JPH0368323B2 - - Google Patents

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to an electromagnetic flowmeter.

(b) Conventional technology Conventionally, electromagnetic flowmeters have been used to apply an excitation current that alternates between positive and negative to an excitation coil, and apply a magnetic field to a fluid pipe through which the fluid to be measured is caused by this excitation current. There is one based on the principle of deriving a signal corresponding to the fluid flow rate from an electrode provided inside the pipe.

(c) Problems to be solved by the invention In recent years, there have been demands for this type of electromagnetic flowmeter to adopt a microcomputer in the signal processing section to improve functionality and reduce costs by simplifying the circuit. strong.

On the other hand, conventional electromagnetic flowmeters employ a constant current source as an excitation current source in order to ensure the stability of their output, which poses a problem in that the excitation circuit becomes complicated and expensive. In addition, separate hold circuits for positive and negative electrodes were required to obtain electrode voltages corresponding to positive and negative changes in the excitation current, and a differential amplifier was also required to determine the difference between the two hold circuits.

The present invention has been made in view of the above-mentioned problems, and uses a non-constant current source to achieve simplification and low cost while still being able to compensate for fluctuations in excitation current. The purpose is to provide an electromagnetic flowmeter that can compensate for fluctuations due to drift and bypass changes.

(d) Means and operation for solving the problem The electromagnetic flowmeter of this invention has excitation coils 3a, 3
A fluid pipe 5 in which an excitation current that alternates between positive and negative is passed through b, and the fluid to be measured is caused to flow by this excitation current.
Electrodes 6a, which are provided in the fluid pipe 5 by applying a magnetic field to
6b, the circuit 4 derives a signal corresponding to the excitation current of the excitation coils 3a, 3b, and the electrode 6
The derived signals from a and 6b are connected to the amplifier 7 and the AC
A first and a second circuit which add a predetermined bypass to the output of the amplifier 7 and the excitation current corresponding signal and amplify them, respectively.
amplifiers 8 and 9, and first and second voltage/frequency converters 10 and 11 that receive the output voltages of the first and second amplifiers 8 and 9 and convert the voltages into pulse signals of proportional frequencies. and first and second photocouplers 12 and 13 that convert the pulse signals from the first and second voltage/frequency converters 10 and 11 into optical signals and then into electrical signals again.
and these first and second photocouplers 12,1
3, and a digital calculation means 18 that divides the count value of the first counter 16 by the count value of the second counter 17. It is configured.

In this electromagnetic flowmeter, the electrodes 6a, 6 of the fluid pipe 5
A signal voltage proportional to the fluid flow is derived from b via an AC amplifier 7. In addition, excitation coils 3a, 3
A signal corresponding to the excitation current b is also derived in circuit 4. Both of these derived signals are amplified by first and second amplifiers 8 and 9 that are biased with a predetermined bias, and their outputs are further amplified by first and second voltage/frequency converters 10 and 11 according to the output voltage. It is converted into a frequency pulse signal. Then, these pulse signals are input to the first and second counters via the first and second photocouplers 12 and 13, and are counted, respectively. Further, the arithmetic processing means 18 such as a microcomputer converts the counted value of the first counter 16, that is, the value equivalent to the interelectrode derived voltage, to the counted value of the second counter 17, that is,
The excitation current is divided by a proportional voltage equivalent value, and the division result value is output as a signal voltage proportional to the true fluid flow rate.

(e) Examples The present invention will be explained in detail below using examples shown in the drawings.

FIG. 1 is a circuit connection diagram of an electromagnetic flowmeter showing an embodiment of the present invention.

