JPH0979877A - Mass flow meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove the mass flow error at a high temperature by compensating the temperature error of a pipe with the temperature coefficient set in advance and measured temperature by a temperature sensor. SOLUTION: The alternating charges of piezoelectric elements 17, 21 are converted 25, 26 into AC voltage e1t , e2t , and the volute signal e3t added 28 with them is inputted to a variable-gain circuit 30. The volute signal e3t is also inputted to a band-pass filter 31, low (high) frequency noises in the signal e3t are removed, and the volute signal e4t is outputted to a Schmidt trigger 32. The pulse signal converted from the signal e4t by the trigger 32 is inputted to an F/V converter and converted into the DC voltage E1t corresponding to the volute frequency. The circuit 30 controls the gain with the voltage V1t in inverse proportion to a passage V1 , the mass flow signal e5t is obtained, and it is V/F-converted 43 and inputted to a micro-computer 44. The temperature signal (t) of a pipe detected by a temperature sensor 40 is also inputted, the temperature affecting the signal e5t is compensated via the signal (t) and the linear expansion coefficient set in advance, and the mass flow signal e6t is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flow meter for measuring a mass flow rate of a measurement fluid by using Karman vortex,
In particular, the present invention relates to a mass flow meter having an improved mass flow rate temperature error.

[0002]

2. Description of the Related Art FIG. 3 is a sectional view showing a section of a vortex detector of a conventional mass flow meter. 10 is a pipeline through which a fluid flows, 11
Is a cylindrical nozzle provided at a right angle to the conduit 10.
Reference numeral 12 denotes a column-shaped vortex generator having a trapezoidal cross section which is inserted at a right angle into the pipe line 10 with a distance from the nozzle 11, one end of which is supported by the pipe line 10 by a screw 13 and the other end of which is formed by a flange portion 14 at the nozzle. It is fixed to 11 by screws or welding.

Reference numeral 15 is a concave portion provided on the flange portion 14 side of the vortex generator 12. Inside the recess 15, a metal base 16, a piezoelectric element 17, and an electrode plate 1 are arranged in this order from the bottom.
8, an insulating plate 19, an electrode plate 20, and a piezoelectric element 21 are arranged in a sandwich shape, and these are pressed and fixed by a metal pressing rod 22. Further, from the electrode plate 18, the lead wire 2
3, the lead wire 24 from the electrode plate 20 is the terminal A,
B has been pulled out.

The piezoelectric elements 17 and 21 are the piezoelectric elements 17 and 2, respectively.
The left side and the right side of FIG. 1 are polarized in opposite directions, and charges of opposite polarities are generated in the upper and lower electrodes with respect to stress in the same direction.

The electric charge generated in the piezoelectric element 17 is applied to the electrode plate 1.
The electric charge generated between the terminal A connected to 8 and the conduit 10 connected via the pedestal 16 and generated in the piezoelectric element 21 is connected to the terminal B connected to the electrode plate 20 and the pressing rod 22. Between the pipe line 10 and the pipe line 10 Stress detection sensor SD
1 is configured as described above.

Next, the charges generated on the two electrode plates 18 and 20 are input to charge amplifiers 25 and 26 as shown in FIG. The output of the charge amplifier 25 and the output of the charge amplifier 26 are added by the adder circuit 28 to obtain an eddy signal. These charge amplifiers 25, 2
6, and the charge converter 29 by the adder circuit 28.
Is composed.

Next, as described above, the stress detection sensor SD1
The operation of obtaining a vortex signal from which noise has been removed by the vortex detector VD1 configured together with will be described with reference to FIG. When the fluid flows into the conduit 10, vibrations due to Karman vortices are generated in the vortex generator 12 in the direction indicated by the arrow F.

Due to this vibration, the vortex generator 12 is shown in FIG.
A stress distribution as shown in FIG. 5A and a stress distribution opposite thereto are repeated, so that the charges + Q and −Q corresponding to the signal stress having the vortex frequency shown in FIG. Repeats occur.

In FIG. 5, for convenience of explanation, the electrode plate 18 or 21 is divided into two parts to the left and right with respect to the paper surface, and one of the upper and lower electrodes is the pedestal 16 or the pressing rod 22.
Is equivalent to.

On the other hand, in the pipeline 10, pipeline vibration that causes noise is also generated. This pipeline vibration is divided into a drag force direction that is the same as the fluid flow direction, a lift force direction that is perpendicular to the fluid flow direction, and a longitudinal component of the vortex generator.

