Abstrict Ultrasonic flow meter of the clamp-on type for the measurement
of the flow rate of a liquid in a pipeline, provided with two piezoelectric
transmitting-receiving transducers, each accommodated in a housing,
which are clamped at both sides of the pipeline axially offset with
respect to each other thereon and which alternately transmit ultrasonic
pulses into the pipeline and receive them therefrom in order to
determine the flow rate from the transit time of the source pulses
in the upstream and downstream direction. The transducers each consist
of a row of adjacent subtransducers, the pulse emission face of
which row is coupled over its entire length in the axial direction
to the pipe wall. The substransducers are activated by control signals,
the phase difference between the control signals at every two adjacent
subtransducers and the distance between the centers thereof being
matched to each other such that the transducer deforms approximately
sinusoidally with a periodic length which corresponds to the wavelength
.lambda. of Lamb surface waves to be generated in the pipe wall
in the A.sub.0 mode. The waves propagate in the axial direction
along the pipe wall and, on the inside thereof, are converted into
longitudinal waves in the adjacent liquid.
Claims We claim:
1. An ultrasonic flow meter for the measurement of the flow rate
of a liquid in a pipeline comprising:
two piezoelectric transmitting-receiving transducers, each having
a housing with means for attachment to a pipeline, said attachment
being on opposite sides thereof, and in axially offset positions
with respect to each other, said transducers alternatively transmitting
ultrasonic pulses into the pipeline and receiving ultrasonic pulses
therefrom in order to determine the flow rate from the transit time
of the sonic pulses in the upstream and downstream directions;
each piezoelectric transmitting-receiving transducer comprising
a row of adjacent subtransducers, the pulse emission face of which
row is coupled over its entire length in the axial direction to
the pipe wall;
means to produce control signals to activate the subtransducers,
the phase difference between the control signals at every two adjacent
subtransducers and the distance between the centers thereof being
matched to each other in a manner such that the transducer deforms
approximately sinusoidally with a periodic length which corresponds
to the wavelength of Lamb surface waves to be generated in the pipe
wall in the A.sub.o mode, which waves propagate in the axial direction
along the pipe wall and, on the inside thereof, are converted into
longitudinal waves in the adjacent liquid.
2. Ultrasonic flow meter according to claim 1 in which the phase
difference between the signals at the respectively adjacent subtransducers
and the distance between the centers thereof are 180.degree. and
.lambda./2 respectively.
3. Ultrasonic flow meter according to claim 1 in which each piezoelectric
transmitting-receiving transducer comprises an oblong piezoelement
in which the subtransducers are formed by means of crosscuts, made
in at least one of the electrodes and situated at said mutual distance
from each other.
4. The meter of claim 1 in which the means for attachment is a
clamp-on means.
Description The invention relates to an ultrasonic flow meter of the clamp-on
type for the measurement of the flow rate of a liquid in a pipeline,
provided with two piezoelectric transmitting-receiving transducers,
each accommodated in a housing intended for the purpose, which are
clamped at both sides of the pipeline axially offset with respect
to each other thereon and which alternately transmit ultrasonic
pulses into the pipeline and receive them therefrom in order to
determine the flow rate from the transit time of the sonic pulses
in the upstream and downstream direction. Such an ultrasonic flow
meter is known from U.S. Pat. No. 4467659.
The piezoelectric transducers used in the ultrasonic flow meter
known from the above-mentioned patent are each at an angle with
respect to the pipe wall and arranged at some distance therefrom
in the housing. In this case, sonic waves of the shear mode type
are transmitted into the surface of the pipe wall. The emission
faces of the piezoelectric transducers may be directed directly
at the pipe wall at an angle or first directly at a reflection wall
of said housing so that the longitudinal mode waves transmitted
by the piezoelectric transducer to the reflection wall are first
converted into shear mode waves which then have a good transfer
to the coupling face with the pipe wall so that sonic waves still
having sufficient energy are transmitted into the latter. A drawback
is, however, that in both cases, energy losses occur as a result
of the propagation in the housing and, additionally in the second
case, as a result of reflection at the reflection wall.
The object of the invention is to limit the energy losses to a
minimum and, at the same time, to provide an ultrasonic flow meter
in which, while a good coupling is maintained between transducer
housing and pipe wall, the waves transmitted into the latter need
to be generated only with very low energy and despite this, can
readily be detected. The object of this invention is also to provide
an extremely simple and cheap ultrasonic flowmeter.
