasonic transit time difference flow velocity measuring method according to the invention;
Fig. 4 is a schematic block diagram illustrating an ultrasonic transit time difference flow velocity measuring system according to the invention;
Fig. 5 is a schematic block diagram illustrating an ultrasonic phase difference flow velocity measuring system according to the invention; and, Fig. 6 is a schematic block diagram illustrating an ultrasonic phase difference flow velocity measuring system according to another embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Firstly, an ultrasonic transit time difference flow velocity measuring method of the invention will be e;{plained in detail referring to the accompanying drawings:
Fig. 3 is a timing chant ou sequence illustrating a flow velocity measuring method. It is known that an ultrasonic carrier frequency f~ is generally selected by considering a noise frequency band caused in a fluid tlow, the security with respect ,,.,. to the directivity diagram of an ultrasonic transducer, an ultrasonic damping factor in fluid, etc.

When a flowvelocity is measured, the selected ultrasonic carrier f~ (Fig. 3, .-.
VI) is amplitude-modulated into a frequency fM (Fig. 3, I) lower than one f~
for a period of i2 (Fig. 3, V) a.nd then transited in a direction similar or contrary to the flow velocity. And, considering a predetermined moment amplitude-modulated as a starting point, a time is measured from the starting point till a designated moment of the amplitude-modulation frequency or signal fMJ while the amplitude-modulated ultrasonic wave is transited/received through a constant interval L
and the received signal is demodulated. The time be defined as ultrasonic transit times tl and t2 propagated in a direction similar or contrary to the flow velocity.
In other words, the amplitude-modulated ultrasonic wave acts as a mark signal for measuring the transit time of the ultrasonic wave. And, because the ultrasonic wave is a kind of sinewave that is continuously transitted and amplitude-modulated for a constant time interval to measure the flow velocity, the ultrasonic frequency band is f~~ f~,~ which is significantly narrower than that of the shorter ultrasonic pulse, ,.-so its damping factor becomes smaller. And, even if the damping factor is too much changed, the processing of the receiving signal is easy and it doesn't make an effect on the measuring of the transit time.
But, when the ultrasonic carrier wave f~ is amplitude-modulated into the amplitude-modulation signal,f~,l, it should be amplitude-modulated at the same phase as that of the amplitude-modulation signal fM, for example a zero phase as shown in Fig. 3, V. Wllen the amplitude-modulated voltage is applied to the ultrasonic transducer, the ultrasonic wave of a type equal to the voltage applied is not transitted, but a first half-period of a modulated ultrasonic wave is distorted in a shape. Furthermore, a signal obtained by receiving/demodulating the amplitude-modulated ultrasonic wave is not corresponding to the shape of the amplitude-modulation signal f,~. Considering these points, the amplitude-modulated signal applied to the ultrasonic transducer is inputted into a demodulator to be ,", demodulated and the amplitude-modulation signal f~,i is detected from the demodulation signal and a moment that the first period of the modulation signal passes over the zero potential is caught using a zero-crossing discriminating circuit.
Herein, the moment caught is considered as a start point for measuring the ultrasonic transit time as shown in Fig. 3, VII and VIII.
Similarly, the amplitude-modulation signal received is also demodulated by the demodulator as pointed out above, the amplitude-modulation signal fM is detected from the demodulation signal and then a moment that the first period of the modulation signal passes over a zero-crossing point is caught to function as a stop signal of the time interval as shown in Fig. 3, X and XI.
As described above, the ultrasonic transit time measuring accuracy can be significantly enhanced, by which only one demodulator demodulates the transmitting/receiving signals and the moments that the first period of the demodulation signal pass over the zero-cross point are used as the time interval measuring start and stop signals.
As shown in Fig. .3, VIII and XI, it is irrelevant to use the moments that one and a half period of the amplitude-modulation signal fM, not the first half period, passes over the zero-crossing point as the time interval measuring start and stop signals. Of course, the delay time is generated at the demodulator, the amplifier, the zero-crossing circuit etc., but it is not necessary to compensate for the delay time, because a system generates the same delay time whenever the flow velocity is measured.
And, the amplitude-modulation signal f~ should catch up with the following conditions:
First condition is what the amplitude-modulation signal fM is significantly higher than a damping pulsation frequency , fp, for example f,Yt » f p. The ultrasonic wave has the damping factor changed due to many factors during the transmitting in fluid. What the damping factor is changed is to make the ultrasonic wave amplitude-modulated. Thus, the amplitude-modulation frequency f~ should be ,..., higher than the damping pulsation frequency fp in that the damping factor pulsates, which is not a noise frequency generated in tZuid. The damping pulsation frequency fp is not high and does not exceed 100Hz, generally.
