JPH055639A - Mass flowmeter - Google Patents

Abstract

PURPOSE:To reduce the error of a mass flowmeter to a binarization error only so as to improve the accuracy of the flowmeter by generating the control signal of the switch of a variable gain amplifier by binarizing the pulse of a vortex frequency. CONSTITUTION:A vortex signal converted from a measured flow rate is changed by means of a digital control signal which is outputted from an adder circuit and used by a variable gain means to change a circuit gain in inverse proportion to the frequency of the vortex signal and the mass flow rate signal corresponding to a measured flow rate is outputted by detecting and rectifying the vortex signal. A pulse signal P1 through a Schmitt trigger 45 is inputted to a PLL circuit 50 and fed back after its frequency is divided by means of a 1/N counter 51, resulting in N-times enlargement of the frequency. A binary counter 52 binarizes the signal P1 and latch circuit 54 latches the binarized signal. The output e3 of the adder circuit is connected to the circuit 54 through resistances R1-Rn and outputted to a detection/rectifier circuit through a variable gain amplifier 56. The amplification degree of the amplifier 56 is inversely proportional to the vortex frequency and changed by means of the output of the circuit 54. Since the control signal of the switch of the amplifier 56 is generated by binarizing the pulse of the vortex frequency, the error of the variable gain means can be eliminated except a binarization error.

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 which detects an alternating force generated in a vortex generator by a Karman vortex corresponding to the flow of a fluid such as gas or liquid and takes out this as a mass flow signal. , And in particular to a mass flow meter modified to reduce the signal / noise (S / N) ratio of the vortex signal.

[0002]

2. Description of the Related Art As a conventional technique for measuring a mass flow rate using a signal due to a Karman vortex, for example, Japanese Patent Application No. 1-251.
There is a mass flowmeter disclosed in No. 095 "Title of Invention: Mass Flowmeter". Therefore, the basic contents of the mass flowmeter disclosed herein will be described below.

FIG. 3 is a block diagram showing the structure of this conventional mass flowmeter. Piezoelectric elements 17, 21 depending on the measuring fluid
The eddy charges Q ₁ and Q ₂ generated in the above are converted into AC voltages e ₁ and e ₂ by the charge amplifiers 25 and 26 via terminals A and B, respectively. The AC voltage e ₂ is added to the AC voltage e ₁ via the volume 27 by the adder circuit 28 and output as the vortex signal e ₃ to the output terminal.

After that, the vortex signal e ₃ is output to the low-pass filter 39 whose corner frequency fc is set to be equal to or lower than the lowest vortex frequency f _min of the vortex signal in the measuring range, where the AC mass flow rate signal is output. It is converted to e ₈ and output. The mass flow rate signal e ₈ is amplified by the amplifier 40, detected and rectified by the detection rectification circuit 41, and output as a DC mass flow rate signal E ₀₂ to the output terminal.

This mass flow rate signal E ₀₂ is output as a pulse signal P _m proportional to the mass flow rate by the voltage / frequency conversion circuit 42. The integrated value of the mass flow rate can be easily obtained by integrating the pulse signals P _m .

Next, the operation of the mass flowmeter configured as described above will be described with reference to FIGS. 4 and 5. Figure 4
Shows the characteristics of the low pass filter when the vortex frequency f is taken on the horizontal axis and the vortex signal e ₃ is taken on the vertical axis. FIG. 5 shows the mass signal ρV on the horizontal axis and the mass flow rate signal e ₈ on the vertical axis. The output characteristics of the low-pass filter of FIG. However, ρ is density and V is velocity.

At the output end of the adder circuit 28, the vortex signal e ₃ from which piping noise and the like are removed is output. This eddy signal is an AC signal proportional to the square of the flow velocity V as shown by e ₃ = K ₃ ρV ² (1).

Here, if the corner frequency fc of the primary low-pass filter 39 is selected to be extremely small with respect to the lowest vortex frequency f _min of the vortex signal as shown in FIG. 4, the AC mass flow rate signal e ₈ Becomes e ₈ = K ₄ ρV ² / f (2) with K ₄ as a constant.

Since V has a proportional relationship with f, the relationship of the expression (2) becomes the relationship of the following expression (3). This relationship is shown in FIG. However, K4 'is a constant. e ₈ = K 4 ′ ρV (3) Therefore, an AC mass flow rate signal e ₈ is obtained at the output end of the low-pass filter 39.

In this case, for example, fc is fc = f _min / 10
, The nonlinear error q at f _min
Is about −0.5%. If the vortex frequency f at the span flow rate is 10f _min , this is -0.0
5% / F. It is a small value corresponding to S. For example, when measuring gas with a diameter of 50 A, the measurement flow rate is 4 to 40 m /
If s, the vortex frequency is 70-700 Hz, and fc
Should be set to fc = 7 Hz.

As can be seen from the above description, in this mass flow meter, the vortex signal is taken out through the low pass filter selected for the predetermined corner frequency, so that the circuit structure is greatly simplified. Mass flow rate can be measured,
Further, since the output of the low-pass filter is an output proportional to the flow velocity, it is possible to avoid the narrowing of the flow rate range due to the saturation of the detection rectification circuit in the subsequent stage. Further, according to this mass flowmeter, since a division circuit having a denominator corresponding to the input signal corresponding to the vortex frequency is not used, it is possible to measure up to the lower limit flow rate at which a vortex is generated if a certain error is allowed. Become.

In such a conventional mass flow meter, the mass flow rate can be measured with a simple structure, the flow rate range can be narrowed due to the saturation of the circuit, and the lower limit flow rate that can be measured Can be lowered.

However, the output e ₃ of the adder circuit 28 is obtained by passing through the low-pass filter 39 having a corner frequency fc to obtain the mass flow rate signal e ₈ shown in the equation (3). Through the low-pass filter 39, when the vortex frequency f increases as shown in FIG. 4, the gain of this circuit decreases in proportion to the vortex frequency f.
When the vortex frequency f becomes large, that is, when the flow rate becomes large, if noise of a low frequency is added, the S / N ratio may be lowered and the signal processing may be adversely affected.

Therefore, in order to solve the above points, the present applicant has filed Japanese Patent Application No. Hei 2-121490 on May 11, 1990.
It proposes "Title of Invention: Mass Flowmeter". The contents of this application will be described below. FIG. 6 is a block diagram showing the configuration of the mass flowmeter according to this application.

The output e ₃ of the adder circuit 28 is converted into a mass signal e ₉ by the variable gain circuit 43 whose gain is controlled by the control voltage Vc and output to the detection rectification circuit 41. Further, the output e ₃ is output to the Schmitt trigger 45 after unnecessary noise is removed by the band filter 44.

The Schmitt trigger 45 pulsates the output of the band filter 44 and outputs it as a pulse signal P ₁ to the frequency / voltage converter 46, where the variable gain circuit 43 is provided.
Is converted into a control voltage Vc for controlling the gain of the. This control voltage Vc is proportional to the frequency of the vortex signal.

Next, the operation of the mass flow meter constructed as above will be described. The amplitude of the output e ₃ of the adder circuit 28 is proportional to ρV ² . Further, the control signal Vc is converted by the Schmitt trigger 45 into a signal proportional to the vortex frequency f of the measurement fluid. The vortex frequency f is the flow velocity V
In the end, the control signal Vc is proportional to the flow velocity V, and Vc = K ₅ V (4) with K ₅ as a constant.

Since the variable gain circuit 43 controls its gain by the control signal Vc so as to be inversely proportional to the vortex frequency f, that is, the flow velocity V, the mass signal e ₉ shown in the following equation is obtained at its output end. e ₉ = e ₃ / Vc = (K ₃ / K ₅ ) ρV (5) Therefore, the mass signal e _{9 in} this case is gain-adjusted in a uniform form over the entire frequency band, and is obtained as a mass signal. Is output. Therefore, the low frequency noise is not emphasized relative to the vortex signal, which contributes to the improvement of the S / N ratio.

Next, the variable gain circuit 43 will be described in more detail with reference to FIGS. FIG. 7 is a block diagram showing a circuit configuration centering on a variable gain circuit, and FIG.
7 is an operation diagram showing an operation state of the analog / digital converter in FIG. 7, and FIG. 9 is a characteristic diagram showing a relationship between the control signal Vc and the gain of the variable gain circuit shown in FIG.

The output e ₃ of the adder circuit 28 is a buffer amplifier 47A, 47B, ... 47N, and resistors R _A , R _B ,.
_N and the switches SW _A , SW _B , ... SW _N are connected in parallel to the (−) input terminal of the adder 48 via a series circuit connected for each corresponding subscript. The (−) input end and the output end of the adder 48 are connected by a resistor R ₀ , and the (+) input end is connected to the common potential point COM.

The gain is determined by the ratio of the resistances R _A , R _B , ... _RN and the resistance R ₀ . In this case, the resistance _{R A, R B, ...... R} N are each 1,2,4,
The values are selected so that the ratio becomes 8, ..., 512, the resistance R ₀ = R _N , and all the switches SW _A , SW _B ,.
When SW _N is on, e ₉ / e ₃ = (512 + 256 + ... + 1). That is, the switches SW _A , SW _B , ... SW _N
The gain can be changed from 1 to 1000 times by properly selecting.

In this case, the switches SW _A , SW _B , ...
... SW control signals _N for controlling the _{S A, S B, ...... S} N are output Q _A of the analog / digital converter 49, Q _B, ......
Get from Q _N. Assuming that there are 10 output terminals Q _A , Q _B , ... Q _N of the analog / digital converter 49, a pulse signal P ₁ at the input terminal of the Schmitt trigger 46 is assumed.
Vortex frequency f ₁ of, f _2, the control signal S for ...... f _1000
A, S _B, corresponding to ...... S _N control signals S _1, S _2, ...
... state of S ₁₀ is high level H, the L b - as shown in Figure 8 when the level to.

In this way, the switches SW _A , SW _B , ...
By controlling ON / OFF of SW _N , the gain of the variable gain circuit 43 can be linearly changed with respect to the control signal Vc as shown in FIG.

[0024]

However, in such a mass flowmeter, the Schmitt trigger 45 is used for the band filter 4.
The output of 4 is pulsed to produce a pulse signal P ₁ of frequency /
Since the voltage converter 46 converts the DC control voltage Vc and the analog / digital converter 49 converts the digital signal, the frequency / voltage converter 46 and the analog / digital converter 49 cause an error. There is.

[0025]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a signal converting means for converting a measured flow rate into a vortex signal and outputting the vortex signal, and an output of the signal converting means for inputting a circuit gain. Variable gain means that is varied by a digital control signal that varies inversely with the frequency of the previous vortex signal, and binarization conversion means that receives the vortex frequency of the previous vortex signal and converts the vortex frequency into a binary signal. The output of the variable gain means is detected and rectified, and the detection and rectification means for outputting a mass flow rate signal corresponding to the measured flow rate is provided, and the above binary signal is output as the above digital control signal.

[0026]

The signal conversion means converts the measured flow rate into the vortex signal and outputs the vortex signal. The output of the signal conversion means is changed by the variable gain means by the digital control signal which changes the circuit gain in inverse proportion to the frequency of the vortex signal. Let On the other hand, the binary conversion means converts the vortex frequency into a binary signal to obtain the digital control signal, which controls the variable gain means. Further, the output of the variable gain means is detected and rectified by the detection rectification means, and a mass flow rate signal corresponding to the measured flow rate is output. As described above, the error factor in the variable gain means, which is a main component of the mass flowmeter, is only the error due to the binarization conversion means, and the error factor can be reduced and the overall accuracy is improved.

[0027]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the main part of the present invention. The pulse signal P ₁ of the Schmitt trigger 45 shown in FIG. 6 and the output e ₃ from the adder circuit 28 are respectively input.

The pulse signal P ₁ has a frequency f _s , and this pulse signal P ₁ is a PLL (phase locked loop).
Input to the circuit 50. The output of the PLL circuit 50 is divided by the 1 / N counter 51 and fed back to the input, whereby the frequency f _s is expanded N times. These circuits are used to improve accuracy when the vortex frequency at the maximum flow rate is low, as will be described later.

The binary counter 52 includes a PLL circuit 5
Reset signal Rs which is output from the output N · f _s and the pulse generating circuit 53 0 is input, the output N · f _s corresponds to the constant pulse period Ts of the reset signal Rs and binarized, the output Q1 , Q2, ..., Qn are output.

The latch circuit 54 includes a binary counter 5
2 outputs Q1, Q2, ..., Qn and clock generation circuit 5
The clock signal CLK from 5 is input, and the binary counter 52 converted into a binary number by this clock signal CLK.
, Qn are output as the outputs D1, D2, ... Dn to the output terminals.

A resistor R _f is connected between the inverting input terminal (−) and the output terminal of the variable gain amplifier 56 whose non-inverting input terminal (+) is connected to the common potential point COM. The output e ₃ from the adder circuit 28 is connected to the end (−) via a series circuit of resistors R ₁ , R ₂ , ..., R _n .

Switches SW ₁ , SW ₂ , ..., SW _n are connected in parallel to these resistors R ₁ , R ₂ , ..., R _n , respectively, and these switches SW ₁ , SW ₂ , …
..., SW _n are outputs D1, D2, ... D of the latch circuit 54
n are inverted and applied by the inverters G ₁ , G ₂ , ..., G _n , respectively, and the opening and closing thereof are controlled.

In this way, the inverse of the variable gain amplifier 56 is
Resistance R connected to the input terminal (-) ₁, R₂, ……, R
_nAnd resistance R_fThe variable gain amplifier 5 with the amplification degree determined by
6 is the output e from the adder circuit 28 applied to its input end
_TenIs output to the detection rectification circuit 41. This amplification is the vortex frequency
In inverse proportion to the number, the digital outputs D1, D2, ... Dn
More variable.

In FIG. 1, the PLL circuit 50 and 1 /
Vortex frequency f by N counter 51_sTo increase N times
Will be described below. For example, the binary counter 52
Has 10-bit output, 0 to 2047
It can be expressed in binary. Vortex circumference at maximum flow rate
Schmitt trigger with low pulse frequency and constant pulse period Ts
The number of pulses output from 45 is much lower than 2047.
If it is about 500, for example,
If output to the binary counter 52 as it is, the upper 2 bits
Will not be used. In other words, the switch of amplifier 56
Switch SW₉, SW _TenWill not work, so disassemble
Noh will be worse.

Therefore, the number of pulses input to the binary counter 52 is increased at a constant pulse period Ts to use all bits to improve the resolution. In addition, in order to increase the number of pulses, the pulse period Ts of the reset signal Rs is
It may be possible to increase the
Data holding time becomes longer, and thus the gain changing cycle also becomes longer, resulting in poor linearity.

Next, the operation of the embodiment configured as described above will be described with reference to the waveform chart shown in FIG. The pulse signal P ₁ having the frequency f _s of the Schmitt trigger 45 is input to the PLL circuit 50 and multiplied by N, and this is input to the binary counter 52. The binary counter 52 is reset when the reset signal Rs is at a high level, but when it goes to a low level (FIG. 2A) at time t ₀ , the output Nf _s of the PLL circuit 50 becomes the data 1 and the binary counter 5
2 is input (FIG. 2 (b)). Binary counter 52
Converts this data 1 into a binary number and outputs it to its output end Q1,
Digitally output as Q2, ..., Qn.

The latch circuit 54 outputs the digital outputs Q1, Q2, ..., While the clock signal CLK is at a high level.
Latch Qn and output at low level. At time t _1, it becomes a low level and outputs D1, D2, ... Dn to its output terminals.
However, it becomes a high level at time t ₂ and these are held as data 1 (FIG. 2 (c)).

At time t ₃ , the reset signal Rs becomes low level (FIG. 2 (a)), so the binary counter 52
Similarly counts the data 2 and outputs it to the latch circuit 54, but at time t ₄ , the clock signal CLK becomes low level, and the latch circuit 54 outputs the corresponding output D1,
D2, ... Dn at the output end of data 2 (Fig. 2 (b))
Output as. At time t ₅ , the clock CLK becomes high level and this data 2 is held (FIG. 2 (c)). Thereafter, these operations are similarly repeated.

Outputs D1, D2, ... Of this latch circuit 54
The binary signal Dn is a switch S as a control signal.
W ₁ , SW ₂ , ..., SW _n are opened and closed so that the amplification degree of the variable gain amplifier 56 is inversely proportional to the vortex frequency.

[0040]

As described above in detail with the embodiments, according to the present invention, since the pulse of the vortex frequency is converted into a binary number and used as the control signal of the switch of the variable gain amplifier, an error may occur. The property is only a conversion error when it is converted into a binary number, and the error in the variable gain means which is an important constituent element of this type of mass flowmeter becomes small, which contributes to the improvement of accuracy as a whole.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram illustrating the operation of the embodiment shown in FIG.

FIG. 3 is a block diagram showing a configuration of a first conventional mass flowmeter.

FIG. 4 is a first graph showing characteristics of the low-pass filter shown in FIG.
FIG.

FIG. 5 is a second graph showing the characteristics of the low-pass filter shown in FIG.
FIG.

FIG. 6 is a block diagram showing a configuration of a second conventional mass flowmeter.

7 is a block diagram showing details of the variable gain circuit shown in FIG.

FIG. 8 is an operation diagram showing an operation state of the analog / digital converter in FIG.

9 is a characteristic showing a relationship between a control signal and a gain of the variable gain circuit shown in FIG.

[Explanation of symbols]

 17, 21 Piezoelectric element 12 Vortex generator 25, 26 Charge amplifier 28 Summing circuit 39 Low-pass filter 41 Detection rectification circuit 43 ... Variable gain circuit 44 Band filter 45 Schmitt trigger 46 Frequency / voltage converter 47A, 47B, ..., 47N buffer Amplifier 48 Adder 49 Analog / Digital converter 50 PLL circuit 51 1 / N counter 52 Binary counter 54 Latch circuit 56 Amplifier

Claims (1)

Claim: What is claimed is: 1. A signal converting means for converting a measured flow rate into an eddy signal and outputting the eddy signal, and an output of the signal converting means is inputted to change a circuit gain in inverse proportion to the frequency of the eddy signal. Variable gain means that is varied by a digital control signal, binarization conversion means that receives the vortex frequency of the vortex signal and converts the vortex frequency into a binary signal, and the output of the variable gain means is detected and rectified to perform the measurement. A mass flowmeter, comprising: a detection rectification unit that outputs a mass flow rate signal corresponding to a flow rate, and that outputs the binary signal as the digital control signal.