JP2707757B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application> The present invention relates to an electromagnetic flowmeter for applying a magnetic field having two different frequencies to a fluid to be measured and measuring the flow rate of the fluid. The present invention relates to an electromagnetic flowmeter improved so that an abnormal output does not occur when the inside becomes empty.

<Prior Art> An industrial electromagnetic flowmeter conventionally employs a commercial frequency excitation method in which excitation is performed using a commercial power supply. The commercial frequency excitation method (a) has a high response speed and can be manufactured at low cost.
(B) There is an advantage that it is hardly affected by low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated by a slurry fluid or a low-conductivity fluid. However, there is a drawback that the zero point fluctuates if left for a long period of time, for example, about one day.

For this reason, a low-frequency excitation method has been adopted in which excitation is performed at a low frequency equal to or lower than half the commercial frequency.
The use of the low-frequency excitation method has an advantage that a stable electromagnetic flowmeter having a zero point can be obtained, as is well known. However, since the excitation frequency is low, the frequency is close to the frequency of the flow noise, and thus is easily affected by the flow noise. In particular, when the flow velocity becomes large, this effect becomes remarkable. Further, when damping is applied in order to reduce the influence of flow noise, there is a disadvantage that the response becomes slow.

Therefore, as proposed in Japanese Patent Application No. 60-197168 (title of the invention: electromagnetic flowmeter), an exciting current component of a commercial frequency and an exciting current component of a lower frequency are simultaneously passed through an exciting coil to form a composite magnetic field. A composite excitation system to be formed has been proposed.

Then, when the measurement fluid flows under the application of the composite magnetic field, a signal voltage including two frequencies is generated. A high-frequency signal voltage discriminated based on the commercial frequency and a low-frequency signal voltage discriminated based on the low frequency are output through a low-pass filter and a high-pass filter having large time constants, respectively. , And outputs having the advantages of the high-frequency excitation method and the low-frequency excitation method.

<Problems to be Solved by the Invention> However, in such an electromagnetic flowmeter, it is necessary to increase the time constant of the low-pass filter on the low frequency side in order to reduce the influence of slurry noise. In order to make the response smooth, the time constant of the high-pass filter is also increased.

In this case, when the inside of the conduit becomes empty, the impedance between the electrodes becomes extremely large as compared with the state of the measurement, so that differential noise is mixed in from the outside via this large impedance. Even if such differential noise is mixed, the differential noise is canceled between the sampling of each signal on the low frequency side, and its effect does not appear in the output due to a large time constant. There is a problem that the differential noise is not canceled between signal samplings, and the influence is enlarged due to a large time constant, resulting in an abnormal output.

<Means for Solving the Problems> In order to solve the above problems, the present invention provides exciting means for supplying a magnetic field having two different frequencies, a first frequency and a second frequency lower than the first frequency. First demodulation means for discriminating and outputting a signal voltage excited by the means and generated in accordance with the flow rate based on the first frequency;
A high-pass filter that performs high-frequency output of the demodulator, a second demodulator that discriminates and demodulates the signal voltage based on the second frequency, a low-pass filter that performs low-frequency output of the second demodulator, and a high-pass filter. Adding means for adding the outputs of the first and second low-pass filters together; and a signal relating to the output of the first demodulating means and the output of the second demodulating means. A deviation calculating means for outputting an empty signal when a value is exceeded, and an output means for receiving a flow rate signal output from the adding means and locking the flow rate signal in a predetermined state by the empty signal. It is.

<Operation> When the measurement fluid becomes empty, the impedance between the electrodes rises, and noise having a predetermined frequency is mixed in between the electrodes.

On the other hand, since the base frequency of the demodulation is different between the first demodulation unit side and the second demodulation unit side, the attenuation characteristics for noise having a predetermined frequency are significantly different.
Therefore, the output signals are taken out from the respective output terminals of the first demodulation means and the second demodulation means, and these deviations are calculated by the deviation calculating means. As a result, when the deviation exceeds a predetermined deviation, An empty signal is output to the output means to forcibly hold the flow signal at a predetermined value. This prevents abnormal output.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention.

Numeral 10 is a conduit for the detector of the electromagnetic flowmeter, which is provided with an insulating lining on its inner surface. 11a and 11b are electrodes for detecting a signal voltage. 12 is an exciting coil,
The magnetic field generated by this is applied to the fluid to be measured.
An excitation current _If is supplied from an excitation circuit 13 to the excitation coil 12.

The excitation circuit 13 is configured as follows. Reference voltage E ₁
The non-inverting input of amplifier Q ₁ via the switch SW ₁ is (+)
Is applied, the output terminal is connected to the base of the transistor Q ₂ in. Inverting input of amplifier Q ₁ together with the emitter of the transistor Q ₂ is connected to the common COM via a resistor R _f (-) to which is connected. Common COM and transistor Q
Switch SW ₂ excitation voltage E _S is between the _second collector and SW ₃
Series circuit and switches SW ₄ and SW ₅ connected in parallel
Is applied through a series circuit of. The excitation coil 12 is connected to a connection point between the switches SW ₂ and SW _{3 and} a connection point between the switches SW ₄ and SW ₅ . Timing signals S ₁ , S ₂ , and S ₃ control the opening and closing of switches SW ₁ , SW ₂ and SW ₅ , and SW ₃ and SW ₄ , respectively.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 14. The common-mode voltage is removed and the impedance is converted by the preamplifier 14, and is output to the connection point 15 via the output terminal. The signal voltage at the coupling point 15 via the switch SW _7, or inverting amplifier Q ₃ and the switch SW ₈
Are applied to the low-pass filters 16 each having a small time constant through the series circuit.

These switches SW ₇ and SW ₈ and the inverting amplifier Q ₃ constitute a low-frequency-side demodulator DEM1.

The signal voltage at the coupling point 15 via the switch SW _9, or is applied to a low pass-receiver 17, each through a series circuit of the inverting amplifier Q ₄ and the switch SW ₁₀ having a small time constant.

Etc. This switch SW _9, such as, SW ₁₀ and the inverting amplifier Q ₄ constituting the demodulator DEM2 the low frequency side.

Then, is opened and closed by the timing signal _{S 7, S 8, S 9} , S 10 from the switch _{SW 7, SW 8, SW 9} , SW 10 are each a timing circuit 18. The output of the low pass duplexer 16 via a low-pass filter 19 having a large time constant, the output of the low pass duplexer 17 are added at each addition point 21 through a series circuit of a variable gain amplifier Q ₅ and a high-pass filter 20 You. Low pass filter 19 is constituted by a resistor R ₁ and capacitor C _1, a high-pass filter
20 is constituted respectively by the resistor R ₂ and capacitor C _2. Further, the addition output at the addition point 21 is a low-pass filter.
Output to the output circuit 23 via 22.

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

The output _NL of the low-pass filter 16 and the output _NH of the low-pass filter 17 are input to the deviation calculation circuit 24, respectively. The deviation calculation circuit 24 compares these outputs N _L and N _H and outputs an empty signal Sem to the output circuit 23 when these deviations ΔE exceed a predetermined value.

The output circuit 23 outputs a flow rate signal S _Q low pass filter 22 convert to the output terminal 25 to a predetermined level.

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

Timing signals S ₁ is turned on as shown in FIG. 2 (b) /
Repeat off, thereby applying the non-inverting input of the amplifier to Q ₁ reference voltage E ₁ to the (+) is controlled. On the other hand, the timing signal S ₂ (FIG. 2 (b)) and S ₃ (FIG. 2 (c)) by the switch SW ₂ and SW ₅ at a low frequency, and the switch SW ₃ and SW ₄ are turned on alternately Therefore, as shown in FIG. 2 (d), an exciting current _If in which a low frequency (period: 2T) and a high frequency (period: 2t) are combined flows.

