JPH0738822Y2 - Electromagnetic flow meter - Google Patents

Description

[Detailed Description of the Invention] [Industrial field of application] The present invention relates to an electromagnetic flowmeter using an AC magnetic field, and in particular, suppresses fluctuations in the measurement output due to frequency fluctuations of an AC power supply (hereinafter referred to as AC power supply). The present invention relates to an electromagnetic flow meter that stabilizes the output.

[Conventional technology]

FIG. 2 is a block diagram showing the configuration of a conventional general electromagnetic flowmeter. In the figure, 1 is a measuring tube through which a fluid to be measured flows, 2 is an exciting coil that is arranged to face the tube 1, and 3a and 3b are perpendicular to the exciting coil 2 while sandwiching the fluid flow. A pair of detection electrodes disposed in the excitation coil 2, and a rectangular wave alternating current synchronized with the frequency of the AC power source is supplied as an exciting current from the exciting current generating circuit 4 to the exciting coil 2. An alternating magnetic field that is orthogonal to the flow of the fluid is formed inside 1. As a result, the measuring tube 1
An electromotive force corresponding to the flow velocity is generated in the fluid flowing in the direction perpendicular to the AC magnetic field, and this electromotive force is detected by the pair of detection electrodes 3a, 3b arranged facing the inner wall surface of the measuring tube 1. This becomes a detection signal corresponding to the flow velocity, that is, a flow rate signal. Then, the flow rate signals from the pair of detection electrodes 3a and 3b are
It is amplified by the AC amplifier circuit 5 including the front-stage amplifiers 5 ₁ and 5 ₂ and the differential amplifier 5 ₃ and input to the sample hold circuits 6 and 7.

Here, the sample and hold circuits 6 and 7 have resistors 6 ₁ and
7 _1, by sampling signals S ₁ and S ₂ on, the switch 6 ₂ and 6 ₂ to OFF, the capacitor 6 _3, 7 _3, consists buffer amplifier 6 _4, 7 ₄ high input impedance, an AC amplifier circuit 5 Sampling signals S ₁ and S _{2 obtained} by synchronizing the rectangular wave flow signal shown in FIG.
(See (b) and (c) of FIG. 3) and hold the sample, and further differentially amplify it by the differential amplifier 8,
It is supposed to be converted into a DC signal. At this time, the sampling timings of the sample and hold circuits 6 and 7 are set so as to be performed at the most stable last stage with the flow rate signal synchronized with the excitation current of the rectangular wave, as shown in FIG.

The direct current flow rate signal thus converted is converted into a digital signal by the analog / digital (A / D) conversion circuit 9 and then taken into the microprocessor (CPU) 10 so that the microprocessor 10 Digital-to-analog (D / A) conversion circuit 1 after performing predetermined arithmetic processing
Through 1, the flow rate data of the fluid is taken out as a measurement output.

By the way, in the electromagnetic flow meter configured in this way,
Minute flow rate signal from differential amplifier 8 (0.4mVp-p / 1m / s)
In case of A / D conversion, in order to cancel the AC power noise, the A / D conversion circuit 9 is composed of a voltage / frequency (V / F) converter 21 and a counter 22 as shown in FIG. , Fifth
The flow rate signal is V / F converted at the timing of the power supply synchronization signal (CLR) shown in FIG. 5 (b) synchronized with the AC power supply shown in FIG. It is designed to be taken into the microprocessor 10 as a digital value. At this time, the DC flow rate signal
Vin (V), the output frequency of the V / F converter 21 is f (Hz), the digital value of the counter 22 is D (CNT), and the cycle of the AC power source is T (s
ec), these have the following relation.

In addition, in the usual arithmetic processing of the microprocessor 10, the cycle T of the AC power supply is fixed (20 msec in the case of 50 Hz), the output is obtained from the digital value D (CNT) obtained from the counter 22, and the output is obtained. Assuming Dout, this has the following relationship.

However, since K ₁ and K ₂ are constant, if the AC power supply cycle is constant for a constant input of the flow rate signal, a constant count value is obtained from the counter 22 and becomes stable as an output.

[Problems to be solved by the invention]

However, in the above-described conventional electromagnetic flowmeter, when the AC power supply cycle fluctuates, the count cycle of the counter 22 also fluctuates, and the count value also fluctuates, resulting in output fluctuation. In general, there is a frequency fluctuation of ± 0.2% for commercial power sources and a few% for private power generation, etc., and there is a problem that the stability of the output is impaired by the frequency fluctuations.

The present invention has been made in view of these points,
The present invention aims to provide an electromagnetic flow meter that suppresses output fluctuations with respect to frequency fluctuations of an AC power source and improves output stability.

[Means for solving problems]

The electromagnetic flowmeter according to the present invention is an electromagnetic flowmeter that detects electromotive force generated when an AC magnetic field is applied to a fluid as a flow rate signal, A / D-converts this flow rate signal, and measures the flow rate of the fluid based on its output. In the flowmeter, an A / D conversion circuit including a V / F converter and a first counter, which V / F-converts the flow rate signal and counts output pulses of the flow rate signal at a cycle T of a power supply synchronization signal synchronized with an AC power supply, The same cycle T as the power supply synchronizing signal
A second counter that counts the reference clock with, a count value Dsig of the first counter, a count value Dref of the second counter, and a frequency of the reference clock.
Calculate fref · (Dsig / Dref) based on fref
Calculate the output frequency fsig from the / F converter and
An arithmetic means for multiplying fsig by a predetermined constant K ₀ to obtain an output D _OUT is provided.

