JPS5811009B2 - electromagnetic flow meter - Google Patents

Description

[Detailed description of the invention] The present invention relates to the overall circuit of an electromagnetic flowmeter.

More specifically, the present invention relates to a two-wire electromagnetic flowmeter in which the entire circuit uses both a signal line and a power line.

In conventionally known electromagnetic flowmeters, a current of about 0.5 to 1.0 A must be continuously passed through the excitation coil to generate a magnetic field in the pipe, so the power consumed by the excitation coil is large. However, it was impossible to configure the entire circuit including the excitation circuit with a two-wire system.

The present invention allows the entire circuit to be configured in a two-wire system by providing a period during which no excitation current is passed through the excitation coil.

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

In the figure, 1 is a pipe through which the fluid to be measured flows, 11
, 12 are electrodes attached to the pipe 1 facing each other, 1
3 and 14 are excitation coils, which constitute an electromagnetic flowmeter transmitter.

21 is a constant current circuit, 22 is a semiconductor switch that turns on and off the current 1W from this constant current circuit 21, and 3 is a control pulse P1. P2. A pulse generator that generates P3, 4 is a capacitor.

The power supply end of the pulse generator 3 and the excitation coil 14゜13
The series circuit of the semiconductor switch 22 and the constant current circuit 21 is as follows:
Both are connected to both ends of the capacitor 4.

5 is an amplifier which inputs the electromotive force generated between the electrodes 11 and 12, 61 and 62 are sampling switches, and 63 and 64 are holding capacitors.

Switches 61 and 62 and capacitors 63 and 64 form a circuit 5H that samples and holds the output signal from the amplifier 5.
1 (SF3).

7 is a differential amplifier that amplifies the difference between the output signals of the sample and hold circuits SH1 and SH2; 8 is an output transistor connected to the output terminal of the differential amplifiers and controls the output current Io; 9 is a constant voltage circuit. .

The power supply end of the amplifier 5, the power supply end of the differential amplifier 7, and one end of the capacitor 4 are all connected to a constant voltage circuit 9.

10 is a two-wire transmission line, and the collector and emitter of the output transistor 8 are connected in series to a feedback resistor Rf, a DC power source Es provided on the receiving side, and a load such as a receiving instrument via the two-wire transmission line 10. connected to the circuit.

The DC power source Es provided on the receiving side is, for example, 24V, and power from there is supplied to the amplifier and transmitter side via the two-wire transmission line 10.

The constant voltage circuit 9 is composed of, for example, a constant current element and a constant voltage element, outputs a constant voltage (Es) of 10V to its output terminal, and connects excitation coils 13 and 14, a pulse generator 3, an amplifier 5, and a differential The amplifier 7 is driven by a constant voltage from this.

The operation of the apparatus configured as described above will be explained below with reference to the time chart of FIG.

First, a driving pulse P1 is output from the pulse generator 3,
As shown in FIG. 2A, the semiconductor switch 22 is driven so that the on time τon is short, for example, about 20 m5, and the off time T is long, for example, 5 seconds.

As a result, an excitation current Iw that is turned on and off flows through the excitation coils 13 and 14 as shown in the opening of FIG.

Here, the maximum value Io of this excitation current is the constant current circuit 21
It can be set by a circuit constant of 0.5A, for example, and maintained at 0.5A.

Furthermore, since the excitation coils 13 and 14 have inductance L, they have a slope at the rise as shown in the figure, and the excitation current I after a time is given by equation (1).

However, R is the resistance of the excitation coils 13 and 14.The inductance L of the excitation coils 13 and 14 is 0.236H, the resistance R is 8.2Ω, Es is 10V, and the maximum current Io is 0.5.
If A, the time t1 until Io=0.5A is approximately 15 m5, and if τon is 20 m5, the time during which the maximum current Io flows is 5 ms.

Also, the time during which the switch 21 is off is 5 se
c, the average current consumption iv in the exciting coil is (
2) The formula is as follows.

The pulse generator 3, the amplifier 5, and the differential amplifier 7 can be made of, for example, a CMO8 or a low power consumption IC, so that the current consumption of these circuits as a whole can be 2 mA or less.

Therefore, it is possible to operate the entire circuit with a power of 10V and 4mA or less.

Between the electrodes 11 and 12, during the period when the excitation current Iw is flowing as shown in FIG. During the period when no current flows, the electrochemical unbalance voltage e2
is occurring.

The amplifier 5 receives the electromotive force e1 and e2 generated between the electrodes 1L12.
The electromotive force e1 is held in the sample and hold circuit SH1 by the drive pulse P2 as shown in FIG. It is held in circuit SH2.

Differential amplifier 7 includes each sample and hold circuit 5H19SH2
The difference e1-e2 between the output signals and the feedback voltage generated at the feedback resistor Rf are amplified and output to the output transistor 8.

Output transistor 8 controls the magnitude of output current Io flowing therein in accordance with the output signal of differential amplifier 7.

Therefore, the load on the receiving side can receive a signal Io proportional to the flow rate with the influence of the unbalanced component e1 removed.

Note that this received signal Io is updated every time an excitation current flows through the excitation coils 13 and 14.

In the above embodiment, the excitation current flowing through the excitation coils 13 and 14 is made by converting the output of the constant voltage circuit 9 (the voltage across the capacitor 4) into a constant current using the constant current circuit 21. , the output of the constant voltage circuit 9 is, for example, DC/
It is also possible to boost the voltage with a DC converter and make it a constant current.

Thereby, the rise time t1 of the excitation current can be reduced.

Further, this exciting current may be made to flow in both forward and reverse directions.

Further, the constant voltage and constant current device may be roughly set, and the circuit may be configured to detect the excitation current and divide the signal voltage.

As explained above, according to the present invention, the period in which the excitation current is passed through the excitation coil is selected to be shorter than the period in which it is not passed, so the average value of the excitation current can be reduced, and the entire circuit can be configured with a two-wire system. .

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the electromagnetic flowmeter of the present invention, and FIG. 2 is a diagram showing its operating waveforms. 1... Pipe, 11, 12... Electrode, 13, 14...
...Exciting coil, 22...Semiconductor switch, 21...
- Constant current circuit, 3... pulse generator, 5... amplifier, SHL. SF3...Sample hold circuit, 7...Differential amplifier, 8...Output transistor, 10...Two-wire transmission line, L...Load.

Claims (1)

[Claims] 1. Circuit means to which power is supplied via a two-wire transmission line and to turn on and off DC current to the excitation coil; power is supplied via the two-wire transmission line to supply current to the excitation coil; circuit means for obtaining and amplifying the difference between the steady value of the signal voltage that occurs between the electrodes when current is flowing and the steady value of the signal voltage that occurs when no current is flowing through the excitation coil; An electromagnetic flowmeter comprising a current control element that controls the current flowing through the two-wire transmission line according to an output signal, and in which a period during which current is flowing through the excitation coil is selected to be shorter than a period when no current is flowing. .