In the figure, 1 is a non-constant current source for excitation, 2 is a switch for switching excitation current polarity, 3a and 3b are excitation coils, and 4 is a resistor for deriving a signal proportional to the excitation current. Excitation coils 3a, 3b and 4
are connected in series and connected to the non-constant current source 1 via the polarity changeover switch 2. Reference numeral 5 denotes a fluid pipe through which the fluid to be measured flows, and an electrode 6 is installed inside the pipe to derive a signal voltage proportional to the fluid flow rate.
a and 6b are provided. The signals derived to the electrodes 6a and 6b are amplified by an AC amplifier 7,
The connections are arranged such that a bias B ₁ is applied to the amplifier 8 . On the other hand, the voltage across resistor 4 is also
A bias B ₂ is adapted to be applied to the amplifier 9. The outputs of the amplifiers 8 and 9 are connected so that the output voltages are applied to the inputs of the voltage/frequency converters 10 and 11.
0 and 11 each output a pulse signal with a frequency proportional to the signal voltage received at the input, and are applied to photocouplers 12 and 13, respectively. The photocouplers 12 and 13 convert the pulse signals from the voltage/frequency converters 10 and 11 into optical signals, transmit them, convert them back into electrical signals, and apply them to one end of the input of gate circuits 14 and 15, respectively. ing. A sampling signal E from a CPU (central processing unit) 18 is applied to the other input ends of the gate circuits 14 and 15.
During the period when this signal is applied, the gate circuits 14, 1
5 is opened, and pulse signals from photocouplers 12 and 13 are derived. The output pulses from the gates 14 and 15 are counted by counters 16 and 17, respectively, and taken into the CPU 18, which divides the counts of both counters 16 and 17. The switching timing of the polarity changeover switch 2 is controlled by the signal a outputted from the CPU 18. In addition, in the above photocoupler 12,1
3 is provided to insulate the fluid flow rate detection system and the signal processing system.

Next, the operation of the embodiment shown in FIG. 1 will be explained. Now, in response to the signal a from the CPU 18, the switching operation of the polarity changeover switch 2 becomes the signal shown in FIG. 2A, and the positive and negative excitation currents shown in FIG. 2B alternately flow through the excitation coils 3a and 3b. When this excitation current flows, a voltage proportional to the fluid flow rate is generated between the electrodes 6a and 6b due to electromagnetic induction, which is amplified by the AC amplifier 7 to obtain the signal voltage shown in FIG. 2C. For example, the output width of the AC amplifier 7 at the sampling period T _{1 of} the sampling signal shown in FIG _.
If the output amplification of AC amplifier 7 is V ₂ , then the bias
The output signal of amplifier 8 to which B ₁ is applied is shown in Figure 2B.
It will be as shown in. i.e. sampling period
When the sampling period is T ₁ , it becomes V ₁ +B ₁ , and when the sampling period is T ₂ , it becomes -V ₂ . When these two signals are applied to the voltage/frequency converter 10 at respective timings, the relationship between the output signal frequencies F ₁ and F ₂ becomes V ₁ +B ₁ =KF _1a and −V ₂ +B ₁ =KF _2a . Then _, as shown in _FIG .
8. The CPU 18 calculates the difference in frequency between sampling periods _T1 and _T2 .
That is, F _1a +F _2a = 1/K _a (V ₁ +V ₂ ) is calculated.

Similarly to the voltage proportional to the fluid flow rate mentioned above, the signal proportional to the excitation current is F _1b + F _2b = 1/K _b (I ₁ + I ₂ ) where _Ka・K _b is the proportionality constant of the voltage/frequency converter. , can be calculated from the frequency difference between the sampling periods T ₁ and T ₂ , that is, the difference in the count value of the counter 17.

Furthermore, (F _1a − F _2a )/KC (F _1b −
F _2b ) is applied to obtain the true signal value proportional to the fluid flow rate. That is, the increase/decrease in the output signal due to the increase/decrease in the excitation current is compensated for by the division process.

Also, since the AC amplifier 7 performs high gain amplification,
Drift is a problem, but if the output of the amplifier 8 is illustrated in consideration of the drift of the AC amplifier 7, it becomes as shown in FIG. (For convenience, the waveform period is shown in the same period as the sampling pulse.) That is, if δV is the drift voltage, the signal voltage V ₁ ,
The drift voltage of the amplifier is superimposed on the voltage obtained by adding bias B ₁ to −V ₂ . Therefore, the input voltage of the voltage/frequency converters 10 and 11 is V ₁ +B ₁ +δV=Kf ₁ −V ₂ +B ₁ +δV=Kf ₂ ∴f ₁ −f ₂ =V ₁ +B ₁ +δV−(−V ₂ +B ₁ + δV)/K = V ₁ +V ₂ /K. That is, according to this embodiment, the Dorft fluctuation and bias fluctuation of the AC amplifier can all be canceled out.

