Abstrict A fluid flow meter comprises a pair of transducers spaced apart
in the direction of fluid flow. A transmitter causes acoustic signals
to be transmitted in both directions through the fluid by the transducers.
A processor determines information relating to the fluid flow by
monitoring the time of flight of acoustic signals received by the
transducers. Part of the space between the transducers defines a
flow path consisting of a flow structure having a plurality of parallel
fluid flow passages which extend axially in the direction of flow,
wherein the cross-sectional diameters of the passages are chosen
such that substantially plane acoustic waves only are transmitted
through the passages by the fluid, and wherein under working conditions
that fluid flows through each passage such that the ratio of the
volume flow rate through the passage to the total volume flow rate
remains substantially constant with respect to the total volume
flow rate.
Claims We claim:
1. A fluid flow meter comprising:
a flow structure defining a substantially annular flow passage
extending along a direction of fluid flow;
a pair of transducers spaced apart along the direction of fluid
flow, said transducers also having an annular form corresponding
to the annular flow passage;
transmitting means for causing acoustic signals to be transmitted
in both directions through the fluid by said transducers; and
processing means for determining information relating to the fluid
flow by monitoring the time of flight of acoustic signals received
by said transducers.
2. A fluid flow meter according to claim 1 wherein said flow structure
presents respective, angled plane faces to incoming acoustic signals.
3. A fluid flow meter according to claim 1 wherein said flow structure
includes a central solid portion defining an inner wall of said
annular flow passage.
4. A fluid flow meter according to claim 3 wherein said central
solid portion is supported by vanes within a surrounding structure.
5. A fluid flow meter according to claim 1 wherein said transducers
comprise piezoelectric elements which vibrate in a radial mode.
6. A fluid flow meter according to claim 1 wherein each transducer
comprises:
means for generating a magnetic field within an annular space;
and
a coil positioned within said annular space whereby or passage
of an electric current through said coil, said coil vibrates to
generate the acoustic signals.
7. A fluid flow meter according to claim 6 wherein said magnetic
field generating means comprises a generally "E" shaped
member, the depth of the arms of the member being approximately
.lambda./2 where .lambda. is the wavelength of the acoustic signals
transmitted through the fluid.
8. A fluid flow meter comprising:
a flow structure defining a substantially annular flow passage
with a constant outer diameter, said flow passage extending along
a direction of fluid flow said flow structure having a portion defining
an inner wall of said annular flow passage, which portion guides
said fluid flow through said annular flow passage;
a pair of transducers spaced apart along the direction of fluid
flow;
transmitting means for causing acoustic signals to be transmitted
in both directions through the fluid by said transducers; and
processing means for determining information relating to the fluid
flow by monitoring the time of flight of acoustic signals received
by said transducers, wherein said annular flow passage has a radial
dimension such as to permit propagation of substantially only plane
wave acoustic signals.
9. A fluid flow meter according to claim 8 wherein the flow structure
presents respective, angled plane faces to incoming acoustic signals.
10. A fluid flow meter according to claim 8 wherein said portion
is a central solid portion defining said inner wall of said annular
flow passage.
11. A fluid flow meter according to claim 10 wherein said central
solid portion is supported by vanes within a surrounding structure.
12. A fluid flow meter according to claim 8 wherein said annular
flow passage is positioned so as to sample an annular region of
waves emitted by said transducers.
13. A fluid flow meter according to claim 8 wherein said transducers
have an annular form corresponding to said flow passage.
14. A fluid flow meter according to claim 8 wherein said transducers
comprise piezoelectric elements which vibrate in a radial mode.
15. A fluid flow meter according to claim 14 wherein each piezoelectric
element is bonded to a respective metal disc.
16. A fluid flow meter according to claim 8 wherein said transducers
are constructed so as to generate an annular sound field which is
input into said annular flow passage.
17. A fluid flow meter according to claim 8 wherein each transducer
comprises:
means for generating a magnetic field within an annular space;
and
a coil positioned within said annular space whereby on passage
of an electric current through said coil, said coil vibrates to
generate the acoustic signals.
18. A fluid flow meter according to claim 17 wherein said magnetic
field generating means comprises tapered pole pieces.
19. A fluid flow meter according to claim 17 further comprising
a non-conducting support diaphragm which carries said electrical
coil.
20. A fluid flow meter according to claim 17 wherein said magnetic
field generating means comprises a generally "E" shaped
member, the depth of the arms of the member being approximately
.lambda./2 where .lambda. is the wavelength of the acoustic signals
transmitted through the fluid.
