JP2001349755A - Vortex detecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically and continuously control a passband of a band pass filter based on a vortex frequency to synchronize the vortex frequency substantially in its center over a wide range irrespective of kinds of fluids and a bore. SOLUTION: A high-pass filter 11 and a low-pass filter 12 constituting the band filter have switched capacitors respectively, and a cut-off frequency is controlled by making switching frequencies thereof variable. A fixed low-pass filter 13 is connected further to an output of the high-pass filter 11 to detect the rough vortex frequency, and the switching frequencies of the respective switched capacitors in the both filters 11, 12 are made variable based on the detected rough vortex frequency. A passband width of the band filter is controlled thereby to bring the detected vortex frequency substantially into the center of the passband width.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vortex detection circuit, and more particularly, to measuring a flow rate by detecting a vortex frequency of a vortex detection signal after passing through a bandpass filter for removing noise other than the vortex detection signal. The present invention relates to a vortex detection circuit.

[0002]

2. Description of the Related Art As is well known, a vortex flowmeter comprises a pipe to be measured through which a fluid to be measured flows, a vortex generator fixed in the pipe to be measured so as to face the flow, and a vortex detector. This is based on the fact that the number of Karman vortices flowing out of a vortex generator per unit time (vortex frequency) is proportional to the flow rate in a predetermined Reynolds number range regardless of gas or liquid. As a vortex detector, a heat change, a lift change, and the like due to a vortex can be detected by a heat sensor, a strain sensor, an optical sensor, a pressure sensor, an ultrasonic sensor, or the like. Such a vortex flow meter is a simple flow meter that can measure the flow rate without being affected by the physical properties of the measurement fluid, and is widely used for measuring the flow rate of gases and liquids. As shown in FIG. 12, in addition to the vortex signal, fluctuations at a frequency lower than the vortex frequency, flow noise at a frequency higher than the vortex frequency, and the like are included. This vortex signal passes through a band-pass filter,
It is necessary to remove noise and the like other than the vortex signal shown in FIG.

A high-precision vortex flowmeter can be provided by providing a band-pass filter which automatically follows the vortex frequency in a vortex signal detection circuit to eliminate signals other than the vortex signal. From such a viewpoint, the present applicant has previously proposed an "band automatic tracking filter" (see Japanese Patent Publication No. 63-32126).

The "automatic band tracking filter" proposed earlier
Is to switch the cutoff frequency of the filter one after another by an analog switch in accordance with the detected vortex frequency, thereby making it possible to eliminate noise and the like other than the vortex frequency. However, since the number of switching stages is limited, it is necessary to increase the pass band width to some extent in order to cope with a wide flow rate range.

A vortex flowmeter using a switched capacitor has been proposed (Japanese Patent No. 3031387). Thus, it is possible to arbitrarily change the bandwidth by changing the corner frequency of the band-pass filter. However, this technique can set the bandpass filter to the bandwidth determined from the flow range determined by the type of the fluid and the diameter, but automatically sets the bandpass filter regardless of the type and the diameter of the fluid. It cannot be synchronized.

[0006]

SUMMARY OF THE INVENTION Accordingly, the present invention solves such a problem and uses a band-pass filter for eliminating noises other than the detected vortex frequency, and a signal passing through the band-pass filter. The purpose is to automatically and continuously control the band based on the vortex frequency and to tune the vortex frequency to a wide range regardless of the type and diameter of the fluid, with the vortex frequency substantially centered.

Another object of the present invention is to make it possible to narrow the pass band width and obtain a higher quality vortex signal.

[0008]

The eddy detection circuit of the present invention comprises:
After passing the vortex detection signal detected by the vortex detector through a band filter that removes noise etc. other than the vortex detection signal,
The flow rate is measured by detecting the vortex frequency of the vortex detection signal. This bandpass filter is composed of a high-pass filter and a low-pass filter connected in cascade. Each of the high-pass filter and the low-pass filter includes one or a plurality of switched capacitors, and is configured so that the cutoff frequency can be controlled by changing the switching frequency. A fixed low-pass filter is further connected to the output of the high-pass filter to detect a rough eddy frequency, and the switching frequencies of the switched capacitors of the high-pass filter and the low-pass filter are varied based on the detected rough eddy frequency. This controls the pass bandwidth of the bandpass filter so that the detected vortex frequency is approximately at the center of the pass bandwidth.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a vortex detection circuit according to the present invention will be described with reference to specific examples. FIG. 1 is a diagram showing the overall schematic configuration of the vortex detection circuit.

The eddy detection signal detected by the eddy detector 1 is amplified by the amplifier circuit 2, and then the noise other than the eddy detection signal is removed through the frequency tracking bandpass filter 3, which is a feature of the present invention. Then, according to the usual techniques,
The waveform is shaped by the waveform shaping circuit 4 and then input to the CPU 5. The CPU 5 displays the input vortex frequency on the display 7, outputs a pulse through the pulse output circuit 6, and outputs an analog signal through the D / A conversion circuit 8.

The frequency tracking bandpass filter 3 which is a feature of the present invention is a bandpass filter composed of a cascade-connected highpass filter 11 and lowpass filter 12 such that the detected vortex frequency is at the center of the bandwidth. Control the pass bandwidth. In order to control the pass bandwidth, a fixed low-pass filter 13 connected to the output of the high-pass filter 11, a waveform shaping circuit 14, and a filter control circuit 1
5 for detecting a rough eddy frequency and controlling a high-pass filter and a low-pass filter cascade-connected to the high-pass filter based on the rough eddy frequency, thereby forming a band constituted by these filters. By tuning the pass filter and passing only the vortex frequency, a good quality vortex signal is detected.

In order to detect the approximate value of the vortex frequency, a signal after leaving the high-pass filter 11 is input to a fixed low-pass filter 13. The characteristics of the band-pass filter constituted by the high-pass filter 11 and the fixed low-pass filter 13 are as shown in FIG. The upper limit fLmax of the illustrated pass band is the fixed low-pass filter 13.
Is set to a fixed value slightly higher than the frequency at the maximum flow rate of the connected vortex flowmeter. The lower limit of the passband is
It is determined by the high-pass filter 11 controlled by the filter control circuit 15. In the figure, the lines denoted by fH1 to fHn indicate that the lower limit of the pass band changes according to the vortex frequency. Note that, as described later, the configuration is such that the cutoff frequency of the high-pass filter does not fall below fH1 even during stoppage. For this reason, as shown by the solid line in FIG. 11, the pass band width (fH1 to fLmax) is widened during stoppage or when the flow rate is low and the vortex frequency is low.

In general, when the vortex frequency is low, the quality of the vortex signal is good. When the vortex frequency is high, the vortex signal tends to include a large amount of flow noise and the like. Also, as shown in FIG.
As the eddy frequency becomes higher, the bandwidth becomes narrower and the performance as a filter improves, so that the eddy frequency can be detected with sufficient accuracy as a filter control signal. In this way, the eddy frequency signal detected as the approximate value is subjected to waveform shaping by the waveform shaping circuit 14, and then input to the filter control circuit 15 for controlling the high-pass filter 11 and the low-pass filter 12, respectively. Control. The details will be described later.

FIG. 6 is a diagram showing an example of the configuration of a fixed low-pass filter that can be used in the vortex detection circuit shown in FIG. When this fixed low-pass filter is Butterworth (maximum flat), R ₅ = R ₆ = R, C ₁₁ = 2C
₁₂ , the cutoff frequency F of the fixed low-pass filter
_{L is, F L = 1 / (√2} · π · C 11 · R) and becomes, cutoff frequency of the fixed filter, as described above, is set to a value slightly higher than the maximum value of the vortex frequency .

The filter control circuit 15 used in the frequency tracking bandpass filter of FIG. 1 is a filter control circuit for re-oscillating a frequency proportional to the vortex frequency in order to tune the high-pass filter 11 and the low-pass filter 12 to the vortex frequency. FIG. 8 shows an example of the configuration.
As shown, by using a high-speed frequency / voltage (F / V) conversion circuit and a voltage / frequency (V / F) conversion circuit, the response of the band-pass filter can be improved. In addition, in order to detect the approximate value of the vortex frequency at high speed, a period is measured using a CPU as shown in FIG.
A rough value of the vortex frequency may be obtained from the reciprocal thereof, and a switching frequency corresponding to the vortex frequency may be oscillated to tune to the vortex frequency.

FIGS. 2 and 3 are diagrams illustrating the configuration of a high-pass filter, respectively. FIGS.
It is a figure which illustrates the structure of each low-pass filter.
Each of them can be used depending on the difference between the primary filter connected in a two-stage cascade and the secondary filter, and the primary filter can be configured in one stage. In any case, the cutoff frequency of the high-pass and low-pass filters can be continuously controlled by the control frequency based on the principle of the switched capacitor. The cutoff frequency of these filter circuits is controlled by the switching frequency Fc1 of the switched capacitors SCP1 to SCP10.
To Fc10 by changing the filter control circuit 15 described above.

A high-pass filter shown in FIGS.
And an equivalent circuit of the switched capacitors SCP1 to SCP10 used in the low-pass filter, as shown in FIG. 7, when the switching frequency is Fc, Ro = 1 / Co · Fc
It is known that this can be expressed as the equivalent resistance Ro.

Therefore, the high-pass filter of FIG.
Frequency F_HIs the first-stage cutoff frequency F_H1= 1 / (2π · C_Two・ Ro) = Co ・ Fc₁/ (2πC_Two) Second cutoff frequency F_H2= 1 / (2π · C_Four・ Ro) = Co ・ F_C2/ (2πC_Four), And if the advantage of the pass frequency range is OdB, C₁= C
_Two, C_Three= C_FourBecomes Also, the high-pass filter shown in FIG.
The cutoff frequency is Butterworth (maximum flat)
In this case, Q = 1 / 良 く 2, and C_Five= C₆= C, Fc
_Four= (1/2) Fc _ThreeThen, the cutoff frequency F at this time
_HIs F_H= 1 / (√2 · π · C · Ro) = Co · Fc_Four/ (√2 · π · C) Here, the frequency of SCP3 was set to 1/2, but it was connected to SCP3.
Assuming that Co is 1/2, the frequencies of SCP3 and SCP4 are the same.
It can be a wave number.

