Abstrict In one embodiment, a first air flow meter includes a pair of ultrasonic
transducers mounted on one wall of a conduit, and an opposite reflective
wall is shaped to focus acoustic signals from one transducer to
the other. In a second embodiment, the opposite wall is separated
into a stepped series of cylindrical reflective surfaces spaced
from the transducers by different distances, each surface being
one-half wavelength from its adjoining surfaces so that constructive
combination of the acoustic signals occurs. For noise of the same
wavelength emitted at sources axially spaced from the transducers,
the reflective surfaces appear to be spaced by approximately 3/4-wavelength
steps so that reflected noise signals tend to cancel.
Claims The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. An acoustic fluid flow meter including a conduit for carrying
fluid flow comprising
a pair of acoustic transducers mounted on one wall of a conduit,
one transducer being located upstream of the other such that acoustic
signals emitted from either transducer reflect from an opposite
wall to the other transducer, the opposite wall comprising one or
more circular cylindrical reflecting surfaces each having a cylinder
axis passing through the transducers so that signals from either
transducer transmitted across the conduit and reflected from the
reflecting surfaces will be focused on the other transducer.
2. An acoustic fluid flow meter including a conduit for carrying
fluid comprising
a pair of acoustic transducers for emitting signals at an operating
wavelength mounted in one wall of a conduit, one transducer located
upstream of the other such that acoustic signals emitted from either
transducer reflect from an opposite wall to the other transducer,
the opposite wall comprising a series of circular cylindrical reflecting
surfaces each having a cylinder axis passing through the transducers
so that signals from either transducer reflected from the reflecting
surfaces will be focused on the other transducer, the reflecting
surfaces being variably spaced from the transducers by about one-half
wavelength multiples as measured along the signal path so that the
reflected signals constructively combine at receiving transducers
while noise emitted at regions upstream or downstream of the transducers
and reflected from the reflecting surfaces will, to some degree,
destructively interfere and thereby be minimized at a receiving
transducer.
Description This invention relates to an acoustic fluid flow meter and particularly
to such a meter using acoustic signals reflected from a conduit
wall. It is well known to measure fluid flow, such as mass air flow,
for example, by acoustic instruments operating in the ultrasonic
range. Such apparatus usually involves a pair of ultrasonic transducers
which alternately emit acoustic signals and, after the signals pass
through the fluid being measured, receive the signals. Electronic
circuitry analyzes the relationship of the transmitted and received
signals to arrive at a measure of the fluid velocity or mass flow.
Specific examples of such apparatus are disclosed in my copending
patent applications "Dual Frequency Acoustic Fluid Flow Method
and Apparatus", U.S. Ser. No. 548994 filed Nov. 7 1983 and
"Method and Apparatus for Measuring Fluid Flow", U.S.
Ser. No. 545258 filed Oct. 25 1983. In such apparatus, as in the
case of any measuring instrumentation, it is desirable to maximize
the signal-to-noise ratio of the received signal and to minimize
any spurious data resulting from false signals. It is also desirable
in order to measure a representative value of fluid flow to sample
a large percentage of the fluid that is being measured. This latter
objective is more easily met by reflective techniques than by direct
transmission from one transducer to another.
It is, therefore, a general object of the invention to provide
a reflective acoustic fluid flow meter having a strong signal reflected
to receiving transducers. It is another object of the invention
to provide such a meter having an improved signal-to-noise ratio.
The invention is carried out by providing a conduit with upstream
and downstream ultrasonic transducers mounted in one wall with the
opposite conduit wall formed for optimum reflection of an acoustic
signal from one transducer to the other and particularly wherein
the reflective wall comprises one or more circular cylindrical segments,
each having a center of curvature passing through the transducers
so that the reflective signal from one transducer is focused onto
the other transducer. The invention is further carried out by utilizing
a reflective surface where individual reflective cylindrical segments
are variably spaced from the transducers by one-half wavelength
distances so that signals from one transducer to another are constructively
combined at the receiving transducer whereas noise from sources
axially spaced from the transducers tends to destructively interfere
after reflection, to thereby minimize noise signals at the receiving
transducer.
The above and other advantages of the invention will become apparent
from the following description and the accompanying drawings wherein
like reference numerals refer to like parts and wherein:
FIG. 1 is a plan view of a reflective acoustic fluid flow meter
according to the invention;
FIG. 2 is a partially sectioned elevational view of the fluid flow
meter of FIG. 1;
FIG. 3 is a cross sectional plan view of a fluid flow meter according
to another embodiment of the invention; and
FIG. 4 is a schematic illustration of the acoustical operation
of the FIG. 3 embodiment of the invention.
This invention has a wide variety of applications; however, it
is disclosed herein in the context of a mass air flow meter in the
induction system of an automotive engine. Thus, the drawings depict
a conduit which is the throttle body or a metering section upstream
of the throttle body. Referring to FIGS. 1 and 2 a housing 10 defines
an inner fluid flow passage 12 which is not circular in cross section
but which has an upper flange 14 that is circular in its outer periphery
to facilitate coupling with conventional circular parts, i.e., an
air cleaner assembly or an air induction hose. An elongated opening
16 in one side of the housing 10 allows upstream and downstream
piezoelectric ultrasonic transducers 18 and 20 respectively, to
be mounted adjacent the inner passage 12. A lateral housing extension
22 contains the transducers 18 and 20 and associated electronic
circuitry, not shown. The inner walls which define the air passage
12 are comprised of two circular cylindrical sections including
a first wall section 24 which subtends an arc of about 250.degree.
and which adjoins a second wall section 26 which completes the passage
12 but which has a radius of curvature about twice that of the first
section 24. The opening 16 containing the transducers 18 and 20
is symmetrically located in the first wall section 24 and is opposite
the second wall section 26 which serves as the reflecting surface
for signals transmitted from one transducer to another.
In operation, an emitted signal is directed across the air passage
12 as indicated by dotted lines 28 in FIG. 2 and reflected by the
second wall section 26 to again cross the passage and focus on the
other transducer. The transducers alternate in their roles as transmitters
and receivers of acoustic signals so that the acoustic energy from
either transducer is focused by reflecting surface 26 onto the other
transducer. To focus the acoustic signals, the reflecting surface
26 has its center of curvature or the axis 30 of the circular cylindrical
segment passing through the transducers 18 and 20. The particular
center of curvature or axis 30 may lie at the surface of the transducers
or preferably at an axis 30 passing through the piezoelectric crystals
of the transducers. The acoustic energy from the transducers is
transmitted through a fan shaped sector indicated by dotted lines
32 of FIG. 1 thereby sweeping through most of the area of the passage
12 to sample a high percentage of the fluid flowing through the
passage. Since the signal emitted from each transducer is focused
onto the other transducer by the reflecting wall 26 a strong acoustic
signal is transmitted.
Typical dimensions for the meter are 55 mm inner diameter of the
first wall section 24 a 51 mm radius of curvature of the reflecting
wall 26 with the center of curvature axis 30 passing 5 mm behind
the opening 16. The transducers are 16 mm in diameter and are spaced
20 mm center-to-center. To optimize the transmission efficiency,
each of the transducers 18 20 is tilted about 10.degree. toward
the other as shown in FIG. 2.
Since the transducers 18 and 20 are efficient receivers of ultrasonic
energy at the preferred frequency of operation (on the order of
35 to 50 kHz), they are able to receive noise of the same frequency.
Of course, the transducers receive noise of other frequencies, but
it is subject to electrical filtering. In the case of automotive
induction systems, as shown in FIGS. 3 and 4 the throttle valve
or blade 34 is a source of noise at the operating frequency. An
idle bypass port 36 is uncovered just as the throttle blade begins
to open and the rush of air past the port produces a whistle at
the operating frequency. FIG. 3 depicts a stepped reflector design
which may replace the simple cylindrical reflector 26 of FIGS. 1
and 2 and which frustrates noise at the operating frequency emanating
at an axial distance from the transducers. According to that design,
the housing 10' is generally circular in its outer periphery. The
inner surface of the housing has a first wall 24' comprising a circular
cylinder and supporting transducers 18 and 20 as in FIGS. 1 and
2. A reflecting wall opposite the transducers comprises a series
of surfaces each comprising a section of a circular cylinder with
its center of curvature or cylinder axis intersecting the transducers.
A central reflective surface 26a directly opposite the transducers
occupies a portion, about one-third, of the reflector surface, and
it is flanked on either side by a cylindrical reflective surface
26b of smaller extent than surface 26a and having the same center
of curvature but a smaller radius of curvature so that a shoulder
or step 38 is formed between the two surfaces. Flanking each of
the reflective surfaces 26b is still another set of reflective cylindrical
surfaces 26c which also have the same center of curvature but a
still shorter radius of curvature. Again another shoulder or step
40 is defined between the surfaces 26b and 26c. The step size or
distance between adjacent surfaces, measured along the signal path,
is one-half wavelength. At a frequency of 35 kHz, that distance
would be 4.38 millimeters. Acoustic signals 28' emitted from a transducer
18 or 20 and reflected from adjacent surfaces will be in phase since
those reflecting from the more distant surface will travel a whole
wavelength farther than the wave reflecting from the adjacent surface.
They will combine at the receiving transducer to form a strong signal.
The step size is not critical since the constructive or destructive
interference of the sound waves is quite effective for small variations
from the ideal phase relationship. For example, for a given wavelength,
the step size may vary +12% for 70% effectiveness.
The diagram of FIG. 4 illustrates the effect of a throttle blade
whistle located about 82 mm downstream of the transducers where
a throttle blade 34 in its closed position contacts the first wall
section 24'. With a reflector design like that of FIGS. 1 and 2
the whistle noise could be focused by the reflector 26 onto either
transducer 18 or 20. However, with the stepped reflector design
of FIG. 3 the step distances of the reflectors as measured along
the paths 42 of the noise is three-quarters of a wavelength at the
operating frequency. Then the noise waves reflected from adjacent
surfaces will be 180.degree. out of phase, thereby destructively
interfering to cancel the noise effects. This circumstance is true
where the whistle noise is incident to the reflector surfaces at
an angle of 48.degree. to the normal plane. Noise sources which
are not exactly positioned to result in the 48.degree. angle will
not be wholly cancelled by the reflected waves; however, effective
noise signals will be greatly diminished even if they are close
to that ideal position. Consequently, the stepped reflector arrangement
is effective not only to focus the signal from one transducer to
another at the operation frequency but also to diminish noise at
the same frequency from axially spaced sources. In FIG. 4 the noise
paths 42 are shown as directed to transducer 18. Similar paths directed
to transducer 20 also exist. Complete noise cancellation will not
occur for both sets of paths but substantial noise diminution will
occur for the latter paths.
Thus it is apparent that the fluid flow meter according to one
embodiment of this invention efficiently transmits an acoustic signal
from one transducer to another while sampling a large area of the
fluid-carrying conduit and, according to another embodiment, has
the additional feature of suppressing noise of the same wavelengths
as the acoustic signals from an axially spaced source. |