Of these, the stress distribution with respect to vibration in the drag direction is as shown in FIG. 5 (b), and positive and negative charges are canceled out within one electrode, and noise charges are not generated. Further, as to the vibration in the longitudinal direction, as shown in FIG. 5C, it is canceled in the electrode, and noise charge is not generated as in the drag direction.

However, the vibration in the lift direction has the same stress distribution as the signal stress, and noise charge is generated. Therefore, the following calculation is performed to eliminate this noise charge. The charges of the piezoelectric elements 17 and 21 are Q ₁ and Q ₂ , the signal components are S ₁ ,
S ₂ , the noise components in the lift direction are N ₁ and N _2, and when the polarization is reversed by the piezoelectric elements 17 and 21, Q ₁ and Q ₂ are expressed by the following equations. However, the vector directions of S ₁ and S ₂ , and N ₁ and N ₂ are the same. _{Q 1 = S 1 + N 1} -Q 2 = -S 2 -N 2

Here, the relationship between the signal component and the noise component of the piezoelectric elements 17 and 21 is shown in FIGS. 5D and 5E (this figure shows the relationship between the noise in the lift direction and the bending moment of the vortex generator with respect to the signal). ), The output of the charge amplifier 25 on the piezoelectric element 17 side is multiplied by N ₁ / N ₂ together with the volume 27 when the output of the charge amplifier 25 on the piezoelectric element 17 side is added by the adder circuit 28. When added to the output of the charge amplifier 26, Q ₁ -Q ₂ (N ₁ / N ₂ ) = S ₁ -S ₂ (N ₁ / N ₂ ), the line noise is removed, and the vortex signal proportional to the measured flow rate is obtained. Can be obtained.

Next, a conversion circuit for calculating the mass flow rate of the fluid by using the vortex signal from which the pipe line noise is removed as described above will be described with reference to FIG. Piezoelectric elements 17, 21
The alternating electric charge generated at is converted into AC voltages e ₁ and e ₂ by the charge amplifiers 25 and 26.

The AC voltage e ₂ is added to the AC voltage e ₁ via the volume 27 by the adder circuit 28, output as the vortex signal e ₃ at the output end thereof, and output to the variable gain circuit 30. This vortex signal e ₃ is given by the following equation, where ρ is the fluid density, V is the fluid flow velocity, and K _{1 is} the proportional constant.

E ₃ = K ₁ ρV ² (1) On the other hand, the vortex signal e ₃ is also output to the band filter 31, where low-frequency or high-frequency noise included in the vortex signal e ₃ is removed to Eddy signal e in the Schmitt circuit 32 of the stage
It is output as ₄ .

The Schmitt circuit 32 converts the vortex signal e ₄ into a pulse signal at a predetermined threshold level and outputs it as a pulse signal P ₁ . This pulse signal P ₁
Is output to a frequency / voltage (F / V) converter 33 and converted into a DC voltage E ₁ corresponding to the vortex frequency.

This DC voltage E ₁ is proportional to the vortex frequency f _V of the vortex signal and is represented by the following equation. E ₁ = K ₂ f _V (2) where K ₂ is a proportional constant.

Further, there is a relationship between the flow velocity V and the vortex frequency f _V , where K _ST is the Strouhal number and d is the diameter of the vortex generator 12, and there is a relationship of f _V = K _ST (V / d). The equation (2) becomes E ₁ = K ₂ K _ST (V / d) (2) ′.

Since the variable gain circuit 30 controls its gain by the DC voltage E ₁ so that it is inversely proportional to the vortex frequency f _V , that is, the flow velocity V, the equation (1) is divided by the equation (2) ′ at its output end. As a form of calculation, a mass flow rate signal e ₅ is obtained as shown in the following equation. e ₅ = e ₃ / E ₁ = (K ₁ / K ₂ ) ρV = (K ₁ / K ₂ K _ST ) dρV (3)

Therefore, the mass flow rate signal e _{5 in} this case is
It is output as a mass flow rate signal whose gain is adjusted uniformly over the entire frequency band. Therefore, low frequency noise is not emphasized relative to the vortex signal S /
A good N signal is obtained.

This mass flow rate signal e ₅ is detected by the detection rectifier circuit 3
4 and is converted into a DC voltage E ₂ here, and then converted into a frequency signal f ₁ by the voltage / frequency conversion circuit 35 and output to the output end 36.

[0023]

However, in the mass flowmeter as described above, when the measurement temperature changes, the area of the conduit 10 and the diameter of the vortex generator 12 change due to thermal expansion. Even when a constant mass flow rate is flowing, the mass flow rate apparently changes, resulting in a temperature error.

This problem will be further described with reference to the explanatory view shown in FIG. FIG. 7A shows a cross-sectional state when the inner wall of the conduit 10 and the vortex generator 12 are at the reference temperature t ₀ , and FIG.
It shows the cross-sectional state of the expansion of the duct 10 and the vortex shedding body 12 when it becomes. The dotted line shows the state at the reference temperature t ₀ , and the solid line shows the state at the temperature t.

Now, the reference temperature t₀Disconnection of pipeline 10 when
Area is S₀, The diameter of the vortex generator 12 is d ₀, Flow velocity V₀, Dense
Degree ρ₀Then, the cross-sectional area of the pipeline 10 at the temperature t is
S_t, The diameter of the vortex generator 12 is d_t, The flow velocity is V_t, Density is ρ_t
Becomes

Therefore, the relationship of ρ ₀ V ₀ S ₀ = ρ _t V _t S _t is obtained from the continuity equation, but the mass flow rate at the reference temperature t ₀ is m ₀ ,
If the mass flow rate at the temperature t is m _t , then m ₀ S ₀ = m _t S _t .

Therefore, assuming that the coefficient of thermal expansion is α, ΔT = t−
If t ₀ , the area S _t is obtained by the relationship of S _t = S ₀ (1 + 2αΔT) (4). Therefore, using this equation (4), m _t = (m ₀ S ₀ / S _t ) = m ₀ / (1 + 2αΔT) ≈m ₀ (1-2αΔT) (5)

Therefore, considering the influence of temperature, the temperature t
In the case of, the formula (1) becomes e _3t = K ₁ ρ _t V _t ² = K ₁ m _t V _t (6).

On the other hand, in consideration of the influence of temperature, the equation (2) 'at the temperature t becomes E _1t = K ₂ V _t = K ₂ K _ST V _t / d _t (7) Where d ₀ is the diameter of the vortex generator at the reference temperature and d _t is the diameter at the temperature t, and d _t = d
₀ (1 + αΔT).

Therefore, the mass flow rate signal e _5t when the temperature is t
From (3), (5) using the formula _{e 5t = e 3t / E 1t} = [K 1 m t V t / K 2 K ST (V t / d t)] = [(K 1 / K _{2 K ST) m t d t} ] = (K 1 / K 2 K ST) m 0 d 0 (1-2αΔT) (1 + αΔT) ≒ (K 1 / K 2 K ST) m 0 d 0 (1-αΔT) (8)

Here, since K ₁ , K ₂ and K _ST are constants that are not influenced by temperature, the mass flow rate signal e _5t is against the expansion of the conduit 10 and the vortex generator 12 due to the temperature change. From the relationship of the equation (8), the mass flow rate changes at the rate of -αΔT.

[0032]

The present invention has, as a structure for solving the above problems, a signal converting means for converting a measured flow rate flowing in a pipe into a vortex signal and outputting the vortex signal, and an amplitude of the vortex signal. Variable gain means that changes the circuit gain in response to the control signal and outputs a division signal, and outputs the previous control signal that is input with the vortex frequency of the previous vortex signal and changes following the vortex frequency. The eddy frequency detecting means, the temperature detecting means for detecting the temperature of the previous pipeline and outputting it as a temperature signal, and the temperature coefficient of the material of the previous pipeline are stored in advance. A signal processing means for inputting a signal and the above-mentioned temperature signal and executing a temperature compensation operation using these signals and outputting a mass flow rate signal is provided.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a sectional view showing a specific configuration of the stress detection sensor shown in FIG. The parts having the same functions as those of the conventional mass flowmeter shown in FIGS. 3 to 6 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

The stress detecting sensor SD2 is different from the stress detecting sensor SD1 in that the temperature sensor 40 is fixed to the upper portion of the piezoelectric element 21 by a pressing rod 46, and the other parts are substantially stress-free. The configuration is the same as that of the detection sensor SD1.

The alternating charges generated in the piezoelectric elements 17 and 21 are converted into AC voltages e _1t and e _2t by the charge amplifiers 25 and 26 in consideration of the influence of temperature. The alternating-current voltage e _2t is added to the alternating-current voltage e _1t via the volume 27 by the adder circuit 28 and output as an eddy signal e _3t at the output end thereof.

The vortex signal e _3t is obtained in the form shown in the equation (6) and is output to the variable gain circuit 30 via the amplifier circuit 42 (the amplification degree is 1 for simplicity). Further, the vortex signal e _3t is also output to the band filter 31, where low-frequency or high-frequency noise included in the vortex signal e _3t is removed and output as the vortex signal e _4t to the Schmitt circuit 32 in the next stage. To be done.

In the Schmitt circuit 32, the vortex signal e _4t is converted into a pulse signal at a predetermined threshold level and output as a pulse signal P _1t . This pulse signal P
_1t is output to the frequency / voltage (F / V) converter 33 and converted into the DC voltage E _1t represented by the equation (7) corresponding to the vortex frequency.

Since the variable gain circuit 30 controls the gain by the DC voltage E _1t so as to be inversely proportional to the vortex frequency f _Vt , that is, the flow velocity V _t , the mass flow rate signal e _5t represented by the equation (8) is output at its output end. To get

The mass flow rate signal e _5t is stored in a random access memory (not shown) of the microcomputer 44 via the V / F conversion circuit 43. Further, the temperature signal t detected by the temperature sensor 40 for measuring the temperature of the conduit 10 and the like is input to the microcomputer 44 after being converted into a digital signal by the temperature conversion circuit 41.

In addition, in the writable non-volatile memory (not shown) of the microcomputer 44, the coefficient of linear expansion α due to the material of the conduit 10 and the vortex generator 12 is preset.

Then, e _5t ≉ (K ₁ / K ₂ K _ST ) m ₀ d ₀ (1-αΔT) is obtained by the following equation shown by the equation (8) obtained by the operation of the variable gain circuit 30. The mass flow rate signal e _5t is affected by the temperature of −αΔT.

Therefore, the microcomputer 44 uses the set linear expansion coefficient α and the measured temperature signal t in the calculation for compensating the influence of this temperature, and sets K _t as a constant in the formula (8). e _6t = for _{(K 1 / K 2 K ST} ) m 0 d 0 (1-αΔT) × K t (1 + αΔT) = K t (K 1 / K 2) m 0 (1-α 2 ΔT 2) ≒ K _t (K ₁ / K ₂ ) m ₀ (9) is calculated and the mass flow rate signal e _6t is compensated for the influence of temperature.
Get. However, ΔT = t−t ₀ .

[0043]

As described above in detail with the embodiments of the present invention, according to the present invention, the temperature error caused by the expansion of the pipeline or the like caused by the temperature is set to the temperature coefficient of the preset pipeline or the like. Since the temperature of the pipeline measured by the temperature sensor is used for compensation, it is possible to eliminate the mass flow rate error at high temperature, and it is expected that the accuracy is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the configuration of the stress detection sensor shown in FIG.

FIG. 3 is a sectional view showing an outline of a section of a vortex detector of a conventional mass flow meter.

FIG. 4 is a block diagram showing a configuration of a charge converter that converts an output signal of the vortex detector shown in FIG. 3 into a voltage signal.

FIG. 5 is an explanatory diagram illustrating an operation of removing noise in the vortex detector.

6 is a block diagram of a conversion circuit that is combined with the vortex detector shown in FIG. 3 to calculate a mass flow rate.

FIG. 7 is an explanatory diagram illustrating a problem of the mass flow meter shown in FIG.

[Explanation of symbols]

 10 Pipe Line 12 Vortex Generator 17, 21 Piezoelectric Element 30 Variable Gain Circuit 31 Bandwidth Filter 40 Temperature Sensor 41 Temperature Conversion Circuit 44 Microcomputer SD1, SD2 Stress Detection Sensor

Claims (1)

[Claims] 1. A signal converting means for converting a measured flow rate flowing through a pipe into an eddy signal and outputting the eddy signal, and a circuit gain corresponding to a control signal to which the amplitude of the vortex signal is input and changing the circuit gain to output a division signal. Variable gain means, vortex frequency detection means for inputting the vortex frequency of the vortex signal and outputting the control signal that changes following the vortex frequency, and temperature for detecting the temperature of the conduit and outputting it as a temperature signal. A temperature coefficient of the material of the pipeline is stored in advance, and the temperature coefficient, the division signal, and the temperature signal are input, and temperature compensation calculation is executed using these to output a mass flow rate signal. And a signal processing means for controlling the mass flowmeter.