Abstrict A transducer is provided for a flow meter wherein the transducer
has an interdigital structure of a polarization structure and/or
of electrodes for the purpose of direct ultrasonic radiation at
an angle .alpha., which is specifiable and electrically controllable,
and wherein disturbances of a flow profile by the installed transducer
are avoided.
Claims I claim as my invention:
1. A flow-through flow meter adapted for use in a pipe line, comprising:
two ultrasonic transducer means for respective transmission and
for respective reception of ultrasonic radiation when installed
in the pipe line, the radiation being directed relative to an axis
of the pipe line obliquely at an angle .alpha. through a medium
flowing in the pipe line between the transducer means so that a
difference between the ultrasonic radiation received in the flow
direction and ultrasonic radiation received counter to the flow
direction may be utilized as a measure of the flow-through quantity
and/or flow-through speed; each of said transducer means being designed
so that when they are mounted in an interior wall of the pipe line
they do not substantially disturb the medium flowing in the pipe
line; each transducer means having associated therewith a material
body and an interdigital structure; a thickness t of the transducer
body being smaller than half a wavelength of the ultrasonic radiation
in the material of the transducer body; a periodicity of the interdigital
structure satisfying a condition
where d is the periodicity of the interdigital structure, and .lambda.
is a wavelength in the flowing medium in the pipe line; and connection
means being provided coupled to AC voltage excitation means with
a frequency f for radiation at an angle .alpha. at the given periodicity
d.
2. The meter of claim 1 wherein the interdigital structure comprises
a comb-like interdigital structure of electrodes on at least one
surface of the body.
3. The meter of claim 1 wherein the interdigital structure comprises
an interdigital structure of polarization associated with the transducer
body.
4. A flow-through flow meter according to claim 1 wherein the interdigital
structure is formed solely by a polarization structure alternating
with said periodicity.
5. A flow-through flow meter according to claim 1 wherein said
interdigital structure comprises a polarization structure associated
with the body which alternates in direction as well as an interdigital
electrode structure, the two structures taken together having the
same periodicity d, and fingers of a one comb-like structure and
fingers of another comb-like structure of the electrode structure
having mutually equal intervals of a dimension d/2.
6. A flow-through flow meter according to claim 1 wherein said
interdigital structure is formed solely by interdigital electrode
arrangements.
7. A flow-through flow meter according to claim 6 wherein on both
sides of the transducer body one interdigital structure is provided,
each of which have a periodicity and both of which are shifted relative
to one another by d/2.
8. A flow-through flow meter according to claim 6 wherein on one
side of the transducer body a doubled interdigital structure with
two comb-like structures each is provided; on the other side of
the transducer body an entire-surface counterelectrode is provided;
four AC voltages with a 90.degree. phase displacement in connection
with the interdigital structure; and a periodicity being given by
an interval between two adjacent fingers of a single one of the
comb-like structures.
9. A flow-through flow meter according to claim 1 wherein an entire-surface
counterelectrode is arranged on a surface of the transducer body
facing the pipe interior.
10. A flow-through flow meter according to claim 1 wherein an interior
surface of the transducer body which, when installed and facing
an interior space of the pipe line, has such a cylindrical curvature
that said interior surface of the transducer body is a continuation
of the pipe interior wall.
11. A flow-through flow meter according to claim 1 wherein on
a surface of the transducer body, which, in the case of an installed
transducer is facing the pipe interior space, an adaptation or matching
piece is placed whose surface facing the pipe interior space is
cylindrically curved and forms a continuation of the tube interior
wall.
12. A method for the operation of a transducer in a flow-through
flow meter according to claim 1 wherein said AC voltage excitation
means permits a controllable variation of the frequency f of an
excitation AC voltage such that an angle .alpha. of the ultrasonic
radiation can be controllably varied to compensate for changes in
a refractive index of the medium flowing in the pipe line.
13. A flow-through flow meter according to claim 1 wherein said
transducer body thickness t is chosen such that an ultrasonic vibration
of said transducer body is a resonance vibration of the body when
excited by said AC voltage excitation means with said frequency
f.
