JP2856521B2 - Electromagnetic flow meter - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an electromagnetic flowmeter, and more particularly to a rectangular wave excitation for realizing a highly accurate and highly stable measurement signal processing in flow measurement with high noise and low flow velocity. The present invention relates to an electromagnetic flow meter of the type.

[Conventional technology]

In a conventional rectangular wave excitation type electromagnetic flow meter, a rectangular wave is applied to a pair of excitation coils provided in a measurement tube to perform excitation, and a rectangular wave-like detection signal obtained from a detection electrode by the rectangular wave excitation. Among them, the signal processing of obtaining measurement data relating to the flow rate by integrating only the stable latter half signal portion except for the unstable first half signal portion including the differential noise was performed.

[Problems to be solved by the invention]

In the above-mentioned conventional method, since the latter half of the rectangular wave detection signal is integrated, its frequency range covers a wide band including the excitation frequency. There was a problem that the effects could not be avoided.

In the electromagnetic flow meter, slurry noise caused by solids affecting the electrodes, flow noise caused by the flow of the liquid to be measured, noise during measurement of low conductive fluids and low flow velocities, etc. These noises cause the measurement accuracy and the stability of the measurement to decrease. The conventional rectangular-wave-excitation-type electromagnetic flowmeter does not give sufficient consideration to solving these problems, and cannot solve it technically.

An object of the present invention is to provide high-precision and stable measurement even under measurement conditions in which slurry noise, flow noise, low-conductive fluid, low-flow velocity noise, etc. are generated, and the accuracy and stability of flow velocity measurement are reduced. It is to provide an electromagnetic flowmeter that can be performed.

[Means for solving the problem]

In order to achieve the above object, a first electromagnetic flow meter according to the present invention has a means for holding a detection signal for a predetermined time after switching an excitation state in synchronization with rectangular wave excitation, and A filter for extracting only a predetermined frequency component related to an excitation frequency from the detection signal, and switching means for switching a center frequency of the signal detection filter to a fundamental frequency or a harmonic frequency in accordance with an object to be measured. The flow rate measurement is performed by using the switched effective value of the frequency component of the fundamental frequency or the harmonic frequency as a measurement signal.

The second electromagnetic flowmeter according to the present invention, in the above configuration, includes a second filter that detects background noise having a center frequency near the predetermined frequency component, and an effective value output of the signal detection filter. And a calculator for calculating the difference between the effective value output of the second filter for detecting the background noise and the flow rate measurement using the output of the calculator as a measurement signal.

The electromagnetic flowmeter according to the present invention, in the above-described configuration, as a means for detecting a change in the characteristics of the filter, a means for intermittently inputting a signal having a constant peak value synchronized with the excitation at a stage preceding the filter, It is characterized in that the measurement signal is normalized based on the output and the flow rate is measured.

The electromagnetic flowmeter according to the present invention is characterized in that, in the above configuration, a filter whose center frequency can be externally controlled by a clock signal is used as the signal detection filter.

[Action]

In the electromagnetic flow meter according to the present invention, a filter for extracting only the frequency component of the fundamental frequency or the harmonic frequency necessary for the flow rate measurement and removing the other frequency components is provided, and by measuring the effective value thereof, the frequency other than the measurement frequency is measured. It excludes the influence of the components. That is, the conventional signal processing is a signal processing on the time axis, but the signal processing of the present invention is based on a signal processing on the frequency axis.

Further, in order to remove a noise component included in the measurement frequency component, a second filter for detecting a background noise component in a frequency band near the measurement frequency is provided, and an effective value of the output is obtained to perform measurement. The difference was determined from the effective value of the frequency to eliminate background noise.

Furthermore, a signal due to differential noise is always included in the frequency composition, and a means for holding a constant value without taking in the measurement signal for a certain time after the excitation switching is provided to eliminate it,
Thereby, the influence of the differential noise is eliminated.

Further, means for switching the center frequency of the filter according to the measurement conditions is provided, whereby low noise and high stability can be realized.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

First, the measurement principle of the electromagnetic flow meter according to the present invention will be described with reference to FIG. In FIG. 1, (A) shows a frequency spectrum of a detection signal obtained by a detector of a rectangular wave excitation type electromagnetic flowmeter. In (A), the horizontal axis represents frequency, and the vertical axis represents signal intensity. (B) shows the pass band characteristic of the signal detection filter, and (C) shows the pass band characteristic of the background noise detection filter. When the excitation frequency in the rectangular exciting method was f _0, the fundamental frequency component 101 peak value of Sx can be obtained in the detection signal, in the same manner Sx _1, S
harmonic component having a peak value of x ₂ 102 (frequency f ₁₎ and 103
(Frequency f ₂ ) is obtained. At the base of each detection signal is a background noise having a continuous noise spectrum.
104 appears, and the background noise 104 usually has a characteristic that the signal strength becomes low and the signal strength becomes large. FIG. 4 shows an example of the actual spectrum of the background noise 104.

In the measurement method of the electromagnetic flow meter according to the present invention, as shown in FIG. 1B, a signal detection filter having a pass band characteristic that passes only the signal of the fundamental frequency component 101 of the detection signal, and FIG. And a background noise detection filter having a pass band characteristic for extracting only the background noise 104a in a predetermined frequency range near the fundamental frequency component as shown in FIG.
101 obtains the difference (signal strength Sx) and background noise 104a (strength S ₀ of the signal) (Sx-S _0), thereby the slurry noise or flow noise, in a low conductivity fluid and low flow rates measured It is configured to extract only a true measurement signal excluding noise and the like. The center frequency of charge passing through the signal detection filter is
f ₀ , and the center frequency of the pass band of the background noise detection filter is f ₀ −Δf.

Next, a specific device configuration will be described with reference to FIG.

Reference numeral 1 denotes a measurement tube through which a fluid to be measured flows. The fluid is a conductive fluid. An electrode 2 is arranged in a measuring tube 1 near the tube wall, and a pair of electrodes 22 are arranged to face each other. The electrodes 2, 2 are electrodes for measuring a flow rate. Further, a pair of exciting coils 3,
Arrange 3 The measurement tube 1 is connected via these exciting coils 3,3.
A rectangular-wave-shaped magnetic field is applied to the conductive fluid therein based on the rectangular wave output from the excitation circuit 4. The electromotive force generated at the electrode 2 at that time is taken out and amplified by the differential amplifier 5. The output signal of the differential amplifier 5 is input to the signal detection unit A by the switching circuit 6 that switches the connection state to the output terminal side of the differential amplifier 5 or the arm potential side.

In the signal detection unit A, two signal processing routes are provided. First, in a first route, the measurement signal passes through a signal detection filter 7, where the signal 101 is extracted. The output signal of the signal detection filter 7 is converted into an effective value by an effective value calculating section 8 is input to the + input terminal of the difference computing unit 11 as the signal of the fundamental frequency component f _0. On the other hand, in the second route, the measurement signal passes through the noise detection filter 9, where the noise 104a in the predetermined frequency range is extracted. The output signal of the noise detection filter 9 is converted into an effective value by the effective value calculation unit 10 and input to the input terminal of the difference calculation unit 11 as background noise. Extracting only signal components by calculating a difference Sx-S ₀ of the difference calculation section 11, two of said signals, is output as a measurement signal from the output terminal 12. The signal processing for obtaining the flow rate described with reference to FIG. 1 is executed by the circuit configuration of the signal detection unit A.

In the above, a narrow band filter is used for the signal detection filter 7 and the noise detection filter 9, and a digital filter or a switched capacitor filter is preferably used.

As an example of the switch capacitor filter, for example, a linear filter (LTC1060) of Linear Technology Co., Ltd., and the like, and similar products are on the market from other companies. These are provided with a terminal for setting a center frequency, and the center frequency of the filter can be set or changed with high accuracy and easily by the frequency of the clock signal supplied thereto. Conventionally, a bandpass filter using an active filter and a passive element has been used. However, there were difficulties in practical use in terms of accuracy, stability, and cost. The realization of means that can be implemented digitally with high accuracy and at low cost has made the method according to the present invention practical.

The switching circuit 6 performs an operation for switching in synchronization with the excitation circuit 4 based on a switching command signal 13 given from the excitation circuit 4. In response to the switching of the excitation, the switching circuit 6 performs an operation of connecting to the ground side when the first half of the detection signal in which the differential noise occurs is present, and connecting to the signal side when there is the latter half that becomes stable. . Thereby, the differential noise generated in the detection signal can be eliminated.

FIG. 3 shows signal waveforms at various parts of the circuit of the electromagnetic flow meter shown in FIG. 3A shows the excitation voltage of a rectangular wave output from the excitation circuit 4, FIG. 3B shows the waveform of the output of the differential amplifier 5, and FIG. 3C shows the signal detection filter. 7 (D) shows the waveform of the output signal of the signal detection filter 7. FIG. The broken-line circle in FIG.
The portion indicated by 14 is an unstable differential noise portion generated in the first half of the detection signal.

As a modified embodiment, when the background noise is low, the signal processing route including the noise detection filter 9 and the effective value calculation unit 10 can be eliminated. Further, the input terminal of the switching circuit 6 is increased by one, and a reference DC signal is input to the input terminal. The measurement signal and the reference DC signal are intermittently switched, and an effective value obtained when the reference DC signal is input. It is also possible to perform a calculation for normalizing the measurement signal with the signal of the calculation unit 8 to correct the fluctuation of the output 12 due to the fluctuation of the characteristic of the signal detection filter 7.

In the above-described embodiment, the measurement signal is obtained by using the signal 101 of the fundamental frequency component and the noise 104a around the signal 101. However, in the same configuration as above, the signals 102 and 103 of the harmonic components are obtained.
The measurement signal can also be obtained by using. In this case, the signal detection filter 7 and the noise detection filter 9 whose pass band characteristics correspond to the frequency passage of the respective harmonic components are used.

Further, the signal detection filter 7 is provided with a configuration capable of changing the pass band, and is provided with a switching unit, and the switching unit arbitrarily switches the pass band of the signal detection filter to a fundamental frequency or a harmonic component. You can also. For the signal detection filter having such a configuration, it is preferable to use, for example, a digital filter whose center frequency can be controlled by an external clock signal. Further, in this case, the configuration of the noise detection filter 9 is also configured such that the pass band can be changed corresponding to the signal detection filter 7, and the filter characteristics are switched by the switching means.

Another embodiment of the electromagnetic flow meter will be described with reference to FIGS. Although concept is similar to FIG. 1, the background noise detection filter 2 Kosonae, background noise S ₀ of each of the noise detection filter frequency band which is located on either side of the fundamental frequency f _0, S ₀ ′
The average value (S ₀ + S ₀ ′) / 2 of the two is used as the value of the background noise, and the background noise can be corrected more accurately. FIG. 6 shows the configuration of the signal processing unit in that case.

In the circuit configuration shown in FIG. 6, a third noise detection filter 9 'and an effective value operation unit 10' are provided in parallel with the first noise detection filter 9 and the effective value operation unit 10 of the embodiment. Is provided. The circuit unit including the noise detection filter 9 and the effective value calculation unit 10 has f ₀ −Δ as described above.
The background noise (signal strength S ₀ ) having f as the center frequency is extracted. The circuit section including the noise detection filter 9 'and the effective value calculation section 10' is a second background noise detection filter circuit section having a pass band having a center frequency of f ₀ + Δf. The second background noise (signal strength S ₀ ′) is extracted. Each output signal of the effective operation units 10 and 10 'is a resistor (resistance value)
2R) is input to the-input terminal of the operational amplifier forming the difference calculation unit 11. Based on the relationship between the values of a plurality of resistors connected to the differential arithmetic unit 11, the difference calculation unit 11, the average value of the values Sx and background noise of the frequency f ₀ of the measuring signal _{(S 0 + S 0 ')} / 2 Is obtained and output to the output terminal 12 as a measurement signal.

Also in the above-described embodiment, the configuration of the filter may be the same as that of the first embodiment.

[Effects of the Invention] As is clear from the above description, according to the present invention, since the measurement signal is extracted by the signal detection filter, it is not affected by noise of frequency components other than the measurement frequency, and noise is not detected. The filter extracts background noise in a predetermined frequency range and removes it from the measurement signal, so that background noise superimposed on the measurement frequency can also be removed, and external noise such as slurry noise and flow noise can be removed. Can also perform stable measurement. Further, according to the signal processing method of the present invention, there is also an effect of reducing stationary noise and noise at low conductivity,
It is possible to achieve higher precision as an electromagnetic flow meter and increase the measurement limit of a low conductive fluid.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of signal processing of an electromagnetic flow meter according to the present invention, FIG. 2 is a circuit diagram showing an embodiment of a signal processing circuit of the electromagnetic flow meter according to the present invention, and FIG. FIG. 4 is a waveform diagram showing an example of background noise, FIG. 5 is a diagram similar to FIG. 1 showing another embodiment, and FIG. 6 is another embodiment. 3 is a circuit diagram of a signal processing circuit of FIG. [Explanation of Signs] 1 ... Measurement tube 2 ... Electrode 3 ... Exciting coil 4 ... Exciting circuit 6 ... Switching circuit 7 ... Signal detection filter 8 ... Effective value calculation unit 9,9 '... Noise detection Filter 10, 10 ': Effective value calculation unit 11: Difference calculation unit

Claims (4)

(57) [Claims] An electromagnetic flowmeter of a rectangular wave excitation type, comprising: means for holding a detection signal for a predetermined time after switching an excitation state in synchronization with rectangular wave excitation; and an excitation frequency based on the held detection signal. And a switching means for switching the center frequency of the signal detection filter to a fundamental frequency or a harmonic frequency according to an object to be measured, and a fundamental frequency switched by the switching means. Alternatively, an electromagnetic flowmeter that performs flow measurement using an effective value of a frequency component of a harmonic frequency as a measurement signal. 2. An electromagnetic flowmeter according to claim 1, wherein said second filter detects a background noise having a center frequency close to said predetermined frequency component, and outputs an effective value output of said signal detecting filter. An electromagnetic flowmeter, comprising: a calculator for calculating a difference from an effective value output of a second filter for detecting ground noise, wherein a flow rate is measured using an output of the calculator as a measurement signal. 3. The electromagnetic flow meter according to claim 1, wherein a means for intermittently inputting a signal of a constant peak value synchronized with the excitation is provided at a preceding stage of the filter as means for detecting a change in the characteristic of the filter. An electromagnetic flowmeter characterized in that a measurement signal is normalized based on the effective value output and a flow rate is measured. 4. The electromagnetic flowmeter according to claim 1, wherein a filter whose center frequency can be controlled by an external clock signal is used as said signal detection filter.