JPH0454890B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an electromagnetic flowmeter used, for example, to measure the flow rate of water and other conductive raw material fluids supplied through pipes in various chemical processes. It is.

[Prior art]

An electromagnetic flowmeter uses Faraday's law as its basic principle, and normally generates an alternating magnetic field perpendicular to the flow whose flow rate is to be measured, and calculates the potential difference generated between opposing electrodes placed perpendicular to the flow and this magnetic field. It is amplified and extracted. The electrical conductivity of fluids is generally not very high, and conventionally the electrical conductivity is about 5μS/cm (resistance of about 200KΩ per cm).
An amplifier with high input impedance (for example, 300 MΩ) and high gain is used so that measurements can be performed even on fluids such as Therefore, even if no current is flowing between the electrodes, for example, if a power distribution board is installed near the measurement location and is passing a large current intermittently, the electromagnetic waves generated from it may cause noise between the electrodes. may occur and be detected as a signal. Although such a signal itself is small, it is common that the normal flow rate is also integrated, so even if the signal is small, when integrated, it appears as a large value, which poses a problem in process control.

[Object and structure of the invention]

The present invention was developed in view of these circumstances, and its purpose is to always set the output to zero when no fluid is flowing, so that the integration counter does not count up regardless of noise between the electrodes. Our goal is to provide an electromagnetic flowmeter that is capable of

In order to achieve such an objective, the present invention
A power supply is connected to the front stage of the amplifier via a high impedance element, so that the amplifier can obtain an output that can be clearly distinguished from when there is no fluid and the gap between the electrodes becomes high impedance. , means for determining whether the above output has been obtained, and means for forcibly reducing the output of the electromagnetic flowmeter to zero when the determined output is obtained. Hereinafter, the present invention will be explained in detail using Examples.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the figure, 1 is a pipe through which fluid flows;
A magnetic field perpendicular to the flow direction of the fluid is formed internally by the excitation coil 2, and opposing electrodes 3 and 4 are provided which are orthogonal to the magnetic field and are insulated from each other. 3,4
The configuration is such that the voltage is applied between the two. A detector 5 is constituted by the conduit 1, the excitation coil 2, and the electrodes 3 and 4. 6 is a constant current source, and 7 is an excitation circuit. The constant current source 6 is invertedly connected to the excitation coil 2 by the signal a sent from the timing signal generation circuit 8, and a square wave alternating current is applied to the excitation coil 2. supply

Electrodes 2 and 3 are connected to non-inverting input terminals of operational amplifiers 9 and 10, respectively. Both operational amplifiers 9 and 10 have their output terminals directly connected to their inverting input terminals to form a so-called voltage follower, and the outputs of both operational amplifiers 9 and 10 are connected via resistors _R1 and _R2 . amplifier 1
It is connected to the inverting input terminal and the non-inverting input terminal of No. 1. Therefore, when fluid is normally flowing in the pipe line 1, an excitation current CL of positive and negative rectangular waves as shown in FIG. If the excitation of is performed alternately,
An output with a similar waveform is obtained from the operational amplifier 9.
On the other hand, the purpose of the operational amplifier 10 is impedance conversion, and as a result, the operational amplifier 11 is also
A similar rectangular waveform output V ₁ is obtained as shown in . In this case, the output voltage V ₁ of the operational amplifier 11 is
The output of operational amplifier 9 is V ₂ , the output of operational amplifier 10 is V ₃ , and the resistance values of resistors R ₁ and R ₂ are respectively
r ₁ , r ₂ , the resistance value r ₃ of the resistor R ₃ connected between the non-inverting input terminal of the operational amplifier 11 and the ground terminal, and the resistance value of the resistor R ₄ connected between the inverting input terminal and the output terminal of the operational amplifier 11 Assuming the resistance value is r ₄ , V ₁ = r ₁ + r ₄ / r ₂ + r ₃・r ₃ / r ₁・V ₃ − r ₄ / r ₁ ×V _2. If r ₁ = r ₂ , r ₃ = r ₄ . Then, it is expressed as V=r ₄ /r ₁ (V ₃ −V ₂ ) (1).

The sample hold circuit 12 includes an operational amplifier 11
The operational amplifier 11
When the output voltage reaches a steady value, the output voltage is sampled and held, and a signal proportional to the excitation current of the excitation coil 2 and proportional to the flow rate of the fluid flowing through the pipe line 1 is obtained as the output. It will be done. Therefore, this signal is further amplified by the DC amplifier 13 to obtain the flow rate output OUT. This flow rate output
OUT is set to 4 to 4 by a voltage current amplifier, for example.
Convert it to a 20mA current and use it as the input signal for the meter.
The flow rate is controlled.

In the above configuration, when there is almost no fluid flowing inside the pipe line 1 and the pipe line 1 becomes empty, the space between the opposing electrodes 3 and 4 becomes almost open, but the space between the electrodes is in a high impedance state. In the conventional configuration, the first stage operational amplifiers 9 and 10
It is unknown which direction the output will go depending on the characteristics of the detector, and in that case, as mentioned above, even though no fluid is flowing, the noise generated in the detector will be picked up and the integration counter will count up. This will cause problems.