21. A gas flow meter comprising:
a flow structure defining a substantially annular flow passage
with a constant outer diameter, said flow passage extending along
a direction of fluid flow said flow structure having a portion defining
an inner wall of said annular flow passage, which portion guides
said fluid flow through said annular flow passage;
a pair of transducers spaced apart along the direction of fluid
flow;
transmitting means for causing acoustic signals to be transmitted
in both directions through the fluid by said transducers; and
processing means for determining information relating to the fluid
flow by monitoring the time of flight of acoustic signals received
by said transducers, wherein said annular flow passage has a radial
dimension such as to permit propagation of substantially only plane
wave acoustic signals.
22. A gas flow meter according to claim 21 wherein the flow structure
presents respective, angled plane faces to incoming acoustic signals.
23. A gas flow meter according to claim 21 wherein said portion
is a central solid portion defining said inner wall of said annular
flow passage.
24. A gas flow meter according to claim 21 wherein said annular
flow passage is positioned so as to sample an annular region of
waves emitted by said transducers.
25. A gas flow meter according to claim 21 wherein said transducers
have an annular form corresponding to said flow passage.
26. A gas flow meter according to claim 21 wherein said transducers
comprise piezoelectric elements which vibrate in a radial mode.
27. A gas flow meter according to claim 21 wherein each transducer
comprises:
means for generating a magnetic field within an annular space;
and
a coil positioned within said annular space whereby on passage
of an electric current through said coil, said coil vibrates to
generate the acoustic signals.
28. A gas flow meter according to claim 27 wherein said magnetic
field generating means comprises a generally "E" shaped
member, the depth of the arms of the member being approximately
.lambda./2 where .lambda. is the wavelength of the acoustic signals
transmitted through the fluid.
Description This invention relates to a fluid flow meter of the kind comprising
a first acoustic transducer upstream of a second acoustic transducer,
the time of flight of acoustic waves between the transducers being
used to measure the flow velocity of a fluid medium flowing between
them.
An ultrasonic fluid movement device which uses this method is described
in EP-A-0347096. This may be used to measure the flow velocity of
gas through a passage of known dimensions. By multiplying the measured
velocity by a velocity dependent co-efficient, the volume flow rate
may be calculated. The device described may be used as part of a
domestic gas meter.
A disadvantage with the prior art, eg. EP-A-0347096 is that the
volume flow rate must be calculated by multiplying the measured
velocity by a velocity dependent coefficient. The relationship between
measured velocity and volume flow rate is non-linear, making this
technique complex and imprecise in practise. It is therefore advantageous
to be able to provide a more precise method of calculating the volume
flow rate.
As described in "Theory of Transit Time Ultrasonic Flowmeters"
(J. Hemp, Cranfield Institute of Technology, 27 Jul., 1981) under
certain flow and acoustic conditions, the phase shift of a plane
wave travelling down a tube due to the flow in the tube will be
proportional to the volume flow rate alone. This is due to the integrating
properties of a plane soundwave.
Compensation for off-axis modes is described in WO93/00570. In
this method a ringaround transmission technique is used in which
every fourth wave packet is inverted with respect to the preceding
packets in order to annul the propagation effects of high order
modes in the duct.
U.S. Pat. No. 4365518 discloses a sonic flow meter in which the
flow path through the meter is divided into a number of elongate
tubes which are sufficiently large with respect to the ultrasonic
wavelength so that the ultrasonic pressure wave is propagated through
each tube without substantial distortion. The prior art is unable
to achieve a linear response over a significant range.
In GB-A-2209216 by providing passages with a diameter below a
certain cut-off point, only plane waves will be transmitted along
the flow path. The volume flow rate can then be directly calculated
without the use of a velocity-dependent co-efficient. Highest accuracy
is achieved when the signal measured follows the direct transmission
path, without being modified by constructive or destructive echoes
which would induce phase errors. Therefore mode suppression also
improves accuracy by ensuring that the signals follow the direct
path. However, the flow passages are arranged in a close-packed
arrangement in the flow tube, so that different fluid flow paths
are set up in passages at varying radial distance from the central
axis of the flow tube. This can lead to errors in the measured flow
rate.
According to a first aspect of the present invention, a fluid flow
meter comprises a pair of transducers spaced apart in the direction
of fluid flow; transmitting means for causing acoustic signals to
be transmitted in both directions through the fluid by the transducers;
and processing means for determining information relating to the
fluid flow by monitoring the time of flight of acoustic signals
received by the transducers, wherein part of the space between the
transducers defines a flow path consisting of a flow structure having
a plurality of parallel fluid flow passages which extend axially
in the direction of flow, wherein the cross-sectional dimensions
of the passages are chosen such that substantially plane acoustic
waves only are transmitted through the passages by the fluid, and
wherein under working conditions the fluid flows through each passage
such that the ratio of the volume flow rate through the passage
to the total volume flow rate remains substantially constant with
respect to the total volume flow rate.
In this arrangement each passage samples a representative fraction
of the total flow rate.
In contrast to the prior art, the present invention can achieve
.+-.1% linearity over 3 decades.
The interstices between the passages are preferably blocked to
prevent fluid flow therealong.
Preferably, each passage has a substantially circular cross-section.
The use of circular cross-section passages leads to a number of
advantages over for example the hexagonal cross-sections used in
GB-A-2209216. In particular, it enables an optimum compromise to
be achieved between the need to minimise pressure drop along the
flow path and to maximise phase shift.
Typically, the invention is achieved by arranging a symmetrical
assembly, with transducers placed centrally at the ends of the flow
path. The flow passages can be arranged with their centres at equal
radial distances from a line connecting the centres of the transducers,
and will therefore sample equivalent flow and acoustic fields, if
the flow is isolated from external influences. Although not essential,
this can be achieved by producing a rotary fluid flow in an inlet
chamber, having substantially no component of velocity in the axial
direction of the flow passages.
According to a second aspect of the present invention, we provide
a fluid flow meter comprising a pair of transducers spaced apart
in the direction of fluid flow; transmitting means for causing acoustic
signals to be transmitted in both directions through the fluid by
the transducers; and processing means for determining information
relating to the fluid flow by monitoring the time of flight of acoustic
signals received by the transducers, wherein part of the space between
the transducers defines a flow path consisting of a flow structure
having a substantially annular flow passage of constant outer diameter,
the transducers also having a corresponding annular form.
In all these cases, it is desirable to prevent acoustic echoes
off the flow structure. This could be achieved, for example, by
positioning the transducers very close to the flow structure or
by using absorbent material on the flow structure. Preferably, however,
the flow structure presents respective angled, plane faces to incoming
acoustic signals.
A fluid flow meter such as described above can be reduced to a
small physical size (e.g. house brick size) and can be produced
at low cost. A unit such as described is highly suitable for domestic
gas metering.
Very low power consumption, which enables long term battery operation,
is achieved through a high electro-acoustic conversion efficiency
and simple data processing.
The design is insensitive to gas composition when used as a gas
meter, and can be implemented for different fluids, including liquids,
for example water.
An example of a fluid flow meter in accordance with the present
invention will now be described with reference to the accompanying
drawings, in which:
FIG. 1 is a block diagram of the overall system;
FIG. 2 is a cross-section of the flow sensor apparatus;
FIG. 3 shows a tapered flow tube design;
FIG. 4 shows an alternative tapered flow tube design;
FIG. 5 is a cross-section through one example of the flow structure
of FIG. 1 along a line A--A;
FIG. 6 is a cross-section similar to FIG. 5 but of a second example;
FIG. 7 is a cross-section similar to FIG. 5 but of a third example;
FIG. 8 is a cross-section similar to FIG. 5 but of a fourth example;
FIG. 9 shows a piezoelectric transducer transmitting acoustic waves
into the passage of FIG. 8;
FIG. 10 is a plan view of FIG. 9 showing the radial oscillations
of the piezoelectric element;
FIG. 11 illustrates a conventional loud speaker;
FIG. 12 is a second example of an annular transducer transmitting
into an annular flow passage; and
FIG. 13 is a cross-section similar to FIG. 5 but of a fifth example.
The flow meter shown in FIGS. 1 and 2 consists of two parts: a
flow sensor (1) and an electronic measurement system (2). The fluid
enters the flow sensor at an inlet (3) and exits at an outlet (4)
after having travelled down a metering tube (5) linking inlet and
outlet chambers (6) and (7).
The flow is probed in the flow sensor using two ultrasonic transducers
(8) and (9) to emit and receive pulses of sound down the metering
tube. The elapsed time .DELTA.t from transmission to reception is
timed in the upstream (+) and downstream (-) directions by the electronic
system 2. From these measurements, the volume flow rate through
the meter is determined as described above.
The electronics system 2 typically consists of a signal generator
which drives the transducer (8) for an upstream measurement, switching
to drive the transducer (9) for a downstream measurement. Acoustic
signals propagate through the metering tube (5) and are received
by the other transducer. Received signals are digitised and fed
to a digital signal processing unit from which a flow rate signal
is output.
