Abstrict An inexpensive flow meter with a special arrangement of a minimum
of transducers always provides an accurate measure of flow of the
fluid in a conduit irrespective of the condition of the fluid. Mode
control switching circuitry including fluid condition detecting
means selectively energizes one or two transmitters in the transducers
each of which then feeds a beam of ultrasonic waves to the fluid.
Receiver means in the transducers selectively receives both the
transmission (transit) mode waves and the reflection (Doppler-shifted)
mode waves in accordance with the current condition of the fluid.
In one embodiment, the beam of ultrasonic waves uses a single path
of travel extending from a transmitter and a receiver while in another
embodiment, it uses dual parallel paths extending from two transducers
and two receivers.
Claims What is claimed is:
1. A Doppler-shift flow meter for use in measuring the velocity
and/or rate of flow of a flowing fluid, said flow meter comprising:
(a) means for generating an oscillatory electric signal having
a frequency in the range of ultrasonic frequency,
(b) a first transducer unit including a first ultrasonic transmitter
having a first sending direction and a first ultrasonic receiver
disposed close to said first ultrasonic transmitter and having a
first receiving direction which forms an angle of substantially
zero with said first sending direction for detecting a Doppler shifted
signal having a frequency higher than the frequency of the input
signal to said first ultrasonic transmitter,
(c) a second transducer unit spaced from said first transducer
unit with the fluid therebetween, and including a second ultrasonic
transmitter having a second sending direction and a second ultrasonic
receiver disposed close to said second ultrasonic transmitter and
having a second receiving direction which forms an angle of substantially
zero with said second sending direction for detecting a Doppler
shifted signal having a frequency lower than the frequency of the
input signal to said second ultrasonic transmitter,
(d) means for obtaining a first Doppler shift (+.DELTA.f.sub.1)
defined by the difference in frequency between the input and output
signals of said first transducer unit and for obtaining a second
Doppler shift (-.DELTA.f.sub.2) defined by the difference in frequency
between the input and output signals of said second transducer unit,
and
(e) signal emphasizing means for producing the difference between
said first and second Doppler shifts (+.DELTA.f.sub.1 +.DELTA.f.sub.2),
thus substantially doubling the value.
2. A flow meter as claimed in claim 1 wherein said first and second
transducer units are disposed in a noise cancellable position in
which first and second bodies of the fluid observed respectively
by said first and second ultrasonic receivers move generally in
symmetry with respect to the central axis of the fluid flow, thus
assuring that the noise component of the Doppler shifted signal
detected by said first ultrasonic receiver assumes characteristics
of frequency variation similar to those assumed by the noise component
of the Doppler shifted signal detected by said second ultrasonic
receiver.
3. A flow meter system as claimed in claim 2 wherein said first
and second transducer units are disposed generally on opposite sides
of the flow path.
4. In an ultrasonic flow meter including transducer means mounted
on the edge of a flow path of a fluid and having ultrasonic transmitting
and receiving means for feeding an ultrasonic signal to the fluid
and receiving the fed-back ultrasonic signals that have traveled
through the fluid, and means electrically coupled with the receiver
transducer means for processing the electric signals thereof into
signals representing the velocity of the flow and/or the rate of
the flow of the fluid, the improvement comprising:
(a) said transducer means including
(i) a first transducer unit and
(ii) a second transducer unit separated by the fluid from said
first transducer unit and disposed downstream relative to said first
transducer unit so that said first transducer unit is aligned with
the path of travel of ultrasonic waves transmitted from said second
transducer unit whereas said second transducer unit is aligned with
the path of travel of ultrasonic waves transmitted from said first
transducer unit whereby in response to a first condition of the
fluid said second transducer unit receives a first transmission
mode ultrasonic signal from said first transducer unit whereas said
first transducer unit receives a second transmission mode ultrasonic
signal from said second transducer unit, and in response to a second
condition of the fluid, said first transducer unit receives a first
reflection mode ultrasonic signal reflected by a reflecting object
or objects in the fluid on the way from said first transducer unit
whereas said second transducer unit receives a second reflection
mode ultrasonic signal reflected by a reflecting object or objects
on the way from said second transducer unit; and
(b) said signal processing means including
(i) first means responsive to said first condition of the fluid
for obtaining a signal representing the time difference between
the time for the ultrasonic waves to travel downstream from said
first transducer unit across the fluid flow path to said second
transducer unit and the time for the ultrasonic waves to travel
upstream from said second transducer unit across the fluid flow
path to said first transducer unit, and
(ii) second means responsive to said second condition of the fluid
for obtaining first and second signals representing respectively
a first Doppler shift as evidenced in said first reflection mode
signal and a second Doppler shift as evidenced in said second reflection
mode signal, and for obtaining a third signal representing the Doppler
shift difference between said first and second Doppler shifts.
5. An ultrasonic flow meter as claimed in claim 4 further comprising
mode control means coupled with said ultrasonic receiving means
for comparing the strength or amplitude of the received signal with
a reference level defining the critical point between said first
and second conditions of the fluid and producing a mode switching
signal when said strength or amplitude of the received signal crosses
said reference level, whereby the flow meter is automatically switched
between the transmission and reflection modes of operation and always
operates in a desired one of said modes in accordance with the condition
of the fluid, thus establishing a continuous measuring of flow irrespective
of a wide variation in parameters defining the condition of the
fluid.
6. An ultrasonic flow meter as claimed in claim 5 wherein the fluid
comprises an industrial waste water and the condition of the fluid
comprises the degree of pollution in the water.
7. An ultrasonic flow meter as claimed in claims 5 or 6 wherein
said mode control means includes timer means for introducing a predetermined
dead time thereby to disable said mode control means from producing
said mode switching signal at the instant when the strength or amplitude
of said received signal crosses said reference level and continues
the disabling until said predetermined dead time has elapsed during
which the strength or amplitude of said received signal is maintained
at a level either higher or lower than said reference level.
8. An ultrasonic flow meter as claimed in claim 5 further including
oscillator means for generating a continuous electric oscillatory
signal having a frequency in the range of the ultrasonic frequency,
modulator means connected between said oscillator means and said
ultrasonic transmitting means and responsive to said mode switching
signal for producing periodically intermittent waves of said electric
oscillatory signal whenever the flow meter is being operated in
said transmission mode and for allowing continuous waves of said
electric oscillatory signal to pass therethrough whenever in said
reflection mode.
9. An ultrasonic flow meter as claimed in claim 8 further including
switching circuit means which is operably connected between said
modulator means and said transducer means only when and whenever
the flow meter is being operated in said transmission mode, and
beam direction control means for periodically switching said circuit
means between a first position in which only the ultrasonic transmitting
means of said first transducer unit is energized to transmit a beam
of ultrasonic waves downstream thereof and a second position in
which only the ultrasonic transmitting means of said second transducer
unit is energized to transmit a beam of ultrasonic waves upstream
thereof.
10. An ultrasonic flow meter as claimed in claim 4 or 9 wherein
said first processing means comprises tuner means coupled with said
ultrasonic receiving means, AGC controlled amplifier coupled with
said tuner means, pulse shaping means coupled with said amplifier
means, counter means coupled with an electric and periodically intermittent
signal in substantially a certain phase or timed relationship with
the signal as supplied to said ultrasonic transmitting means and
also coupled with the output signal from said pulse shaper in substantially
a certain phase or timed relationship with the signal as received
by said ultrasonic receiving means for measuring the time difference
therebetween, and producing the corresponding digital code, memory
means coupled with said counter means, and arithmetic and logic
means coupled with said memory means.
11. An ultrasonic flow meter as claimed in claim 10 wherein said
second processing means comprises mixing means, low-pass filtering
means, frequency-to-voltage converting means, analog-to-digital
converting means, memory means and arithmetic and logic means.
12. An ultrasonic flow meter as claimed in claims 5 6 8 or 9
wherein said first transducer unit includes a transmitter and a
receiver disposed close to and downstream relative to said transmitter,
and said second transducer unit includes a transmitter and a receiver
disposed close to and downstream relative to said transmitter whereby
in the transmission mode, said transmitter of said first transducer
unit transmits a beam of ultrasonic waves, which travels along a
first path and is received by said receiver of said second transducer
unit whereas said transmitter of said second transducer unit transmits
a beam of ultrasonic waves, which travels along a second path which
is generally parallel to said first path and is received by said
receiver of said first transducer unit, and in the reflection mode,
the geam of ultrasonic waves from said transmitter of said first
transducer unit is reflected on the way and received by said receiver
of said first transducer unit whereas the beam of ultrasonic waves
from said transmitter of said second transducer unit is reflected
on the way and received by said receiver of said second transducer
unit.
13. An ultrasonic flow meter as claimed in claim 10 wherein said
second processing means comprises mixing means, low-pass filtering
means, frequency-to-voltage converting means, analog-to-digital
converting means, memory means and arithmetic and logic means.
14. An ultrasonic flow meter as claimed in claim 9 wherein said
first transducer unit includes a transceiver and a transmitter disposed
close to and upstream relative to said transceiver, and said second
transducer unit includes a transceiver and a receiver disposed close
to and downstream relative to said transceiver, whereby when in
the transmission mode, an ultrasonic beam is transmitted by said
transceiver of said first transducer unit and travels across the
fluid path and downstream along a path and is received by said transceiver
of said second transducer unit during downstream beam direction
periods whereas during upstream beam direction periods an ultrasonic
beam is transmitted by said transceiver of said second transducer
unit and travels across the fluid path and upstream along the same
path as during downstream beam direction periods and is received
by said transceiver of said first transducer unit, and when in the
reflection mode, an ultrasonic beam is transmitted by said transmitter
of said first transducer unit and is reflected on the way and received
by said transceiver of said first transducer unit whereas an ultrasonic
beam is transmitted by said transceiver of said second transducer
unit and is reflected on the way and received by said receiver of
said second transducer unit.
15. An ultrasonic flow meter as claimed in claim 9 wherein said
first transducer unit includes a transceiver and a receiver disposed
close to and downstream relative to said transceiver ad said second
transducer unit also includes a transceiver and a receiver disposed
close to and downstream relative to said transceiver, whereby when
in the transmission mode, an ultrasonic beam travels along a single
path between said transceivers in the direction across and downstream
for first beam direction periods and in the opposite direction or
second beam direction periods, and when in the reflection mode,
an ultrasonic beam from said transceiver of said first transducer
unit is reflected on the way and received by said receiver of said
first transducer unit whereas an ultrasonic beam from said transceiver
of said second transducer unit is reflected on the way and received
by said receiver of said second transducer unit.
16. An ultrasonic flow meter as claimed in claim 4 wherein said
first transducer unit includes a transmitter transducer and a receiver
transducer, and said second transducer unit includes a transmitter
transducer and a receiver transducer.
17. An ultrasonic flow meter as claimed in claim 4 wherein said
first transducer unit includes a transmitter transducer and a transceiver
transducer, and said second transducer unit includes a receiver
transducer and a transceiver transducer.
18. A ultrasonic flow meter as claimed in claim 4 wherein said
first transducer unit includes a transceiver transducer and a receiver
transducer, and said second transducer unit includes a transceiver
transducer and a receiver transducer.
19. An ultrasonic flow measurement system for measuring the velocity
of the flow and/or the rate of the flow of a fluid to be tested
comprising:
(a) means for supplying ultrasonic signals through the fluid and
receiving ultrasonic signals that have traveled through the fluid,
the received signal being either a reflection mode signal generated
by reflection of the supplied signal by a reflecting object or objects
in the fluid or a transmission mode signal which is a supplied signal
that has traveled without being reflected, said means including
a first transmitter transducer for transmitting an ultrasonic signal
through the fluid along a first path, a first receiver transducer
disposed adjacent to said first transmitter transducer and aligned
with a second path generally parallel to and adjacent to said first
path for receiving ultrasonic signals incident thereon along the
second path, a second transmitter trransducer for transmitting an
ultrasonic signal through the fluid along the second path, and a
second receiver transducer disposed adjacent to said second transmitter
transducer and aligned with said first path for receiving ultrasonic
signals incident thereon along the first path;
(b) first signal processing means operatively coupled with said
first and second receiver transducers for processing the reflection
mode signals into signals representing the velocity of flow of the
fluid, said first signal processing means including means for producing
first and second beat signals representing the first Doppler shift
and second Doppler shift, respectively, means coupled with said
beat signal producing means for producing the difference signal
representing accurately the signal component of the first and second
Doppler shifts while cancelling substantially the noise component
thereof, and means coupled with said differential means for correcting
the signal component by introducing a predetermined arithmetic and
logic operation thereto whereby the resultant signal represents
the velocity of flow and/or the rate of flow;
(c) second signal processing means operatively coupled with said
first and second receiver transducers for processing the transmission
mode signals into signals representing the velocity of flow and/or
flow rate of the fluid, said second signal processing means including
means for producing first and second time signals representing the
first time of ultrasonic propagation or travel from said first transmitter
transducer to said second receiver transducer, and the second time
of ultrasonic propagation or travel from said second transmitter
transducer to said first receiver respectively, time difference
means for producing a signal representing the time difference between
the first time and second time, and means coupled with said time
difference means for correcting the time difference signal by introducing
a second predetermined arithmetic and logic operation whereby the
corrected signal represents the velocity of flow and/or the rate
of flow of the liquid; and
(d) mode switching means for switching the system between the reflection
mode and the transmission mode, said mode switching means including
means for detecting a critical level of pollution determined by
the number of said reflecting objects existing in a unit volume
of the fluid by comparing the amplitude or strength of the received
ultrasonic signal with a reference level, means coupled with said
detecting means for producing a switching signal whereby the system
is automatically switched between the two modes of operation in
accordance with the number of the reflecting objects contained in
the fluid.
Description FIELD OF THE INVENTION
This invention relates to an ultrasonic flow meter for measuring
the velocity of the flow (change of position/unit time) and/or the
flow rate (volume/unit time).
The term "particle" refers, herein to acoustic reflecting
object in the fluid, such as "air bubble" and/or "solid
particle".
BACKGROUND OF THE INVENTION
Sing-around method, phase-difference method, time-difference method
are wellknown techniques for measuring the velocity of flow and/or
the flow rate of clear water. Particularly, the time-difference
technique has been frequently used. This system typically includes
a pair of transducers disposed obliquely in a facing relationship
and positioned upstream and downstream in the path of travel of
the liquid so that the velocity vector of the liquid changes the
propagation speed of the ultrasonic wave along a path from the transmitter
to the receiver transducer.
In one mode, the upstream-positioned transducer transmits ultrasonic
signals to the downstream-positioned transducer to obtain the time
for the acoustic wave traveling or being propagated downstream while
in another mode, the downstream-positioned transducer transmits
ultrasonic signals to the upstream-positioned transducer to obtain
the time for the acoustic wave traveling or being propagated upstream.
The velocity of the liquid is determined from the time difference
between the upstream and downstream modes of propagation.
However, any of the above-mentioned three techniques is a transmission
method in which it is assumed that ultrasonic waves can travel through
the liquid medium between the sound transmitter and receiver and
reach the latter without significant loss or attenuation.
Air bubbles and solids contained in the liquid are obstacles to
ultrasonic waves and scatter them. For this reason, the transmission
method is applicable only to clear water which contains few or no
air bubbles and/or solids.
The Doppler shift or reflection method is also utilized to measure
the velocity and/or the flow rate of fluid. In a typical Doppler
shift ultrasonic flow meter, an acoustic transmitter and receiver
are mounted on the opposite sides of a test conduit in which fluid
flows. The transmitter transmits ultrasonic waves which then travel,
for example, upstream with respect to the direction of the fluid
containing air bubbles and/or solids, and are scattered by those
particles. Part of the scattered or reflected waves comes into the
field of the receivers and is observed as a Doppler-shifted signal.
The value of the Doppler shift relates to the velocity or average
velocity of those particles in the fluid near the central axis which
have received the transmitted waves and reflected them toward the
receiver. It is clearly understood that this method cannot be applied
to flow measurement of clear water which does not include air bubbles
or solids.
Although the existence of particles or reflecting objects is a
pre-requisite for the Doppler-shift flow measurement, the prior
art Doppler-shift flow meters are not applicable to very polluted
water containing a large number of particles.
There exists a great need for continuous measurement of flow of
a liquid in which pollution degree varies with time, such as the
industrial waste water of the type for which the total amount of
pollutants is regulated. To achieve this in the prior art, at least
two types of flow measuring equipment are required. This complicates
the transportation and installation work. Further, the operator
must monitor the level of pollution and decide which type of the
equipment to switch into operation. This requires high skill on
the part of the operator.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an ultrasonic flow
meter capable of measuring the flow of the fluid irrespective of
the condition thereof.
Another object of this invention is to provide an ultrasonic flow
meter capable of measuring the velocity of flow and/or rate of flow
of fluid with low degree pollution.
Another object of this invention is to provide an ultrasonic flow
meter of the Doppler-shift type capable of measuring the velocity
of flow and/or rate of flow of highly polluted fluid.
Yet another object of this invention is to provide an ultrasonic
flow meter capable of measuring the flow of fluid with high accuracy.
Still another object of this invention is to provide an ultrasonic
flow meter which can continuously measure the velocity of flow and/or
rate of flow of the fluid.
A further object of the invention is to provide an ultrasonic flow
meter having two modes of operation in accordance with the condition
of the fluid.
A still further object of the invention is to provide an ultrasonic
flow meter which detects a transmission mode ultrasonic signal in
response to a first condition of the fluid and also detects a reflection
mode ultrasonic signal in response to a second condition of the
fluid.
Another object of this invention is to provide an ultrasonic flow
meter which is inexpensive to manufacture.
A further object of this invention is to provide an ultrasonic
flow meter which is easy to transport and install and remove.
A further object of this invention is to provide an automatic flow
meter which does not require a person's attention.
A still further object of the invention is to provide an ultrasonic
flow meter including two signal processing means which selectively
operate in response to both low and high degree of pollution of
water.
In accordance with one aspect of this invention, there is provided
a reflection type or Doppler shift ultrasonic flow meter including
an acoustic transmitter and an acoustic receiver that is disposed
very close to the transmitter. The beam of ultrasonic waves from
the transmitter is reflected in the fluid by particles therein and
then returns to the acoustic receiver. Since the distance of travel
of the waves from the transmitter and the receiver through fluid
is relatively short. Accordingly, the flow meter system is applicable
to those fluids that contain a large number of particles such as
air bubbles and solids.
In accord with a further aspect of the invention, there is provided
an ultrasonic Doppler shift flow meter including two sets of acoustic
transmitter and receiver with one set positioned downstream the
other and in facing relationship thereto. This second system improves
the signal-to-noise ratio of the mentioned system.
In accordance with a further aspect of this invention, there is
provided an ultrasonic flow meter having Doppler shift measuring
means and second means for measuring the time of propagation for
the ultrasonic wave from a downstream-positioned acoustic transmitter
to an upstream-positioned acoustic receiver and third means for
measuring the time of propagation from an upstream-positioned transmitter
to a downstream-positioned receiver. These transmitters and receivers
are disposed as described with respect to the above second system.
The ultrasonic flow meter automatically switches between transmission
and reflection modes of operation. Accordingly, for less polluted
fluid, the second and third means operate. While, for more polluted
fluid, the Doppler shift measuring means operates, whereby the flow
meter continuously provides a good measure of the flow velocity
and/or flow rate of the fluid, whatever the degree of pollution
of the fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages will become
more apparent from the description by reference to the accompanying
drawings in which:
FIG. 1 is a block diagram of an embodiment of this invention;
FIG. 2 is a time chart illustrating principal waves in the embodiment
in FIG. 1;
FIG. 3 is a block diagram of another embodiment of the invention;
FIG. 4 is a time chart illustrating principal waves in the embodiment
in FIG. 3 and in the reflection mode signal processing section D
in FIG. 6;
FIG. 5 is a block diagram of another embodiment of this invention;
FIGS. 6A, 6B, and 6C are block diagrams of another embodiment of
this invention;
FIG. 7 is a time chart illustrating principal waves in the transmission
mode signal processing section C in FIG. 1;
FIGS. 8A, 8B, and 8C are block diagrams of another embodiment of
this invention;
FIGS. 9A and 9B are block diagrams of another embodiment of this
invention; and
FIG. 10 is a block diagram showing a portion of another embodiment
of this invention wherein the tuners 19 26 and 32 may be connected
to the amplifiers 21 28 and 34 in FIG. 8 respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a block diagram showing the arrangement of circuit components
in one embodiment of this invention, wherein a transmitting transducer
6 is fixed adjacent a receiving transducer 8 on the inclined surface
of a plastic wedge 3 positioned in close contact with the outer
wall of a pipe 2. An oscillator 100 generates an electrical signal
having a frequency ft which is supplied to the transducer 6 that
transmits an ultrasonic signal having a frequency ft and a waveform
as shown in curve (A) of FIG. 2. The ultrasonic signal is transmitted
through the plastic wedge 3 while it forms a beam having an axis
4 that crosses the central axis 5 of the pipe 2 at an angle of .alpha..
The ultrasonic signal hits a solid particle 12 that flows near the
wall of the pipe 2 and part of the signal is reflected to be received
by the transducer 8. When the transducer 6 supplies a sinusoidal
ultrasonic signal as shown in curve (A) of FIG. 2 the transducer
8 receives an ultrasonic signal having a waveform as shown in curve
(B) of FIG. 2 having a frequency fr which is lower than the frequency
ft of the transmitted ultrasonic signal because of the Doppler effect
due to a vector component of the velocity of the solid particle
12 on the axis 4 in the direction toward the central axis 5. In
addition, as indicated by t.sub.1 in curve (B) of FIG. 2 the received
ultrasonic signal has a delay time with respect to the transmitted
ultrasonic signal. The delay time is equal to the time required
for the ultrasonic signal transmitted to pass into the pipe 2 through
the plastic wedge 3 reach the solid particle 12 by going through
the fluid 1 and be reflected by that particle to be received by
the transducer 8. When the solid particle 12 moves away, similar
solid particles successively appear on the axis 4 to keep the supply
of the sinusoidal signal shown in curve (B) of FIG. 2.
Curve (C) of FIG. 2 shows the waveform of a signal reflected from
another solid particle 12' which is also present on the axis 4 but
is closer to the central axis 5 than the particle 12 and if the
distribution of flow rate is substantially uniform, the frequency
fr' of the reflected signal is almost equal to the frequency fr
of the signal reflected from the solid particle 12 present close
to the particle 12'. But since the ultrasonic signal is reflected
by the solid particle 12 only a very weak ultrasonic signal reaches
the solid particle 12', and in addition, the signal reflected from
the solid particle 12' is sometimes blocked by the solid particle
12 and in consequence, the transducer 8 receives an extremely attenuated
signal from the solid particle 12'.
Accordingly, in the circuit configuration shown in FIG. 1 the
frequency of the reflected signal received by the transducer 8 is
determined by the Doppler shift caused by a change in the rate of
the fluid flowing in the vicinity of the wall of the pipe 2 not
by the fluid flowing in the center of the pipe 2. In curve (C) of
FIG. 2 the delay time t.sub.2 is required for the ultrasonic signal
transmitted to go to and come from the solid particle 12', and is
greater than the delay time t.sub.1 since the solid particle 12'
is more remote from the transducer 6 than the solid particle 12.
The ultrasonic signal received by the transducer 8 is supplied
to a mixer 110 in the form of an electrical signal having a frequency
of fr. The mixer 110 also receives an electrical signal having a
frequency ft from the oscillator 100 and supplies a low-pass filter
120 with an output signal as shown in curve (D) of FIG. 2 having
a frequency (ft-fr) in addition to the electrical signals having
frequencies of ft and fr. The low-pass filter 120 delivers an output
composed of a signal component having a frequency (ft-fr) which
is much lower than ft or fr. As already mentioned, the frequency
(ft-fr) is determined by the Doppler shift caused by a change in
the velocity of the fluid near the wall of the pipe 2 and the relation
between the (ft-fr) and fluid velocity is expressed by the following
equation: ##EQU1## wherein V=the velocity of the fluid near the
wall of the pipe, .alpha.=the angle formed by the axis of the beam
of the ultrasonic signal from the transmitter 6 and the central
axis of the pipe, C=the propagation speed of the ultrasonic signal
for V=0 and thus K=constant.
Then the output signal from the low-pass filter 120 is supplied
to a frequency discriminator 130 which produces an output voltage
proportional to the frequency difference (ft-fr) which is further
supplied to a corrector 140 which adjusts the voltage output with
the constant K and makes a necessary correction based on the actually
measured distribution of flow velocity, to thereby produce an output
voltage that represents the average flow velocity in the pipe 2.
The corrector 140 provides an output voltage that indicates the
flow rate (volume/unit time) by making an additional correction
in which the average flow velocity (distance of travel/unit time)
is multiplied by a constant for the crossectional area of the pipe
2.
In the illustrated embodiment of ultrasonic flow meter according
to this invention, the transmitter and receiver transducers (6 and
8) of only one set are positioned adjacent each other so the only
Doppler shift that can be utilized is that caused by the differential
frequency (ft-fr) which represents the difference between the frequency
ft of an ultrasonic signal transmitted from the transducer 6 and
the frequency fr of an ultrasonic signal received by the transducer
8. One defect of this system is that as the flow rate of the fluid
decreases, so does the differential frequency (ft-fr), making accurate
measurement of the flow rate difficult. Another defect is that although
when considered as an average particle the solid particle 12 that
reflects ultrasonic signals moves parallel to the wall of the pipe
2 or the central axis 5 some, in fact move in a slightly serpentine
path instead of taking a linear course that is completely parallel
to the central axis. A vector component of the velocity of the particle
12 in the direction of the axis 4 that is produced by such serpentine
motion of the particle 12 becomes an error-causing factor in the
frequency range of the ultrasonic signal reflected from the particle
12 and causes the frequency of the ultrasonic signal received by
the transducer 8 to fluctuate, i.e. become higher or lower than
fr. As a result, the measured value of flow velocity and corresponding
flow rate fluctuates to provide an output that contains a significant
error.
Therefore, another purpose of this invention is to provide an ultrasonic
flow meter that uses two sets of transmitting and receiving transducers
and determines the difference of output signals delivered from the
two receiver transducers to amplify the shift of frequency due to
the Doppler shift and offset a noise component in the frequency
range due to such factors as the serpentine motion of solid particles
in the fluid.
FIG. 3 is a block diagram showing the arrangement of circuit components
in another embodiment of this invention, wherein like numerals identify
like components; a second transmitting transducer 11 is fixed adjacent
a second receiving transducer 10 on the inclined surface of a plastic
wedge 9 so that the axis 4' of the ultrasonic signal beam transmitted
forms an angle .alpha. with respect to the central axis 5 of the
pipe 2. The ultrasonic signal having a frequency ft transmitted
from the first transmitting transducer 6 is reflected from the solid
particle 12 and is received by the first receiving transducer 8
which converts said signal into an electrical signal. Since the
axis 4 of the ultrasonic signal beam is such that a vector component
of the velocity of the fluid 1 in the direction of the axis 4 points
toward the central axis 5 the frequency fr of the reflected wave
having the Doppler shift due to the movement of the solid particle
12 in the direction of the central axis is lower than the frequency
ft of the ultrasonic signal transmitted. Accordingly, when the first
transmitting transducer supplies a sinusoidal wave having a frequency
ft, the first receiving transducer 8 delivers a sinusoidal signal
having a frequency fr and a waveform as shown in curve (A) of FIG.
4. But actually, because of the vector component of the velocity
of the solid particle 12 in the direction of the axis 4 that is
caused by the serpentine motion of the particle 12 the frequency
of the reflected wave received at the first receiving transducer
8 i.e. the frequency of the electrical signal supplied from the
first receiver transducer 8 fluctuates, i.e. becomes greater or
smaller than fr by the amount of .DELTA.f.sub.N. An amplifier 150
amplifies such electrical signal and supplies the amplified signal
to the mixer 110. The gain of the amplifier 150 is so adjusted by
an automatic amplitude adjusting unit or AGC element 160 that it
keeps delivering an output signal having a constant amplitude. In
the embodiment illustrated, even if there is a change in the relative
position of the first transducer 6 and the first receiver transducer
8 or in the nature of the solid particle 12 in the fluid 1 an electrical
signal having a frequency fr and a constant amplitude is kept supplied
to the mixer 110 thus minimizing any false measurement due to the
change in the amplitude of the signal supplied to the frequency
discriminator 130.
The mixer 110 and low-pass filter 120 operate in the same manner
as described in connection with the first embodiment of this invention
by reference to FIG. 1 and the low-pass filter 120 supplies the
frequency discriminator 130 with a low-frequency signal having a
frequency ft-(fr.-+..DELTA.f.sub.N) and a waveform as shown in curve
(B) of FIG. 4. In response to such low-frequency signal, the frequency
discriminator 130 produces an output signal as shown in curve (C)
of FIG. 4. In curve (C) of FIG. 4 .DELTA.E is an output signal
that is produced as a result of the Doppler shift (ft-fr) due to
the movement of the solid particle 12 in the direction of the central
axis 5 which is detected by the first transmitter transducer 6 and
the first receiver transducer 8 and .DELTA.E.sub.N is the effective
value of a ripple component that is produced as a result of a frequency
change .DELTA.f.sub.N that is caused by the serpentine motion of
the solid particle 12.
The second transmitting transducer 11 and the second receiving
transducer 10 operate in the same manner as the first transmitting
transducer 6 and the first receiving transducer 8 and the second
receiving transducer 10 produces an electrical signal having a frequency
fr'. In the illustrated arrangement of the second transmitting transducer
11 and the second receiving transducer 10 the axis 4' of the ultrasonic
signal beam is so formed that a vector component of the velocity
of the fluid 1 in the direction of the axis 4 points in a direction
opposite to that of the central axis 5 and hence, the frequency
fr' of the reflected wave having the Doppler effect due to the movement
of the solid particle 13 is higher than the frequency ft of the
ultrasonic signal transmitted. Accordingly, the second receiving
transducer 10 provides a sinusoidal signal that becomes higher or
lower than the frequency fr' by the amount .DELTA.f.sub.N' as shown
in curve (D) of FIG. 4. In response to such electrical signal, the
second amplifier 150', automatic amplitude adjusting unit 160',
mixer 110', low-pass filter 120' and frequency discriminator or
frequency-voltage converter 130' operate in the manner entirely
the same as when they operate to process the signal from the first
receiving transducer 8. The second frequency discriminator 130'
is supplied with a low-frequency signal having a frequency (fr'.-+..DELTA.f.sub.N')-ft
as shown in curve (E) of FIG. 4 thereby producing an output signal
as shown in curve (F) of FIG. 4. In curve (F) of FIG. 4 .DELTA.E'
is the output signal produced by the Doppler shift fr'-ft that is
detected by the second transmitting transducer 11 and the second
receiving transducer 10 and .DELTA.E.sub.N' is the effective value
of a ripple component that is produced as a result of frequency
change that is caused by the serpentine motion of the solid particle
13. When fluid of uniform composition flows through an ordinary
pipe, the distribution of flow velocity is symmetrical with respect
to the central axis 5 so the absolute values of .DELTA.E and .DELTA.E'
are substantially equal to each other, and so are the absolute values
of .DELTA.E.sub.N and .DELTA.E'.sub.N.
