JP2769990B2 - Flow measurement method and flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow meter for measuring the flow velocity of gas or liquid in a pipeline using Karman vortex.

[0002]

2. Description of the Related Art In principle, various types of flow meters for measuring the flow rate in a pipeline are used, and a throttle flow meter, a pitot tube flow meter, and an area type flow meter for measuring a differential pressure before and after a throttle are used. Total
An electromagnetic flow meter, an ultrasonic flow meter and the like can be mentioned, and a relatively new one is a vortex flow meter. This utilizes a phenomenon in which, when an object 1 such as a cylinder or a prism is placed in a flow, two rows of vortices 2 are alternately discharged downstream as shown in FIG. This vortex is called Karman vortex, and the velocity of the vortex is measured by the fact that the frequency of the vortex is proportional to the flow velocity.

That is, if the vortex emission frequency, which is the number of vortices emitted per unit time, is f, the flow velocity is v, and the diameter of a cylinder inserted in the flow is d, the following equation is established. f = Sv / d

Here, S is a dimensionless constant called the Strouhal number, which is about 0.2 for a cylinder. Karman vortices are generated for any shape that is not streamlined. For example, if the cross section is a square prism, d
Is the length of one side, S is about 0.16. The value of the Strouhal number is substantially constant over a wide range of the kinematic viscosity coefficient of the fluid, and the above relationship holds regardless of whether the fluid is a liquid or a gas.

In this method of detecting the emission frequency of the Karman vortex, it can be considered that the wake of the column is meandering, and the force caused by the meandering may be measured. Therefore, a resistive strain sensor, a capacitive displacement sensor, a piezoelectric sensor and the like are actually used. This vortex flowmeter has no particular defect except for the lower limit of the flow rate that can be measured from the aspect of vortex stability, and has high durability because there is no movable part mechanically.
Due to its advantages such as being unaffected by the temperature, pressure, composition, etc. of the fluid, its application is expanding.

Japanese Patent Application Laid-Open No. 5-180673 discloses a vortex flowmeter as means for measuring the vortex emission frequency.
One using an optical fiber is disclosed. In this method, a polarization-maintaining optical fiber is provided in a pipe, light of two orthogonal polarization modes is incident on the optical fiber, and the frequency at which the phase difference between the two optical modes changes is detected. This utilizes a phenomenon in which the refractive index changes when the optical fiber is bent and deformed by eddy currents, and the phase difference between these two polarization mode lights changes in orthogonal two polarization mode lights.

The polarization-maintaining optical fiber is formed by deforming the core or clad into an elliptical shape by applying a method such as drawing off both side surfaces in the course of the manufacturing process and then drawing, or imparting a strain in the diameter direction by other methods. This is a special optical fiber that allows transmission while maintaining the polarization plane of the incident light. As a specific configuration of a flow meter utilizing this, a laser oscillator such as a HeNe laser is used as a light emitting side, a quarter wavelength number for converting linearly polarized light into circularly polarized light, and an analyzer (Wollaston prism) is used as a light receiving side. ),
It is composed of a photodetector such as a photodiode, and a spectrum analyzer that performs frequency analysis is provided as a device for processing a detection signal.

[0008]

The method of detecting Karman vortex with the above-mentioned optical fiber is different from the method of using a resistive strain sensor, a capacitive displacement sensor, a piezoelectric sensor, or the like. It has the advantage that it does not need to be provided in the pipeline. Therefore, there is an advantage that an explosion-proof specification for measuring a flammable fluid can be easily achieved. However, the method described in the above-mentioned Japanese Patent Application Laid-Open No. 5-180673 uses a special optical component, a spectrum analyzer, and the like, and therefore requires a large-scale apparatus and extremely high cost. Accordingly, an object of the present invention is to provide an inexpensive optical fiber while using the optical fiber for detecting Karman vortices while maintaining the above-mentioned advantages for safety.

[0009]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems. In a flow rate measuring method for generating a Karman vortex in a pipe and measuring the flow rate based on the number of vortex discharges, the present invention relates to a method for measuring a flow rate of a vortex. By arranging an optical fiber therein, passing the intermittent light having a frequency higher than the vortex emission frequency to the optical fiber, and detecting a phenomenon in which the width of the light pulse passing through the optical fiber is reduced by the force applied to the optical fiber by the vortex, A flow measurement method characterized in that the number of vortex shedding is measured. The optical fiber is also characterized in that it is a plastic optical fiber.

Also, a Karman vortex generator arranged in a conduit, an optical fiber disposed downstream of the Karman vortex generator, both ends of which are outside the conduit, and an optical fiber connected to one end of the optical fiber and having a fixed width. A pulse light, a light radiator, a light detector connected to the other end, an arithmetic circuit that generates an output corresponding to the pulse width of a signal from the light detector, and a frequency of an output fluctuation of the arithmetic circuit. And a frequency measuring circuit for measuring the flow rate. In addition, the arithmetic circuit that generates an output corresponding to the pulse width of the signal from the photodetector according to the time difference between the signal corresponding to the light incident on the optical fiber and the detection signal of the light from the photodetector. It is also characterized by a circuit for obtaining a signal and a circuit for integrating this signal.