In the above embodiment, the biases B ₁ and B ₂ are as follows:
Of course, different values may be used, but the circuit can be further simplified by using the same reference power source.

(F) Effects of the Invention According to the electromagnetic flowmeter of the present invention, in addition to deriving a signal proportional to the fluid pipe flow rate, a signal proportional to the excitation current is also derived, and both derived signals are pulsed by a voltage/frequency converter. Since the pulse signal is converted into a signal, this pulse signal is counted by a counter, and the arithmetic processing section divides both counts, it is possible to compensate for the influence of fluctuations in the excitation current. In particular, since a non-constant current source can be used in the electromagnetic flowmeter, the circuit can be simplified and costs can be reduced, which has great practical effects.

Furthermore, since both the signal from the AC amplifier and the signal proportional to the excitation current are applied to the voltage/frequency converter through an amplifier with bias added to each, even if the excitation signal is reversed in polarity, The signal amplitude can be derived as a frequency difference, AC
Amplifier drift fluctuations and bias fluctuations can also be canceled out. Therefore, unlike conventional electromagnetic flowmeters, there is no need for hold amplifiers or differential amplifiers for positive and negative signals, and the circuit can be simplified. In addition, the flow rate corresponding signal and the excitation current corresponding signal are set to the first and second signals, respectively.
converts it into a pulse signal with a frequency corresponding to the voltage value of the voltage/frequency converter, counts those pulse signals with a first and second counter, and divides both output values with a digital calculation means. Because of this, the circuit configuration is simpler than when constructed using analog circuits, and accuracy is also improved. Furthermore, the first and second
Since the fluid flow rate detection system and the signal processing system are insulated by the photocoupler, the signal processing system is not affected by noise from the detection system.

[Brief explanation of drawings]

Fig. 1 is a circuit connection diagram of an electromagnetic flowmeter showing an embodiment of the present invention, Fig. 2 is a waveform diagram of each part to explain the operation in the embodiment shown in Fig. 1, and Fig. 3 is a diagram similar to that shown in Fig. 1. FIG. 3 is a waveform diagram for explaining drift compensation of the AC amplifier in the example circuit shown in FIG. 1: Non-constant current source, 2: Polarity switch, 3
a, 3b: excitation coil, 4: resistance, 5: fluid pipe,
6a, 6b: electrode, 7: AC amplifier, 8, 9: amplifier, 10, 11: voltage/frequency converter, 12,
13: Photocoupler, 14, 15: Gate circuit, 1
6, 17: Counter, 18: CPU.

Claims (1)

[Claims] 1. An excitation current that alternately changes between positive and negative is passed through the excitation coils 3a and 3b, and a magnetic field is applied to the fluid tube 5 through which the fluid to be measured is caused to flow by this excitation current. In an electromagnetic flowmeter that derives a signal corresponding to the fluid flow rate from provided electrodes 6a, 6b, the circuit 4 derives a signal corresponding to the excitation current of the excitation coils 3a, 3b, and the electrodes 6a, 6b.
an AC amplifier 7 for amplifying the derived signal of the
First and second amplifiers 8 and 9 amplify the output of the AC amplifier 7 and a signal corresponding to the excitation current by applying a predetermined bias, respectively, and the output voltages of the first and second amplifiers 8 and 9 are first and second voltage/frequency converters 10, 11 that convert into pulse signals with a frequency proportional to , and convert the pulse signals from these first and second voltage/frequency converters 10, 11 into optical signals. first and second photocouplers 12, 1 that convert the signal into an electric signal again.
3, and these first and second photocouplers 12,
first and second counters 16 and 17 that count upon receiving pulse signals from 13; and digital calculation means 18 that divides the count value of the first counter 16 by the count value of the second counter 17. An electromagnetic flowmeter characterized by the following.