Inlet chamber (6) is a cylindrical cavity into which fluid incoming
through inlet (3) is injected tangentially in order to produce a
rotary fluid flow within the chamber (6) having no component of
velocity in the axial direction of the metering tube (5). The purpose
of doing this is to remove or reduce flow influences upstream of
inlet (3) which could affect flow velocity in metering tube (5).
Metering tube (5) is thus effectively decoupled from external disturbing
influences in the incoming flow, and the fluid flow through the
tube is rotationally symmetrical about a line 32 connecting centres
of the transducers.
An inner tube holder (10) can be shaped so as to reflect any signal
away from the direct path so that echoes reflecting from it do not
interfere with the direct path signal until the measurement has
been made. This can be done by having angled faces with the small
cross-section to scatter the signal and deflect it towards a longer
interference path or an absorbing path.
Echoes are also experienced due to reflection of the acoustic signal
from the ends (11) of the metering tube 5 back to the transducer
(8 or 9). This may be avoided by designing the metering tube (5)
as shown in FIGS. 3 or 4. The metering tube (5) comprises a number
of flow passages (6162) or (7172). The metering tube ends (1111')
are tapered to reflect the signal 12 away from the transducer and
away from the direct path, so that echoes reflecting from it do
not interfere with the direct path signal during the measurement
period.
The transducer directivity can be tailored to minimise the power
of the signal that is not travelling down the metering tube.
The volume flow rate through the meter is derived from the measured
velocity. If the velocity was uniform across the flow, the volume
flow rate would simply be:
where Q is the volume flow rate, U the uniform flow velocity and
A the cross-sectional area of the flow region.
However, due to the effect of viscosity, the velocity is not uniform,
and in a circular pipe assumes a developing parabolic distribution
from the inlet. This velocity profile is further modified by the
onset of turbulence at the higher flow rates.
Under these conditions, the measured velocity cannot simply be
multiplied by the cross-sectional area to arrive at a value for
the volume flow rate. This needs to be corrected.
Unfortunately, the velocity profile assumed in the pipe is a function
of many variables, and displays a non-linear behaviour. Standard
averaging methods attempt to correct this shortcoming, but are expensive
and imprecise in practice.
This meter uses the integrating properties of a plane sound wave.
A plane wave travelling down a tube will experience a phase shift
due to the flow in the tube. Under certain flow and acoustic conditions,
this phase shift is proportional to the flow rate alone, as discussed
in "Theory of Transit Time Ultrasonic Flowmeters" (J.
Hemp, Cranfield Institute of Technology, 27 Jul., 1981 pp 142-144).
In order to guarantee that the time of flight .DELTA.t is measured
by a plane sound wave only, we need to remove the higher order modes
by time domain separation and/or by operating below the cut-off
frequency of the metering tube.
In the present meter, the metering tube 5 is designed so as to
transmit only plane waves at the frequency used to probe the flow.
For every set of frequency, flow rate, pipe wall impedance and pipe
geometry, there corresponds a cut-off frequency below which sound
will only propagate in its first mode or as a plane wave.
At frequencies higher than the cut-off frequency, higher order
modes propagate, the different modes travelling with different group
velocities. This difference in velocities can also be used to isolate
the plane wave transmission by differentiating in the time domain
between the modes of propagation.
In a circular pipe, the dominant parameter upon the cut-off frequency
is the pipe diameter. Hence, we can consider the problem in terms
of a cut-off diameter. It is defined as the diameter below which
the sound will only propagate as a plane sound wave at the given
frequency.
The relationship between the cut-off pipe dimensions and the cut-off
frequency is roughly linear and need not be known precisely. For
a perfectly rigid circular pipe, this requirement translates into
using a diameter smaller than 0.586 .lambda.. Where .lambda. is
the wavelength of the signal propagating in free space. This requirement
can be relaxed if viscous damping, wall flexibility and surface
texture are considered. All these aspects act to limit or prevent
higher mode propagation, thereby achieving plane wave only propagation
in a pipe above this calculated cut-off frequency. For example,
one can design a pipe lined with acoustic absorbent material, which
effectively absorbs the higher order modes reflections and prevents
their propagation.
The present meter has a metering tube (5) with an overall diameter
much greater than the cut-off diameter for the chosen frequency
of operation. However, it is subdivided into a number of parallel
axially extending passages whose individual diameters are well below
the cut-off diameter at 40 kHz. In cross-section, the metering tube
(5) may be of close-packed structure, as illustrated in FIG. 5
which is a cross-section of the metering tube (5), showing a number
of passages (515253). The interstices 33 between the passages
515253 are blocked.