[0011]

FIG. 1 is a view showing a method of the present invention, in which a Karman vortex generator 4 is placed in a conduit 3 and an optical fiber 5 is placed downstream thereof.
And both ends communicate with the outside of the pipeline. A light radiator 6 composed of a light emitting diode or the like is coupled to one end of the light radiator 6 and a photodetector 7 composed of a phototransistor or the like is coupled to the other end. Then, the light radiator 6 generates intermittent light as shown in FIG. The frequency (the number of pulses per unit time) is, for example, several tens of kHz. Then, in the photodetector 7 on the light receiving side, a phenomenon is observed in which the width of the detected light pulse is reduced according to the force applied to the optical fiber. In the present invention, utilizing this phenomenon, it is detected that the width of the light pulse that has passed through the optical fiber changes periodically according to the emission frequency of the Karman vortex,
From this, the vortex emission frequency is extracted by signal processing.

The decrease in the width of the light pulse that has passed through the optical fiber is caused by the delay of the rise of the pulse by R as shown in FIG. This phenomenon has not been published so far and is the phenomenon discovered by the present inventors for the first time, but the theoretical basis is unknown at present. In this phenomenon, only the rise of the pulse passing through the optical fiber is delayed, and the fall does not change, and the optical pulse is not delayed with a time lag. For this reason, if the width of the light pulse of the light radiator is gradually reduced while a force is applied to the optical fiber, only the rise is delayed, so that no light passes through the optical fiber.

It is known that the bending of an optical fiber causes an increase in light attenuation. This is because the condition of light reflection between the core and the cladding of the optical fiber changes, and the numerical aperture substantially decreases. To do that. The phenomenon used in the present invention is different from this. Unlike the method of measuring the light intensity, the method of the present invention is hardly affected by other factors such as temperature conditions, and can reliably detect with sufficient sensitivity even if the length of the optical fiber itself is short.

Optical fibers include quartz, multi-component glass, and plastic materials as materials, and the plastic material is suitable for the flow meter of the present invention. Plastic optical fiber is made of high refractive index plastic such as polymethyl methacrylate for the core and low refractive index plastic such as fluororesin for the cladding, and the wire thickness is about 0.5mm to 2.0mm. Are manufactured for optical sensors and for transmitting optical signals in equipment. The light propagates in multiple modes, and has a large diameter and a large numerical aperture of about 0.5, so that light can be easily transmitted using a light emitting diode as a light source.

The vortex emission frequency f is, for example, the flow velocity v
= 2 m / S, the diameter of the cylinder d = 10 mm = 0.01 m, and the number of Strouhals S = 0.2, f = 40 Hz,
Since the intermittent frequency (the number of pulses / second) of the light incident on the optical fiber is, for example, several kHz or more, the vortex emission frequency can be measured with sufficient accuracy by changing the width of each pulse. Usually, a rectangular wave, that is, a pulse having a duty ratio of 50%, is easy to manufacture the oscillator.

FIG. 4 shows the waveform of an electric signal or light at each stage in the measuring method of the present invention. FIG. 4A shows the pulse waveform of light incident on the optical fiber. On the other hand, FIG. 2B shows an optical signal detected at the other end of the optical fiber, where a delay R occurs at the rise of the pulse according to the force applied to the optical fiber, and the pulse width is reduced. Therefore, a signal corresponding to the vortex emission frequency can be obtained by measuring this pulse width and putting it into an arithmetic circuit that generates an output corresponding to the change. For example, FIG.
If the output of (b) is made to have a constant amplitude, an output corresponding to the change in the area of the pulse wave, that is, a change in the pulse width can be taken out by entering the integration circuit. However, even if the change in the delay R of the rise of the pulse is small, the change can be captured with high sensitivity by temporarily converting the pulse into a pulse corresponding to the delay R itself. This involves generating a signal corresponding to the light incident on the optical fiber, that is, a signal corresponding to the time difference between the signal of FIG. 4A and the detection signal of the light from the photodetector, ie, the signal of FIG. It should be done. (C) is a waveform obtained in this way, and if this is put into an integrating circuit, an output corresponding to a change in pulse width as shown in (d) is obtained.

The waveform of FIG. 4D has a value whose frequency is proportional to the discharge frequency of the vortex, that is, the flow rate. In order to measure the frequency, the frequency may be converted into a pulse corresponding to the frequency, and the number of pulses per fixed time may be measured by a gate circuit. The accumulated flow rate can be measured by accumulating the pulse number itself. In order to convert the waveform of FIG. 4D into a pulse having an appropriate width, a constant output may be produced when the input is larger than a certain threshold value A as shown in FIG. 4E.

