JP2942140B2 - Coriolis 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 Coriolis flowmeter, and more particularly, to a Coriolis flowmeter which determines a Coriolis force proportional to a mass flow rate acting on a measuring tube from a time difference of a detection signal at a vibration detecting position of the measuring tube. It relates to a flow meter.

[0002]

2. Description of the Related Art When a measuring tube is supported at both ends and a central portion of the supported measuring tube is alternately driven in a direction perpendicular to the tube axis,
When the fluid moves, a phase difference occurs between the inflow side and the outflow side of the measurement tube around the center of the measurement tube. This phase difference is based on the Coriolis force and is a value proportional to the mass flow rate. Coriolis flow meters that detect the phase difference and measure the mass flow rate are well known. Thus, the phase difference is
When the measurement tube is stationary, the measurement is performed from the time difference between the detection positions when the two detection positions defined on the measurement tube pass through the reference surface, with the plane passing through the tube axis of the measurement tube as the reference plane. Conventionally, the time difference is obtained by shaping the waveforms of the displacement detector signals of the measurement tubes attached to the symmetrical positions on the inflow side and the outflow side of the measurement tubes into square waves, respectively, and calculating the time difference between the rise times of the shaped square waves. It is obtained from the number of clocks of a constant frequency counted between them.

[0003]

As described above, the Coriolis flowmeter supports the measuring tube at two points, and the supporting point is a node with respect to the supported measuring tube at a predetermined frequency (fundamental frequency) around the supporting point. The mass flow meter obtains a mass flow rate by detecting the Coriolis force acting on the fluid flowing through the measurement tube by the vibration. However, the phase difference signal of the measuring tube caused by the Coriolis force is extremely small compared to the driving amplitude of the measuring tube. Therefore, the time difference detected in proportion to the phase difference signal is also very small. In order to measure this time difference with high resolution, it is necessary to increase the clock frequency. However, since the Coriolis flowmeter is mounted between the upstream and downstream pipes, it is easily affected by external vibration and noise caused by the pipes, and stable time difference measurement cannot be performed by simply using a clock having a high resolution.

The present invention provides a Coriolis flow meter having a time difference detecting means capable of obtaining an averaged stable time difference with high sensitivity without a noise effect without using a clock for the time difference measurement. The purpose is to do so.

[0005]

In order to solve the above-mentioned problems, the present invention provides (1) a measuring pipe through which a fluid flows, and a supporting member for supporting the measuring pipe at two points in a flow direction at an interval. ,
Driving means for driving the measurement tube around the support member at a fundamental frequency, a first detector and a second detector for detecting vibration of the measurement tube at a symmetric position between the support members of the measurement tube, A Coriolis flowmeter having time difference detecting means for detecting a time difference on a reference time axis from the vibration signals detected by the first detector and the second detector and obtaining a mass flow rate in proportion to the time difference. Means for multiplying the sine wave signal of the fundamental frequency detected by the first detector by N, and a first circuit for converting the multiplied sine wave signal into a triangular wave signal synchronized with the sine wave signal
A waveform conversion circuit, a second multiplier circuit for multiplying the sine wave signal of the fundamental frequency detected by the second detector by N,
A second waveform conversion circuit that converts the multiplied sine wave signal into a triangular wave signal synchronized with the sine wave signal, a subtraction circuit that subtracts the output of the second waveform conversion circuit from the output of the first waveform conversion circuit, A cycle measuring circuit for measuring the cycle of the fundamental frequency, and a multiplying circuit for multiplying the measured cycle of the fundamental frequency by the output of the subtraction circuit.

[0006]

The fundamental sine wave signals having different phases detected at two detection positions defined in the measuring tube are converted into sine wave signals having N times the frequency, and the multiplied sine wave signal is converted into a sine wave signal. Convert to synchronized triangular wave signal. The triangular wave signals having different phases are subtracted to form a trapezoidal wave signal having an amplitude proportional to the phase difference N times the phase difference obtained from the basic sine wave signal. The time difference is obtained by multiplying the trapezoidal wave signal by the period of the basic sine wave signal.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing a Coriolis flow meter according to the present invention, a basic structure of a Coriolis flow meter will be described first. FIG. 4 is a basic structural view for explaining a Coriolis flowmeter according to the present invention, in which reference numeral 20 denotes a Coriolis flowmeter main body,
1 is a base, 22 is a measuring tube, 23 and 24 are support members, 25
Is a driving means, 26 is a first detector, and 27 is a second detector.

The Coriolis flowmeter main body 20 includes a measurement tube 22 through which a fluid flows, and a first support member 23 fixed to a base 21 supporting the measurement tube 22 at two points separated by a distance L in the flow direction.
, A second support member 24, a driving unit 25 for driving the measuring tube 22 at a central portion of the supported measuring tube 22 at a predetermined fundamental frequency in a direction perpendicular to the axis OO, It comprises a first detector 26 and a second detector 27 for detecting a phase displacement generated in the measuring tube 22 by a force.

The driving means 25 comprises, for example, a magnetic body 25a fixed to the measuring tube 22 and a coil 25b fixed to the base 21 in opposition to the magnetic body 25a.
b, a sine-wave driving current is applied to
Is an arrow ± ω around the support members 23 and 24 at the fundamental frequency.
_1, it is driven in omega ₂ directions.

The first and second detectors 26 and 27 are composed of magnets 26a and 27a having the same principle structure and the same standard, and detection coils 26b and 27b. The magnets 26a and 27a are connected to the inlet of the measuring tube 22. The detection coils 26b and 27b are fixed to the base 21 at symmetric positions from the outlet.

The Coriolis flowmeter main body 2 having the above configuration
Is driven by the driving means 25 at a fundamental frequency, the Coriolis force proportional to the vector product of the mass flow rate m acting on the measuring tube 22 and the fundamental frequency f is applied to the first and second detectors 26 and 27. A sine wave signal having a small phase difference due to the above is detected. Fluid flows in the direction of arrow Q, and
Is driven in the directions of + ω ₁ and + ω ₂ , the first detector 26
Has a phase delay proportional to the Coriolis force with respect to the sine wave signal of the second detector 27. vice versa,
When driven in the −ω ₁ and −ω ₂ directions, they have opposite phase delays.

The present invention detects a phase delay, that is, a phase difference φ as a time difference ΔT, and its principle will be described. Assuming that a sine wave signal output from the first detector 26 at a fundamental frequency f = ω / 2π is y ₁ and a sine wave signal output from the second detector 27 is y ₂ , y ₁ = A ₁ sin ωt ( 1) y ₂ = A ₂ sin (ωt + φ) (2) where A ₁ and A ₂ are amplitude, ω is drive angular velocity, and t is time.

When the equations (1) and (2) are each divided by the amplitude, sin ωt = y ₁ / A ₁ (3) sin (ωt + φ) = y ₂ / A ₂ (4) Equations (1) and (2) In order to obtain a triangular wave signal synchronized with the sine wave signal represented by the following equation, for example, taking the inverse sine function of equations (3) and (4), sin ^-1 (sin ωt) = sin ^-1 (y ₁ / A ₁ ) = ωt (5) sin [sin (ω + φ)] = sin ⁻¹ (y ₂ / A ₂ ) = ωt + φ (6)

From equations (5) and (6), φ = sin ⁻¹ (y ₂ / A ₂ ) −sin ⁻¹ (y ₁ / A ₁ ) (7) On the other hand, the fundamental frequency of the sine wave signal is represented by f (period) T), the time difference ΔT between the sine wave signals y ₁ and y ₂ is given by ΔT = (φ / 2π) × (1 / f) = (φ / 2π) × T (8). By substituting equation (7) into the equation, the time difference ΔT can be obtained. Similarly, the time difference ΔT ₁ when synchronized with the fundamental frequencies of y ₁ and y ₂ and multiplied is also given by ΔT ₁ = ΔT = (φ ₁ / 2π) / T ₁ .

Although the inverse sine function is a triangular wave synchronized with the sine wave signal, the peak value of the triangular wave signal obtained by the equations (5) and (6) is small. As a result, the level of the phase difference φ signal obtained by the equation (7) is also small, and it is difficult to measure this signal with high accuracy.

For this reason, the input signals (1) and (2)
When the sine wave signals y ₁ and y ₂ of the fundamental frequency in the equation are converted into sine wave signals of N times synchronized with the input signal, from equation (8), ΔT = (φ / 2π) × T = (φ ₁ / 2π ) × T ₁ (9) where T: period of input signal (basic frequency) φ: phase difference between input sine wave signals y ₁ and y ₂ T ₁ : period of sine wave signal multiplied by N φ ₁ : multiplied by N The phase difference between the sine wave signals y ₁ and y ₂ is obtained. Here, since φ ₁ = Nφ (10) T = NT ₁ (11), from equation (9), the time delay ΔT is ΔT = ((φ ₁ / 2π) × T) × 1 / N (12) Is obtained, and the phase difference signal φ ₁ when multiplied by N is expanded N times to the phase difference signal φ in the case of the fundamental frequency.

Embodiment 1 (corresponding to claim 1) FIG. 1 is a block diagram for explaining an embodiment of a time difference detection circuit of a Coriolis flowmeter according to the present invention.
1, 5 are input terminals for the first and second sine wave signals, 2, 6 are first and second integrating amplifier circuits, 3, 7 are first and second N-multiplier circuits, and 4, 8 are first and second A waveform conversion circuit, 9 is a subtraction circuit,
Reference numeral 10 denotes a cycle measuring circuit, 11 denotes a multiplying circuit, and 12 denotes a 1 / N step-down circuit.

The block diagram shown in FIG. 1 is for realizing the equation (12). The first sine wave signal of the fundamental frequency output from the first detector of the Coriolis flowmeter is supplied to the input terminal 1.
To enter. Since the first sine wave signal contains harmonics, the high frequency is removed by the first integration amplifier circuit and amplified to a predetermined voltage level. The amplified first sine wave signal is
The signal is input to the first N multiplier circuit 3 and is converted into an N-fold sine frequency signal.

The multiplication circuit is, for example, a well-known PLL (Phas
e-Locked Loop) circuit, PC (phase comparator), L
It is configured by a feedback loop including a PF (low-pass filter), a VCO (voltage controlled oscillator), and a frequency divider. In the PLL circuit, the input signal frequency is multiplied to a frequency proportional to the frequency division ratio of the frequency divider.

The multiplied first sine signal is input to the first waveform conversion circuit 4 and is converted into a triangular wave signal synchronized with the N-multiplied first sine signal frequency. As a circuit method of the waveform conversion circuit, a method of combining a rectangular wave conversion circuit for converting a sine wave signal into a rectangular wave and an integration circuit, or an inverse sine wave conversion circuit may be used.

For the second sine wave signal from the second detector input to the input terminal 5, the same as the above-described waveform conversion operation for the first sine wave detection signal, the second sine wave signal is the second sine wave signal. Through the integrating amplifier circuit 7 and the second waveform conversion circuit 8, the second
A triangular wave signal synchronized with the sine wave signal obtained by multiplying the frequency signal of the sine wave signal by N is obtained.

The first waveform conversion circuit 4 and the second waveform conversion circuit 8
Are subtracted by the subtraction circuit 9. The subtraction output is a trapezoidal wave signal having a voltage level whose amplitude is proportional to the phase difference φ ₁ (= Nφ). The voltage level of this trapezoidal wave signal is N times larger than the voltage level when no multiplication is performed.

On the other hand, the period of the fundamental frequency signal is measured by the period measuring circuit 10 for the output signal of the second integrating amplifier circuit 6 whose waveform has been shaped. The cycle measuring circuit 10
For example, the voltage level when the integrator signal operating at the rise of the square wave signal synchronized with the fundamental frequency signal coincides with the fall or the number of clocks input during the rise and fall of the square wave signal, etc. It is measured in analog and digital.

The measured period T ₁ is input to the multiplication circuit 11, where the multiplication circuit 11 multiplies the period T ₁ by the phase difference signal φ ₁ to obtain a value N proportional to the time difference between the first and second input signals.
ΔT is obtained. The output signal of the multiplication circuit 11 is reduced by the
By reducing to 1 / N by 2, a highly accurate time difference signal proportional to the Coriolis force can be obtained. However, decreasing circuit 12 is not preferable on the signal operating at which decreasing the phase difference signal phi _1, which is N times, do not require, except when determining the accurate time difference.

The time difference measuring means of FIG. 1 having the above configuration will be described with reference to Table 1 and FIG. Table 1 shows the relationship between the sine wave and the inverse sine wave with respect to the phase of the sine wave signal.
FIG. 4 is a waveform diagram illustrating the principle of phase difference detection. The first sine wave signal input from the input terminal 1 is waveform-shaped by the first integration amplifier circuit 2, and the waveform-shaped first sine wave signal is multiplied by N by the N-multiplier circuit 3, as shown in FIG. Show
It becomes a periodic signal of sinx. The sine wave sinx multiplied by N is 2
This is a periodic function having peak values at π / 4 and 6π / 4 and zero-crossing at an integral multiple of π. Also, sine wave sinx
Sin ^-1 (sinx), which is the inverse function of, has a peak value of ± π / 2 at 2π / 4 and 6π / 4, and becomes a triangular wave signal that crosses zero at a position of an integral multiple of π.

[0026]

[Table 1]

On the other hand, after amplifying and shaping (6) the second sine wave signal inputted from the input terminal 2, each sine wave signal multiplied by N (7) is obtained by multiplying the sine wave of the first detector 1 by N. The sine wave signal and the phase difference Nφ
As shown in (3), it is represented on the vertical axis. Therefore, the inverse sine wave sin ^-1 (sinx + φ) has a phase difference Nφ, and sin ^-1 (sin x
It becomes the same triangular wave signal as x).

FIG. 2 (4) shows sin ^-1 (sinx) of FIG. 2 (2).
2 (3) is a trapezoidal periodic function that zero-crosses at a phase of an integral multiple of substantially π / 2 obtained by subtracting sin ^-1 {sin (x + φ)} by a subtraction circuit 9, and the height of the trapezoid Is 2Nφ, the time delay of falling is Δt, and the height of the trapezoid changes in proportion to the phase difference Nφ. As described above, since the phase difference signal Nφ is detected as the phase difference signal Nφ having a constant height at a time when the phase is substantially π, an averaged phase difference Nφ is detected, and therefore, the period T _{1 of the} fundamental frequency f Are also averaged, and a time difference signal free from noise is detected.

As is clear from the above description, the sine wave si
nx and the inverse sine wave sin ^-1 (sinx) are periodic functions having the same period, and since the inverse sine wave sin ^-1 (sinx) is a triangular wave,
Even if sinx is directly converted into a triangular wave, a similar trapezoidal phase difference signal can be detected. That is, the first and second conversion circuits 4 and 8 are converted into a triangular wave signal having the same cycle as the sine wave signal, and the other circuits are configured in exactly the same manner as the circuit shown in FIG. A time difference signal ΔT is obtained. Similarly, the phase difference N can also be obtained by a trapezoidal wave obtained by clipping the top of the triangular wave with a constant voltage.
φ can be detected.

FIG. 3 shows an experimental example of the output characteristics of the phase difference signal of the subtraction circuit 9 shown in FIG. 1. FIG. 3 (1) shows a case where the fundamental frequency f = 500 Hz, and FIG. Rate N = 1
In the case of 0, FIG. 3 (3) is a phase difference signal having a trapezoidal wave height when the multiplying factor is N = 20, and the voltages of all the triangular waves are the same 19.2 V _PP .

As is apparent from the figure, for the phase difference signal level of 0.486 V in the case where the fundamental frequency f = 500 Hz in FIG. 3A where the measuring tube is driven, N = 1 in FIG.
In the case of 0, Nf = 500 Hz, it is 4.72v, which is about 10 times, and in the case of N = 20, Nf = 10000 Hz in FIG. 3 (3), it is 9.44v, which is 20 times. It can be seen that a signal level can be obtained.

As described above, by multiplying the fundamental frequency by N times, the voltage level of the phase difference signal between the detection signals becomes N
And the resolution of the time difference signal ΔT is improved by N times, and the reliability is also improved in accordance with this magnification.

[0033]

As is apparent from the above description, according to the present invention, the phase difference proportional to the Coriolis force generated when the measuring tube passes through the reference plane at the pair of detection positions of the measuring tube is represented by a trapezoidal wave. And the height of the trapezoidal wave is smaller than the height of the fundamental frequency for driving the measuring tube.
Since the sensitivity is increased in proportion to the multiplication number N of the fundamental frequency, the sensitivity becomes N times higher, and the constant level voltage height of the trapezoidal wave is measured. Therefore, a highly reliable phase difference can be detected. Therefore, the Coriolis force obtained by multiplying the phase difference by the period of the fundamental frequency also has a highly sensitive and highly reliable value.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining an embodiment of a time difference detection circuit of a Coriolis flowmeter according to the present invention.

FIG. 2 is a waveform diagram illustrating the principle of phase difference detection.

FIG. 3 shows an experimental example of a phase difference signal output characteristic of the subtraction circuit 9 shown in FIG.

FIG. 4 is a basic structural diagram for explaining a Coriolis flowmeter according to the present invention.

[Explanation of symbols]

1,5 ... input terminals of first and second sine wave signals, 2,6 ... first and second integral amplifier circuits, 3,7 ... first and second N-multiplier circuits, 4,8 ... first and second Waveform conversion circuit, 9 subtraction circuit,
10: cycle measuring circuit, 11: multiplying circuit, 12: 1 / N multiplying circuit.

Claims (1)

(57) [Claims] 1. A measuring tube through which a fluid flows, a supporting member for supporting the measuring tube at two points in a flow direction at an interval, and driving means for driving the measuring tube around the supporting member at a fundamental frequency; A first detector and a second detector for detecting vibration of the measurement tube at a position symmetrical between the support members of the measurement tube; and a reference based on the vibration signals detected by the first and second detectors. In a Coriolis flowmeter having a time difference detecting means for detecting a time difference on a time axis and obtaining a mass flow rate in proportion to the time difference, the time difference detecting means may be the first type.
A first frequency multiplier for multiplying the sine wave signal of the fundamental frequency detected by the detector by N, a first waveform converter for converting the multiplied sine wave signal into a triangular wave signal synchronized with the sine wave signal, A second multiplying circuit for multiplying the sine wave signal of the fundamental frequency detected by the second detector by N, and a second waveform conversion circuit for converting the multiplied sine wave signal into a triangular wave signal synchronized with the sine wave signal; A subtraction circuit for subtracting an output of the second waveform conversion circuit from an output of the first waveform conversion circuit, a cycle measurement circuit for measuring a cycle of the fundamental frequency, and a cycle of the measured fundamental frequency and the subtraction circuit. A Coriolis flowmeter comprising a multiplication circuit for multiplying an output.