JPS6128284B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rectangular wave excitation type electromagnetic flowmeter, and aims to realize this type of electromagnetic flowmeter that operates with low power consumption.

Generally, in an electromagnetic flowmeter using a rectangular wave excitation method, the period in which current is flowing through the excitation coil is selected to be equal to the period in which it is not flowing, and the steady value of the signal voltage when the excitation current is flowing is After sampling and holding the steady-state values of the signal voltages when no current is flowing, a signal corresponding to the flow rate is detected from the difference. Therefore, an electromagnetic flowmeter using a rectangular wave excitation method is not affected by electrochemical unbalanced voltage that occurs between electrodes, which is a problem with a DC excitation method, and is free from 90° noise, which is a problem with an AC excitation method. It has features such as being unaffected by common mode noise caused by dirt and electrode contamination. On the other hand,
It is necessary to apply a current of about 0.5 to 1.0A to the excitation coil by turning it on and off, which not only consumes a lot of power, but also
There are drawbacks such as reduced reliability due to heat generated by the switch element for turning the current on and off, and the need to take measures to prevent heat generation.

Here, the present invention effectively eliminates the above-described drawbacks of conventional devices by controlling the amplitude I and repetition rate of the excitation current in relation to the flow velocity of the fluid to be measured, and provides an electromagnetic flowmeter with low power consumption. This is what we are trying to achieve.

FIG. 1 is a block diagram showing an embodiment of an electromagnetic flowmeter according to the present invention. In the figure, 1
is a rectangular wave excitation circuit, which includes a DC voltage source 2, a current control circuit 3 connected in series to this DC voltage source 2, and a current I connected in series to this current control circuit.
It has a switch element 4 that turns on and off. 5
is an electromagnetic flowmeter transmitter, which includes an excitation coil 6, a pipe 7 through which a fluid to be measured flows, and electrodes 81 and 82. 9 is an amplifier that amplifies the electromotive force generated between the electrodes 81 and 82; 10 is a controller that inputs a signal from the amplifier 9; I and repetition period T are controlled. 11
is an arithmetic circuit which receives signals related to the amplitude I and repetition period T of the excitation current flowing through the excitation coil 6.

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

In this device, the controller 10 receives a signal corresponding to the electromotive force generated between the electrodes 81 and 82 applied via the amplifier 9, that is, an unbalanced voltage generated when no excitation current is flowing, and a pipe 7. When the flow velocity of the fluid to be measured becomes low, that is, in the low flow velocity region, the amplitude I of the excitation current is changed as shown in Fig. 2A. control is performed so that the ratio t/T of the time (time width) during which the current is flowing and the repetition period T becomes small. Furthermore, as the flow velocity of the fluid to be measured increases, that is, in the high flow velocity region, the amplitude of the exciting current I
is controlled so that the ratio t/T between the current flowing time t and the repetition period T is increased.

Between the electrodes 81 and 82 of the transmitter 5, there is an electromotive force e corresponding to the excitation current I flowing through the excitation coil 6, the flow velocity V of the fluid to be measured flowing through the pipe 7, and an unbalanced voltage e.
A sum voltage e _S with _B is generated, and this relationship can be expressed as (1)
As shown in the formula.

e _S =K・D・I・V+e _B However, K: Constant D: Distance between electrodes 81 and 82 e=K・D・I・V As described above, the controller 10 is connected to the amplifier 9
The amplitude I and repetition period t/T of the excitation current are controlled according to the signal e _I from the
_B (unbalanced voltage) and the difference voltage _e (=
e _S −e _B ) is controlled so that it always has a constant value regardless of the flow velocity V. When controlled like this,
Equation (1) can be expressed as equation (2).

I=e/K・D・1/v =Ko・1/v (2) However, Ko=e/K・D, which is a constant value.As is clear from equation (2), in the device according to the present invention, The amplitude I of the current is inversely proportional to the flow velocity V of the fluid to be measured; the lower the flow velocity, the larger the amplitude I of the excitation current, and the higher the flow velocity, the smaller the amplitude I of the excitation current.

The arithmetic circuit 11 receives a signal related to the amplitude I of the excitation current, and can obtain a signal e _p related to the flow velocity V by performing reciprocal calculation.

Here, the temporally averaged excitation current Iav supplied from the DC power supply 2 is, if the current width t is constant.
It is expressed by equation (3).

Iav=I・t/T (3) The controller 10 greatly controls the repetition period T of the excitation current in proportion to the amplitude I, that is, inversely proportional to the flow velocity V, as shown in equation (4). Accordingly, the averaged excitation current Iav is as shown in equation (5).

T=k・I (4) Iav=t/k=const (5) From equation (5), the average power supplied from the DC power supply 2 is a constant value determined by the current width t, and this current width is By making appropriate selections, power consumption can be kept low.

In this device, the amplitude I of the excitation current becomes infinite when the flow velocity v=0, as is clear from equation (2), so the operation is stopped at a low flow velocity below a certain value.

According to this device, while keeping power consumption low, the amplitude I of the excitation current increases in the low flow rate region, making it possible to improve the S/N ratio, and in the high flow rate region, the excitation current is repeatedly Period T
Since this decreases, responsiveness can be improved. Therefore, according to the present invention, it is possible to realize an electromagnetic flowmeter that operates efficiently with a low-capacity power source.

Note that in the above embodiment, the excitation current is
As shown in the figure, the pulse waveform changes to one polarity, but as shown in FIG. 3 or 4, the pulse waveform may change to both positive and negative polarities. Furthermore, although the controller and the arithmetic circuit are shown as separate block diagrams here, they can be used in common if they are constructed of, for example, a sample-and-hold circuit and a microprocessor. In addition, here, the signal related to the flow velocity V was obtained by calculating the reciprocal of the amplitude value I of the excitation current, but from equation (4), the repetition period T and the amplitude value I are in a proportional relationship. The flow velocity V may be obtained by calculating the reciprocal of this period T.

As described above, according to the present invention, it is possible to realize a rectangular wave excitation type electromagnetic flowmeter that operates efficiently with low power consumption.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the configuration of one embodiment of the device of the present invention, Fig. 2 is a waveform diagram of the excitation current in the device of Fig. 1, and Figs. It is a waveform diagram of a current. 1... Rectangular wave excitation circuit, 2... DC voltage source, 3
...Current control circuit, 4...Switch element, 5...
Transmitter, 6... Excitation coil, 7... Pipe, 8
1, 82...electrode, 9...amplifier, 10...controller, 11...arithmetic circuit.

Claims (1)

[Claims] 1. An electromagnetic flowmeter comprising a rectangular wave excitation circuit, an excitation coil to which excitation current is supplied from the rectangular wave excitation circuit, and a pair of electrodes provided on a pipe through which a fluid to be measured flows. A current control circuit and a switch element connected in series with the excitation coil are input with a signal generated between the pair of electrodes, and a signal generated between the electrodes when no excitation current is flowing and an excitation current are passed. so that the difference between the signal that sometimes occurs between the electrodes is a constant value,
The amplitude value I of the excitation current is controlled through the current control circuit, and the excitation current is supplied with a predetermined time width through the switch element, and the repetition period T is controlled in proportion to the amplitude value I of the excitation current. An electromagnetic flowmeter comprising: a controller that performs a reciprocal operation on the amplitude value I or repetition period T of the excitation current; and an arithmetic circuit that calculates a reciprocal of the amplitude value I or repetition period T of the excitation current and outputs a signal related to the flow velocity of the fluid to be measured.