Although not shown in the figure, in order to prevent the switching frequency from becoming zero during stop, a resistor is previously inserted in parallel with the switched capacitor so that the cutoff frequency becomes slightly lower than the minimum vortex frequency. Alternatively, even if the vortex frequency becomes zero, it can be configured to oscillate at the minimum frequency.

On the other hand, the low-pass filter is connected downstream of the high-pass filter, and has a cutoff frequency F _L1 of FIG.
Is, if the gain in the passband is OdB, Fc ₅ = Fc ₆ ,
Next Fc ₇ = Fc _8, cut-off frequency _{F L1 = 1 / (2 π} · C 7 · Ro) of the first stage _{= Co · Fc 5 / (2} πC 7) cutoff frequency of the second stage F _L2 = 1 / (2 π · C ₈ · Ro) = Co · Fc ₇ / (2 πC ₈ ). In order to set the cutoff frequency of the low-pass filter in FIG. 5 to Butterworth (maximum sharing),
It is sufficient to Q = 1 / √2, when the _{Fc 9 = Fc 10, C 10} = 2C 9, cutoff frequency F _L at this _{time, F L = 1 / (√2} · π · C 10 · Ro) = Co · F _c9 (√2 · π · C ₁₀ ), and a high-pass filter and a low-pass filter are connected (cascaded) to form a band-pass filter with an arbitrary bandwidth. When Fc is increased, the equivalent resistance value Ro is decreased, and the cutoff frequency of the high-pass and low-pass filters can be increased. FIG. 10 is a diagram showing the characteristics of the band-pass filter. Although FIG. 10 shows stepwise, in practice, the cutoff frequency can be continuously changed. As shown in the figure, a rough eddy frequency is detected, and based on this rough eddy frequency, a high-pass filter and a low-pass filter connected in cascade are controlled, so that the band-pass filter formed by these filters becomes a vortex. The pass bandwidth is continuously varied so that only frequencies are passed.

[0021]

According to the present invention, the approximate eddy frequency is detected, and the switching frequency of each of the switched capacitors of the high-pass filter and the low-pass filter is varied based on the detected approximate eddy frequency. The pass band is automatically and continuously controlled based on the vortex frequency, so that the vortex frequency can be tuned to a wide range irrespective of the type and diameter of the fluid, with the vortex frequency being substantially centered.

Further, the present invention makes it possible to narrow the pass band width and obtain a higher quality eddy signal.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall schematic configuration of a vortex detection circuit.

FIG. 2 is a diagram illustrating a configuration of a high-pass filter.

FIG. 3 is a diagram illustrating another configuration of the high-pass filter.

FIG. 4 is a diagram illustrating a configuration of a low-pass filter.

FIG. 5 is a diagram illustrating another configuration of the low-pass filter.

FIG. 6 is a diagram illustrating an example of a configuration of a fixed low-pass filter that can be used in the vortex detection circuit illustrated in FIG. 1;

FIG. 7 is a diagram showing an equivalent circuit of the switched capacitor SCP.

FIG. 8 is a diagram illustrating a filter control circuit.

FIG. 9 is a diagram illustrating another example of the filter control circuit.

FIG. 10 is a diagram illustrating characteristics of a band-pass filter including a high-pass filter and a low-pass filter.

FIG. 11 is a diagram illustrating characteristics of a band-pass filter including a high-pass filter and a fixed low-pass filter.

FIG. 12 is a diagram showing an input waveform of a band-pass filter.

FIG. 13 is a diagram showing a bandpass filter output waveform.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 vortex detector 2 amplifier circuit 3 frequency tracking bandpass filter 4 waveform shaping circuit 5 CPU 6 pulse output circuit 7 display 8 D / A conversion circuit 11 high-pass filter 12 low-pass filter 13 fixed low-pass filter 14 waveform shaping circuit 15 filter control circuit

Claims (1)

[Claims] 1. A vortex detection signal detected by a vortex detector,
In a vortex detection circuit that measures a flow rate by detecting a vortex frequency of the vortex detection signal after passing through a band filter that removes noise and the like other than the vortex detection signal, the band filter includes a high-pass filter and a cascade connection with the high-pass filter. The high-pass filter and the low-pass filter are each provided with one or more switched capacitors, and configured to be able to control the cutoff frequency by varying the switching frequency thereof, A fixed low-pass filter is further connected to the output of the high-pass filter to detect a rough eddy frequency, and based on the detected rough eddy frequency, varies the switching frequency of each of the switched capacitors of the high-pass filter and the low-pass filter. And the detected vortex frequency is A vortex detection circuit comprising controlling a pass band of the band filter so as to be substantially at the center of the band width.