Signal voltage second view of attachment point 15 (e), since it is sampled by the timing signal S ₇ and S ₈ shown in (f),
Voltage shown in FIG. 2 (g) is obtained at the output side of the switch SW _7. The 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 node 15 is shown in FIG.
Because it is sampled by a timing signal S _9, S ₁₀ at the timing shown by (i), the output side of the switch SW ₉ is output signal voltage shown in FIG. 2 (j), the signal voltage is a variable gain amplifier Q The size is adjusted at ₅ and output to the addition point 21 via the high-pass filter 19.

Each signal voltage added at the addition point 21 is a low-pass filter.
Is smoothed is output to the output circuit 23 as a flow rate signal S _Q 22.

When the measurement fluid is filled inside the conduit 10, electromagnetic
Since the impedance between 11a and 11b is low,
The voltage due to noise appearing at the output terminals 6 and 17 is extremely low. Thus, the output N _L and N _H is empty signal Sem from its output end located in the deviation allowable value of the output deviation calculation circuit 24 is not outputted, Therefore the flow rate signal S _Q is level converted from the output circuit 23 Output as is.

On the other hand, when the measurement fluid inside the conduit 10 becomes empty, the impedance between the electrodes 11a and 11b becomes extremely high, and noise having a predetermined frequency becomes large. This noise is output to the demodulators DEM1 and DEM2, respectively, but since the functions of discriminating the frequencies of these demodulators DEM1 and DEM2 are greatly different, different outputs _NL and N Output as _H.

When the deviation between these outputs _NL and _NH exceeds a predetermined value, the deviation calculation circuit 24 outputs an empty signal Sem to the output circuit 23 from its output end assuming that the inside of the conduit 10 is empty, The output circuit 23 is, for example, a flow signal S immediately before the empty signal Sem is output.
Fix to _Q value, or swing to the 0% side or 100% side.

In the above description, the outputs _NL and _NH are set to the low-pass filters 16 and 17 respectively.
Is detected at the output terminal of the demodulator DE.
It may be detected from each output terminal of M1 and DEM2.

Also, when slurry noise is mixed, the output N _L
There is a deviation in N _H , which can be distinguished from, for example, the output N _L
The interior of the conduit 10 when the difference between this and the like when the value of N _H respectively detected sequentially in time is greater than a predetermined value can be distinguished by determining as being empty.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, even if the fluid inside the conduit becomes empty and large noise is mixed in, the demodulator on the low frequency side of the two-frequency excitation is used. By utilizing the difference between the frequency discrimination function of the demodulator on the high frequency side and the demodulator on the high frequency side, abnormal flow rate output can be suppressed by a simple configuration based on the output difference related to these outputs.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 is a waveform diagram showing waveforms of respective parts of the embodiment in FIG. 10 ... conduit, 12 ... excitation coil, 13 ... excitation circuit, 14 ...
... Preamplifier, 15 ... Connection point, 16, 17 ... Low pass filter,
18 ... Timing circuit, 19 ... Low-pass filter, 20 ...
... High-pass filter, 21 ... Addition point, 23 ... Output circuit,
24: Deviation calculation circuit, DEM1, DEM2: Demodulator.

Claims (1)

(57) [Claims] A first frequency and a second frequency lower than the first frequency;
An exciting means for supplying magnetic fields having two different frequencies, a first demodulating means for discriminating and outputting a signal voltage excited by the exciting means and generated in accordance with the flow rate based on the first frequency; A high-pass filter that high-pass-waves the output of the demodulator, a second demodulator that discriminates and demodulates the signal voltage based on the second frequency, and a low-pass filter that low-pass-waves the output of the second demodulator;
An adder for additively combining the outputs of the high-pass filter and the low-pass filter; and a signal associated with an output of the first demodulator and an output of the second demodulator, and calculating a deviation between them. A deviation calculating means for outputting an empty signal when the deviation exceeds a predetermined value; and an output means for receiving a flow signal output from the adding means and locking the flow signal in a predetermined state by the empty signal. An electromagnetic flowmeter comprising: