JPH063382B2 - Electromagnetic flow meter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electromagnetic flowmeter for applying a magnetic field to a fluid to be measured and measuring its flow rate, and particularly to its excitation method and its accompanying signal processing method. The present invention relates to an improved electromagnetic flow meter.

<Prior Art> An industrial electromagnetic flow meter has conventionally adopted a commercial frequency excitation method in which a commercial power source is used for excitation. The commercial frequency excitation method has a high response speed and can be manufactured at low cost.
(B) It has the advantage that it is less susceptible to low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated by a slurry fluid or low-conductivity fluid. In addition, there is a drawback that the zero point fluctuates if it is left for a long period of time, for example, for about one day.

For this reason, a low frequency excitation method has been adopted in which excitation is performed at a low frequency which is 1/2 of the commercial frequency or lower. As is well known, the low frequency excitation method has an advantage that a stable electromagnetic flowmeter having a zero point can be obtained. However, since the excitation frequency is low, it is close to the frequency of the flow noise, so that it is easily affected by the flow noise, and especially when the flow velocity becomes large, this effect becomes remarkable. In addition, if damping is applied to reduce the influence of flow noise, the response becomes slow.

Therefore, as proposed in Japanese Patent Application No. 60-197168 (invention name: electromagnetic flowmeter), an exciting current component of a commercial frequency and an exciting current component of a lower frequency are simultaneously applied to an exciting coil to generate a composite magnetic field. A composite excitation method to form is proposed.

Then, when the measurement fluid is caused to flow under the application of the composite magnetic field, a signal voltage including two frequencies is generated. The high-frequency signal voltage discriminated based on the commercial frequency and the low-frequency signal voltage discriminated based on the low frequency are output via the low-pass filter and the high-pass filter having large time constants, respectively, and these outputs are also output. The output having the advantages of the high frequency excitation method and the low frequency excitation method is obtained by adding and outputting.

<Problems to be solved by the invention> However, in such an electromagnetic flow meter, it is necessary to set a large time constant of the low-pass filter on the low frequency side in order to reduce the influence of slurry noise, and accordingly, the output is increased. The time constant of the high-pass filter will also be large in order to smooth the response of.

In this case, if differential noise unrelated to slurry noise mixes in, the effect does not appear in the output due to the large time constant on the low frequency side, but the effect of this differential noise on the high frequency side due to the large time constant. Therefore, there is a problem that the zero point fluctuates for a considerable period of time, which is stable in the long term but is determined by a large time constant.

<Means for Solving Problems> In order to solve the above problems, the present invention provides an exciting means for supplying a magnetic field having two different frequencies, a first frequency and a second frequency lower than the first frequency, and A first demodulation means for discriminating and outputting a signal voltage excited by the excitation means and corresponding to the flow rate based on the first frequency, a high-pass filter for high-pass filtering the output of the first demodulation means, and a signal voltage Second demodulation means for discriminating and demodulating based on the second frequency, low-pass filter for low-pass filtering the output of the second demodulation means, and addition means for additively synthesizing each output of the high-pass filter and the low-pass filter. A noise discriminating means for detecting noise included in the signal voltage and changing the first frequency in relation to this noise.

<Example> Hereinafter, an example of the present invention is described based on a drawing. FIG. 1 is a block diagram showing one embodiment of the present invention.

Reference numeral 10 denotes a detector conduit of an electromagnetic flow meter, which has an insulating lining on its inner surface. 11a and 11b are electrodes for detecting a signal voltage. Reference numeral 12 is an exciting coil, and the magnetic field generated by this is applied to the fluid to be measured. An exciting current _If is supplied from the exciting circuit 13 to the exciting coil 12.

The exciting circuit 13 is configured as follows. Reference voltage E ₁
Is the non-inverting input terminal (+) of the amplifier Q ₁ via the switch SW _1.
And its output is connected to the base of transistor Q ₂ . The emitter of the transistor Q ₂ is connected to the common COM via the resistor R _f and is also connected to the inverting input terminal (−) of the amplifier Q ₁ . An exciting voltage Es is applied to the switch S between the common COM and the collector of the transistor Q _2.
It is applied through a series circuit of W ₂ and SW _{3 and} a series circuit of switches SW ₄ and SW ₅ connected in parallel with the series circuit. The exciting coil 12 is connected to the connection points of the switches SW ₂ and SW ₃ and the switches SW ₄ and S ₄ .
Connected to the connection points of W ₅ , respectively. Timing signal S ₁ ,
S ₂ and S ₃ are switches SW ₁ , SW ₂ and SW ₅ , SW ₃ and S, respectively.
Controls the opening and closing of W ₄ . In this way, for example, a composite exciting current having a low frequency of 6.25 Hz and a high frequency of 200 Hz can be obtained.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 14. The preamplifier 14 removes the common mode voltage and converts the impedance, and outputs the result to the coupling point 15 via the output terminal. The signal voltage at node 15 is passed through switch SW ₇ or inverting amplifier Q ₃
And a switch SW _{8 connected} in series to a low-pass filter 16 having a small time constant.

The signal voltage at the connection point 15 is applied to the low-pass filter 17 having a small time constant via the switch SW ₃ or via the series circuit of the inverting amplifier Q ₄ and the switch SW ₁₀ . The switches SW ₇ , SW ₈ , SW ₉ , and SW ₁₀ are timing signals S ₇ from the timing circuit 18, respectively.
Are opened and closed in _{S 8, S 9, S 10} . The low-pass filter 16 passes through a low-pass filter 19 having a large time constant, and the output of the low-pass filter 17 is a variable gain amplifier Q ₅ and a high-pass filter 20.
Are added at the addition points 21 via the series circuits of.
The addition output at the addition point 21 is output to the output end 23 via the low-pass filter 22 as a signal voltage proportional to the flow rate.

The variable amplifier Q ₅ is for adjusting the output voltage V _L of the low-pass filter 19 and the output voltage V _H of the high-pass filter 20 to be equal.

The output of the low pass filter 16 is input to the noise discrimination circuit 24. The noise discriminating circuit 24 detects noise such as slurry noise appearing in the output of the low-pass filter 16 on the low frequency side, and outputs a frequency change signal S _f corresponding to the magnitude thereof. Timing signal S ₁
To change.

The noise discrimination circuit 24 is composed of a comparator Q ₆ , a comparator Q ₇ , and an OR gate Q ₈ . The output of the low-pass filter 16 is applied to the other end of each input of the comparator Q ₆ having the reference voltage E ₂ applied to one end of the input and the comparator Q ₇ having the reference voltage E ₃ applied to one end of the input. , Its outputs are applied to the inputs of an OR gate Q ₈ . The output of the OR gate Q ₈ is output as the frequency changing signal S _f .

Next, the operation of the electromagnetic flow meter shown in FIG. 1 will be described with reference to the waveform chart shown in FIG.

The timing signal S ₁ is repeatedly turned on / off as shown in FIG. 2 (a), whereby the application of the reference voltage E ₁ to the non-inverting input terminal (+) of the amplifier Q ₁ is controlled. On the other hand, the timing signals S ₂ (FIG. 2 (b)) and S ₃ (FIG. 2 (c)) are used to switch SW ₂ and SW ₅ , and SW ₃ and SW ₄ at low frequencies.
Are alternately turned on, so that an exciting current I _f in which a low frequency (cycle: 2T) and a high frequency (cycle: 2t) are combined as shown in FIG.

Since the signal voltage at the connection point 15 is sampled by the timing signals S ₇ and S ₈ shown in FIGS.
The voltage shown in FIG. 2 (g) is obtained at the output side of the switch SW ₇ . A voltage smoothed by the low pass filter 16 is obtained at the output side of the low pass filter 19.

Further, the signal voltage at the connection point 15 is shown in FIG.
Since it is sampled by the timing signals S ₉ and S ₁₀ at the timing shown in (i), the signal voltage shown in FIG. 2 (n) is output to the output side of the switch SW ₉ , and this signal voltage is output to the variable gain amplifier Q. The size is adjusted at ₅ and output to the addition point 21 via the high-pass filter 19.

The signal voltages added at the addition point 21 are smoothed by the low pass filter 22 and output to the output terminal 23.

The time constant of the low-pass filter 16 on the low frequency side, which has substantially the same frequency band as the slurry noise, is selected to be small.
If there is slurry noise at the output end, it appears.
Therefore, the noise discrimination circuit 24 detects this.

When the slurry noise is greater than the reference voltages E ₂ and E ₃ , the frequency change signal S _f outputs a high level signal, for example, and sets the frequency of the timing signal S ₁ from the timing circuit 18 to a large value. When the noise does not exceed the reference voltages E ₂ and E ₃ , a low level signal is output and the frequency of the timing signal S ₁ from the timing circuit 18 is set to a small value.

When the slurry noise is small, the frequency on the high frequency side of the two frequencies is lowered to, for example, about 25 Hz, so even if there is variation due to differential noise, it is possible to suppress the slow variation of the zero point caused by this. Moreover, since the slurry noise is originally small, its influence does not appear on the high frequency side and the low frequency side.

On the other hand, when the slurry noise is large, the high frequency of the two frequencies is increased to, for example, about 200 Hz, so that the high frequency side is separated from the slurry noise frequency band,
It is hard to be affected. On the other hand, since the low frequency side is output through the low pass filter having a large time constant, the influence of slurry noise does not appear. When the frequency is increased, the fluctuation of the zero point due to the differential noise on the high frequency side becomes large, but when the slurry noise is large, the flow rate of the fluid is also large, so the differential noise causes the zero point to have a large time constant centered on zero. Even if it fluctuates during the time determined by, the fluctuation of the zero point is masked by the large flow rate instruction value and is averaged over time so that there is substantially no error, and a substantially stable output is obtained at the addition point 21. To be Also, the influence of differential noise does not appear in the output because of the large low-pass filter on the low frequency side.

Therefore, by changing the frequency on the high frequency side depending on the magnitude of the slurry noise, it is possible to obtain a two-frequency excitation type electromagnetic flowmeter that is substantially unaffected by the slurry noise and the differential noise.

The input signal of the above noise discrimination circuit may be not only on the low frequency side but also on the high frequency side or the output after addition. Further, although the noise discrimination circuit outputs the frequency change signal by discriminating the magnitude of the noise, the noise discrimination circuit may detect the change of the noise. Further, these circuits may be realized by software using a microcomputer.

<Effects of the Invention> According to the present invention as described above in detail with reference to the embodiments, the high frequency side of the two frequencies is changed in relation to the slurry noise. The electromagnetic flowmeter can have a stable zero point without being affected by differential noise.

[Brief description of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 is a waveform diagram showing the waveform of each part of the embodiment in FIG. 10 ... Conduit, 12 ... Excitation coil, 13 ... Excitation circuit, 14
... Preamplifier, 15 ... Coupling point, 16, 17 ... Low pass filter, 18 ... Timing circuit, 19 ... Low pass filter,
20 ... High-pass filter, 21 ... Addition point, 24 ... Noise discrimination circuit, S _f ... Frequency change signal,

Claims (1)

[Claims] 1. A first frequency and a second frequency lower than the first frequency.
Exciting means for supplying magnetic fields having three different frequencies, first demodulating means for discriminating and outputting the signal voltage excited by the exciting means and corresponding to the flow rate based on the first frequency, and the first demodulating means. A high-pass filter for high-pass filtering the output of the demodulating means, a second demodulating means for discriminating and demodulating the signal voltage based on the second frequency, and a low-pass filter for low-pass filtering the output of the second demodulating means,
The high-pass filter and the low-pass filter are provided with an adding means for additively combining the respective outputs, and a noise discriminating means for detecting noise included in the signal voltage and changing the first frequency in relation to the noise. An electromagnetic flow meter characterized by: