Abstrict A Karman vortex flow meter comprises a vortex generating post 4
disposed in a fluid conduit 1 a plurality of ultrasonic wave transmitters
6a, 6b provided in a wall of the conduit downstream of the post,
and an ultrasonic wave receiver 8 provided in a wall of the conduit
opposing the transmitters. The transmitters are either juxtaposed
at a horizontal sectional face of the conduit and generated ultrasonic
waves simultaneously toward the receiver, or they are provided in
series in an axial direction of the conduit while the receiver is
provided opposite the transmitter on the downstream side. In the
latter embodiment the transmitters are connected to a change-over
circuit which switches from the downstream transmitter to the upstream
one when the detected frequency of the Karman vortex streets in
the fluid exceeds a predetermined value.
Claims What is claimed is;
1. A Karman vortex flow meter, comprising: a vortex generating
post disposed in an elongate conduit, in which fluid to be measured
flows, perpendicularly to a flow of the fluid, a plurality of ultrasonic
wave transmitters provided on a wall of said conduit on a downstream
side of said vortex generating post in orientations perpendicular
to the flow of the fluid, and an ultrasonic wave receiver provided
on said wall of said conduit opposing said transmitters, wherein
said transmitters are disposed in a juxtaposed relationship on the
wall of said conduit at a cross-sectional face thereof perpendicular
to an axis of the conduit and are connected to an oscillating circuit
device for causing said transmitters to generate ultrasonic waves
simultaneously toward said receiver, and said receiver is connected
to a receiving circuit device for detecting a generation frequency
of Karman vortex streets generated by said vortex generating post.
2. A Karman vortex flow meter as claimed in claim 1 wherein said
ultrasonic wave receiver is provided at a location spaced downstream
by a predetermined distance from said ultrasonic wave transmitters.
3. A Karman vortex flow meter, comprising: a vortex generating
post disposed in an elongate conduit, in which fluid to be measured
flows, perpendicularly to a flow of the fluid, a plurality of ultrasonic
wave transmitters provided on a wall of said conduit on a downstream
side of said vortex generating post in orientations perpendicular
to the flow of the fluid, and an ultrasonic wave receiver provided
on said wall of said conduit opposing said transmitters, wherein
said transmitters are disposed in series upstream and downstream
in an axial direction of said conduit and are connected to a change-over
circuit for alternatively rendering said transmitters effective.
4. A Karman vortex flow meter as claimed in claim 3 wherein said
ultrasonic wave receiver is provided in an opposing relationship
to the downstream transmitter.
5. A Karman vortex flow meter as claimed in claim 3 wherein said
transmitters are alternatively rendered effective automatically
when the generation frequency of Karman vortex streets of the fluid
becomes higher than a predetermined value.
6. A Karman vortex flow meter as claimed in any one of the preceding
claims, wherein the fluid is gas.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a Karman vortex flow meter which detects
a generation frequency of Karman vortex streets with an ultrasonic
wave to measure a flow velocity or flow rate of fluid flowing in
a conduit.
2. Description of the Prior Art
Conventionally, a Karman vortex flow meter of the type mentioned
is disclosed, for example, in Japanese Utility Model Publication
No. 48-17010 Japanese Patent Publication No. 55-11206 or Japanese
Patent Publication No. 56-34046. Those devices are each constituted
such that an ultrasonic wave generated by an ultrasonic wave transmitter
is received by a receiver.
It is proposed in Japanese Patent Publication No. 56-34046 to dispose
a transmitter and a receiver at displaced locations side taking
into consideration that an ultrasonic wave which is emitted from
the transmitter and propagated across fluid flowing in the conduit
is deflected to the downstream side due to a flow velocity of the
fluid.
Since the conventional Karman vortex flow meters have such a construction
as described above, if the receiver is disposed, for example, at
a position at which it can receive an ultrasonic wave propagating
thereto through a flow with its highest sensitivity when the flow
velocity of fluid flowing in the conduit is comparatively low, then
when the flow of the fluid becomes high, an ultrasonic wave which
is deflected by the flow cannot be received with a high sensitivity.
Therefore, it is a common practice that the output power of an
ultrasonic wave of the transmitter is enhanced or that the location
of the receiver is modified so as to facilitate reception of the
ultrasonic wave.
However, when the first method is taken, in other words, if a voltage
higher than a rated value is applied to the transmitter to increase
the output power of an ultrasonic wave, then the transmitter will
sometimes suffer from deterioration in characteristic or from failure.
Further, since a power supply voltage (terminal voltage of a battery)
is normally comparatively low in such a case as the Karman vortex
flow meter applied to detect an intake air amount of an engine for
an automobile, the voltage to be applied to the transmitter cannot
readily be raised to a level higher than the power supply voltage.
If the second method is taken, in other words, if the receiver is
disposed at a position where the reception sensitivity thereof is
in the highest condition when the flow velocity is high, then when
the flow velocity becomes low, the reception sensitivity is deteriorated.
SUMMARY OF THE INVENTION
The present invention has been made to eliminate such subjects
as described above, and it is therefore an object of the present
invention to provide a Karman vortex flow meter which can increase
an ultrasonic wave to be generated without applying an excessively
high voltage to a transmitter even if a power supply voltage itself
is low. It is also another object of the present invention to provide
the flow meter which can receive an ultrasonic wave from a transmitter
with a high sensitivity irrespective of the flow velocity of fluid.
In a first aspect of the present invention, a Karman vortex flow
meter comprises a vortex generating post disposed in a conduit,
in which fluid to be measured flows, perpendicularly to a flow of
the fluid, a plurality of ultrasonic wave transmitters provided
on a wall of the conduit on the downstream side of the vortex generating
post for generating ultrasonic waves simultaneously, and an ultrasonic
wave receiver provided at a portion of the wall of the conduit opposing
the transmitters. Since the plurality of ultrasonic wave transmitters
transmit ultrasonic waves of the same frequency simultaneously toward
the receiver, the ultrasonic waves to be transmitted toward the
receiver can be increased as a combined intense ultrasonic wave
without applying an excessively high voltage to the transmitters,
whereby the flow meter can accomplish detection of Karman vortex
streets even in a flow of a high velocity in such a condition as
being wide in dynamic range and high in reliability.
In a second aspect of the present invention, a Karman vortex flow
meter comprises a vortex generating post disposed in a conduit,
in which fluid to be measured flows, perpendicularly to a flow of
the fluid, a first ultrasonic wave transmitter provided on a wall
of the conduit on the downstream side of the vortex generating post,
a second ultrasonic wave transmitter provided on the wall of the
conduit downstream of the first transmitter, an ultrasonic wave
receiver provided on the wall of the conduit opposing the second
transmitter, and a change-over means for changing over the effective
ultrasonic wave transmitter between the first and second ultrasonic
wave transmitters in response to a velocity of the fluid in the
conduit. Since the transmitter on the downstream side is rendered
effective by the change-over means when the flow velocity of the
fluid is low, whereas the transmitter on the upstream side is rendered
effective when the flow velocity becomes higher than a predetermined
flow velocity, an ultrasonic wave transmitted from a transmitter
can be received with a high sensitivity by the receiver, and accordingly,
an ultrasonic wave generated from a transmitter can be received
with a high sensitivity by the receiver irrespective of the flow
velocity of the fluid in the conduit, and consequently, the Karman
vortex flow meter is high in reliability.
The above and other objects, features and advantages of the invention
will become apparent from the following description and appended
claims, taken in conjunction with the accompanying drawings which
show a preferred embodiment of the invention by way of example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a horizontal sectional view of a Karman vortex flow meter
according to an embodiment of the present invention;
FIG. 2 is a vertical sectional view taken along line II--II of
FIG. 1;
FIG. 3 is a horizontal sectional view of another embodiment; and
FIG. 4 is constructional view of an entire Karman vortex flow meter
according to another embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, embodiments of the present invention will be
described with reference to the drawings. FIG. 1 is a horizontal
sectional view of a Karman vortex flow meter of an embodiment in
accordance with the first aspect of the present invention, and FIG.
2 is a vertical sectional view taken along line II--II of FIG. 1.
In those figures, reference numeral 1 denotes a conduit which is
formed as a unitary member by plastic molding and in which fluid
to be measured flows, and a sound absorbing material 2 for absorbing
an ultrasonic wave is formed on an inner face of a wall of the conduit
1. Reference numeral 3 denotes a rectifier disposed at an upstream
portion of the conduit 1 for rectifying and introducing fluid into
the conduit 1 4 is a vortex generating post disposed perpendicularly
to a flow of fluid in the conduit 1 and 5 is a Karman vortex street
generated by the vortex generating post 4. Reference characters
6a and 6b denote two ultrasonic wave transmitters disposed on the
downstream side of the Karman vortex street generating post 4 in
a perpendicular direction to a flow of fluid, and the ultrasonic
wave transmitters 6a and 6b are accommodated in accommodating holes
7a and 7b formed in the wall of the conduit 1. Reference numeral
8 denotes an ultrasonic wave receiver opposed in an equally spaced
relationship to the transmitters 6a and 6b for receiving ultrasonic
waves from the transmitters, and ultrasonic wave receiver 8 is accommodated
in an accommodating hole 9 formed in the wall of the conduit 1.
Reference numeral 10 denotes a cap for the accommodating holes 7a
and 7b, and 11 is a cap for the accommodating hole 9. Reference
numeral 12 denotes an oscillating circuit device for driving the
transmitters 6a and 6b simultaneously, and 13 is a receiving circuit
device for amplifying a received signal of the receiver 8 and detecting
a generation frequency of Karman vortex streets.
Subsequently, operation will be described. In order to detect a
generation frequency of Karman vortex streets 5 generated in a flow
of fluid in the conduit 1 ultrasonic waves are generated simultaneously
from the transmitters 6a and 6b into the Karman vortex streets.
Those ultrasonic waves propagate across the flow while being modulated
by the Karman vortex streets and are thus received by the receiver
8. Such ultrasonic waves from the transmitters 6a and 6b propagate
with some expansion, and when the velocity of the flow is low, the
ultrasonic waves are little deflected by the flow, but if the flow
becomes high in velocity, then the ultrasonic wave will be deflected
to the downstream side and simultaneously be modulated excessively
by the Karman vortex streets which are intensified corresponding
to the flow velocity. If such condition is reached, in case the
ultrasonic waves received are weak, then the reception becomes difficult.
In the above structure according to the present invention, since
ultrasonic waves are transmitted simultaneously from the two transmitters
6a and 6b, an intense ultrasonic wave is obtained, and consequently,
even if the flow velocity rises, reception of an ultrasonic wave
is still easy, and stabilized detection of Karman vortex streets
can be accomplished.
FIG. 3 shows a horizontal sectional view of a Karman vortex flow
meter of another embodiment in accordance with the first aspect
of the present invention wherein the position of the receiver 8
is disposed on the downstream side of the transmitters 6a and 6b.
With the Karman vortex flow meter, as the receiver 8 is disposed
on the downstream side by a distance from the transmitters 6a and
6b taking it into consideration that an ultrasonic wave which propagates
across a flow in the conduit 1 is deflected to the downstream side
due to the flow velocity of the fluid, detection of Karman vortex
streets in a flow of a higher flow velocity than that in the preceding
embodiment is enabled.
FIG. 4 shows a constructional view of an entire Karman vortex flow
meter in accordance with the second aspect of the present invention.
In the figure, reference character 1' denotes a conduit in which
fluid to be measured flows, 4' is a vortex generating post disposed
substantially perpendicularly to fluid flowing in the conduit 1',
6a' is a first ultrasonic wave transmitter provided on a wall of
the conduit 1' for generating an ultrasonic wave into Karman vortex
streets generated on the downstream side of the vortex generating
post 4', 6b' is a second ultrasonic wave transmitter disposed at
a portion of the wall of the conduit on the downstream side of the
first ultrasonic wave transmitter, and 8' is an ultrasonic wave
receiver disposed on the wall of the conduit 1 opposing the transmitter
6b'. Reference numeral 14 denotes a detecting circuit device, which
is constituted from an oscillator 15 for exciting the first and
second ultrasonic wave transmitters 6a' and 6b', a phase shifter
16 a phase comparator 17 a band pass filter 18 a low pass filter
19 and amplifiers 20 and 21. Reference numeral 22 denotes an output
terminal of the detecting circuit device 14 and a vortex signal
output is obtained from the output terminal 22. Reference numeral
23 denotes a change-over means for the two ultrasonic wave transmitters
6a' and 6b', and the change-over means 23 can change over the effective
ultrasonic wave transmitter such that, when the frequency of the
vortex signal output of the output terminal 22 is low, the second
ultrasonic wave transmitter 6b' on the downstream side may be excited
by the oscillator 15 but if the frequency of the vortex signal
output becomes higher than a predetermined level, then the first
ultrasonic wave transmitter 6a' on the upstream side may be excited
by the oscillator 15.
Subsequently, operation of the present invention will be described.
When the flow velocity of fluid flowing in the conduit 1' is low,
the second ultrasonic wave transmitter 6b' is rendered effective
by the change-over means 23 and an ultrasonic wave generated from
the transmitter 6b' propagates in the fluid while being deflected
little by the flow velocity and is thus received by the receiver
8' on the opposing side. However, if the flow velocity becomes high,
an ultrasonic wave propagating in the fluid is deflected so that
it becomes difficult to receive the ultrasonic wave by the receiver
8'. Therefore, the first ultrasonic wave transmitter 6a' on the
upstream side which is so disposed taking it into consideration
that an ultrasonic wave is deflected is rendered effective to enable
reception by the receiver 8'. In particular, since the generation
frequency of Karman vortex streets generated on the downstream side
of the vortex generating post 4' increases in proportion to the
flow velocity of the fluid to be measured, if the vortex signal
output frequency (Karman vortex street generation frequency) obtained
from the output terminal 22 becomes higher than a predetermined
value, then the effective ultrasonic wave transmitter is changed
over from the second ultrasonic wave transmitter 6b' to the first
ultrasonic wave transmitter 6a' by the change-over means 23.
Accordingly, an ultrasonic wave generated from a transmitter can
be received with a high sensitivity by the receiver irrespective
of the flow velocity of the fluid in the conduit, and consequently,
the Karman vortex flow meter is high in reliability. |