Note: Descriptions are shown in the official language in which they were submitted.
CA 02279257 1999-07-30
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
CA 02279257 1999-07-30
,~ 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
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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
-,
CA 02279257 1999-07-30
,.,, 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
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,,~ 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
CA 02279257 1999-07-30
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.
6
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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
7
CA 02279257 1999-07-30
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-
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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.
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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
CA 02279257 1999-07-30
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
CA 02279257 1999-07-30
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 ~'
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-,
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
13
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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
CA 02279257 1999-07-30
""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
21
1,
CA 02279257 1999-07-30
,_" 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
16
CA 02279257 1999-07-30
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
17
CA 02279257 1999-07-30
.- 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
18
CA 02279257 1999-07-30
,,-,. 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
19
.- As a result,
= 0 . 0406g'?
CA 02279257 1999-07-30
.-,. 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
CA 02279257 1999-07-30
,.-. 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
21
CA 02279257 1999-07-30
,,~., 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)
22
CA 02279257 1999-07-30
,,.~ 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:
3
' ~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
CA 02279257 1999-07-30
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
24
CA 02279257 1999-07-30
,,-. 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
-, ;
CA 02279257 1999-07-30
,... 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
21
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
CA 02279257 1999-07-30
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
CA 02279257 1999-07-30
.~-.. 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