According to the invention this is achieved in an ultrasonic flow
meter of the type mentioned in the introduction in a manner such
that the piezoelectric transmitting-receiving transducers each consist
of a row of adjacent subtransducers, the pulse emission face of
which row is coupled over its entire length in the axial direction
to the pipe wall, the subtransducers being activated by control
signals, the phase difference between the control signals at every
two adjacent subtransducers and the distance between the centres
thereof being matched to each other in a manner such that the transducer
deforms approximately sinusoidally with a periodic length which
corresponds to the wavelength .lambda. of Lamb surface waves to
be generated in the pipe wall in the A.sub.o mode, which waves propagate
in the axial direction along the pipe wall and, on the inside thereof,
are converted into longitudinal waves in the adjacent liquid.
In an advantageous embodiment of said ultrasonic flow meter the
said phase difference between the signals at the respectively adjacent
subtransducers and the distance between the centres thereof are
180.degree. and .lambda./2 respectively.
In a further advantageous embodiment of said ultrasonic flow meter
each piezoelectric transducer comprises an oblong piezoelement in
which the subtransducers are formed by means of crosscuts, made
in at least one of the electrodes and situated at said mutual distance
from each other.
An ultrasonic flow meter of the inserted type is known from the
European Patent Application 0040837. In this type of flow meter
the wall of the flow pipe is interrupted in order to fit the two
transducers of the flow meter which are situated opposite each other.
The sonic waves are transmitted directly to and received directly
from the flowing liquid. In this case, the position of the oppositely
situated transducer is critical since the beam of sonic waves emitted
is narrow. This is largely compensated for because the angle of
incidence of the waves into the liquid can be controlled by frequency
adjustment. Nevertheless, the position of the receiving transducer
remains critical and strict account has to be taken of this in siting
the two transducers in this case.
On the other hand, in the flow meter of the clamp-on type according
to the invention, the siting is not critical and the transducers
to be clamped securely to the outside of the flow pipe can be offset
with respect to each other without this affecting the measurement.
This is explained in further detail below.
The invention will be explained in more detail on the basis of
an exemplary embodiment with reference to the drawings, wherein:
FIG. 1 gives a sectional view of two transmitting-receiving transducers,
used in the ultrasonic flow meter according to the invention, which
are fitted at both sides of a pipeline axially offset with respect
to each other thereon;
FIG. 2 gives a sectional view of a single transmitting-receiving
transducer of FIG. 1 sited on the pipe wall;
FIG. 3 gives a diagrammatic sectional view of a part of a transmitting-receiving
transducer according to the invention;
FIG. 4 gives a further diagrammatic view of a transmitting-receiving
transducer according to the invention; and
FIG. 5 gives a block diagram of the electronic control unit used
in the ultrasonic flow meter according to the invention.
FIG. 1 indicates how two transducer housings 3 are mounted on a
pipeline 5 through which a liquid 6 is flowing, at sites on the
pipe wall which are axially offset with respect to each other. Each
housing 3 contains a transducer 1 or 2 respectively consisting of
a row of subtransducers. Said transducers 1 2 are fitted on the
base face of the housing sited on the pipe wall, while, for example,
the rest of the housing is filled with an acoustically damping material
4. On the coupling side of each transducer 1 2 there is a coupling
wall 8 of material which is as thin as possible. This may be, for
example, a layer of stainless steel of a few tenths of a millimetre
in order to achieve the acoustic coupling with as little transfer
loss as possible.
The ultrasonic flow meter according to the invention also includes
a standard electronic control unit. This control unit supplies the
alternating energization of the transmitting-receiving transducers,
the synchronization, the reception and processing of the received
pulse signals.
As indicated in FIG. 1 the transducer 1 generates ultrasonic pulses
which produce Lamb waves in the A.sub.o mode in a part of the pipe
wall. These are shear mode waves which propagate mainly at the surface
of the outside and inside of the pipe wall since the amplitude of
this type of waves is a maximum at the surface of the pipe wall.
These waves can be generated with relatively little energy in the
transducer and can also readily be detected by means of the other
transducer. The shear mode waves propagating in the surface of the
pipe wall on the inside produce in the liquid longitudinal mode
waves which propagate in the downstream and upstream direction through
the liquid and are received by the transducer 2. The angle of incidence
(.pi./2-.alpha.) of the waves in the liquid is determined only by
the geometry of the pipe wall and the material constants of pipe
wall and liquid. The transducer 2 then transmits sonic pulses under
the influence of the control unit through the liquid which are received
by the transducer 1. The ultrasonic flow meter then determines the
difference in the transit times of the sonic pulses in the upstream
and downstream direction and determines the flow rate V of the liquid
from these in the normal manner.
FIG. 2 indicates diagrammatically how said shear mode Lamb waves
at the surface of the outside and inside of the pipe wall propagate
at a velocity V.sub.b. At the inside of the pipe wall, longitudinal
or compression pressure waves are then generated in the liquid by
said Lamb waves. Said longitudinal waves form well defined rays
which propagate through the liquid at an angle of (.pi./2-.alpha.)
with respect to the normal to the pipe wall. When said longitudinal
waves strike a part of the tube wall, these liquid pressure waves
are converted again into Lamb surface waves and then received by
the transducer 2.
The transmitting-receiving transducers 1 2 are, according to the
invention, constructed from a number of subtransducers arranged
in the form of a row. In contrast to the known embodiments, said
subtransducers are in turn sited by means of their emission faces
on the base of the transducer housing on the tube wall. The adjacent
subtransducers are activated by control signals which differ in
phase. The wavelength of the Lamb surface wave to be generated in
the tube wall is determined by the phase relationship of the control
signals and also the spatial distance between successive subtransducers
or elements.
In the simplest form, for example, the subtransducers or elements
can be fitted separately in a row next to each other and are activated
by control signals which successively have a phase difference of
180.degree.. In this connection the control signals may alternately
differ in phase by 180.degree. or the elements may alternately be
activated by a fixed voltage and by a signal of particular amplitude.
It is also possible to polarize the separate elements alternately
in opposite directions and to activate them all with the same signal.
The mutual distance of successive centres of the subtransducers
in this case corresponds to half the wavelength of the Lamb surface
waves in the A.sub.o mode to be generated in the pipe wall.
In a further embodiment, it is possible to construct the transducer
not from separate elements but from one whole element, a pattern
of electrodes on both sides or on one side of said element being
sufficient to cause the whole unit to react as separate transmitting-receiving
subtransducers.
FIG. 3 indicates this embodiment according to the invention in
which the subtransducers are formed by making cross cuts in one
electrode 7 at a mutual distance of .lambda./2. Here .lambda. is
the wavelength of the Lamb surface waves in the A.sub.o mode to
be generated in the pipe wall. The cross cuts in the top electrode
7 divide the piezoelectrode into a number of imaginary subelements
or substransducers. A square-wave voltage is applied across the
subtransducers as is indicated in FIG. 4. The piezoelement then
deforms approximately sinusoidally as indicated in FIG. 3 with a
periodic length corresponding to the wavelength .lambda. of the
Lamb waves in the A.sub.o mode.
It is also possible to provide the subtransducers or the the said
cross cuts at a distance from each other other than the said .lambda./2.
By matching the phase difference between the control signals to
this and by adjusting it to a value other than 180.degree., the
required Lamb surface waves in the A.sub.o mode can again be generated.
A very cheap and simple flow meter is ultimately obtained by means
of all these above-mentioned embodiments of the transducer.
As a result of this siting of the transducers with their emission
faces directly on the base of the transducer housing and then directly
on the pipeline and by matching the distance between the subtransducers,
it is possible to generate and receive in a selective manner Lamb
surface waves in the A.sub.o mode in the tube wall. With respect
to the known transducers used in industry, these ultrasonic waves
are generated with very low energy in the transmitting transducer
and they are also received in an unambiguous manner by the receiving
transducer. The amount of noise and parasitic waves is extremely
low. As a result, the energy ratio between the transmitted electric
pulse and the received electric pulse is large, the electrical energy
at the same time being converted in an efficient manner into mechanical
power and vice versa.
Because surface waves are produced at the receiving side over a
relatively large region on the outside surface of the pipe wall,
the definition of the position of the receiving transducer is not
critical. After installing and securely clamping the transducers,
the fixed transit times of the sonic signal in the transducer and
in the solid walls are allowed for electronically in the control
unit by the adjustment of dead times.
The ultrasonic flow meter according to the invention can be used
on pipelines of different material. Matching to said material takes
place by altering the mutual distance between the cross cuts in
the electrode of the transducer.
Reference is now made to FIG. 5 in which a block diagram of the
said electronic control unit is indicated. The pulse generator 12
is controlled from the central clock circuit or microprocessor 10
so as to alternately energize the upstream transducer 1 and the
downstream transducer 2. At the same time, the transmitting-receiving
switch 13 is energized from the microprocessor. After switching
over the transmitting-receiving switch 13 the pulses received are
fed to the amplifier 14 and then to the time measuring circuit 15.
In the microprocessor 10 which is equipped with a memory 11 the
determination of the flow rate is then carried out on the basis
of the data fed in by the input unit 16 such as dead time, pipe
material, pipe diameter, etc. An indication of the flow rate is
then delivered via the output circuit 17 to the output 18. |