[0018]

EXAMPLE The method of the present invention was used to measure the flow rate of a 100 mm diameter tube. As shown in FIG. 1, a cylinder is provided as a Karman vortex generator 4 in a tube 3 and an optical fiber 5 is provided downstream thereof.
Were provided in parallel. The optical fiber used was a multi-mode plastic having a diameter of 1.0 mm. GaAlA which emits infrared light having a wavelength of 890 nm as the light radiator 6
An s-based light emitting diode was used. On the other hand, as the photodetector 7, a photo IC in which a photodiode, an amplifier circuit, and the like were integrated on one chip and housed in a package was used.

FIG. 5 shows an example of a circuit for detecting a vortex emission frequency. The light emitting diode 12 is caused to emit light by a pulse output from the oscillator 11 as shown in FIG. The light that has passed through the optical fiber 5 is a photo IC 13
And generates a signal in which the output voltage appears only when detecting the light as shown in FIG.

On the other hand, the signal from the oscillator 11 passes through the inverter 14 and is converted into a signal in which a voltage appears when the light emitting diode is not emitting light, that is, a signal in which the waveform of FIG. The signal is input to the NOR circuit 15 together with the signal from the photo IC 13. As a result, a signal corresponding to a time when these two inputs are not present, that is, a signal shown in FIG. 3C corresponding to a delay in pulse rising due to passage through the optical fiber is obtained.

The signal from the NOR circuit passes through the integration circuit 16 to obtain a voltage waveform corresponding to the periodic fluctuation of the pulse width as shown in FIG. This output is passed through a peak hold circuit 17 and divided by a voltage divider 18 to a value such as 90% of the peak value, and is input to a comparator circuit 19 together with the direct output of the NOR circuit. 4 corresponding to the time exceeding the output voltage from the voltage divider 18.
A pulse output as shown in (e) is obtained. That is, the role of the peak hold circuit 17 is to obtain a pulse of an appropriate width by comparing the output voltage from the integration circuit 16 with the peak value. Voltage divider 18
Is larger, that is, as the output voltage of the peak hold circuit 17 becomes closer to itself, the output pulse width from the comparator circuit 19 becomes smaller. The peak hold circuit is reset at regular intervals so as to deal with fluctuations in the peak value itself.

The output of the comparator circuit 19 is input to a frequency measurement circuit 20 that performs pulse counting, and the output from the frequency measurement circuit 20 becomes a target flow rate measurement value. The frequency measurement circuit is generally known, and repeatedly counts the number of input pulses for each gate for a predetermined time.

With the above configuration, the pulse frequency of the oscillator 11 is 25 kHz, the duty ratio is 50%, and the relationship between the vortex generation frequency and the flow rate is measured. Was.

[0024]

According to the present invention, the emission frequency of the Karman vortex can be measured by placing only an optical fiber in the fluid to determine the flow rate. Therefore, since the electric parts do not come into contact with the fluid, an explosion-proof structure can be provided, which is suitable for flow measurement in a petrochemical plant or the like. Moreover, according to the method of using the optical fiber of the present invention,
It can be manufactured at very low cost without using any expensive optical components for the incidence and detection of light with the optical fiber, and without using an expensive device such as a spectrum analyzer in the electric circuit for processing the detection signal.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a flow measurement method according to the present invention.

FIG. 2 is a diagram illustrating the Karman vortex

FIG. 3 is a diagram showing a state of a light pulse according to the present invention;
(A) shows incident light, (b) shows detection light

FIGS. 4A to 4E are diagrams showing states of an optical pulse signal and a detection signal thereof according to the present invention, wherein FIGS.

FIG. 5 is a diagram showing an example of a measurement circuit in the flow meter of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Object 2 Vortex 3 Pipeline 4 Vortex generator 5 Optical fiber 6 Optical radiator 7 Photodetector 11 Oscillator 12 Light emitting diode 13 Photo IC 14 Inverter 15 NOR circuit 16 Integrating circuit 17 Peak hold circuit 18 Voltage divider 19 Comparator circuit 20 Frequency measurement circuit

Claims (4)

(57) [Claims] In a flow rate measuring method for generating a Karman vortex in a pipe and measuring a flow rate based on the number of vortex shedding, an optical fiber is arranged in a vortex flow, and a frequency higher than the vortex shedding frequency is provided in the optical fiber. And measuring the number of vortex discharges by detecting a phenomenon in which the width of a light pulse passing through the optical fiber decreases due to the force applied to the optical fiber by the vortex. 2. The method according to claim 1, wherein the optical fiber is a plastic optical fiber. 3. A Karman vortex generator disposed in a conduit, an optical fiber disposed downstream of the Karman vortex generator and having both ends outside the conduit, and a pulse having a fixed width connected to one end of the optical fiber. A light radiator that continuously generates light, a light detector connected to the other end, and an arithmetic circuit that generates an output corresponding to a pulse width of a signal from the light detector,
A flow rate measuring circuit configured to measure a frequency of a change in output of the arithmetic circuit. 4. An arithmetic circuit for generating an output corresponding to a pulse width of a signal from a photodetector, according to a time difference between a signal corresponding to light incident on the optical fiber and a light detection signal from the photodetector. 4. The flowmeter according to claim 3, comprising a circuit for obtaining a signal and a circuit for integrating the signal.