An output signal as shown in curve (C) of FIG. 4 from the first
frequency discriminator 130 and an output signal as shown in curve
(F) of FIG. 4 from the second frequency discriminator 130' are supplied
to a differential amplifier 170 which subtracts the latter signal
from the former to supply the corrector 140 with an output signal
as shown in curve (G) of FIG. 4. The first and second receiving
transducers 8 10 are so positioned that they are subjected to the
Doppler shifts in opposite directions because of the movement of
the solid particle 12 and 13 in the direction of the central axis
5. Hence, as shown at .DELTA.E in curve (C) of FIG. 4 and at -.DELTA.E'
in curve (F) of FIG. 4 output signals having opposite polarity
are produced as a result of the movement of the solid particles
12 13 in the direction of the central axis at different speeds,
and they change in opposite directions in response to a change in
the speed. Consequently, an output signal .DELTA.E+ .DELTA.E' as
shown in curve (G) of FIG. 4 is produced as the signal for the difference
between the two signals, and the resulting voltage output is almost
twice that produced from the circuit using only the first receiving
transducer 8.
To eliminate a noise component resulting from the serpentine motion
of the solid particles 12 13 symmetrical with respect to the central
axis 5 the first receiving transducer 8 and the second receiving
transducer 10 are so arranged that they are subjected to the Doppler
shift in the same direction. In addition, the serpentine motion
of the solid particle 12 and that of the solid particle 13 contained
in the fluid 1 at a position close to the wall of the pipe 2 that
is opposite to the wall close to which the particle 12 is present
have many velocity components symmetrical with respect to the central
axis 5 and are related to each other. As a result, when the ripple
component of the output signal from the first frequency discriminator
130 increases as shown, for example, at (a) in curve (C) of FIG.
4 there is a great possibility that the output signal from the
second frequency discriminator 130' also changes in the same way
as shown at (a') in curve (F) of FIG. 4. Consequently, in the signal
shown in curve (G) of FIG. 4 that indicates the difference between
the two signals, the ripple components .DELTA.E.sub.N and .DELTA.E.sub.N',
i.e. the noise components in frequency range due to the serpentine
motion of the solid particles 12 13 are offset.
The phenomena described above are formulated as follows: When a
vector component of the velocity of the solid particle 12 in the
direction of the central axis 5 and a vector component of the velocity
of the solid particle 13 in the same direction are V.sub.1 and V.sub.2
respectively, the following relations are established: V.sub.1 =u.sub.1
+.DELTA.u.sub.1 and V.sub.2 =u.sub.2 -.DELTA.u.sub.2 wherein u.sub.1
and u.sub.2 are vector components of the average velocities of the
solid particles 12 13 respectively, in the direction of the central
axis 5 and .DELTA.u.sub.1 and .DELTA.u.sub.2 are vector components
of the change in the velocities of the solid particles 12 13 respectively,
due to the motion of the particles.
As already known, the relation between the Doppler shift and the
velocity of an object to be measured is represented by the following
formula: ##EQU2## wherein .DELTA.f=the Doppler shift, ft=the frequency
of the signal transmitted, fr=the frequency of the signal received,
C=the propagation speed of the ultrasonic signal for V=0 V=the
velocity of the object to be measured, and .alpha.=the angle formed
between the direction in which the signal transmitted or received
propagates and the direction in which the object to be measured
moves.
Therefore, the Doppler shift .DELTA.f.sub.1 detected by the first
transmitting transducer 6 and the first receiving transmitter 8
in FIG. 3 is represented as follows: ##EQU3##
Likewise, the Doppler shift detected by the second transmitting
transducer 11 and the second receiving transducer 10 is represented
by: ##EQU4##
The difference between the two Doppler shifts obtained by the differential
amplifier 170 is: ##EQU5## wherein the first term in the right side
represents the Doppler shift based on the average velocity of the
particles 12 13 and the second term represents the Doppler shift
due to the serpentine motion of the particles. Therefore, when the
solid particles 12 13 move along serpentine courses in complete
symmetry with respect to the axis 5 .DELTA.u.sub.1 is equal to
.DELTA.u.sub.2 in the formula above, so that the second term in
the right side is zero, indicating that the noise component in frequency
range is completely offset.
If the average velocities of the solid particles 12 13 are equal
in the direction of the central axis 5 u.sub.1 is equal to u.sub.2
in the formula above, so that the first term in the right side is
(4ft/C)u.sub.1 cos .alpha., indicating that the Doppler shift due
to the average velocities of the solid particles in the direction
of the central axis 5 is doubled. FIG. 5 is a block diagram showing
the arrangement of circuit components according to still another
embodiment of this invention, wherein like numerals identify like
components. When a control signal from a control unit 210 actuates
a switching unit 220 for supplying an electrical signal from the
first receiving transducer 8 to the amplifier 150 the frequency
discriminator 130 operates in the same manner as described in connection
with the second embodiment by reference to FIG. 3 to provide an
output signal as shown in curve (C) of FIG. 4. An analog-digital
converter 180 converts the output signal into a digital signal which
is supplied to a memory 190. Then, the control unit 210 makes another
supply of a control signal which actuates the switch unit 220 for
supplying an electrical signal from the second receiving transducer
10 to the amplifier 150. In the same manner as described above,
the frequency discriminator 130 supplies the analog-digital converter
180 with an output signal as shown in curve (F) of FIG. 4 which
is converted by the converter 180 to a digital signal for storage
in the memory 190. Then, the control unit 210 sends a command to
an arithmetic and logic unit 200 which reads from the memory 190
the digital signal that corresponds to the output signal from the
first receiving transducer 8 and the digital signal that corresponds
to the output signal from the second receiving transducer 10 and
subtracts the latter signal from the former. Such digital arithmetic
operation provides a digital signal that corresponds to the output
signal shown in curve (G) of FIG. 4. The corrector 140 receives
the digital signal from the arithmetic unit 200 and makes necessary
corrections, i.e. a correction based on the distribution of flow
velocity (distance of travel/unit time) particle velocity, and a
correction with a constant providing an indication of flow rate
(volume of flow/unit time).
One advantage of the embodiment shown in FIG. 5 is that it need
use only one amplifier 150 one mixer 110 and one frequency discriminator
130 since these units are used on a time-shared basis (in comparison,
two units of each component are necessary in the embodiment of FIG.
3).
In the second and third embodiments of this invention (FIGS. 3
and 4), the axis 4' of the ultrasonic signal beam from the second
transmitting transducer 11 is positioned on the extension of the
axis 4 of the ultrasonic signal beam from the first transmitting
transducer 6. However the second transmitting and receiving transducers
11 10 need not be so positioned as to satisfy this relation. Alternatively
the first and second transmitting transducers 6 11 may be arranged
on the line that crosses the central axis 5 at a right angle, with
the fluid 1 being interposed between the two. In other words, the
two transmitting transducers are disposed on opposite sides of the
pipe or conduit 2. One advantage of this arrangement is that the
positions at which the ultrasonic signal is reflected from the solid
particles 12 13 are symmetrical with respect to the central axis
5 and in consequence, most reliable offsetting of the frequency
changes due to the serpentine motion of the solid particles 12
13 is achieved.
However, it should be noted that the space relationship between
the two transducer units (one comprises the transmitter 6 and receiver
8 while the other unit comprises the transmitter 11 and the receiver
10) as illustrated in FIG. 3 also produces a substantial noise cancelling
effect, in cooperation with the signal emphasizing processing as
given in the differential amplifier 170. Briefly, this space relationship
is also a noise cancellable arrangement unless the distance between
the two transducer units is far longer than the radius of the pipe
2 (d>>r).
In the second and third embodiments, the angle at which the axis
4' of the ultrasonic signal beam from the second transmitter transducer
11 crosses the central axis 5 is equal to the angle at which the
axis 4 of the ultrasonic signal beam from the first transmitter
transducer 6 crosses the central axis 5 but the two angles may
differ from each other and even in such a case, by adjusting the
gain of the differential amplifier 170 with respect to individual
input signals, ripple component produced by a change in frequency
can also be offset satisfactorily and the output signal that is
produced by the Doppler shift due to the movement of the solid particles
12 13 in the direction of the central axis 5 is substantially twice
the output produced from the embodiment of FIG. 1.
Referring now to FIGS. 6A, 6B and 6C there is shown yet another
embodiment of this invention. The illustrated embodiment basically
comprises an ultrasonic transmitter and receiver section designated
A, an oscillator section designated B, a transmission mode signal
processing section designated C, a reflection mode signal processing
section designated D, a mode switching control section designated
E, mode switching sections designated F.sub.1 and F.sub.2 and an
output section designated G.
The ultrasonic signal transmission and reflection section A comprises
a wedge shaped plastic member mounted on the wall 2 of the pipe
in which the tested fluid 1 flows in the direction generally designated
by an arrow V, an upstream-positioned transducer 6 secured on the
inclined portion of the wedge 3 for transmitting ultrasonic signals
downstream in the fluid medium along the path of signal propagation
4 which intersects the axis of the pipe 5 or the direction of the
fluid flow with a predetermined angle .alpha. less than 90 degrees,
an upstream-positioned receiving transducer 8 also secured on the
inclined portion of the wedge 3 and disposed adjacent to the transducer
6 and in alignment with the path of travel 7 generally parallel
and adjacent to the path 4 a wedge shaped plastic member 9 mounted
on the pipe wall 2 and disposed in opposed relationship to the plastic
wedge 3 a downstream-positioned receiving transducer 10 secured
on the inclined portion of the wedge 9 the path of travel 7 and
a downstream-positioned transmitting transducer 11 disposed adjacent
to the downstream-positioned receiving transducer 10 for transmitting
ultrasonic waves upstream along the path of propagation 7. Solids
or air bubbles are schematically exemplified by dots designated
12 and 13.
Oscillator section B comprises an oscillator 14 for generating
electric signals which are converted by the transmitting transducers
6 and 11 to ultrasonic signals having a frequency ranging preferably
from 500 KHz to 1 MHz, and a modulator 15 operatively connected
between the oscillator 14 and the transmitting transducers 6 and
11 for amplitude-modulating the sinusoidal carrier waves from the
oscillator 14 in accordance with pulses or modulating signals from
a pulse generator 16.
Transmission mode signal processing section C comprises an ultrasonic
beam direction switching circuit 17 including switches S.sub.1 and
S.sub.2 for switching the direction of the ultrasonic beam between
upstream and downstream, a controller 18 connected to the direction
switching circuit 17 and the modulator 15 for controlling the direction
switching circuit 17 a tuner 19 connected to the movable contact
of the switch or relay S.sub.2 an amplifier 21 gain-controlled
by an automatic gain control element 20 a pulse shaper 22 connected
to the gain controlled amplifier 21 a time interval counter 23
connected to the pulse shaper 22 and the modulator 15 for measuring
the time of ultrasonic propagation, a memory 24 connected to the
counter 23 and an ALU 25 connected to the memory 24 and the beam
direction switching circuit 18.
The reflection mode signal processing section D comprises a first
tuner of the staggered type 26 an amplifier 28 connected to the
tuner 26 and gain-controlled by an automatic gain controlling element
27 a first mixer 29 connected to the gain-controlled amplifier
28 and the oscillator 14 for producing heat signals, a first low-pass
filter 30 connected to the mixer 29 a first frequency-to-voltage
converter or frequency discriminator 31 connected to the low-pass
filter 30. This constitutes a first channel of the processing section
D for detecting a first type Doppler shift (decrease in frequency)
in the reflection mode. The reflection mode signal processing section
D also comprises a second type Doppler shift (increase in frequency)
detecting channel including a second tuner of the staggered type
32 which is connected to an amplifier 34 having an automatic gain
control element 33 which amplifier is connected to a second mixer
35 which is connected to a second low-pass filter 36 which is connected
to a second frequency discriminator 37. The outputs from the first
and second channels are connected to a differential amplifier 38
which is connected to an analog-to-digital converter 39 which is
connected to an ALU 40.
A mode switching control section E comprises an adjustable reference
voltage circuit 41 for supplying the reference voltage to a comparator
42 which also receives the signal from the amplifier 21. The output
of the comparator 42 is fed to a timer 43 which introduces a predetermined
delay or dead time in response and is connected to a mode switching
control circuit 44.
A first mode switching section F.sub.1 comprises a switching circuit
45 including switches S.sub.3 through S.sub.6. A second mode switching
section F.sub.2 comprises a switching circuit 46 including a switch
S.sub.7.
An output or display section G comprises a display register 47
connected to the movable contact of the switch S.sub.7 in the mode
switching circuit 46 and a display 48 connected to the register
47.
The output of the mode switching control section E is connected
to the first and second mode switching circuits 45 and 46 the beam
direction controller 18 pulse generator 16 and the comparator
42.
A movable contact of the switch S.sub.1 is connected to the modulator
15 beam direction switching controller 18 a stationary or fixed
contact II of the switch S.sub.3 and a stationary contact II of
the switch S.sub.4. A stationary contact a of the switch S.sub.1
is connected to a stationary contact I of the switch S.sub.4. Another
stationary contact b of the switch S.sub.1 is connected to a stationary
contact I of the switch S.sub.3. A movable contact of the switch
S.sub.2 is connected to the tuner 19. A stationary contact a of
the switch S.sub.2 is connected to a stationary contact I of the
switch S.sub.6. Another stationary contact b of the switch S.sub.2
is connected to a stationary contact I of the switch S.sub.5. A
stationary contact II of the switch S.sub.5 is connected to the
second tuner 32. A stationary contact II of the switch S.sub.6 is
connected to the first tuner 26 and the tuner 19.
The movable contacts of the switches S.sub.3 S.sub.4 S.sub.5
and S.sub.6 are connected to the transmitting transducer 6 the
transmitting transducer 11 the receiving transducer 10 and the
receiving transducer 8 respectively.
A stationary contact I of the switch S.sub.7 is connected to the
processor 25 in the transmission mode signal processing section
C while another stationary contact II of the same switch is connected
to the processor 40 in the reflection mode signal processing section
D. A movable contact of the switch S.sub.7 is connected to the display
controller 47.
The construction of this embodiment and its internal connections
have been described. The operation of the embodiment will be described
in conjunction with FIGS. 4 and 7 illustrating the principal waveforms
in the transmiwsion mode signal processing section C and the reflection
mode signal processing section D, respectively.
The mode of the flow meter system is controlled or selected by
the mode switching control section E. For clear or less polluted
fluid, the mode switching control section E generates a first mode
switching signal at the output of the mode switching control circuit
44. This mode switching signal is fed to the mode switching sections
F.sub.1 and F.sub.2 in which the switches S.sub.3 through S.sub.6
respond to connect or bridge their respective movable contacts to
the stationary contacts I, and maintain this position as far as
the system is operated in the transmission mode. The first mode
switching signal enables the beam direction controller 18 to produce
a control signal as shown in curve (A) of FIG. 7 switched periodically
between high and low levels. For high level periods Us', the switches
S.sub.1 and S.sub.2 in the beam direction switching circuit 17 are
energized and placed in an upstream position wherein the respective
movable contacts are connected to the stationary contacts a whereas
for periods Ds' during which the control signal is low, the switches
S.sub.1 and S.sub.2 are deenergized and placed in the downstream
position wherein the respective movable contacts are connected to
the stationary contacts b.
The first mode switching signal from the mode switching control
section E further enables the pulse generator 16 in the oscillator
section B to produce pulses or modulating waves in order to perform
the modulation in the oscillator section. Preferably, the modulating
waves are periodic rectangular pulses having a pulse width in the
range of 1 microsecond through 5 milliseconds and a pulse repetition
rate in the range of 20 through 500 PPS. A carrier or sinusoidal
wave generated by the oscillator 14 and having a relatively high
frequency in the order of 500 KHz through 1 MHz is amplitude-modulated
by the modulator 15 in accordance with the pulses from the pulse
generator 16. In this manner, the oscillator section B produces
a modulated or intermittent (pulsed) signal as far as the system
is operated in the transmission mode.
The modulated signal is fed to the beam direction switching circuit
17 which is now periodically switched between upstream and downstream
positions by the controller 18. Curve (B) of FIG. 7 illustrates
an envelope of the modulated signal appearing at the stationary
contact a of the switch S.sub.1 while curve (C) of FIG. 7 illustrates
an envelope of the modulated signal appearing at the other stationary
contact b of the same switch S.sub.1.
In the upstream position, the modulated signal is passed through
the contact a of the switch S.sub.1 and through the movable contact
of S.sub.4 in the mode switching section F.sub.1 and fed to the
transmitting transducer 11. The transducer 11 converts the electric
signal to an ultrasonic signal and transmits it upstream to the
tested fluid 1 along the line of travel 7 in alignment with the
receiving transducer 8. Accordingly, part of the beamlike ultrasonic
signal is detected and converted by the transducer 8 into a sinusoidal,
intermittent (modulated) electric signal as illustrated in FIG.
7. The intermittent wave is then passed through the stationary contact
I of the switch S.sub.6 and the stationary contact a of the switch
S.sub.2 to the tuner 19. The tuner rejects the noise component in
the received, intermittent wave and supplies the tuned wave to the
amplifier 21 which amplifies it to a level appropriate for subsequent
signal processing and supplies the amplified signal to the pulse
shaper 22. In this respect the amplifier 21 is automatically gain-controlled
by the AGC element 20 so that the output signal of the amplifier
is maintained at a sufficient level or amplitude regardless of whether
the signal from the tuner 19 temporarily decreases in amplitude
when air bubbles or solids in the fluid momentarily come into path
of travel of the ultrasonic beam. The pulse shaper 22 may include
a Schmitt trigger circuit which is triggered to produce a rectangular
pulse in response to each leading edge of the intermittent signal.
Curve (F) of FIG. 7 illustrates a rectangular pulse train produced
by the pulse shaper 22.
It is noted from curve (B) of FIG. 7 and curve (F) of FIG. 7 that
the time difference between one of the pulses in curve (B) of FIG.
7 and the corresponding pulse in curve (C) of FIG. 7 as exemplified
by a time t.sub.1 represents the time of travel of the ultrasonic
signal from the transmitting transducer 11 upstream through the
fluid medium 1 along the path 7 to the receiving transducer 8.
Of course, the time difference t.sub.1 includes time lag in response
in the electric circuit elements such as the transducers 11 and
8 tuner 19 amplifier 21 and pulse shaper 22. However, the time
lag in these circuit elements is generally constant so that appropriate
subtractive correction can easily be made by the ALU 25.
The time counter 23 includes a high frequency clock and starts
counting the clock pulses each time it receives a pulse from the
modulator 15 and continues counting until the corresponding pulse
from the pulse shaper 22 is received. In this manner, the time difference
as indicated by t.sub.1 is temporarily stored in the counter 23.
Immediately after the counter stops counting it transfers the count
in a digital form to the memory 24. The counter 23 repeats this
operation, i.e. counts the next time difference between a second
pulse applied to the transducer 11 and the corresponding pulse from
the pulse shaper 22 and transfers the count to the memory 24.
The memory stores these counts in order as they are successively
transferred from the counter 23.
The pulse train from the modulator 15 is also fed to the beam direction
controller 18 which has already been enabled by the first mode switching
signal from the mode switching controller 44. The beam direction
controller 18 when enabled, counts the pulses from the modulator
15. When the count reaches a predetermined value resulting from
the predetermined elapse of time defining the end of an upstream
(or downstream) beam direction period, the beam direction switching
controller 18 produces and supplies a signal as indicated by high-to-low
level transition in curve (A) of FIG. 7 to the processor 25. Responsively,
the processor 25 rapidly fetches from the memory 24 a group of time
differences t.sub.1 stored in a digitally coded form therein, and
calculates the average E.sub.1 thereof. It is noted that the average
value represents the average time of ultrasonic propagation in the
downstream direction through the fluid medium 1 from the transducer
11 to the transducer 8 as derived from data during the predetermined
period (U period in FIG. 7) which is set in the beam direction switching
controller 18.
The above mentioned signal from the beam direction switching controller
18 is also fed to the beam direction switching circuit 17 which,
in turn, switches the switches S.sub.1 and S.sub.2 to move and connect
the respective movable contacts to the stationary contacts b, thus
establishing a downstream direction of beam propagation.
Accordingly, the pulses from the modulator 15 (in fact, the sinusoidal
electric waves as amplitude-modulated by rectangular pulses from
the pulse generator (16) are now fed through the stationary contact
b of the switch S.sub.1 and the stationary contact I of the switch
S.sub.3 to the transmitting transducer 6 from which the converted
ultrasonic waves are propagated and travel downstream in the fluid
medium 1 along the path 4 down to the receiving transducer 10. The
output signal of the transducer 10 is fed through the stationary
contact I of the switch S.sub.5 and the stationary contact b of
the switch S.sub.2 to the tuner 19 after which the signal is processed
in a similar manner as described. The time counter 23 counts the
time difference which however, in this case represents the time
of the ultrasonic propagation in the downstream rather than upstream
direction, as required as the ultrasonic signal travels downstream
along the path 4 from the transmitting transducer 6 to the receiving
transducer 10. An example of this type time difference is indicated
by t.sub.2 curve (F) of FIG. 7 as well as curve (E) of FIG. 7 in
comparison with curve (C) of FIG. 7 illustrating the envelope of
the signal as applied to the transmitting transducer 6.
The memory 24 stores the downstream propagation data by successively
receiving the counts from the counter 23. The beam direction controller
18 detects the predetermined elapsed time as described to produce
a beam direction switching signal as indicated by low-to-high level
transition of wave in curve (A) of FIG. 7. The signal controls the
beam switching circuit to switch back into the upstream position
so that the movable contacts of the switches S.sub.1 and S.sub.2
depart from the first stationary contacts b and engage the second
stationary contacts a.
The signal from the beam direction controller 18 is also fed to
the processor 25 which, thereupon, reads out the t.sub.2 data i.e.
a group of digital codes and calculate the average t.sub.2 thereof
which is then subtracted from the average t.sub.1 previously calculated.
In this manner, the processor 25 produces an average velocity of
flow of the tested fluid 1 over a period or cycle half of which
is the predetermined time in the beam direction controller 18.
Preferably, the processor includes a parallel-to-serial converter
from which the digital data representing the average velocity of
flow of the tested fluid are serially or bit by bit fed through
the stationary contact I of the switch S.sub.7 to a serial-to-parallel
converter in the display register 47 which, thereupon controls
the display 48 to visually display the flow velocity of the tested
fluid. The serial transfer of data between the processor 25 and
the display register or controller 47 minimizes the number of contacts
of the switch S.sub.7 in the mode switching section F.sub.2 as well
as the number of transmission lines connected therebetween, thus
facilitating a remote display.
The processor 25 may preferably include flow rate (volume of flow/unit
time) measuring function which calculates the flow rate by multiplying
the velocity of flow (length/unit time) by the sectional area across
which the tested fluid passes, thus permitting the display 48 to
indicate a direct representation of the volume of flow per unit
time.
The operation in the transmission mode has been described. A description
of the switching operation into the reflection mode and the operation
in the reflection mode will follow.
As the tested fluid 1 becomes more polluted or cloudier, it introduces
a greater loss or attenuation to the ultrasonic waves traveling
therethrough along the paths 4 and 7 thereby decreasing the amplitude
of the received signal. When the input signal to the amplifier falls
down to an amplitude or level sufficient to nullify the feed-back
action of the AGC element 20 which, for less-polluted fluid, normally
maintains the output signal level of the amplifier 21 then the
output signal of amplifier 21 decreases. As shown in FIG. 6 the
output pulse signal of amplifier 21 is monitored by the comparator
42 in the mode switching control section E which samples and holds
the peak of each pulse thereof and compares it with the reference
voltage from the adjustable voltage generator 41. Upon detection,
the comparator 42 produces and supplies a signal to the timer 43
(dead time introducer). If the signal from the comparator 42 exists
or continues beyond the predetermined time interval selected in
the timer 43 the timer signals the mode switching control circuit
44 which, thereupon generates a mode switching control signal whereby
the system is switched into the reflection mode.
The timer 43 introduces a dead time and functions to prevent the
frequent occurrence of switching between the transmission and reflection
modes resulting from temporary variations in the level of pollution
in the tested fluid 1.
The reference voltage produced by the voltage generator 41 may
be manually adjustable so that a desired switching level of pollution
at which the mode change occurs is selected.
The mode switching signal from the mode switching control circuit
44 is fed to the mode switching circuits 45 and 46 in which the
movable contacts of the switches S.sub.3 through S.sub.6 and S.sub.7
disengage from the first stationary contacts I and are connected
to the second stationary contacts II.
The mode switching signal is further fed to the beam direction
switching control circuit 18 and the pulse generator 16. Responsively,
the beam direction switching circuit 17 is placed into a disabled
condition. The pulse generator 16 stops the generation of pulses
and instead supplies a high or "1" state level to the
modulator 15. As a result, the carrier from the oscillator 14 is
no longer modulated.
In response to the mode switching signal, the comparator 42 also
stops the "sample and hold" function as described. In
this manner, the system is switched into the reflection mode from
the transmission mode. It is noted here that the beam direction
switching circuit 17 including the switches S.sub.1 and S.sub.2
does not participate in the operation of the system during the reflection
mode.
In the reflection mode, the sinusoidal signal or carrier from the
oscillator 14 is not modulated by pulses, and is fed, as a "continuous
wave", to both of the transmitting transducers 6 and 11 passing
through the second stationary contact II of the switch S.sub.3 and
the second stationary contact II of the switch S.sub.4 respectively.
Accordingly, the transducer 6 transmits a continuous ultrasonic
signal through the tested fluid 1 along the path 4. The fluid 1
is now a relatively polluted one containing a considerable number
of air bubbles and/or solids as generally indicated by 12 and 13
in FIG. 6. The continuous ultrasonic signal from the transducer
6 is thus scattered by such particles adjacent to the wall of the
pipe 2 as they are crossing the path of ultrasonic propagation 4.
Part of the scattered waves or reflected waves, then advance along
a path 7 generally parallel and adjacent to path 4 and reaches the
receiving transducer 8 disposed adjacent to the transmitting transducer
6. In this respect, the receiving transducer 8 is preferably placed
slightly downstream (rather than upstream) relative to the transmitting
transducer 6 in order to receive a maximum fraction of the scattered
waves as a reflection mode signal because the path of the ultrasonic
waves is shifted downstream by the flow of the liquid.
Before proceeding with a description of the operation of the embodiment
it should be noted that the ultrasonic signal transmitted from the
transmitter is a beam or bundle of ultrasonic waves comprising a
number of substantially collimated, or parallel waves with very
low divergence. Accordingly, the parallel waves individually impinge
upon a plurality of particles (solids and/or air bubbles) in the
normal fluid. However, unless otherwise specified, the description
on the reflection mode has dealt and will deal with a simplified
case wherein the waves are reflected by a single particle, in order
to a facilitate the understanding of one aspect of this invention.
Also it should be noted that the transmitter 11 is disposed very
close to the receiver 10.
The angle determined by the path of travel 7 extending between
the transmitter and the reflecting particle 13 and the path of backward
travel from that particle to the receiver 10 is very small and always
has a substantial constant, small degree of angle irrespective of
the position of that particle relative to the transducers 10 and
11. This is also applied to another transducer set (6 and 8).
Turning back to the operation of the embodiment, the receiving
transducer 8 observes a first type Doppler shift (decrease in frequency)
in the ultrasonic wave incident thereupon.
The received ultrasonic signal is converted by the receiving transducer
8 to an electric signal as exemplified in curve (A) of FIG. 4.
In a similar manner, the second transmitting transducer 11 transmits
a continuous ultrasonic signal along the path 7. The signal is similarly
scattered by the air bubbles and/or solids 13 passing near the wall
of pipe 2. Part of the scattered ultrasonic wave falls upon the
receiving transducer 10 disposed adjacent to the transmitting transducer
11.
The transducer 10 thereupon converts the incident ultrasonic signal
to an electric signal as schematically shown in curve (D) of FIG.
4. This signal assumes a second type Doppler shift, i.e. an increased
frequency relative to the frequency of the sinusoidal signal as
applied to the transmitting transducer 11 in accordance with the
velocity of the flow of the fluid (the velocity of the particle).
The first type Doppler shifted sinusoidal signal from the transducer
8 and second type Doppler shifted sinusoidal signal from the transducer
10 are fed through the respective stationary contacts II of the
switches S.sub.6 and S.sub.5 to the first and second tuners 26 and
32 respectively.
The tuners 26 and 32 and the amplifiers 28 and 34 used in the
reflection mode signal processing section D operate on the received
signal in a manner similar to that of the tuner 19 and the amplifier
21 in the transmission mode signal processing section C.
After being tuned or filtered and amplified, the Doppler shifted
signal of the first type is fed to the mixer 29 in which it is mixed
with the sinusoidal signal from the carrier oscillator 14 to produce
beats having two frequency components, i.e. the difference between
the input signal frequencies, and the addition thereof. The low-pass
filter 30 rejects the addition and only permits the difference type
beat signal as schematically shown in curve (B) of FIG. 4 to pass
therethrough.
In a similar manner, the second type Doppler shifted signal is
mixed by the mixer 35 with the sinusoidal reference signal from
the oscillator 14 and then filtered by the low-pass filter 36. The
filtered beat signal is schematically shown in curve (E) of FIG.
4.
The frequency-to-voltage converter 31 receives the beat signal
from the low-pass filter 30 and converts it to a voltage in proportion
to the input frequency.
The voltage signal is schematically shown in curve (C) of FIG.
4. Similarly, the second frequency-to-voltage converter 37 receives
the second beat signal from the low-pass filter 36 and converts
it to a voltage in proportion to the input signal frequency. It
is preferred, however, that the voltage signal outputted from the
second converter 37 be in polarity opposite to the voltage signal
from the first converter 31. For example, the first converter 31
outputs a positive voltage while the second converter 37 outputs
a negative voltage although the absolute value of either output
signal voltage is in proportion to the value of the input signal
frequency. The voltage signal from the second converter 37 is schematically
shown in curve (F) of FIG. 4.
Description now turns to an analysis of the motion of the flowing
fluid and particles contained and entrained therein and the influence
of the noise velocity component on the flow meter system.
In general, solids and air bubbles entrained in the fluid flowing
in a pipe moves at a velocity vector consisting of a first velocity
component parallel to the axis of the pipe or the general flow of
the liquid and a second velocity component normal thereto. The first
component provides a signal with the Doppler shift type flow measurement
whereas the second component introduces a noise (or error) thereto.
Assuming, for example that the solid 12 as shown in FIG. 6B and
a body of fluid between that solid and the wall of pipe 2 adjacent
to the ultrasonic receiver 8 moves inwardly toward the axis 5 such
a second velocity vector imparts a second-type Doppler shift (increase
in frequency) to the receiver 8 because this vector includes a vector
component in the same direction as the direction of propagation
of the reflection mode ultrasonic signal from solid or sound source
12 to the receiver 8. On the other hand, an outward movement of
the solid 12 introduces a first-type Doppler shift or decrease in
frequency to the observing receiver 8.
The second velocity component is very time dependent. In other
words, at a particular time an inward velocity vector component
is imparted to the solid 12 and, at another time an outward vector
component is provided thereto.
Owing to the time varying characteristics of the second velocity
component normal to the axis 5 or the general direction of the flow
of fluid, the signals received by the receivers 8 and 10 fluctuate
in frequency with respect to time. The frequency of the received
signal at the receiver 8 is indicated by fr.+-..DELTA.f.sub.N in
curve (A) of FIG. 4. The term fr is the center frequency determined
by the time-independent or stationary velocity component of the
velocity vector of the solid 12 i.e. signal component, while the
second term .DELTA.f.sub.N is an error due to the time-varying velocity
component i.e. noise component.
Similarly, the frequency of the received signal at the receiver
10 is indicated by fr'.+-..DELTA.f'.sub.N in curve (D) of FIG. 4.
Accordingly, the output voltage of the frequency-to-voltage converter
31 also comprises the stable voltage component E (curve (C) of FIG.
4) representing the signal component of the Doppler shift, and the
fluctuating component .DELTA.E.sub.N representing the noise component
of the Doppler shift due to the second velocity vector component
normal to the axis of fluid flow. Similarly, the second frequency-to-voltage
converter 37 outputs a voltage consisting of the stable component
E' and fluctuating component .DELTA.E'.sub.N as shown in FIG. 4(F).
It is found that the above mentioned second velocity components
imparted to solids in the fluid because of random aspect distributed
in an approximately symmetrical pattern with respect to the center
axis of flow. In FIG. 6B, the solid or air bubble 12 is located
at a point (in fact, passes by this point at the reflection time)
which is almost symmetrical with respect to the location of the
solid 13 with respect to the axis 5. The above law of symmetry of
the second velocity or noise component holds that the second velocity
component of the solid 12 and that of the solid 13 are opposite
in direction and nearly the same in size. Correspondingly, the noise
component of the Doppler shift .DELTA.f.sub.N due to the second
or noise velocity component of the solid 12 as evidenced in the
receiver 8 is of the same type and is nearly the same in amount
as the noise component of the Doppler shift .DELTA.f.sub.N ' due
to the second velocity component of the solid 13 as evidenced in
the receiver 10. If statistically considered, .DELTA.f.sub.N =.DELTA.f.sub.N
'= 0 when particles are infinitely many.
In accordance with these principals, the differential amplifier
38 subtracts the Doppler shift in the form of voltage (-.DELTA.E'+.DELTA.E.sub.N
') in the second channel from the Doppler shift in the form of voltage
(.DELTA.E+.DELTA.E.sub.N) in the first channel thereby to emphasize
or double the signal component and cancel or offset the noise component
as schematically shown in curve (G) of FIG. 4 thus improving signal-to-noise
ratio. A curve a in curve (G) of FIG. 4 more specifically illustrates
the voltage signal from the first F/V converter while a sub-curve
a' in curve (C) of FIG. 4 more specifically illustrates the voltage
signal from the second F/V converter. After subtracting the noise
component there is almost no noise.
The output signal of the differential amplifier has a high-signal-to-low-noise
ratio and, is then fed to an analog-to-digital converter from which
the corresponding digital signal is fed to the ALU 40.
The ALU 40 similar to the transmission mode signal ALU 25 corrects
the signal and obtains data representing with high accuracy the
velocity of flow (distance/unit time) and/or rate of flow (volume
of flow/unit time) of the tested fluid and transfers it serially
through the second contact II of the switch S.sub.7 to the display
control 47 which operates in the same manner as in the transmission
mode.
The description will now turn to the operation for automatically
switching from the reflection mode back to the transmission mode.
When the system is operated in the reflection mode, the output
signal of the transducer 8 that is fed to the first tuner 26 is
also supplied to the tuner 19. Accordingly, the mode switching control
section E is operated in the same manner as in the transmission
mode, except that the comparator 42 continuously compares the output
from the amplifier 21 with the reference voltage from the voltage
generator 41 without sampling and holding each peak of the output
signal. Hence, upon detecting a stable increase in the amplitude
of the output signal from the receiving transducer 8 the section
D switches the system from the reflection mode to the transmission
mode.
More specifically, the mode switching control 44 transfers a signal
to the switches S.sub.3 through S.sub.7 thus establishing a connection
between their respective movable contacts and the first stationary
contacts I. The signal also serves to put the beam direction controller
18 into operation and enables the pulse generator 16 to supply pulses
to the modulator 15.
In response to the switching signal, the comparator 42 also functions
back to sample and hold each peak of signal to be compared.
FIGS. 8A, 8B and 8C are a block diagram illustrating still another
embodiment of the inventon. In this embodiment, a mode switching
circuit 45 comprises switches S.sub.3 through S.sub.6. A beam direction
switching circuit 17 comprises switches S.sub.1 and S.sub.2. A movable
contact of a switch S.sub.1 is connected to a modulator 15 a beam
direction controller 18 and second stationary contacts II of respective
switches S.sub.4 and S.sub.5. A stationary contact a of the switch
S.sub.1 is connected to a first stationary contact I of the switch
S.sub.5 and a stationary contact b of the switch S.sub.2. A stationary
contact b of the switch S.sub.1 is connected to a first stationary
contact I of the switch S.sub.3 and a stationary contact a of the
switch S.sub.2. A movable contact of the switch S.sub.2 is connected
to a tuner 19. A second contact II of the switch S.sub.3 is connected
to a first tuner 26 in the reflection mode signal section D, While
a second contact II of the switch S.sub.6 is connected to a second
tuner 32 and a tuner 19.
Movable contacts of the switches S.sub.3 through S.sub.6 are connected
to a transmitting and receiving transducer 8', a transmitting transducer
6 a transmitting and receiving transducer 11' (hereinafter to be
referred to simply "transceiver transducer"), and a receiving
transducer 10 respectively.
In other respects, the construction of the present embodiment is
identical with that of the previous embodiment in FIGS. 6A, 6B and
6C. Accordingly, like numerals in FIGS. (6A, 6B, 6C) and (8A, 8B,
8C) represent like elements.
In the transmission mode, the movable contacts of switches S.sub.3
and S.sub.4 are respectively connected to the first stationary contacts
I thereof. Further, the movable contacts of switches S.sub.1 and
S.sub.2 are cyclically connected alternatively to either one of
the contacts a and b thereof at desired periods.
In a period during which the movable contacts of the switches S.sub.1
and S.sub.2 are connected to the stationary contacts a thereof,
a pulse train from the modulator 15 is fed through the stationary
contact a of the switch S.sub.1 and through the first contact I
of the switch S.sub.5 and to the transceiver transducer 11' which
responds as a transmitter to transmit an ultrasonic signal along
the path 7.
In response to the ultrasonic signal, the transceiver receiver
8' operates as a receiver and, thus converts the ultrasonic signal
into an electric signal which is, then, fed through the first stationary
contact I of the switch S.sub.3 and the stationary contact a of
the switch S.sub.2 to the tuner 19.
In another period during which the movable contacts of the switches
S.sub.1 and S.sub.2 are connected to the stationary contacts thereof,
the pulse train from the modulator 15 is fed through the stationary
contact b of the switch S.sub.1 and through the first contact I
of the switch S.sub.3 and to the transducer 8' which responds as
a transmitter to transmit an ultrasonic signal. At this time, the
transceiver transducer 11' operates as a receiver rather than a
transmitter as described above, and, thus, supplies the output signal
through the first stationary contact I of the switch S.sub.5 and
the stationary contact b of the switch S.sub.2 to the tuner 19.
In the reflection mode, the movable contacts of the switches S.sub.3
through S.sub.6 are respectively connected to the second stationary
contacts II thereof. In this position, switching circuit 17 does
not participate in the operation of the system.
As described in connection with the embodiment in FIG. 6 a continuous
sinusoidal signal from the carrier 14 is not modulated by pulses
as described, and is fed through the second stationary contact II
of the switch S.sub.4 to the transmitter transducer 6 as well as
the transceiver transducer 11' via the second stationary contact
II of the switch S.sub.5.
The ultrasonic signal from the transducer is then reflected as
described, and received by the transducer 8' serving now as a receiver
which then converts the ultrasonic signal into an electric signal.
The electric signal is fed through the second stationary contact
II of the switch S.sub.3 to the first tuner 26.
Similarly, in response to the ultrasonic signal from the transceiver
transducer 11', which is operating as a transmitter, the receiving
transducer 10 outputs a corresponding electric signal which is fed
through the second stationary contact II of the switch S.sub.6 to
the second tuner 32 in the reflection mode signal processing section.
It is noted that the ultrasonic beam of the embodiment as illustrated
in FIG. 8B uses a single path of travel (as indicated by 7) in the
transmission mode while the ultrasonic beam of the embodiment in
FIGS. 9A and 9B uses either of two paths of travel (as indicated
by 4 and 7). The single path arrangement is highly advantageous
because the distance of travel for the ultrasonic beam from the
transmitter to the receiver through the fluid does not change at
all between the upstream and downstream travels of the beam.
Also, it is noted that the embodiment as illustrated in FIGS. 8A,
8B and 8C uses a transducer combination of two transceivers (8',
11'), one transmitter (6) and one receiver (10).
Of course, other transducer combinations that use only one path
of travel of the ultrasonic beam in the transmission mode can be
designed and employed. An example of such combintion is a transducer
combination of two transceivers and two receivers. This example
is shown in FIG. 10 wherein a first transceiver is designated by
8', a first receiver disposed close to and slightly downstream relative
to the first transcriver is designated by 6 a second transceiver
separated by the fluid from the transducers 6 and 8' is designated
by 11', and a second receiver disposed close to and slightly downstream
relative to the second transceiver is designated by 10. Three tuners
designated by 19 26 and 32 in FIG. 10 may be connected to the amplifiers
21 28 and 32 in FIG. 8 respectively. In this combination and arrangement
of transducers, the first and second transceivers 8' and 11' alternately
and complementarily transmit an ultrasonic beam into the fluid and
receive the transmission mode signal whenever the system is being
operated in the transmission mode. In the reflection mode, both
of the first and second transceivers 8' and 11' always operate as
"transmitters" and the first and second receivers 6 and
10 receive the reflection mode signals originally transmitted from
the first and second transceivers 8' and 11', respectively. Yet
another transducer combination utilizing a single path of travel
of ultrasonic beam (such as the path 7 in FIG. 8) may be two transceiver
transducers and two transmitter transducers.