14. A flow-through flow meter system, comprising: a pipe line;
two ultrasonic transducer means for transmission and reception of
ultrasonic radiation installed in the pipe line and wherein said
ultrasonic radiation is directed relative to an axis of the pipe
line at an angle .alpha. through a medium flowing between the transducer
means in the pipe line; each of said transducer means being designed
so that they conform to an interior wall of the pipe and do not
substantially disturb the medium flowing in the pipe line; each
transducer means having an interdigital structure associated with
a piezo-electric body of the transducer, a thickness of the transducer
body being smaller than half a wavelength of the ultrasonic radiation
in a material of the transducer body; a periodicity of the interdigital
structure satisfying a condition
where d is the periodicity of the interdigital structure, and .lambda.
is a wavelength in the flowing medium in the pipe line; and means
for coupling an excitation voltage to the transducer means of frequency
f which corresponds with ultrasonic radiation at an angle .alpha.
given periodicity d.
15. A flow-through flow meter according to claim 13 wherein said
transducer body thickness t is chosen such that an ultrasonic vibration
of said transducer body is a resonance vibration of the body when
excited by said AC voltage excitation means with said frequency
f.
Description BACKGROUND OF THE INVENTION
The present invention relates to a flow-through flow meter having
ultrasonic transducers for a respective transmission or respective
reception of ultrasonic radiation. The transducers are mounted in
association with a pipe such that the ultrasonic radiation passing
through the flowing medium is analyzed. A difference between the
ultrasonic radiation received in the flow direction and the ultrasonic
radiation received counter to the flow direction may be utilized
as a measure of the flow-through quantity and/or flow-through speed.
From state of the art publications such as "Flow, its Measurement
and Control in Science and Industry", Vol. 1 Part 2; and "Flow
Measuring Devices", pp. 897-915 Industry Society of America,
Pittsburgh (1974) (both incorporated herein by reference), a number
of flow-through flow meters operating with ultrasound are known.
This latter comprehensive publication refers to many older printed
publications. In particular, FIG. 3 (p. 905) and FIG. 4 (p. 907),
illustrate arrangements comprising transmitting and receiving transducers
whose radiation direction is directed obliquely to the axis of the
pipe line, in the interior of which the flowing agent is to be measured
with regard to speed and/or flowing quantity. All these embodiments
are of such a type that the interior surface of the pipe wall exhibits
an interference or fault due to a wedge-shaped depression or a wedge-shaped
projection. Such disturbances or interferences of the pipe interior
wall, however, are very undesirable, since, on the one hand, they
cause disturbances in the flow profile, and hence in the flow resistance,
and, on the other hand, they give rise to deposits at this location,
whereby these deposits, moreover, also impair the propagation of
the ultrasonic radiation.
In order to avoid such difficulties, constructions have been proposed
which are illustrated in the above-cited publication in FIG. 6 (p.
908). An unaltered smooth pipe interior wall is present therein,
and the transducers for transmission and reception are again mounted
with the transmission- or receiving-direction, respectively, obliquely
directed relative to the pipe axis. In accordance with one sample
embodiment, a notch in the exterior wall of the pipe line is provided
in which the respective transducer is mounted. This transducer must
radiate, or receive, respectively, its ultrasonic radiation through
the pipe wall, whereby, at the boundary surface between the pipe
interior wall and the pipe interior volume, a refraction of the
propagating ultrasonic radiation occurs which, moreover, is dependent
upon changes in the refractive index of the medium flowing in the
pipe line.
In another embodiment a transmission member for the ultrasonic
radiation is inserted obliquely into the pipe wall, the actual transducer
member being applied to the exterior end face of said transmission
member. In the region of the interior wall of the pipe line, this
transmission member has a surface which is a smooth continuation
of the pipe interior wall. In the case of a transmission member
of this type, the previously cited effect of radiation refraction
takes place.
