JP2867609B2 - Mass flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention detects an alternating force generated in a vortex generator by a Karman vortex corresponding to a flow of a fluid such as a gas or a liquid, and uses this to detect a mass. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flowmeter that extracts a flow signal, and more particularly to an improved mass flowmeter that can effectively remove low-frequency noise when superimposed on a vortex signal.

<Prior Art> FIG. 8 is a block diagram showing a first example of a mass flow meter disclosed in Japanese Patent Publication No. 1-25405, and an outline of the weight flow meter will be described below.

It converted by the eddy detection sensor 1 to an alternating signal e _t of the vortex frequency proportional to flow rate to, and converted into a pulse output signal E _P the alternating signal e _t by pulsing circuit 2, an analog of the pulse output signal E _P into an analog voltage E _V which is proportional to the flow rate by the F / V converter circuit 4 for converting the voltage E _V.

Further, the alternating signal e _t of the vortex detection sensor 1 to obtain an analog signal E _D that varies in amplitude in response to the square of the flow velocity at its output is detected by the detection circuit 3.

Arithmetic circuit 5 is input and the analog signal E _D proportional to the square of the analog voltage E _V and a flow rate proportional to the flow velocity, to calculate the mass flow rate E _M by calculating it like proportions of.

FIG. 9 is a block diagram showing another conventional mass flow meter using the vortex detection sensor 1. As shown in FIG.

Alternating signal e _t is the output of the eddy detection sensor 1 is input to the signal conversion circuit 6, wherein the alternating signal e _t is converted into an AC signal e _A of the vortex frequency. If vortex frequency f of the alternating signal e _t is the flow rate of the fluid flowing through the conduit is V, the _{f = K 1 V ... (1} ) a K ₁ is a constant.

In the AC signal e _A , K ₂ is a constant, ρ is a fluid density, and ω =
Assuming 2πf and t as time, e _A = K ₂ ρV ² sinωt (2) Therefore, the following equation (3) is obtained from the equations (1) and (2).

e _A = (K ₂ / 4π ² K ₁ ² ) ρω ² xsinωt (3) By outputting this AC signal e _A to the integration circuit 7 in the next stage, the output terminal thereof has an integration output e represented by the following equation. _{Get B.}

_{e B = e A dt = (} K 2 / 4π 2 K 1 2) ρωx (cosωt-K 3) ... (4) However, K ₃ is a constant of integration.

When detection rectifier this integration output e _B by the detection circuit 8, the detection output e _C is, e _C = become ^{_{(K 2 K 4 / 4π 2}} K 1 2) ρω ... (5), the detection output e _C is It becomes a signal proportional to the mass flow rate.

<Problem to be Solved by the Invention> However, the conventional mass flow meter shown in FIG. 8 obtains the mass flow rate by calculation, but the circuit configuration is complicated, and the detection circuit 3 has the square of the flow velocity. since correspondingly in the configuration to obtain an analog signal E _D of amplitude changes, there is a disadvantage that the dynamic range of the circuit is narrowed.

Further, the conventional mass flow meter shown in FIG. 9 has a configuration in which ρV ² is integrated by an integrating circuit to obtain a mass flow rate, and the mass flow rate is obtained at the output end of the vortex detection sensor 1 as shown in FIG. When the low frequency noise e _N to an alternating signal e _t for is superimposed, the output terminal of the integrating circuit 7 as shown in Figure No. 10 (b), the integration output e _B
Alternating signal e _t of the ^- low frequency noise e _N is superimposed on the ^- (Usually the frequency is about 1 / 7-1 / 10 of the vortex frequency) results in is relatively emphasized, the detection circuit 8 detection output e _C is an alternating signal e _t ^- rather low frequency noise e _N ^- becomes as proportional to the amplitude, there is a problem that not from that corresponding to the mass flow rate.