This offers the advantage of keeping the flow velocities low, thereby
reducing the flow resistance. It also reduces flow noise and improves
the accuracy of the method.
In contrast to the close-packed structure, as shown in FIG. 5 a
structure as shown in FIGS. 6 or 7 is preferred. The interstices
33 in FIG. 6 are blocked off and do not contain fluid. The structures
shown in FIGS. 6 and 7 may be contrasted with the close-packed structure
of FIG. 5 in the following way. The central axis line (32) connecting
the transducers is shown in FIGS. 5 to 7 and is normal to the plane
of the page. The passages (515253) in FIG. 5 are each at different
radial distances from the line 32. This causes different flow and
acoustic paths to be sampled by each passage. In contrast, all the
passages e.g. 6162 in FIG. 6 and e.g. 7172 in FIG. 7 are at equal
radial distances from the line 32. The flow and acoustic paths in
these passages are equivalent, and this causes a reduction in errors.
In all these examples the ratio q.sub.n /Q is constant for all
working values of Q, where q.sub.n is the volume flow rate through
the passage being probed, and Q is the total volume flow rate between
the input and output chambers 67. A measurement of q then gives
a value for Q.
An alternative flow passage is illustrated (in cross-section) in
FIG. 8. An annular flow passage of constant outer diameter 82 contains
fluid, and encloses a central solid portion 81 in the form of a
massive rod. The portion 81 is supported by vanes (not shown) within
the surrounding structure. The distance d is below the cut-off diameter,
in order to suppress radial off-axis modes.
Circumferential off-axis modes are suppressed by ensuring equivalent
transmitter/receiver response round the circumference. This can
be achieved by suitable design of transducer.
Although not shown, various combinations of the FIG. 8 example
with the examples of FIGS. 5 and 6 are possible.
An example of a suitable type of transducer is shown in FIG. 9
in which the annular flow passage 82 samples an annular region of
the waves emitted by piezoelectric transducer 130. The transducer
comprises a piezoelectric element 133 which is bonded to a metal
disk 131 shown in cross-section in FIG. 9. As shown in the plan
view of FIG. 10 the piezoelectric element 133 vibrates in a "radial"
mode, indicated at 141 which causes the metal disk to vibrate as
indicated by dotted lines 132 in FIG. 9.
A second example of a suitable transducer for use in conjunction
with an annular flow passage is shown in FIG. 12. The transducer
is similar in construction to a conventional permanent magnet moving
coil loudspeaker shown in FIG. 11. Permanent magnet 141 carries
flux lines as shown at 142. Coil 143 attached to cone diaphragm
145 vibrates in the magnetic field in the annular space 146 between
the poles of the permanent magnet 141 in response to the electric
current passing through it, as shown at 147.
In the transducer of FIG. 12 the pole pieces 152153 are tapered
to increase the magnetic field in the region between them. A light,
non-conducting support diaphragm 154 carries a spiral coil of current
conductors 155 on its surface. The spiral coil lies in the magnetic
field between the pole pieces 152153 and vibrates in response
to a current through the coil. This provides an annular sound field
157 which may be input into the annular flow passage 82 and which
does not vary around the circumference of the annulus, as required.
It is preferable for the distance 159 to be approximately .lambda./2
where .lambda. is the wavelength of sound in the gas.
FIG. 13 illustrates, in cross-section, a further alternative flow
passage, comprising a slot 121 dimensions h and w (where w is much
greater than h). (As in FIGS. 8 and 10 fluid flows into the page).
h is below the cut-off diameter, and off-axis modes in the w direction
are suppressed by suitable design of transducer, as in the case
of the annular passage.
In this example, the cross-sectional area "A" of the
metering tube 5 is chosen so that the maximum mean flow velocity
Umax, as defined by: ##EQU1## produces a sub-cyclic phase shift,
i.e.: ##EQU2## where f is the frequency of the acoustic waves, c
is the speed of sound and l is the distance between the transducers.
This improves the linearity of the meter and facilitates the measure.
It can be shown by analysis that reducing the phase shift .DELTA..theta.
due to the flow to .DELTA..theta.<.pi. ensures the independence
of the measurement to flow velocity profile and increases the linearity
of the flow meter. Keeping the phase shift sub-cyclic allows the
same acquisition window to be used upstream and downstream, and
thereby reduces the sensitivity to transducer response/start-up.
However, perfectly acceptable performance has been achieved using
an area A which does not produce a sub-cyclic phase shift. |