.,.
Second condition is what a Garner period should be contained more than 20th times in an amplitude-modulation period, for example fM <_ fCl20. The condition concerns the amplitude-modulation of the carrier fC, in which the phase of the carrier f~ at the start point of the amplitude-modulation is not always uniform, even if the carrier f~ is amplil:ude-modulated at a zero-crossing point as shown in Fig.
3, V. For it, the amplitude-modulated ultrasonic wave raises the transient phenomena and distorts the waveform in the interval of a first one-fourth period of the amplitude-modulation signal fM. In order to prevent a wave distorted portion from exceeding one-fourth period, the carrier f~ should include at least five periods in the first one-fourth period of the amplitude-modulation signal fM. Thus, the signal of the carrier f~ should exist over 20 (=4 x 5) in one period of the amplitude-modulation signal fM. In addition, it is preferable that the frequency of the carrier f~ is higher than that of the amplitude-modulation signal fM in order to filter the amplitude-modulation signal, f~,I from the pulsating frequency of the carrier f~.
Third condition is what a continuous time of amplitude-modulated signals desirably exceeds at least five periods of the amplitude-modulation signal fM
(~/ fM), if the amplitude-modulated signal is demodulated to detect the amplitude-modulation signal fM. Ii'the amplitude-modulated signal having the amplitude-modulation period to be repeated two or three times is demodulated, the outputting signal of the demodulator is distorted.
Fourth condition is what if the ultrasonic wave is transited/received in turns in a direction similar or contrary to the flow velocity, it is desirable that the continuous time of the amplitude-modulated ultrasonic wave does not exceed one-second of the ultrasonic transit time. The example is as follows:
L
~_ll~ I~~C ~ U
f:LIC
..,. As described above, the amplitude-modulation signal fN~ satisfying with four conditions is selected by the following expression:
1 ~' -, f ~ W 10 ( ~ m~~.~c L U max ~ f ,i~l ~ 0 . 05 f c ~ ~7 Wherein, Cmax is a maximum sound velocity that can be expected in fluid, and vmax (= umax ~osa ) is a maximum flow velocity measuring value.
It is preferable in selecting the amplitude-modulation signal fM satisfying with the expression (6) that the relatively lower frequency is selected as far as possible, because the transient phenomena happen when the voltage applied to the ultrasonic transducer is rapidly changed. It is desirable that the amplitude-modulation percentage rra does not exceed 50%. According to the experiments, the amplitude-modulation percentage m of 25 - 30% is very reasonable. The ultrasonic damping factor pulsates at the lower frequency fp, the changing ratio of which is generally about 50%. Thus, if m > 50%, it is afraid that the amplitude-modulated wave is cut off. For example, assumed that L = 1 Om, a = 45°, Cmax =
1500m/s, f~
= SOOKHz, fp « 1507 < fM <_ 25 ~ 10' Hz. Thus, fM can be selected in the range of 10 to 20KHz. Considering the transient phenomena of the ultrasonic wave, it is not necessary to select thc: higher frequency of the amplitude-modulation signal fM.
Fig. 4 is a schematic block diagram illustrating the configuration of a system according to one embodiment of the invention for realizing a method of measuring a flow velocity as described above.
Ultrasonic transducers 1 and 2 are connected to a transducer switching circuit 3 to be switched into the transiting or receiving state. An outputting amplifier 18 excites the ultrasonic transducer 1 or 2. A receiving amplifier 19 amplifiers the signals from the ultrasonic transducer 1 or 2, which is a narrow band amplifier that has the function of the automatic gain control(AGC) and amplifies only the frequency band of an amplitude-modulation signal.
An amplitude-modulator 17 amplitude-modulates an ultrasonic carrier signal fC. A carrier oscillator 1=i generates an ultrasonic carrier signal f~. A
modulating oscillator- 14 generates a modulation signal ,f,~,~ lower than the carrier signal ~ .
Herein, both of the carrier oscillator 13 and the modulating oscillator 14 are ",, sinewave oscillators. A demodulator 20 demodulates the amplitude-modulated signal to detect the modulation frequency f~~,~. A narrow band amplifier 21 is a narrow band amplifier that amplifies the modulation signal fM. A zero-crossing circuit 22 outputs a square pulse when a first period of the outputting signal fM
from the narrow band amplifier 21 passes over the zero point. A time interval measuring apparatus 7 measures the time interval between two pulses. An arithmetic logic unit 8 computes a flow velocity based on an ultrasonic transit time difference flow velocity measuring expression. A switch circuit 23 permits the outputting signal of the modulation frequency fM from the modulating oscillator 14 to be passed therethrough in a given time interval. A zero-crossing circuit 15 generates a square pulse when first one period of the modulation signal fM
passes over the zero-point. A monostable multivibrator 16 is operated by the zero-crossing circuit I S to generate a pulse of a given length.