Of course the respective examples need modified switching circuits
as modified from the switching circuits 45 and 17 in FIG. 8A. Such
modification of switching circuits is obvious to those skilled in
the art.
FIGS. 9A and 9B are block diagram showing circuit components of
this invention according to another embodiment, wherein the mode
switching circuit 45 of the mode switch section F.sub.1 is composed
of only one switch S.sub.3 and the beam direction switching circuit
17 of the transmission measurement mode signal processing section
C is composed of switches S.sub.1 and S.sub.2. The movable contact
of the switch S.sub.1 is connected to the pulse modulator 15. The
fixed contact a of the switch S.sub.1 is connected to the movable
contact of the switch S.sub.3 and the downstream-positioned transmitter
transducer 11. The fixed contact b of the switch S.sub.1 is connected
to the fixed contact II of the switch S.sub.3 and the upstream-positioned
transmitting transducer 6. The movable contact of the switch S.sub.2
is connected to the tuner 19. The fixed contact a of the switch
S.sub.2 is connected to the upstream-positioned receiver transducer
8 and the fixed contact b of the switch S.sub.2 is connected to
the downstream-positioned receiving transducer 10.
The reflection measurement mode signal processing section D is
composed of a mixer 29' connected to the oscillator 14 and amplifier
21 a low-pass filter 30' connected to said mixer, a frequency-to-voltage
converter 31' connected to said filter, an analog-digital converter
49 connected to said frequency-to-voltage converter, a memory 50
connected to said analog-digital converter, and an ALU 40' connected
to said memory. The beam direction switching control 18 of the transmission
mode signal processing section C is connected to the pulse generator
16 rather than the mode switching control 44. The components common
to both FIGS. (6A, 6B, 6C) and (9A, 9B) are identified by like symbols.
For operation in transmission mode, the movable contact of the
switch S.sub.3 is kept connected to the fixed contact I and as in
the first embodiment, the movable contacts of the switches S.sub.1
and S.sub.2 are alternately connected to the respective fixed contacts
a and b at given intervals. Therefore, in a period of measurement
when the movable contacts of the switches S.sub.1 and S.sub.2 are
connected to the respective fixed contacts a, the pulse train from
the modulator 15 is supplied through the fixed contact a of the
switch S.sub.1 to the downstream transmitting transducer 11 which
converts the pulse train into an ultrasonic signal. The ultrasonic
signal is supplied to the upstream receiving transducer 8 which
converts the signal into an electrical signal to be supplied to
the tuner 19 through the fixed contact a of the switch S.sub.2.
In a period of measurement when the movable points of the switches
S.sub.1 and S.sub.2 are connected to the respective fixed contacts
b, the pulse train from the pulse modulator 15 is supplied through
the fixed contact b of the switch S.sub.1 to the upstream-positioned
transmitter transducer 6 which converts the pulse train into an
ultrasonic signal. The ultrasonic signal is supplied to the downstream-positioned
receiving transducer 10 which converts the signal into an electrical
signal to be supplied to the tuner 19 through the fixed contact
b of the switch S.sub.2.
The subsequent operation in the transmission measuring mode is
identical with what is described in connection with the embodiment
in FIGS. 6A, 6B and 6C.
For operation in reflection measuring mode, the movable contact
of the switch S.sub.3 is kept connected to the fixed contact II
and the movable contacts of the switches S.sub.1 and S.sub.2 are
connected alternately to the respective fixed contacts a and b at
given intervals. As in the case of the embodiments in FIGS. 6 and
8 the pulse modulator 15 delivers a sinusoidal signal continuously.
In a period of measurement when the movable contacts of the switches
S.sub.1 and S.sub.2 are connected to the respective fixed contacts
a, the sinusoidal signal from the modulator 15 is supplied to the
upstream-positioned transmitter transducer 6 through the fixed contact
a of the switch S.sub.1 and the fixed contact II of the switch S.sub.3
while at the same time, the signal is supplied to the downstream-positioned
transmitting transducer II through the fixed contact a of the switch
S.sub.1. The upstream-positioned receiver transducer 8 receives
the reflected wave of the ultrasonic signal sent from the upstream-positioned
transmitting transducer 6 and converts such wave into an electrical
signal which is supplied to the tuner 19 through the fixed contact
a of the switch S.sub.2. The electrical signal delivered from the
downstream-positioned receiving transducer 10 is neutral to the
operation of the system because the fixed contact b of the switch
S.sub.2 is open. The tuner 19 automatic gain control 20 and the
amplifier 21 operate in an identical manner to the first tuner 26
automatic gain control 27 and amplifier 28 of the embodiments in
FIGS. (6A, 6B, 6C) and (8A, 8B, 8C), and the mixer 29' is supplied
with a sinusoidal signal having the Doppler shift due to a reduction
in frequency. The mixer 29' is also supplied with a sinusoidal signal
from the oscillator 14 so it combines the two sinusoidal signals
and delivers a first beat signal. As in the embodiments in FIGS.
(6A, 6B, 6C) and (8A, 8B, 8C), the first beat signals are supplied
to the low-pass filter 30' and frequency-to-voltage converter 31'.
In the period of measurement described above, the frequency-to-voltage
converter 31' produces an analog voltage of positive polarity that
is proportional to the frequency of the first beat signal having
the Doppler shift due to a reduction in frequency. The analog-digital
converter 49 converts the analog voltage to a digital code which
is forwarded to the memory 50. The memory 50 stores the digital
code till the end of the subsequent period of measurement.
In the period of measurement described above, by counting a train
of pulses produced from the pulse generator 16 the beam direction
switching control 18 detects the lapse of the given period of measurement
and supplies the beam direction switching element 17 with a control
signal serving as a beam direction switching signal to thereby connect
the movable contacts of the switches S.sub.1 and S.sub.2 to the
respective fixed contact b.
The above sequence starts the subsequent period of measurement
wherein the sinusoidal signal delivered from the pulse modulator
15 is supplied to the upstream-positioned transmitter transducer
6 through the fixed contact b of the switch S.sub.1 while at the
same time, the signal is supplied to the downstream-positioned transmitting
transducer 11 through the fixed contact b of the switch S.sub.1
and the fixed contact II of the switch S.sub.3. The electrical signal
from the downstream-positioned receiving transducer 10 is supplied
to the tuner 19 through the fixed contact b of the switch S.sub.2.
The electrical signal from the upstream-positioned receiving transducer
8 is neutral to the operation of the system because the fixed contact
a of the switch S.sub.2 is open. The automatic gain control 20 and
the amplifier 21 operate in a manner identical to the operation
of the first tuner 32 automatic gain control 33 and amplifier 34
of the embodiments in FIGS. 6 and 8 and the mixer 29' delivers
a second beat signal having the Doppler shift due to a reduction
in frequency, and the frequency-to-voltage converter 31' produces
an analog voltage of negative polarity proportional to the frequency
of the second beat signal. The analog-digital converter 49 converts
said analog voltage of negative polarity to a digital code which
is forwarded to the memory 50 for storage. Then, the beam direction
switching control 18 detects the lapse of the given period of measurement
and supplies a control signal to the beam direction switching element
17 which connects the movable contacts of the switches S.sub.1 to
S.sub.2 to the respective fixed contacts a for starting the next
period of measurement. The control signal is also supplied to the
arithmetic unit 40' which then reads a digital code representing
the analog voltage of positive polarity that corresponds to the
first beat signal and digital code representing the analog voltage
of negative polarity that corresponds to the second beat signal,
and provides a serial digital code for the flow rate of the fluid
1 formed on the basis of the result of subtraction of the latter
signal from the former. For the operation of the other circuit components,
see the description of the embodiment given by reference to FIGS.
6A, 6B and 6C.
In the last embodiment illustrated above, the switches S.sub.1
S.sub.2 the tuner 19 automatic gain control 20 and amplifier 21
are used both in transmission measuring mode and in reflection measuring
mode. In addition, the beam direction switching operation indispensable
to transmission measuring mode is also effective in reflection measuring
mode, and the mixer 29', low-pass filter 30' and frequency-to-voltage
converter 31' are operated on a time-shared basis. Accordingly,
one great economical advantage of this embodiment is that the number
of the above mentioned six components required is half the number
required in the embodiments in FIGS. (6A, 6B, 6C) and (8A, 8B, 8C).
In the embodiments of this invention, it is recommended that the
switches S.sub.1 to S.sub.7 be composed of mercury-wetted contact
relays that are more reliable than the conventional relay element.
Alternatively, analog switches composed of a semi-conductor element
may replace the mechanical contact type switching elements.
The system as exemplified in the latter three embodiments is capable
of automatic selection between a measurement in transmission mode
and a measurement in reflection mode depending upon the concentration
of contaminants. Therefore, even if there is a change with time
in the concentration of contaminants in the fluid, two separate
units, one for measuring in transmission mode and the other for
measuring in reflection mode, need not be used as in the conventional
technique, and the flow rate of the fluid can be simply measured
by one unit. This eases transportation, installation and operation
of the system. Another advantage of automatic switching between
the two measuring modes in that it achieves extended and continuous
measurement and recording of the flow velocity (distance of travel/unit
time) and/or flow rate (volume/unit time) of fluid wherein the concentration
of contaminants or pollutants varies considerably with time. What
is more, in the system of these embodiments, two sets of transmitter
and receiver transducers which are indispensable to a measurement
in transmission mode are positioned to face each other separated
by the fluid and are also used effectively for measurement in reflection
mode so as to suppress noise output due to the noise motion of particles
and/or the fluid (zig-zag motion of particles, drift of fluid and
etc.) and to double the signal output of measurement due to a velocity
vector in the direction of fluid channel.
Accordingly, the system as described and shown respectively in
FIGS. (6A, 6B, 6C), (8A, 8B, 8C), and (9A, 9B) provides a continuous
measuring of the flow rate and/or flow velocity of the fluid with
an improved S/N ratio and its performance is demonstratively beyond
that of the prior-art ultrasonic flow measurement system.
Although the above have been described and shown, various other
modifications and changes could be made without departing from the
spirit and scope of the invention.
Thus, some of the advantages provided by the system of this invention
are automatic continuous and accurate (high S/N ratio of signal)
measurement of flow, applicability to fluid with a wide variation
in number of contaminant particles and a compact size that is easy
to transport and to install. |