In an additional embodiment the transducer member is mounted on
a coupling block which is secured externally to the pipe wall, whereby
likewise the refraction occurs.
These various embodiments for the mounting of the transducer, which
have already been known for quite some time, have, as already partially
discussed, disadvantages of various types. The embodiments comprising
transmission members (FIG. 6 of the publication) are, above all,
also less sensitive, since the ultrasound passes through a considerable
path distance in a medium, which is not influenced by the respective
conditions in the flowing medium.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide, for a flow-through
flow meter as described above, additional measures which avoid the
disadvantages of the state of the art, such as the formation of
flow obstacles and/or deposit spaces, and/or eliminate reductions
in the measuring insensitivity.
This object is achieved, for the flow-through flow meter in accordance
with the invention, by designing the transducers such that when
they are mounted in an interior wall of the pipe line they do not
substantially disturb the medium in the pipe line. Each transducer
has an interdigital structure associated with a body of the transducer
and wherein a thickness of the transducer body is smaller than half
a wavelength of the ultrasonic radiation in a material of the transducer
body. A periodicity of the interdigital structure satisfies a condition
d=.lambda./cos.sup..alpha. where d is periodicity of the interdigital
structure and .lambda. is a wavelength in the flowing medium in
the pipe line. Connection means are provided to couple an AC excitation
voltage with frequency f for providing radiation at an angle .alpha.
for the given periodicity d.
The invention proceeds from the consideration that these problems,
in principle, are to be solved in that the ultrasonic radiation,
which is to be necessarily transmitted and received obliquely, preferably
at an angle .alpha..apprxeq.45.degree. to the axis of the pipe line,
is directly transmitted and received by the surface of the pipe
interior wall, or a surface which is at least essentially equal
to this surface. This fundamental solution pertaining to this inventive
principle consists in installing in the pipe wall one such ultrasonic
transducer, respectively, whose transmitting or receiving surface
at least essentially is a continuation of the surface of the pipe
interior wall. It is preferably even curved like the pipe interior
wall, and nevertheless exhibits a transmission or a receiving direction,
respectively, for the ultrasonic radiation which has the angle .alpha.
of the provided transmission direction of the pipe interior space.
In the case of this inventive transducer, no refractive effects
as in the case of the arrangements known according to FIG. 6 of
the above-captioned publication occur. Instead of a curvature of
the entire transducer member, a curvature solely of the interior
surface also suffices. An additional possibility for the disturbance-free
continuation of the pipe interior wall consists in placing an adaptation
(or matching) piece of plastic material on the transducer which
is planar in the direction of the transducer and, in the direction
of the pipe interior, is cylindrically concavely curved (like the
pipe interior wall).
In contrast with the transmission members for transducers radiating
in a normal direction which are mentioned above and which correspond
to the state of the art, refractive influences and their change
in the case of agents having a different sonic velocity are virtually
of no significance. For the components of the sonic radiation which
lie in planes which are parallel to the pipe axis, the inventive
adaptation piece acts like a plane-parallel plate. For components
which lie in planes which are perpendicular to the pipe axis, the
refractive angles are very small, since the radius of curvature
of the pipe in relation to the transducer dimensions is great.
The individual transducers employed in the invention have a periodic
structure according to the equation
with .lambda.=wavelength in the medium of the pipe interior space,
and d=the distance between two adjacent fingers of a comb of the
periodic interdigital structure, with an additional finger of the
opposite comb present in this spacing.
This leads to a radiation from the surface of the transducer member,
which radiation is obliquely directed with the angle .alpha. relative
to the surface of the transducer. This periodic structure can be
of an alternating polarity over the length (parallel to the pipe
axis) of the transducer member in the material of the transducer
member. Consequently, the polarization of the same, which is parallel
to the thickness of the transducer member, is directed to be alternatingly
anti-parallel with this periodic direction. Another embodiment of
the inventive principle provides, on the surface of the transducer
member, not facing the pipe interior space, an interdigital structure
as the electrode arrangement. This electrode-interdigital structure
consists of two inter-engaging comb structures with fingers and
a bus bar, respectively. Interdigital structures of this type are
known, moreover, from piezo-ceramic transit time arrangements for
another purpose.
In a particularly advanced solution in accordance with the invention,
a transducer is provided which has a periodically alternating polarization
as well as a correspondingly periodically designed interdigital
structure on the surface of the transducer member not facing the
pipe interior space. This embodiment of the invention, designed
to be doubly alternating in the case of excitation with two AC voltages
which are phase-shifted relative to one another by 90.degree., has
a transmitting and receiving direction at an angle .alpha. to the
surface, however, still only in one single direction. The periodicity
d according to the above equation applies therein for the distance
of such next-adjacent fingers, to which also a repeating polarization
corresponds.
An additional, particularly advanced solution has a transducer
with a doubled interdigital structure on the one side of the transducer
member and a uniform polarization of the transducer member. The
excitation proceeds with four AC voltages, which are phase-displaced
respectively by 90.degree., of which one in each instance is placed
between one of the four comb structures of the doubled interdigital
structure and the common counter electrode of the rear side. This
transducer also likewise still radiates only in one direction by
the angle .alpha..
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 1a illustrate a first embodiment of a transducer for
a flow-through flow meter in accordance with the invention, whereby
this transducer has an interdigital structure;
FIG. 2 illustrates a second embodiment with periodicity of the
polarization;
FIG. 3 illustrates an additional embodiment with an interdigital
structure present on both sides;
FIG. 4 illustrates an additional embodiment with a doubled interdigital
structure;
FIG. 5 illustrates an embodiment with an interdigital structure
as well as periodicity of the polarization; and
FIG. 6 illustrates an adaptation piece.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 1 designates, in section, the pipe line in which the
medium to be measured--not illustrated here--flows, and through
the interior space of which ultrasound is emitted or radiated in
a manner known per se, with the angle .alpha. from a transducer
2 to a transducer 3 and, for the purpose of difference formation,
from transducer 3 to the transducer 2. The transducer 2 with a
plate-shaped body K of piezo-electrical ceramic material, is installed
in the wall of the pipe 1 in such a manner that the pipe interior
wall has as small as possible an interruption of its shape. In particular,
the interior side 4 of the member K of the transducer 2 can have
a curvature corresponding to the pipe interior wall.
The transducer 2 has, on the interior side 4 an electrode coating
5 as is conventional for piezo-ceramic components, which coating,
corresponding to the respective flowing medium, is sufficiently
resistant, or is protected by an acoustically non-interfering coating,
respectively. This electrode coating 5 is preferably electrically
conductively connected with the pipe in the form of a connection
line.
On the exterior surface 6 of the transducer 2 there is an interdigital
structure, known per se, consisting of two comb-like structures
interengaging with their fingers, whereby the fingers of each comb
structure are connected with one bus bar each. The fingers 7 illustrated
in cross-section in the representation of FIG. 1 belong to the
one comb structure, and the fingers 8 belong to the other comb structure.
FIG. 1a illustrates, for the purpose of better explanation, the
diagram of such an interdigital structure 9 with the fingers 7 and
8 of a respective comb structure. 10 and 10' designate the bus bars
interconnecting the respective fingers 8 or 9.
The arrows 11 designate the permanent polarization of the piezo-electric
ceramic material of the transducer 2. This polarization direction
can also be oppositely directed to the arrows 11 without anything
changing in the method of operation of this embodiment according
to FIG. 1. The transducer 3 can have precisely the same embodiment
as the transducer 2.
The dimension d indicates the periodicity of this interdigital
structure. It is also clearly recognizable that, between the two
adjacent fingers, respectively, of the one comb structure (7 10),
one finger 8 respectively, of the other comb structure (8 10')
is disposed.
As indicated in FIG. 1a for the purpose of clarity, the finger
7 is connected to the connection of the comb structure and an AC
voltage U.sub.1 which voltage is connected between the counterelectrode
5 of the interior side and the bus 10; and the finger 8 is connected
to the connection 18 of the other comb structure and a second AC
voltage U.sub.2 which voltage is connected between the counterelectrode
5 and bus 10'. Both AC voltages U.sub.1 and U.sub.2 have the same
ultrasonic transmitting and receiving frequency f of the transducers
2 and 3 they have the same amplitude, and are phase-displaced relative
to one another by 180.degree.. This condition and the above-indicated
angular relation with the periodicity d provides an ultrasonic radiation
12 radiated as illustrated with the angle .alpha. from the member
K having a prescribed value, said ultrasonic radiation 12 having
a normal (except for the marginal regions) plane wave front relative
to the illustrated arrow. The same sharp directional characteristic
also applies to the receiving behavior of the transducers 2 and
3.
Basically, the interdigital structure 9 can be applied also on
the pipe interior side, and on the body of the transducer 2 or 3
(the counterelectrode 5 can also be applied on the exterior side).
However, for reasons of durability of the transducers 2 and 3 alone,
the selection as illustrated of the sides for the interdigital structure
and counterelectrode is preferable.
Merely for the purpose of completeness, the embodiment illustrated
in FIG. 1 also emits an ultrasonic wave in the direction indicated
by 12', which, however, runs to a dead end in the pipe interior
and has no useful significance in terms of the inventive flow measurement.
FIG. 2 illustrates, in an enlarged illustration, only the transducer
22 in a specific embodiment for an inventive flow meter such as
is to be installed in the pipe wall instead of a transducer 2 and
3.
FIG. 2 illustrates the embodiment with alternating polarization
in the body K of the transducer 22; and 111 and 112 designate the
respectively opposite polarization directions of the overall polarization
of the piezo-ceramic body K of the transducer 22. Such a periodic
structure with the periodicity interval d (in order to satisfy the
above equation for radiation at the angle .alpha.) is realized by
means of a preceding polarization with a corresponding raster or
grid shaped electrode. 5 and 5' designate total-surface electrode
coatings of the surfaces 4 and 6 of the transducer member K, whereby
details regarding such an electrode coating 5 have already been
disclosed above. Between these two electrodes, i.e., at the connections
71 and 81 one single excitation AC voltage U.about. with the specified
frequency f is applied. The periodicity of the polarization 111
112 guarantees the damanded ultrasonic radiation at the angle .alpha..
The receiving characteristic of such a transducer 22 according to
FIG. 2 has a corresponding angle .alpha..
FIG. 3 again shows in an enlarged illustration the specific embodiment
of a transducer 32 to be employed for the invention, which, like
the transducers 2 and 3 is to likewise be installed in the pipe
line wall 1.
11 again designates the polarization of the body K of this transducer
32. This transducer 32 has, on its two surfaces of its ceramic body,
one interdigital structure 9 each, such as is illustrated in FIG.
1a; i.e., such an interdigital structure is also present on the
pipe interior side. 7 and 8 again designate the fingers of the respective
comb structure with the one bus bar 10 visible in the sectional
illustration. d indicates again the periodicity according to the
above equation. As is also taken into account in the illustration
of FIG. 3 the fingers 7 of the interdigital structure of the one
surface 4 of the transducer member are electrically connected with
the similarly designated fingers 7 of the interdigital structure
of the opposite surface 6. Thus, the directly opposite fingers 7
8 with the respectively opposite connections 71 or 81 are interconnected
for the excitation AC voltage U.about.. Due to this connection--offset
by d/2--of the opposite fingers, or comb structures 7 and 8 respectively,
of the interdigital structures, the connection of a single AC voltage
of the provided excitation frequency f suffices here in order to
attain the radiation 12 (and 12') at the angle .alpha. to the surface
4 4'.
FIG. 4 illustrates an embodiment of a transducer for an inventive
flow meter in which this transducer 42 on its one surface, has,
as schematically illustrated, a doubled interdigital structure with
the periodicity d. For the rear side of the transducer 42 there
suffices, as the counterelectrode, a total-surface metallization
(as in the examples of FIGS. 1 and 3).
As illustrated, the doubled interdigital structure 91 consists
of a total of four comb structures with one bus bar 10 10', 10",
10'" each. The two bus bars 10 and 10' are disposed on the
one marginal side of the doubled interdigital structure. The bus
bar 10 has the illustrated fingers 7 and the illustrated fingers
7' belong to the bus bar 10". The fingers 7 and 7' of the two
comb structures alternate with one another, as is apparent from
FIG. 4. Engaging interdigitally thereto, i.e., into the comb structures
with the fingers 7 and 7', are the comb structures with the fingers
8 and 8', whereby as illustrated, the fingers 8 are connected with
the bus bar 10' and the fingers 8' are connected with the bus bar
10'". The connections of the individual comb structures, or
of the bus bars 10 through 10'", respectively, are referenced
43 44 45 and 46. Between the connection 43 and the non-illustrated
total-surface counterelectrode of the rear side of the transducer
42 a first AC voltage U.sub.43 with the provided ultrasonic frequency
f is to be connected. Between the connection 44 and the cited counterelectrode
a second AC voltage U.sub.44 is to be connected which has a +90.degree.
phase displacement (or shift) relative to the voltage U.sub.43.
Between the connection 45 and the counterelectrode an AC voltage
U.sub.45 with a +180.degree. phase displacement (or shift) relative
to U.sub.43 is to be connected. Finally, between the connection
46 and the counterelectrode an AC voltage U.sub.46 with a +270.degree.
phase displacement relative to U.sub.43 is to be connected. These
four AC voltages U.sub.43 through U.sub.46 have the same frequency
and the same amplitude. The permanent polarization of the body of
the transducer 42 is, in the thickness direction of the same, equally
great in the entire body and equidirectional;i.e. uniform.
The transducer 42 according to FIG. 4 has, as an advantage relative
to the transducers according to FIGS. 1 through 3 only one radiation
direction 12 with the angle according to the abve equation. In the
case of an embodiment of a transducer 42 according to FIG. 4 the
additional radiation direction, illustrated in broken lines, referenced
12' in FIGS. 1 and 2 is eliminated. In the case of an embodiment
according to FIG. 4 disturbances or interferences due to ultrasonic
power radiated in a direction 12' are not be to reckoned with; namely,
such back reflections which could originate from curved pieces,
screwed connections, etc. of the pipe line.
Correspondingly, this transducer according to FIG. 4 also has only
a one-sided received sensitivity in solely the direction 12. Thus,
this transducer is also insensitive to disturbances brought about
by multiply reflected sound waves which would impinge in the pipe
line always from the direction 12'.
The elimination of the radiation direction 12' leads to a corresponding
amplification of the beam 12 given an equally great electrical AC
voltage power expended, and, moreover, leads to a correspondingly
increased receiving sensitivity.
FIG. 5 illustrates a further embodiment of the invention whereby
the transducer 52 in its piezo-ceramic body K has a periodicity
of polarization and, on one side, again preferably the exterior
side 6 of the transducer 52 installed in the pipe, also has an interdigital
structure 9 according to FIG. 1a. The alternating polarization pertaining
to this interdigital structure 9 is indicated by 211 and 212. It
is important to recognize that adjacent fingers 7 or adjacent fingers
8 of the one, or of the other, comb structure, respectively, in
the body K of the transducer, have a respectively opposite polarization
direction 211 and 212 and that the same polarization direction
211 or 212 is again present, respectively, only for the respective
second finger 7 or respective second finger 8 of the one or of the
other comb structure. In the present case of FIG. 5 the interdigital
structure 9 adapted or matched to the alternating structure of
the indicated polarization, has the periodicity d, as indicated
in FIG. 5.
For the practical embodiment of the transducer 52 according to
FIG. 5 as well as also for the transducer 42 according to FIG.