A switch circuit 24 is switched by the pulse of the monostable multivibrator 16 to allow the outputting; signal of the modulation oscillator 14 to be applied to the amplitude-modulator 17. A switch circuit 25 allows an ultrasonic modulated output to be applied to the demodulator 20 and then is switched to permit the outputting signal from the receiving amplifier 19 to be inputted into the amplitude-modulator 20. A voltage attenuator 27 adjusts the outputting voltage of the outputting amplifier 18. A switch circuit controller 26 controls the switch circuits 3 and 23 and 25.
The operation of the ultrasonic flow velocity measuring system as shown in Fig;. 4 will be explained in detail below with reference to Fig. 3.
The carrier oscillator 13 and the modulation oscillator 14 are first oscillated to generate sinewaves of the ultrasonic carrier frequency f~ and the modulation frequency fM, respectively, as shown in Fig. 3, VI and I. When a flow velocity measuring; moment is reached, the switch circuit controller 26 applies a square pulse of a length ii (referring to Fig;. 3, II) to the switch circuit 23. The switch circuit 23 permits the signal of the modulation frequency f~,~ from the modulation oscillator .... 1 ~ to be inputted to the zero-crossing circuit 1 ~. Then, as the operation potential level of the zero-crossing circuit 1 ~ is set at a low level "-", the zero-crossing circuit 1 ~4 ""y 15 generates a square pulse (referring to Fig. 3, III), when first one-half period of the outputting signal from the modulation oscillator 14 passes through the zero point (U = 0). The square pulse is inputted into the monostable multivibrator 16 and the monostable multivibrator 16 generates a square pulse of a length i., (Fig.
3, IV).
The switch circuit 24 is switched by the square pulse of i, to permit the signal of the modulation frequency fM from the modulation oscillator 14 to be inputted to the amplitude-modulator 17. Thus, the signal of the ultrasonic carrier frequency f~ is amplitude-modulated for the time of i., as shown in Fig. 3, VI. Like it, the ultrasonic carrier frequency fC is always supposed to be amplitude-modulated into the same phase of the modulation frequency fM.
The amplitude-modulated signal from the amplitude-modulator 17 is amplified by the outputting amplifier 18 and then applied to the ultrasonic transducer 1. The ultrasonic transducer 1 transits the amplitude-modulated ultrasonic wave through fluid to the transducer 2.
At the same time, the outputting signal of the outputting amplifier 18 is inputted through the voltage attenuator 27 and the switch circuit 25 to the demodulator 20 to detect the modulation signal fM (Fig. 3, VII). The narrow band amplifier 21 amplifies the modulation signal demodulated by the demodulator 20 and applies the amplified signal to the zero-crossing circuit 22. The zero-crossing circuit 22 generates a shorter square pulse (Fig. 3, VIII) at the moment that first one-half period "-"of the modulation signal fM passes through the zero-point.
The shorter square pulse is inputted into the time interval measuring apparatus 7 to function as a time measuring start signal.
Thereafter, the switch circuit 25 cuts off the input to the attenuator 27 and forces the outputting signal from the receiving amplifier 19 to be applied to the demodulator 20. In other words, the ultrasonic wave amplitude-modulated that the transducer 1 emits transits through an intec-val L, is received by the transducer 2 and ,,.. amplified by the receiving amplifier 19. The outputting signal (Fig. 3, IX) from the receiving amplifier 19 is applied through the demodulator 20 and the amplifier 1, ,_" to the zero-crossing circuit 22. The zero-crossing circuit 22 generates the shorter square pulse (Fig. 3, XI) and applies it to the time interval measuring apparatus 7 to function as a time measuring stop signal.
Therefore, the timE; interval measuring apparatus 7 measures the time interval t 1 between the first and second square pulses from the zero-crossing circuit 22.
After finishing the measurement of the time interval t 1, the transducer switch circuit 3 is switched to connect the transducer 2 to the outputting amplifier 18.
Then, the switch circuit 25 is connected to the attenuator 27 and the switch circuit 23 is again switched. And, next operations are repeated in the same procedures as the measuring ones of the time interval t 1. Therefore, a time t 2 is measured till the amplitude-modulated ultrasonic wave is transited from the transducer 2 and received by the transducer 1.
The time intervals t 1 and t ~ are inputted into the flow velocity arithmetic logic unit 8 to compute the flow velocity based on the flow velocity measuring expression (3). The flow velocity arithmetic logic unit 8 outputs a signal corresponding to the flow velocity V. The outputting signal of the flow velocity V
is provided to a flow rate measuring arithmetic logic unit (not shown), if the system is a flow rate measuring system.
Herein, important things are as follows: it has features in that in order to measure the time intervals t 1 and t ~, the amplitude-modulated outputting signal inputted into the transducer 1 (or 2) and the signal received by the transducer 2(or 1 ) pass through one demodulator and the zero-crossing circuit, and the start and stop pulse signals inputted into the time interval measuring apparatus 7 are shaped into a square pulse.