[Action]

Therefore, in the present invention, based on the count value Dsig of the first counter, the count value Dref of the second counter and the frequency fref of the reference clock, fref · (Dsig
/ Dref) is calculated to obtain the output frequency fsig from the V / F converter, and this output frequency fsig is multiplied by a predetermined constant K ₀ to obtain the output D _OUT. The fluctuation of _OUT can be suppressed.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to examples.

FIG. 1 is a block diagram of a main part configuration showing an embodiment of an electromagnetic flowmeter according to the present invention. The electromagnetic flowmeter of this embodiment is
The electromotive force obtained between the detection electrodes 3a and 3b in FIG. 2 is detected as a flow rate signal, and this flow rate signal is converted into a DC signal via the AC amplification circuit 5, the sample hold circuits 6 and 7, and the differential amplifier 8. Along with the conventional example shown in FIG. 2, the A / D conversion circuit 9 converts the digital signal into a digital signal, and then the signal is taken into the microprocessor 10. The D conversion circuit 9 is composed of the V / F converter 12 and the counter 13, and the power supply synchronization signal CLR synchronized with the AC power supply is used.
The flow rate signal Vin from the differential amplifier 8 is V / F converted at a cycle T of, and the output pulse is counted by the counter 13. Further, a counter 15 for counting a reference clock of a constant frequency generated from the reference clock generation circuit 14 at the same timing as the power supply synchronizing signal CLR is provided, and the count value of each of the counters 13 and 15 is set to the microprocessor 10 as a digital value. By capturing, a constant output is obtained by performing a correction calculation on the fluctuation of the AC power supply cycle based on these count values. In the figure, the same reference numerals indicate the same parts, and the V / F converter 12 and the counter 13
Is similar to the conventional V / F converter 21 and counter 22.

Therefore, according to the configuration of the above embodiment, the flow rate signal Vin
The output frequency of the V / F converter 12 that converts V to F is fsig (Hz),
The digital value of the counter 13 that counts the output pulse is Dsig (CNT), the frequency of the reference clock of the reference clock generation circuit 14 is fref (Hz), and the digital value of the counter 15 that counts the reference clock is Dref (CNT). Then,
These have the following relationship.

Therefore, the V / F conversion period of the V / F converter 12, that is, the power supply synchronization signal
To obtain the output frequency fsig without being affected by the period T of CLR, However, by performing arithmetic processing of K as a constant by the microprocessor 10, its output Dout is Dout = K ₀ · fsig (5) However, K ₀ is a constant. This makes it possible to suppress output fluctuation due to fluctuations in the power supply frequency without being affected by AC power supply noise.

The correction calculation process of the above-described embodiment is effective not only for power supply frequency fluctuations but also in the following cases. That is, it is effective for a system in which the V / F period fluctuates when the V / F conversion is performed in a period that is an integral multiple of the noise period in order to cancel the noise having a certain frequency characteristic.

[Effect of device]

As described above, according to the present invention, the count value Dsig of the first counter and the count value Dre of the second counter are
Based on f and the frequency fref of the reference clock, fref
(Dsig / Dref) is calculated and output frequency from V / F converter
fsig is obtained, and the output frequency fsig is multiplied by a predetermined constant K ₀ to obtain the output D _OUT , and the fluctuation of the output D _OUT due to the fluctuation of the power supply frequency can be suppressed. Stable output can be obtained without receiving. Further, since the power supply synchronization signal detection circuit detects the voltage at the normal voltage level, fluctuations in the power supply voltage are converted into frequency fluctuations, but there is an effect that the fluctuations in the power supply voltage also become stronger.

[Brief description of drawings]

FIG. 1 is a block diagram of a main structure of an electromagnetic flowmeter according to an embodiment of the present invention, FIG. 2 is a block diagram showing a structure of a conventional general electromagnetic flowmeter, and FIG. FIG. 4 is an explanatory diagram showing the flow timing signal and the sampling timing relationship by the sample and hold circuit, FIG. 4 is a block diagram showing the configuration of the A / D conversion circuit in FIG. 2, and FIG. 5 is the timing relationship between the AC power supply and the power supply synchronization signal. It is an explanatory view shown. 9 ... Analog / digital (A / D) converter, 10 ... Microprocessor, 12 ... Voltage / frequency (V / F) converter, 13 ... Counter, 14 ... Reference clock generator, 15
……counter.

Claims (1)

[Scope of utility model registration request] 1. An electromotive force generated when an alternating magnetic field is applied to a fluid is detected as a flow rate signal, and this flow rate signal is converted into an analog signal.
In an electromagnetic flow meter that measures the flow rate of fluid based on its output after digital conversion, voltage / frequency that converts the flow rate signal to frequency / frequency at the cycle T of the power supply synchronization signal synchronized with the AC power supply and counts the output pulse An analog / digital conversion circuit including a converter and a first counter, a second counter that counts a reference clock at the same cycle T as the power supply synchronization signal, a count value Dsig in the first counter, and a second counter Based on the count value Dref in the counter and the frequency fref of the reference clock, fref · (Dsig / Dref) is calculated to obtain the output frequency fsig from the voltage / frequency converter,
An electromagnetic flowmeter, comprising: an arithmetic means for multiplying the output frequency fsig by a predetermined constant K ₀ to obtain an output D _OUT .