4 (for otherwise equal conditions of angle .alpha., the thickness
of the body of the transducer, the excitation frequency f, etc.)
intervals twice as narrow are present therein between the adjacent
fingers 7 and 8 (or 7 7' and 8 8' according to FIG. 4) of the
entire interdigital structure between the fingers 7 7', or between
the fingers 8 8' of the respective comb structure. The interdigital
structure to be produced of the transducer 42 and 52 according to
FIGS. 4 and 5 thus requires twice as high a resolution of the fingers
7 7', 8 and 8'.
For connection of the interdigital structure 9 of the transducer
52 the fingers 7 are connected by a first common connection 53
and the fingers 8 are connected with a second common connection
54. For excitation of the transducer 52 the two frequency and amplitude
equal AC voltages U.sub.11 and U.sub.12 are to be applied as illustrated.
However, U.sub.11 and U.sub.12 (in contrast with FIGS. 1 1a) need
to have a relative phase displacement of only 90.degree.. The 90.degree.
phase displacement (or phase quadrature) has the advantage that
such a phase displacement can be electronically produced in a simple
fashion with a capacitor.
Also, the transducer 52 according to FIG. 5 has only one radiation
direction 12 with the angle .alpha. according to the above equation,
so that, here again, that which has been stated regarding FIG. 4
applies.
FIG. 6 illustrates a sectional view of an adaptation or matching
piece 61 as already explained above, such as is employed with equal
advantage for the transducer 2 (as illustrated) as well as for all
remaining transducers 3 22 32 42 and 52 with a planar transducer
body. The reference sign 1 is directed to the pipe.
From the above condition d=.lambda./cos .alpha., there results
.alpha.=arccos (c/f.multidot.d) with c=sonic speed and f=frequency
of the exciting voltage U.
A further advantage of the inventive transducer is that, through
mere frequency change of the excitation AC voltage U, the radiation
angle i.e., the angle .alpha. of the direction 12 can be electrically
subsequently adjusted or controlled. This is important not only
for an imprecise original installation, but also, for example, for
a temperature-conditioned change of the refractive index of the
flowing medium. In the case of the invention, a simple automatic
fine tuning for optimum transmission and reception can be provided.
At the optimum conditions, the relation c=d.multidot.f.multidot.cos
.alpha. applies, with constructively prescribed d and .alpha.. This
means that the value of the sonic speed is c.about.f and can be
ascertained from the value f.
The value c of the sonic speed is required in the final (known
for ultrasonic flow meters) numerical evaluation, and results here
in a simple fashion.
An additional feature of the invention is that the transducer body
K is given a thickness t such that the value of the resonance frequency
of a thickness vibration mode of the plate-shaped body K is equal
to the frequency f of the AC excitation voltage. The frequency f
of the excitation voltage is given by the predetermined angle .alpha..
The simplest way of finding the necessary thickness t for resonance
at a given frequency of the transducer is by experiment. This value
of resonance frequency of said transducer body K is lower than the
respective value of a resonance frequency of an equally shaped plate
which has no interdigital structure according to FIG. 2 or 3 as
described in the above disclosure. The reason for this lower frequency
is that the transducer body with said interdigital structure is
nearly free of Poisson effects which otherwise stiffens a body with
respect to its resonance frequency.
A transducer body with the above mentioned value t for its thickness
has a higher amplitude of ultrasonic vibration. That higher amplitude
corresponds to a higher efficiency for transducing excitation voltage
into mechanical ultrasonic vibration and vice versa. According to
the effect that the phase .phi. between excitation voltage and mechanical
vibration changes 180.degree. in the neighborhood of the value f
of resonance frequency, there is a certain restriction to this special
embodiment of the invention to cases in which a change of the value
of the resonance frequency f, i.e. according to temperature changes,
is small.
Although various minor modifications may be suggested by those
versed in the art, it should be understood that I wish to embody
within the scope of the patent warranted hereon, all such embodiments
as reasonably and properly come within the scope of my contribution
to the art. |