Therefore, in this embodiment, a power supply that generates a voltage +V is connected to the non-inverting input terminal of the operational amplifier 9 through the resistor _R5 , and a voltage is connected to the non-inverting input terminal of the operational amplifier 10 through the resistor ^R6 . A power supply that generates -V is connected. The resistance values r ₅ and r ₆ of the resistors R ₅ and R ₆ are both within 100KΩ when the fluid is flowing through the pipe 1 and the resistance value between the electrodes 3 and 4 is often within 100KΩ. However, in terms of accuracy, it is set to a sufficiently high value, for example, about 100 MΩ, so that it can be ignored. As a result, when the fluid stops flowing and the electrodes 3 and 4 are in an open state, the above power supply voltage +V is generated almost unchanged at the output of the operational amplifier 9, and -V is generated almost unchanged at the output of the operational amplifier 10. appear. Therefore, if the circuit constants such as the resistance values of the resistors R ₁ to R ₄ are appropriately selected, for example, if r ₁ = 2r ₄ in equation (1), the distance between electrodes 3 and 4 will be reduced as described above. When it is in the open state, the output of the operational amplifier 11 is approximately -V
It can be clearly distinguished from the output when the fluid is flowing normally, as shown in FIG. 2b.

The present embodiment further includes a comparator 14 formed of an operational amplifier that receives the output of the operational amplifier 11 and compares it with a reference voltage V ₀ , and a switch circuit 15 that operates in response to the output of the comparator 14. We are prepared.

Here, the reference voltage V ₀ of the comparator 14 is set to a voltage slightly higher than -V, and when the output V ₃ of the operational amplifier 11 swings to -V, the output V ₄ of the comparator 14 is set to the voltage slightly higher than -V. It swings to the positive side as shown in Figure 2 d.
The switch circuit 15 includes a normally-off analog switch SW ₁ connected between the output terminal of the operational amplifier 11 and the sample-hold circuit 12, and a normally-off analog switch SW 2 connected between the sample-hold circuit 12 and the ground terminal _. The output of the comparator 14 is sent out as a control signal for the analog switch _SW1 , and is also inverted by the inverter 16 and sent out as a control signal for the analog switch _SW2 . Therefore, as described above, if the comparator 14 sends out a positive judgment output indicating that fluid has stopped flowing, the switch
SW ₁ opens and switch SW ₂ closes,
As a result, the input of the sample old circuit 12 becomes zero potential regardless of the detection output from the detector 5.

As described above, +V and -V power supplies are connected to the front stages of operational amplifiers 9 and 10 through resistors _R5 and _R6 , respectively, and when fluid stops flowing, the output _V3 of operational amplifier 11 is approximately -V. Although the present invention is not limited to this example, it is possible to connect a high impedance element only to either the front stage of the operational amplifier 9 or the front stage of the operational amplifier 10. Furthermore, it is only necessary that the operational amplifier 11 obtains an output that can clearly distinguish when the fluid stops flowing from when the fluid normally flows.

In the above-described embodiment, as a means for determining that the output of the operational amplifier 11 has become approximately equal to the output when the fluid has disappeared and the gap between the electrodes has become high impedance, Although the comparator 14 is made of an operational amplifier and has a reference voltage set to be approximately equal to the output when the impedance becomes high, the present invention is not limited thereto. For example, in recent years, computers have often been used in process control, and it goes without saying that the above-mentioned determination may be performed using software using this computer. Moreover, it is needless to say that the switch circuit 15 is not limited to the switch circuit 15, and any structure may be used as long as it is a means for forcibly reducing the flow rate output OUT to zero when the above judgment output is obtained.

〔Effect of the invention〕

As explained above, according to the present invention, a power supply is connected to the front stage of the amplifier via a high impedance element, and when the fluid is gone and the impedance between the electrodes becomes high, it is clearly distinguished from when the fluid is flowing. means for causing the amplifier to obtain an output that can be obtained, and for determining that the output has been obtained;
By providing a means for forcibly reducing the output of the electromagnetic flowmeter to zero when the judgment output is obtained,
When there is no fluid, the output is always zero regardless of the presence or absence of noise between the electrodes, and the integration counter does not count up, allowing accurate measurements.
It is also extremely useful for process control.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
The figure is a timing chart for explaining its operation. 1... Pipe line through which fluid flows, 2... Excitation coil, 3,
4... Counter electrode, 9, 10, 11... Operational amplifier, 1
4...Comparator as determination means, 15...Switch circuit as means for zeroing the output, _R5 , _R6 ...High resistance resistors, +V, -V...Power supply voltage.

Claims (1)

[Claims] 1. An excitation coil that forms an alternating current magnetic field perpendicular to the flow, electrodes facing each other in a direction perpendicular to the alternating current magnetic field across the flow, and an amplifier that amplifies the potential difference between the electrodes. In an electromagnetic flowmeter, a power source is connected to the front stage of the amplifier via a high impedance element having an impedance sufficiently higher than the resistance of the fluid flowing between the electrodes, and the voltage of this power source is applied to the voltage between the electrodes when the fluid runs out. When the impedance becomes high, the output of the amplifier is set to a value that can be clearly distinguished from the output when fluid is flowing between the electrodes, and the output of the amplifier is set to a value that can be clearly distinguished from the output when fluid is flowing between the electrodes. A determination means that determines that the output is almost equal to the output when there is almost no flow and sends out a determination output, and a determination means that forcibly makes the output of the electromagnetic flowmeter zero when the determination output is obtained from this determination means. An electromagnetic flowmeter characterized by comprising means for: 2. As a means of determination, a comparator is provided which has a reference voltage approximately equal to the output of the amplifier when almost no fluid flows between the electrodes, and whose input is the output of the amplifier, and as a means of forcing the output to zero. , a patent characterized in that a switch circuit is provided for opening a contact connecting the output terminal and the output of the amplifier and closing a contact connecting the output terminal and the ground terminal by the output of the comparator. An electromagnetic flowmeter according to claim 1.