<Means for Solving the Problems> The present invention mainly aims at solving the above problems.
A vortex signal converting means for converting a vortex generated by the flow velocity of the measurement fluid into a vortex signal, and the vortex signal is inputted and the high cut frequency is selected sufficiently smaller than the vortex frequency of the vortex signal to be measured, and the low cut frequency is a low cut frequency change signal. A band-pass filter that outputs a mass flow rate signal corresponding to the vortex signal, and a frequency detection circuit that receives the vortex signal, detects the frequency of the vortex signal, and outputs a low-cut frequency change signal according to its magnitude. It is something to do.

<Operation> The vortex signal conversion means converts the vortex generated by the flow velocity of the measurement fluid into a vortex signal.

The vortex signal is input to the bandpass filter and signal processing is performed.This vortex signal has a high cut-off frequency of the bandpass filter that is sufficiently smaller than the vortex frequency of the vortex signal to be measured. The low-cut frequency of the band-pass filter is changed according to the vortex frequency of the vortex signal by the low-cut frequency change signal to reduce the low-frequency noise input to the vortex signal.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention.

Reference numerals 17 and 18 denote piezoelectric elements.These piezoelectric elements 17 and 18 are provided in the vortex generator, and charge generated due to the vortex generated by the vortex generator when the measurement fluid flows. Q ₁ ,
It detects the Q ₂ output to the charge converter CCV.

The charge converter CCV converts the charges Q ₁ and Q ₂ into an AC voltage signal e ₁ corresponding to the number of vortices and outputs the voltage signal e ₁ to the temperature correction circuit TCC.

The temperature correction circuit TCC also receives a temperature signal T from a temperature measuring element TMS installed inside the vortex generator, and the temperature correction circuit TCC responds to the voltage signal e1 with the piezoelectric elements 17, ₁
Detection sensitivity with respect to temperature of 8, and outputs the band-pass filter BPF that is, as the vortex signal e ₂ by correcting the temperature change of the piezoelectric constant.

Bandpass filter BPF is set sufficiently smaller than the vortex frequency f to be measured its high-cut frequency f _A, thereby converting the vortex signal e ₂ to the mass flow rate signal e _3. Also, the low cut frequencies f _B and f _C are the low cut frequency change signals S ₁ ,
Switched by the magnitude of the vortex frequency f in S _2, to reduce low frequency noise e _N contained in the vortex signal e _2.

The mass flow rate signal e ₃ is converted to mass flow rate signal E ₄ DC is outputted via the amplifier AM1 in the detector / rectifier circuit DRC.

This mass flow signal E ₄ is supplied to the voltage /
The frequency converter VFC converts it into a frequency signal, and the frequency / frequency converter FFC has a 50% duty pulse train signal F
Converted to 50. The frequency of the pulse train signal F50 is proportional to the mass flow rate.

The temperature compensation circuit vortex signal e ₂ which is the output of the TTC is outputted to Akuteibu bandpass filter ABF is here unnecessary noise removal, the output of Schmitt trigger SMT
Is output to

Schmitt trigger SMT and outputs the detected vortex frequency f of the vortex signal e ₂ to the frequency / voltage converter EVC. Frequency / voltage converter FVC outputs as the eddy voltage E ₅ is converted into a voltage signal corresponding to the detected vortex frequency f.

CO1 and CO2 are both comparators, and the eddy voltage E ₅
Is input and compared with a predetermined reference low cut frequency,
The low cut frequency change signals S ₁ and S ₂ for switching the low cut frequencies f _B and f _C of the bandpass filter BPF according to the frequency of the eddy voltage E ₅ are output. This embodiment shows a case of two-stage switching.

Further, the vortex voltage E ₅ is also input to the comparator CO3, where it outputs a control signal S ₃ at its output when the smaller the value is not observed to be due to vortex frequency, switch
Turn off SW1 to perform zero cut. Specifically, the threshold voltage of the comparator CO3 is set to a value that changes in proportion to the minimum frequency of the eddy frequency to be measured.

Further, the comparator CO4 render it off by sending a control signal S ₄ to the switch SW1 when the small vortex voltage E ₅ of the output of the detector / rectifier circuit DRC not admitted to the vortex signal. Specifically, the threshold voltage of the comparator CO4 is set to a value that changes in proportion to the minimum mass flow rate of the mass flow rate to be measured. As a result, a stable zero point is obtained by removing the influence of the offset generated in the process from the charge converter CCV to the detection / rectifier circuit DRC.

Next, details of the detection unit including the piezoelectric element and the char converter shown in FIG. 1 will be described with reference to FIGS.

In FIG. 2, reference numeral 10 denotes a pipe through which a fluid flows, and 11 denotes a pipe 10
Is a cylindrical nozzle provided at a right angle to. 12 is a nozzle
Numeral 11 denotes a columnar vortex generator having a trapezoidal cross-section inserted at right angles to the conduit 10 with an interval, one end of which is supported by the conduit 10 by a screw 13 and the other end of which is screwed to the nozzle 11 by a flange portion 14. Alternatively, it is fixed by welding. Reference numeral 15 denotes a concave portion provided on the flange portion 14 side of the vortex generator 12. Inside the concave portion 15, a metal pedestal 16, a piezoelectric element 17,
The electrode plate 18, the insulating plate 19, the electrode plate 20, and the piezoelectric element 21 are arranged in a sandwich shape, and these are pressed and fixed by a metal pressing rod 22.

Further, lead wires 23 are drawn out from the electrode plate 18 and lead wires 24 are drawn out from the electrode plate 20 to the terminals A and B, respectively.

The piezoelectric elements 17, 21 are polarized in opposite directions on the left and right sides, respectively, of the piezoelectric elements 17, 21 with respect to the plane of the paper, and generate electric charges of opposite polarities on the upper and lower electrodes with respect to the stress in the same direction.

The electric charge generated in the piezoelectric element 17 is obtained between the terminal A connected to the electrode plate 18 and the pipe 10 connected through the pedestal 16, and the electric charge generated in the piezoelectric element 21 is connected to the electrode plate 20. Between the terminal B and the conduit 10 connected to the pressing rod 20.

The charges generated on the two electrode plates 18 and 20 are input to charge amplifiers 25 and 26 as shown in FIG. Charge amplifier 25
The output of the charge amplifier 26 and the output of the charge amplifier 26 are added via a volume 27 by an adding circuit 28, and an AC voltage signal is obtained as a vortex signal.
get e ₁ . These charge amplifiers 25, 26 and summing circuits
28 constitutes a charge converter CCV.

Next, an operation of obtaining a vortex signal from which noise has been removed by the vortex detector configured as described above will be described with reference to FIG.

When the fluid flows through the pipe 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, a repetition of the stress distribution as shown in FIG.
In FIG. 21, repetition of charges + Q and -Q corresponding to the signal stress having the vortex frequency shown in FIG. In FIG. 4, the electrode plate 18 or 21 is divided into two parts on the left and right sides with respect to the plane of the drawing, and one of the upper and lower electrodes corresponds to the pedestal 16 or the pressing rod 22 for appropriate explanation.

On the other hand, the pipeline 10 also generates pipeline vibration that becomes noise. The pipeline vibration is divided into a drag direction in the same direction as the fluid flow, a lift direction perpendicular to the fluid flow, and a longitudinal component of the vortex generator. Among them, the stress distribution with respect to the vibration in the drag direction is as shown in FIG.
The positive and negative charges are canceled in each of the electrodes, and no noise charges are generated. Also, as shown in FIG. 4 (c), the vibration in the longitudinal direction is canceled in the electrode and no noise charge is generated as in the case of the drag direction.

However, the vibration in the lift direction has the same stress distribution as the signal stress, and generates noise charges. Therefore, the following calculation is performed to eliminate this noise charge. Piezoelectric elements 17, 21
Each of charge _{Q 1, Q 2, S 1} , S 2 a signal component, the lift direction of the noise component and N _1, N _2, when the reversed polarization in the piezoelectric elements 17 and 21 Q _1, Q ₂ the following It is shown by the formula.

Q ₁ = S ₁ + N ₁ −Q ₂ = −S ₂ −N ₂ However, the vector directions of S ₁ and S ₂ and N ₁ and N ₂ are the same. Here, the relationship between the signal component and the noise component of the piezoelectric elements 17 and 21 is shown in FIGS. 4D and 4E (this diagram shows the relationship between the noise in the lift direction and the bending moment of the vortex generator with respect to the signal). As shown in FIG. 3, when the output of the charge amplifier 25 on the piezoelectric element 17 side is added by the adding circuit 28, the charge on the piezoelectric element 21 side is multiplied by N ₁ / N ₂ together with the volume 27 as shown in FIG. When added to the output of the amplifier 26, Q ₁ −Q ₂ (N ₁ / N ₂ ) = S ₁ −S ₂ (N ₁ / N ₂ ), and the line noise is removed, and an AC voltage signal proportional to the measured flow rate is obtained. e ₁ can be obtained.

Next, the bandpass filter BPF shown in FIG. 1 will be described in detail with reference to FIGS. 5 and 6.

Vortex signal e ₂ is inputted to the resistor R ₁ and capacitor C ₁ 1-order low-pass filter constituted by a LP1, capacitor C ₂ and the output is impedance converted by Baffua amplifier OA1
At one end. The other end of capacitor C ₂ is a capacitor
Inverting input of the operational amplifier OA2 via the C ₃ (-) to which is connected. Non-inverting input terminal (+) output terminal of the connected the operational amplifier OA2 to the common potential point COM is the capacitor C ₂ and C ₃
Of which is connected via a capacitor C ₄ to the connection point Y. Further, the output end the resistance R ₂ of the operational amplifier OA2, a series circuit of a resistor R ₃ and the switch SW _2, the inverting input series circuit in parallel with the resistor R ₄ and the switch SW ₃ (-) to be connected I have. These switches SW ₂ and SW ₃ are connected to comparators CO 1 and C
It is opened and closed by the low cut frequency change signals S ₁ and S ₂ from O2.

Further, a resistor is provided between the connection point Y and the common potential point COM.
A series circuit of R _5, switches SW ₄ and the capacitor C ₅ and the resistor R _6,
A series circuit of the switch SW ₅ and the capacitor C ₆ and the resistance R ₇ are connected in parallel. And these switches
SW ₄ and SW ₅ are opened and closed by low cut frequency change signals S ₁ and S ₂ , respectively.

Next, the bandpass filter BPF configured as above
Will be described with reference to a characteristic diagram shown in FIG.

The horizontal axis indicates the vortex frequency, and the vertical axis indicates the gain of the bandpass filter BPF. _{Also, C A, C B, C} C is low cut frequencies f _A, respectively, f _B, shows a gain curve with f _C (both not shown). Then, the band-pass filters C _A , C _B , and C _C are respectively selected corresponding to the vortex frequencies f _VA , f _V2 , and f _V3 .

The low-cut frequencies f _B and f _C are determined by secondary high-pass filters that are switched by switches SW ₄ and SW ₅ opened and closed by low-cut frequency change signals S ₁ and S ₂ , respectively.

As is apparent from FIG. 6, the vortex frequencies f _VA , f _V2 ,
f _V3 and the respective high cut frequencies (not shown) of the band pass filters C _A , C _B , C _C have the following relationship.

f _VA > High cut frequency of band pass filter C _A f _V2 > High cut frequency of band pass filter C _B f _V3 > High cut frequency of band pass filter C _{C When} the vortex signal e ₂ passes through such a band pass filter BPF, Become like Alternating signal of vortex detection sensor (vortex signal)
Is proportional to the square of the flow velocity, and the flow velocity is proportional to the vortex frequency f. Thus, alternating signal = K × density × f ² where K is a proportional constant. The frequency of this eddy signal is higher than the high cut frequency of the bandpass filter BPF as shown in FIG. 6 (in this frequency band, the gain of the bandpass filter is 1 / f (f
Is the frequency). Therefore, when the vortex signal passes through the band-pass filter, the amplitude of the vortex signal is reduced by 1 / f, and the output signal is as follows: output signal = K × density × f = K × density × flow velocity This signal is a signal corresponding to the mass flow rate.

Thus, the vortex signal e2 is converted into the mass flow signal e3 by passing through the band-pass filter BPF.

In this case, when the eddy frequency to be measured is further increased, the gain on the low frequency side remains high unless the characteristics of the bandpass filter BPF are changed, and the susceptibility to low frequency component noise is increased. . Therefore, the low-cut frequency of the band-pass filter BPF is switched according to the frequency of the vortex signal to lower the low-frequency gain, thereby reducing the effect of low-frequency noise.

In this way, it is possible to obtain the mass flow rate signal e ₃ that reduced low frequency noise as shown in Figure 7.

<Effect of the Invention> As described above in detail with the embodiment, according to the first aspect of the present invention, the high cut frequency is selected to be sufficiently smaller than the eddy frequency of the eddy signal to be measured, and the low cut frequency is set to the low cut frequency. Since the eddy flow signal is input to the bandpass filter controlled by the change signal, a mass flow signal from which the influence of low-frequency noise has been removed can be obtained with a simple configuration.

According to the second aspect of the present invention, the minimum amplitude and the minimum vortex frequency are selected in advance for the expected vortex signal, so that the stability of the zero point can be improved. .

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of one embodiment of the present invention, FIG. 2 is a cross-sectional view showing an outline of a cross section of a vortex detecting section of the mass flow meter in FIG. 1, and FIG. 3 is shown in FIG. FIG. 4 is a block diagram showing the configuration of the charge converter, FIG. 4 is an explanatory diagram for explaining the operation of removing noise in the vortex detector, FIG. 5 is a circuit diagram showing the specific configuration of the bandpass filter shown in FIG.
FIG. 6 is a characteristic diagram showing the characteristics of the bandpass filter shown in FIG. 5, FIG. 7 is a waveform diagram showing how the low-frequency noise is reduced by the bandpass filter shown in FIG. 1, and FIG. FIG. 9 is a block diagram showing an example of the configuration of the mass flow meter of FIG. 9, FIG. 9 is a block diagram showing another example of the configuration of the conventional mass flow meter, and FIG. 10 is a problem of the mass flow meter shown in FIG. FIG. DESCRIPTION OF SYMBOLS 1 ... Eddy detection sensor, 2 ... Pulsing circuit, 3 ... Detection circuit, 4 ... F / V conversion circuit, 5 ... Operation circuit, 6 ... Signal conversion circuit, 7 ... Integration circuit, 8 ... … Detector circuit, 10 ……
Pipe line, 12: Vortex generator, 17, 21: Piezoelectric element, CCV ...
Charge converter, TCC ... Temperature compensation circuit, BPF ... Pand pass filter, DRC ... Detection / rectifier circuit, VFC ... Voltage / frequency converter, FFC ... Frequency / frequency converter, CO1
~ CO4 ... Comparator, ABF ... Active bandpass filter, SMT ... Schmitt trigger.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A 1-267418 (JP, A) JP-A 63-256823 (JP, A) JP 1-25405 (JP, B2) (58) Field (Int.Cl. ⁶ , DB name) G01F 1/86

Claims (2)

(57) [Claims] 1. A vortex signal converting means for converting a vortex generated by the flow velocity of a measurement fluid into a vortex signal, and a high cut frequency to which the vortex signal is inputted is selected to be sufficiently smaller than a vortex frequency of the vortex signal to be measured. A band-pass filter whose frequency is controlled by a low-cut frequency change signal to output a mass flow rate signal corresponding to the vortex signal, and the vortex signal is input, the frequency of the vortex signal is detected, and the low-cut frequency change signal is determined by the magnitude thereof. And a frequency detection circuit for outputting a signal. 2. A switch provided on the output side of the bandpass filter for opening and closing the mass flow signal, and detecting whether a frequency of the vortex signal is smaller than a lower limit of a vortex frequency to be measured. Frequency lower limit detecting means for transmitting a control signal to turn off the switch means when the mass flow rate signal is below the lower limit. And an amplitude detecting means for transmitting a control signal for turning off said switch means.
Mass flow meter according to the item.