The expression (~) well-known as a phase difference flow velocity measuring expression depends on the square of the sound velocity C-. In the expression (~), Ocp also is a phase difference between the ultrasonic waves transited in the ,... directions similar and contrary to the flow velocity. Except for the glow velocity measuring method of the ~°xpression (5), a phase difference glow velocity measuring directions similar and contrary to the flow velocity. Except for the flow velocity measuring method of the expression (5), a phase difference flow velocity measuring expression that does not depend on the sound velocity C could be derived.
The phase difference 0 ø , between the ultrasonic transiting wave and the received wave next to be transited toward the flow velocity direction and the phase difference 0 ~ 2 between the ultrasonic transiting signal and the received signal next to be transited in a direction contrary to the flow velocity are as follows:
~ ~'1=2~f ,C+u C7-a) .iJ ~~=2~f L (7-b>
Wherein, a = Vcc~sa, and L is an interval between ultrasonic transducers.
The difference between the reciprocal numbers of the phase differences 0 ~ , and 0 ~ 2 is as follows:
1 - 1- - 2 YcoSCr i1 W i1 ui ? 2 zfL
Wherein, V is as i:ollows:
cos ( ~ ~ 1 ~ c~? ) C9) The flow velocity measuring method is highly worth being used, because it is not necessary to measure the sound velocity, separately, even under the condition that the sound velocity is significantly changed. But, only if the measuring error of the phase differences D ~ , and D ø 2 are very small enough to be ignored, the flow velocity could be measw~ed based on the expression (9).
For example, D ~G 1 = 2.0rad and O ~2 = 2.2rad. Assumed that the phase differences are measlu-ed in the range of the error of 0.5%, the measured phase differences are as follows:
,... ~~,'=2.0 (1 +0.005)=2.01 0~2'=2.2(1 -0.005)=2.189 .- As a result, = 0 . 0406g'? 8 ~5 -1 ~ ; _ ~l ~ ,>
But, the actual value is as follows:
~ 10 - 2 I y = 0 . 045454 S
Therefore, the error is as follows:
0 . 0~06~'?S - 0 . 04545 _ _ 0 .10~ = 1 a . ;~ '' 0 . 0~5:~~>
That is to say, thE; phase difference was measured in range of the error of 0.5%, but the error between the differences of the reciprocal numbers with respect to the phase differences was increased more than 20 times. Thus, the flow velocity measuring error might have become more than 10%.
In order to realize the phase difference flow velocity measuring method not depending on the sound. velocity C, the phase difference must be very precisely measured.
It appears the following problem from the expression (7). As the interval L
is increased, the sound velocity C is lowered and the ultrasonic frequency is increased, the phase difference D ~ l,~ is too much increased more than n. Of course, if L, C and a are given, the ultrasonic frequency f, that enables the phase difference D ~ not to exceed the measuring range ~t of a general phase difference discriminator, could be selected, but it must be far higher than a noise frequency band generated in fluid.
For example, assumed that the inner diameter D of a natural gas pipe is equal to 0.3m, C ~ 420m/s, V == 30m/s, ec = 45~ and L = 0.4?~m, the ultrasonic frequency f~that does not exceed the phase difference n is as follows:
C ~ ycos~x_ _ :~20 . 30 ~ cos-l~' ?,~L '?;~ - 0. !'?~
Such like a frequency band is included in a noise frequency one.
Furthermore, It makes it impossible to manufacture a compact transducer that ,,-,. transits the sound wave of 165Hz.
In order to be escaped out of the noise band, if the ultrasonic carrier frequency fc is selected to be 40KHz, the phase difference in said examples is as follows:
4~0 + 30 cos cr - 2=~ 1. 522 - ~ rad > i 6 T -:-I N
Herein, 768n can not be measured by the general phase difference discriminator.
In order to resolve these problems, the invention considers an ultrasonic frequency fC escaped far away from the noise band as a carrier and amplitude-modulates it into a frequency fnl lower than the ultrasonic frequency fC , transits it in the directions similar and contrary to the flow velocity and measures the phase differences between the transiting signal and the received signal as follows:
First, the amplituG',e-modulation frequency fM is selected so that the phase differences ~ ~, Ml and 0 ~M~ between the transiting wave of an amplitude-modulated signal and received and demodulated signals next to be transited in the directions similar and contrary to the flow velocity satisfy with the following conditions:
-~ ~',vn=2~f,~r ~ =n~Z+b~ (10-a) C ma,~c + U
~ ~,~=2~rf~u _ =n,~+a»
(10-b) C min U max Wherein, n = coast (l, ', 3, ....); a < 1.0, b < 1.0, Cln~l,c and Cain are maximum and minimum sound velocities in fluid and vma~ = Vmaxcoset, which is a maximum tlow velocity measuring range.
In this case, as n~, is previously known, the phase differences 0 ~~ ~,I1 and 0 ~, ~,I~ are supposed to be measured, only if an and bn is measured and next add ~~ nn thereto. Herein, an is .a maximum measuring limit and bn is a lowest measuring limit. Because it is unstable if a == 1 and b = 0, it is desirable that a is selected to be .- As a result, = 0 . 0406g'?

.-,. equal to 0.95 and b is selected to be equal to 0.2.
The n that satisfies with expression ( 10) is as follows:
The following relative expression from the expression ( 10) is given.
yL -f- b _ C' min U ma.~c YL -~" a C~ mat ~ U ma~c Wherein, n is as follows:
_ a( C min U max ) - b( C max - U max ) C max C min + ~ U ma.~c The modulation frequency fM based on such like obtained n is as follows:
f :vl = n + a ( C' min - U max ) 2~, or, f :l~l - L ~ ( C max + U ma~c ) ~ 1 ~-b ) Therefore, the carrier f~ is amplitude-modulated into the selected modulation frequency fM, and the amplitude-modulated signal is transited/received. If the phase differences 0 ~ ~,T1 anc~. 0 ~M~ between the modulation frequencies f,~ are measured in the range of a constant error 8M, the calculation results of the phase differences 0 ~ ~,il and 0 ~~ ~.~ are as follows:
~ ~,vn~ = n;~r+ b~r(1 ~ ~,~r) X12-a) 4 ~~~2~ = n;~+ a~r(1 -- ~:~r) C1?-b) Wherein, an = D ~ ytVl l and brc = 0 ~ MM-,, which are a phase difference that the phase difference discriminator can measure. Multiplying the phase difference by f~ l n,f,~ becomes a value that divides the phase differences ~
~~, ~ 1 and 0 ~, ~~ between the carriers into n.
O1~-a) .~ ~,vn~ X J ~ - m 1 ~ a ,.. %~ f ,u I ~ y O 13-b - m ~ -t-n f :~r ,.-. Wherein, ~3 < 1.0, y < 1.0 and m 1 and m2 are integers ( 1, 2, 3, 4, ...).
If the phase differences 0 ~, C 1 and D ~ C2 are measured as described above, it is noted that mln + (3~z and m~~t + yet are obtainable.
The values that the discriminator measures the phase difference between the carriers are as follows:
~ ~c,w~ =ail - ~~) (14-n) ~ ~ c,~r~' = YT( 1 ~ o ~) ( 14-b) If the m 1 n and ~71~ ~, are added to the measured values, the difference between a phase upon the transiting of the carrier wave and the phase of the received signal next to be transited in directions similar and contrary to the flow velocity are as follows:
~J ~ ci.~ - y~ 1 ~r+ air( 1 ~ o" ~) C 1~-a) ~J ~ cz ~ - y~ ~ ~r + ~7~( 1 - o ~ ) C 1 ~-b ) The phase differences ~ ~ C 1' and ~ c~ ~, ~ obtained like above are substituted into the flo~.v velocity measuring expression to compute the flow velocity as follows:
cos a ~ Ji 1 ' 4 ~ ~ ) C 1G ) ~c~ ~c~
If the phase difference of the carriers is measured like the above method, the measuring error is reduced tens or hundreds times over the error 8~ of the phase difference discriminator.
_ 4~c~~ -!a~'ci az8~ _ _ o~ C1i-a) ~='~~- ~ ~~ rnl~~r+~3~z 1+ mt .~~ c~ ' W ~G~~~ ~~ ~l~-b) o~~~- Q ~~ - 1 I yn~
Wherein, rnl and m~ » l, ~3 and y < 1Ø Thus, 8~ ~,, ~, and 8~ ~, ', are too ~- much smaller than 8~ .
As described above, according to the invention, because the phase difference ,,~., is accurately measured when the ultrasonic wave is transited and received, the flow velocity could be measured based on the phase difference flow measuring expression that does not depend upon the sound velocity. Also, even if L and V
are larger, C is lower and the phase difference between the ultrasonic waves exceeds far away from ~ rads, floe flow velocity could be easily measured.
For example, when the flow velocity of natural gas flowing in a pipe having an inner diameter of 30C)mm is measured, it is assumed that Cm;" = 420m/s, Cma.~ _ 450m/s, L = 0.425m, Vr~~ cosa = 30m/s and the ultrasonic carrier frequency f~
is selected at 40KHz by considering the noise in the pipe. Assuming that the measuring range of the phase difference discriminator is selected to be 0 ~ ~, b~ =
0.2rc, for example b = 0.2 when the phase difference becomes minimum in the range, and a~
= 0.95n, for example a = 0.95 when the phase difference becomes maximum in the range. Therefore, the modulation frequency f,,~ is as follows:
,.. __ 0 . 95 420 - 30 ) - 0 . 2 ( 450 + 30 ) 450 - 420 + 2 ~ 30 - 3 . 05 Assuming that n is selected at 3 and stored at the memory of the system, f :N ~ 3 ~ . 4-~ ( 420 - 30 ) =1839 . 62'~~
Assuming that fM is selected at I830Hz, dining transiting the ultrasonic wave amplitude-modulated at the amplitude-modulation signal f,:t of 1830Hz in the directions similar and contrary to the flow velocity, the received signal is demodulated to detect the ampliW de-modulation signal f,,~. Then, if the phase difference between the phase of the amplitude-modulation signal fM of the transiting side and the receiving signal phase is measured, the results will be as follows:
If the flow velocity Vcosa is equal to 20m/s and C is equal to 450m/s, .~ ~ nrl = 2 ~ f M C + a - 2 'r1830 _~ ~ ~~ ~0 = 10 . 3 i 28 ~ 7 . . .
=3.z + 0.301i8r (n=3) .~ ~,~,rz=2zf,~r CLU =3~+0.60893 (ya=3) ,,.~ Herein, it is' known that the phase difference that the discriminator can measure is 0.30178n anal 0.60893n. Assumed that the phase differences are the measuring error is performed in the range of ~ 1 %, the computed phase difference is as follows:
' = 3 ~ + 0 . 30178 ~( 1 + 0 . O l ) =10 . 382328 rad = 3 ~r + 0 . 60893 ~r( 1- 0 . O l ) =11. 31865 rad Next procedure is as follows:

' ~f ~ =10 . 3823 . 40 ~ 10 _ l 2 . 235819 ~ . .
4 ~ :gin ' ~ f ~ 21830 Herein, m 1 (=72 ) is stored at the memory of the system.
~ J f'- = I 1. 31865 40 ' 10 3 = ; 8 . 75056 ~ -,~r 21830 Herein, m~(=78) is stored at the memory of the system.
The actual phase difference between the carriers is as follows:
...
~ 2'~ f ~ C + v = 226 . 7294 I02 = 72 .1'1021216 ~c Wherein, it is noted that rnl(=72) was coincident with the stored value and the phase difference 0 ~ ~M1 between the carriers that could be directly measured is equal to 0.17021276.
4 ~ cz = 2 ~ f ~ CL v = 24'7 . 8205182 = r 8 . 883'12094 ~' Wherein, m~(=78) was coincided with the stored value, and the phase difference 0 ~~ ~~,I~ between the carriers is equal to 0.88372094.
If the phase differences D ~, ~~,I1 and D ~~ ~M~ are measured in the range of the error of =~ 1 %, = 0 . 54 rad, !J ~ c,,~r> ~ = 2 . r 48 rad The calculating results of the phase difference t1 ~, ~ 1 and ,~ ~,~~~ are as follows:
= 72 a + 0 . 54 = 226 . i 346 i rad ' = 78 ~ + 2 . 748 = 24 r . 7922 rad These phase differences are substituted into the flow velocity measuring expression to compute the flow velocity as follows:
Y~ cos a= ~z f ~L(- d ~ ~, - ~ ~1 '. ) _ ~r40 ~ 10 3 ~ 0 . 424 ( 10 3 - 10 - ~ .
x.226.. . 0 . ~ i r 9. . ) - 19 . 9 i m/ s The first flow velocity Vcosa was equal to ZOm/s, but the actual measured flow velocity became 19.95m/s. Thus, the measuring error became about -0.15%.
That is to say, the phase differences were measured two times in the range of 1 %. Nevertheless, as a result, the flow velocity measuring error was reduced by 0.15%.
Such like an error reduced reason is why the measuring errors of the phase difference 0 ~ ~, and D S~ ~2 are significantly decreased.
,, d ~ c~. ~ - ~ ~ c~ 226 . 7346 i - 226 .'72941 d ~ ~t - 4 ~ ~ - 226 . ~ 2941 .~, = 0 . 000023 23 = 0 . 0023 The phase difference 0 ~' cMl was measured at F~ (=1 %). Nevertheless, the measuring error D ~ ~l was reduced ml/~i ( = 72/0.1702 ~ 423) times (referring to the expression 17). It is assumed that the phase differences 0 ~ ~1, O ~MM2 ~ ~' cMl and 0 ~ ~M2 are measured at the error of 1 % from the above example, but actually, it is norma that the phase difference is measured at the error of 0.5%.
As described above, according to the invention, the flow velocity of gas, in which the flow velocity is high and the sound velocity is low, could be accurately measured based on the phase difference method irrelevant to the sound velocity change in a pipe of a larger inner diameter.
In Fig. 5, a schematic block diagram illustrating the configuration of a system for realizing a method of measuring a flow velocity based on a phase difference method is shown as one embodiment of the invention.
Ultrasonic transducers l and 1' are an ultrasonic receiving transducer to ,,-. receive an ultrasonic wave and ultrasonic transducer 2 is an ultrasonic transiting transducer to transit ultrasonic waves at a wider directivity angle. A carrier oscillator 13 and a modulating wave oscillator 14 generate an ultrasonic carrier frequency f~ and an amplitude-modulation frequency f~ , respectively. An amplitude-modulator 17 amplitude-modulates an ultrasonic carrier frequency f~.
An outputting amplifier 18 excites the ultrasonic transducer 2. Receiving amplifiers 19, 19' amplifier the signals from the ultrasonic transducers 1, 1', respectively.
Demodulators 20, 20' demodulate the amplitude-modulated signal to detect the modulation frequency ~f~. Narrow band amplifiers 21, 21' amplify the signals outputted from the demodulator 20, 20'. Phase difference discriminators 28, 28' detect the phase differE:nces D c~ MM1 and 0 ~MM2 between the amplitude-modulation waves fM. Phase difference discriminators 31 detect the phase differences 0 ~ ~~,I1 and D ~ CM2 between the carriers f~. Amplifier-limners 30, 30' amplify and limit thf: amplitude-modulated signals to a predetermined level.
Phase shifters 29, 29' is needed in forcing the output of the phase difference discriminators 28, 28' to be adjusted to zero, if the flow velocity V is zero.
An arithmetic logic unit 32 computes the phase differences D ~ C 1 and 0 ~~ C~
between the carriers f~ and then the flow velocity according to the invention.
The ultrasonic flow velocity measuring system is operated as follows:
The amplitude-modulator 17 amplitude-modulates the carrier frequency f~
generated by the carrier oscillator 13 into the modulation frequency f~,1 generated by the modulation oscillator 14. The amplifier 18 amplifies the amplitude-modulated signal and supplies it to the transiting ultrasonic transducer 2. If the transducer 2 transits the amplitude-modulated signal in the directions similar and contrary to the flow velocity, the receiving transducer 1 receives the signal transited in the directions similar and contrary to the flow velocity V and converts it into electrical signals. The outputting signal from the receiving transducer 1 is amplified -~ by the receiving amplifier 19 for amplifyin; the frequency band of f~ -~-f~,~ and is inputted into the demodulator 20. At the output of the demodulator 20 the -, ;

,... amplitude-modulation signal fNt is generated. The signals is inputted through the phase shifter 29 into the narrow band amplifier 21. The narrow band amplifier again filters the amplitude-modulated signal and applies it to the phase difference discriminator 28 of a lover frequency fM. The discriminator 28 detects the signal corresponding to the phase difference of D ~, ~~ that is smaller than ~ and inputs its outputting signal into the arithmetic logic unit 32 that computes the phase difference and the flow velocity.
The ultrasonic wave transited in the flow velocity direction is received by the receiving transducer 1' and the phase difference 0 ~~ M~,jl is detected through a receiving amplifier 19', a demodulator 20', a narrow band amplifier 21', the discriminator 28' as mentioned above. At the same time, the outputting signal from the receiving amplifier 19' is amplified to a saturated state by the amplifier-limiter 30' and inputted into the phase difference discriminators 31'. The phase difference discriminators 31' generates the signals corresponding to the phase differences ~ ~ CM1 and 0 c~~ CM2 and inputs them to the arithmetic logic unit 32.
The arithmetic logic unit .i2 is supposed to force the integers of n, fM, f~, L, and cosy to be inputted thereinto in advance and obtains m 1 and m~ according to the expression ( 13 ), calculates the phase differences D ~, C 1 and 0 ~, C~
of carriers according to the expression ( 15 ) and computes the flow velocity V according to the expression ( 16). Such like obtained flow velocity may be used in computing the flow rate, if it is adapted. to a tlowmeter.
There is a case to measure the sound velocity C in another way. For example, if a tlowmeter for measuring a volume flow rate is installed to measure the mass flow rate of gas, the gas pressure and temperature are separately measured. In this case, the sound velocity could be computed using the measuring results of gas pressure and temperature. If the liquid tow rate is sometimes measured, there is a case that the sound velocity C in liquid may be previously known with being not changed. In this case, the ultrasonic wave transited in the directions similar-and contrary to the flow velocity are received and the phase difference OcpC
between the ?6 receiving signals is measured, so that the flow velocity V could be measured based on the expression (5). At this time, if ~cpC » ~, the phase difference ~cp~ is measured as follows: in order to amplitude-modulate the ultrasonic carrier f~
into the modulation frequency fM, the modulation frequency fM is selected as follows:
,~
min _ ~ .
f ,~r s sL v m~.~ cos cr ~ 1~
Wherein, Cmin is a lowest sound velocity that can be expected in fluid.
The phase difference OcpM between the receiving signals of such like selected amplitude-modulated frequencies does not exceeds n in the maximum flow velocity measuring value.. The amplitude-modulated signal received is demodulated, so that the phase difference ~cpM between the modulated frequencies is measured and then m is obtained by the following expression ( 19).
4 r~,~x ~~' - = m~-fi-a= ~ ~'' ~1~3) .~r ~z Wherein, a < 1Ø
The an in the expression ( 19) is a part that is supposed to be able to measure the phase difference between the carriers. At the same time, the phase difference aTz between the carrier signals is measured and OcpC is calculated by the following expression.
~J g~ ~= m ~-f- air (?~) Next, the Ocp~ is substituted into the expression (~) to compute the flow velocity V. Herein, the phase difference to be measured is an. When the absolute error Dan in measuring the arc is equal to 8ayarc ( 8a.~ is a relative en-or.), the measuring error of Ocp~ is as follows:
__ o~ a;,- a ~r __ ~ a;:
(mrt-a)~r 1+m/a Therefore, 8~~< << 8~.~ and the accuracy of the flow velocity calculation is r~ enhanced. Another embodiment of a system for realizing a method for measuring a flow velocity by such .ike a method is shown as a schematic diagram in Fig.
6.
~7 .~-.. Referring to' Fig. 6, the reference numbers are referenced by the same numbers to the same parks as those of Fig. 5. Only, a flow velocity arithmetic logic units is supposed to ford; the integers of fM, f~, L, and cosy to be inputted therein in advance and to compute the flow velocity based on the expressions ( 18), ( 19) and (5).
Accordingly, the invention can amplitude-modulate an ultrasonic wave and measure a flow velocity i:n a higher reliability based on an ultrasonic time difference method in a larger river, a larger sluice way channel, a pipe of a larger inner diameter. Also, the invention provides a phase difference flow velocity measuring method not depending upon a sound velocity, using a general phase difference discriminator having the phase difference measuring range of ~, even if the phase difference exceeds nrad.
~S

Claims

1. A time difference flow velocity measuring method of measuring the times that the ultrasonic wave is transited in the directions similar and contrary to a flow velocity and computing the flow velocity comprising steps of:

amplitude-modulating an ultrasonic carrier of a frequency f C into an amplitude-modulation frequency f M smaller than the carrier one f C for over a period ~ (=5/ f M), whenever the ultrasonic transit time is measured;
transiting the amplitude-modulated signal in the directions similar and contrary to the flow velocity;
demodulating the received amplitude-modulated signals next to be transited in the directions similar and contrary to the flow velocity to detect the amplitude-modulation signal f M;
measuring the time between moments that the ultrasonic carrier f C is amplitude-modulated into the amplitude-modulation frequency f M and the amplitude-modulation signal f M is detected from the received signal; and, substituting the measured time differences into a time difference flow velocity measuring expression and computing the flow velocity, in which the frequency of the amplitude-modulation signal f M is determined by the following expression:
wherein, f p is a maximum frequency in that the damping factor pulsates at the time of transiting the ultrasonic wave in fluid, C max is a maximum sound velocity in fluid, L is an ultrasonic transit distance, V max is a maximum flow velocity that can be expected in the interval L and .alpha. is an angle that the transit distance L and the direction of the flow velocity form.

2. The ultrasonic flow velocity measuring method as claimed in Claim 1, in which:

the method of measuring a time that the ultrasonic wave is transited comprises steps of:
inputting the amplitude-modulation voltage f M increased from a zero phase toward the "+" phase into an amplitude-modulator and then inputting the amplitude-modulated outputting voltage into an ultrasonic transducer while inputting the outputting voltage into a demodulator in turns to detect the amplitude-modulation signal f M, determining as an ultrasonic transit time measuring start timing point a moment that a first period or one and a half period of the amplitude-modulation signal passes over the zero-crossing potential, demodulating the signal, in which the amplitude-modulated ultrasonic wave is transited through the interval distance L and then received by another ultrasonic transducer, by said demodulator and detecting the amplitude-modulation signal f M, determining as an ultrasonic transit time measuring stop timing point a moment that a first period or one and a half period of the amplitude-modulation signal passes over the zero-crossing potential, and measuring ultrasonic transit times using the ultrasonic transit time start and stop timing points.

3. A phase difference flow velocity measuring method of transiting/receiving an ultrasonic wave at a constant angle .alpha. in the directions similar and contrary to the flow velocity and using the ultrasonic phase difference changed proportional to the flow velocity comprising steps of:
amplitude-modulating an ultrasonic wave of a frequency f C into an amplitude-modulation frequency f M, lower than the one f C while transiting it in the directions similar and contrary to the flow velocity, continuously;
demodulating the ultrasonic signal received next to be transited through the internal L in the directions similar and contrary to the flow velocity to detect the signal of the amplitude-modulation frequency f M;
measuring a phase difference .DELTA.~ M1 between the amplitude-modulation signals f M when the signal of the amplitude-modulation frequency f M is detected and emitted in a direction similar to the flow velocity and a phase difference .DELTA. ~ M2 the amplitude-modulation signals f M received and demodulated next to be transited in a direction contrary to the flow velocity;
obtaining multiples m 1 and m 2 of .pi. excluding phase different components .beta..pi. and .gamma..pi. measured by a phase discriminator from phase differences .DELTA. ~, C1 and .DELTA. ~ C2 between a phase of an ultrasonic wave f C upon transiting and a phase of received signal f C by the following expression;
wherein, .beta. < 1.0 and .gamma. < 1Ø
storing m 1 and m 2, measuring the phase different components .beta..pi. and .gamma..pi., adding m1 .pi. and m2 .pi. to the measured results to calculate the phase differences .DELTA. ~ C1 and .DELTA. ~C2 and computing the flow velocity based on the following expression;
selecting the amplitude-modulation frequency f M as follows:
storing n; and .DELTA. ~ M1 = n.pi. + a.pi. .DELTA. ~ M2 = n.pi. + a.pi.

measuring the phase differences a.pi. and b.pi. that a phase difference discriminator can measure in the above expression and adding n.pi. thereto to obtain the phase differences .DELTA. ~ M1 and .DELTA. ~ M2.

wherein, a (<1.0) is a factor for selecting a maximum measuring range (a.pi.) max of a phase difference discriminator, which is 0.95, and b (<1.0) is a factor for selecting a maximum measuring range (b.pi.) max of a phase difference discriminator, which is closely equal to 0.2, C max and C mix are maximum and minimum sound velocities that can be expected, ~ max (=V max cos.alpha.) is a maximum flow velocity measuring range.

4. A method of measuring the phase difference .DELTA. ~ C between signals that an ultrasonic wave in the directions similar and contrary to the flow velocity is transited and then received, if the sound velocity C is separately measured or constant, and computing the flow velocity V by the following expression:

(IMG) comprising steps of:
amplitude-modulating an ultrasonic wave f C into an amplitude-modulation frequency f M, transiting the amplitude-modulated signal in the directions similar and contrary to the flow velocity, demodulating the received signals next to be transited, measuring a phase difference .DELTA. ~ M (<
.pi.) between them and obtaining a multiple m that exceeds the .pi. of the phase difference .DELTA. ~ C by the following expression; and (IMG) measuring the phase difference a.pi. between the received ultrasonic signals f C at the same time by the phase discriminator to obtain a.pi., adding m.pi.
thereto to obtain .DELTA. ~ C and calculating the flow velocity according to said expression, wherein the amplitude-modulation frequency f M is selected by the following expression:

(IMG)