JPS6257205B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention displaces a vibrating member by pressure fluctuations of a Karman vortex generated on both downstream sides of a columnar object inserted into a fluid, and detects a vortex frequency from the vibration frequency of the vibrating member. Regarding the Karma Vortex Flowmeter.

[Conventional technology]

In general, this type of flowmeter has the advantage of a simple structure because it directly detects pressure changes in the vortex as displacement or force. It is important to improve sensitivity. Among conventional devices of this type, one with relatively good sensitivity is a device in which the vibrating member is formed of a plate-like member made of a light resin or the like, and the vibrating member is held rotatably around a rotating shaft. This device has the drawback that the plate-shaped member is displaced in proportion to the pressure of the vortex, so if the pressure of the vortex increases in a high region, excessive displacement or force will be applied, making it impossible to accurately detect the vortex or causing damage. . In addition, for example, a device that detects strain by bending a plate-like member is
There is a device shown in 36933.

[Problems to be solved by the invention]

Although this conventional device can stably detect vortices even in high flow velocity regions, it has the disadvantage that it cannot detect vortices in low flow velocity regions because it uses bending or flexural vibration of the plate. An object of the present invention is to eliminate the drawbacks of these conventional devices and provide a Karman vortex flowmeter that can stably detect vortices in a wide flow velocity range.

[Means to solve the problem]

In order to achieve such an objective, the present invention
The vibrating member that responds to pressure changes in the vortex is formed into a substantially first-order vibration system, and the first-order natural frequency of the vibrating member is approximately equal to the lowest frequency of the vortex to be detected. .

[Effect]

As a result of various studies and experiments conducted by the present inventors, it was discovered for the first time that the magnitude of pressure change in a vortex increases approximately in proportion to the square of the flow velocity. Therefore, based on this experimental result that the magnitude of the pressure change in the vortex increases approximately in proportion to the square of the flow velocity, the present invention transforms the vibrating member that responds to the pressure change in the vortex into an approximately first-order vibration system. The vortex is formed by a vibrating member whose primary natural frequency is approximately equal to the lowest frequency of the vortex to be detected. In other words, when an excitation force with a constant amplitude is applied, the frequency response of the primary vibration system reaches a maximum at approximately the resonant angular frequency _ωo , and increases to the square of the excitation angular frequency ω as the excitation angular frequency ω increases. decreases inversely. What is important here is that, as described above, through experiments conducted by the present inventors, it has been found that the pressure change in the vortex increases approximately in proportion to the square of the flow velocity. In other words, since the vortex frequency is proportional to the flow velocity, it can be assumed that the vortex pressure change increases approximately in proportion to the square of the vortex frequency. Therefore, by configuring the vibrating member that responds to the pressure change of the vortex by a vibrating member that is formed into an approximately first-order vibration system and whose primary natural frequency is approximately equal to the lowest frequency of the vortex to be detected. , due to the combination of the amplitude characteristic of the vibrating member, which decreases in inverse proportion to the square of the excitation angular frequency ω, and the excitation force (vortex pressure change), which increases approximately in proportion to the square of the vortex frequency. The amplitude of the vibrating member can be substantially constant.

【Example】

An embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is an overall configuration diagram showing an embodiment of the present invention, Fig. 2 is a sectional view taken along line A-A of the vortex generator, Fig. 3 is an enlarged cross-sectional view of the vortex detection section, and Fig. 4 is a plane view of the vibrator. FIG. 5 is a diagram showing the operating characteristics of the vibrator. In FIG. 1, 1 is a pipe, 2 is a pair of vortex generators for generating Karman vortices, and 3 is a vortex detection mechanism. Numerals 41 and 42 are slits provided on both sides of the vortex generator 2 for guiding pressure changes of the vortex. In FIG. 4, 5 is a vibrator, which includes a diaphragm 6 on which the pressure of the vortex acts, and a pair of span bands for holding the diaphragm 6 on a line-symmetrical axis that includes its center of gravity and causing torsional vibration. 71, 72 and a frame portion 8 serving as a fixed end of the span band are made of a single metal plate, and the mass of the diaphragm 6 is kept balanced with respect to its central axis. In addition, 9
1 and 92 are punched parts. In Figure 3,
A housing 10 houses the vibrator 5, and is composed of a lower plate 11 and an upper plate 12. The lower plate 11 and the upper plate 12 are provided with grooves (not shown) facing each other in a shape corresponding to the shape of the vibrator 5. The vibrator 5 is held, and a vibration chamber 14 and a storage chamber 15 for a span band are formed. Vibration chamber 1
4 is roughly divided into two chambers, an upper chamber 16 and lower chambers 171, 172, by the diaphragm 6 of the vibrator 5, and furthermore, the chamber formed by the diaphragm 6 and the lower plate 11 is divided into two by the protrusion 18. Yotsute room 171, 172
It is divided into two halves, with rooms 171 and 172
communicate with the openings 41 and 42 of the vortex generator 2 via pressure-conducting holes 191 and 192, respectively.
The purpose of the protrusion 18 is to transmit the pressure change of the vortex from the opening 41 or 42 to the diaphragm 6 without loss, and the gap between the protrusion 18 and the diaphragm 6 is made as small as possible, about 0.1 mm. Furthermore, for the same purpose, the gap between the periphery of the diaphragm 6 and the vibration chamber 14 is also kept to the same extent. Further, a substantially constant tension is applied to the span bands 71 and 72. Note that 20 is the diaphragm 6
21 is a processing circuit for a displacement detection signal. In the above configuration, now the opening 4 of the vortex generator 2
When the vortex 22 is generated on the 2 side, the pressure in this area is lower than that in the opening 41 on the opposite side.
A pressure difference occurs between chambers 1 and 42, and a pressure difference occurs between chambers 171 and 172 that communicate with the chambers 171 and 172.
This pressure difference causes the diaphragm 6 to rotate clockwise. Next, when a vortex is generated on the opposite side of the vortex generation 2,
The diaphragm 6 now rotates counterclockwise. That is, the diaphragm 6 vibrates in one reciprocating motion in response to the generation of a pair of vortices. In this way, the vibrator 5 vibrates in accordance with the frequency of the vortex, but in the present invention, the natural frequency of torsion determined by the moment of inertia of the diaphragm 6 and the torsional spring constants of the span bands 71 and 72 is The lengths and widths of the bands 71 and 72 and the dimensions of the diaphragm 6 are made to approximately match the lowest frequency of the vortex to be measured. When an excitation force with a constant amplitude is applied to such a primary vibration system, the frequency response will be approximately at the resonance angle, although it depends on the size of the damper, as shown in Figure 5A, which is well known from vibrational theory. It reaches a maximum at frequency ω _o ,
It has a characteristic that as the excitation angular frequency ω increases, it decreases in inverse proportion to the square of the excitation angular frequency. Here, in the figure, the vertical axis is the amplitude ratio (dynamic amplitude x/static amplitude a _s
t ), and the horizontal axis is dimensionless and expressed as the ratio between the angular frequencies ω and ω _o . On the other hand, through experiments conducted by the present inventors, it was found that the magnitude of the pressure change in the vortex increases approximately in proportion to the square of the flow velocity. By the way, the vortex frequency is proportional to the flow velocity. Therefore, as shown in FIG. 5B, it can be said that the pressure change of the vortex increases approximately in proportion to the square of the vortex frequency. In this case, in the present invention, since the natural angular frequency ω _o of the vibrator 5 is made to match the lowest frequency of the vortex, the vibrator 5 receives an excitation force that increases in proportion to the square of the vortex frequency. As a result, the amplitude of the diaphragm 6 of the vibrator 5 becomes approximately constant as shown in FIG. 5C. According to the present invention, by lowering the resonant frequency of the vibrator and matching the characteristics of the pressure change of the vortex, a large displacement can be obtained in the low flow rate region (low frequency region), and a large displacement can be obtained in the low flow region (low frequency region). area)
This has the advantage of preventing excessive displacement. Note that it is desirable to add appropriate damping to this vibration system in order to eliminate instability during resonance. Further, the configuration of the vibrator is not limited to this embodiment, and the point is that the first-order natural frequency may be made to match the lowest frequency of the vortex.

【Effect of the invention】

As described above, according to the present invention, the plate-shaped member that is displaced by the pressure of the vortex is configured into a substantially first-order vibration system, and the first-order natural frequency is lowered to match the lowest frequency of the vortex. Since the frequency response of the vibration system and the pressure change of the vortex are complementary to each other, a nearly constant displacement amplitude can be obtained over the entire vortex frequency range, and the sensitivity is improved in the low region. It is practically effective because it does not cause excessive displacement even in high flow areas, provides stable vibration, and does not cause damage.

[Brief explanation of the drawing]

Fig. 1 is an overall configuration diagram of an embodiment of the present invention, Fig. 2
The figures are a cross-sectional view taken along the line AA in FIG. 1, FIG. 3 is an enlarged cross-sectional view of the vortex detection section, FIG. 4 is a plan view of the vibrator, and FIG. 5 is a diagram of the operating characteristics of the vibrator. 5... Vortex generator, 5... Vibrator, 6... Vibration plate.

Claims (1)

[Claims] 1. A vibrating member that vibrates in response to pressure fluctuations occurring alternately near both sides of a Karman vortex generator inserted into a fluid flow, and the vibration frequency of the vibrating member is used to determine the flow of the fluid. In a Karman vortex flowmeter for measuring flow rate, the vibrating member is formed into a substantially first-order vibration system, and the first-order natural frequency thereof is substantially equal to the lowest frequency of the vortex to be detected. A Karman vortex flowmeter characterized by: