JPH0450508Y2 - - Google Patents

Description

[Detailed description of the invention] <Field of industrial application> The present invention relates to an electromagnetic flowmeter, and particularly relates to an invention useful for use in a liningless type electromagnetic flowmeter that does not have an insulating lining material in the conduit. .

<Prior Art> Generally, in an electromagnetic flowmeter, the inner surface of the conduit is lined with an insulating lining material in order to prevent the signal voltage generated in the conduit in response to the flow rate from being short-circuited by the conductive conduit. However, recently, electromagnetic flowmeters without linings have been proposed in order to prevent accidents caused by deformation of the lining material. An example of such a conventional electromagnetic flowmeter is shown in FIG. A conductive conduit 1 is filled with a fluid to be measured 2, and a magnetic field B is applied across the conductive conduit. When the fluid to be measured 2 flows, a signal voltage is generated at the measurement electrodes 3a and 3b. This signal voltage is applied to the non-inverting input terminals of the feed amplifiers 4a, 4b and amplified, and the voltage appearing at the output terminals 5a, 5b causes the measuring electrodes 3a,
A current is passed through the ground electrode G through the power supply electrodes 6a and 6b fixed to the conductive conduit 1 in the vicinity of the conductive conduit 3b, thereby forming a potential distribution in the conductive conduit 1. The potential near the measurement electrode of the potential distribution formed in this way is detected by tube potential electrodes 7a, 7b fixed to the conductive conduit 1 between the measurement electrodes 3a, 3b and the power supply electrodes 6a, 6b. It is fed back to the inverting input terminals of the feed amplifiers 4a and 4b and stabilized in a balanced state.

Next, the internal configuration of the feed amplifier 4a is shown in FIG. 2 (the configuration of the feed amplifier 4b is also exactly the same). The feed amplifier 4a is composed of a preamplifier 8a and output transistors 9a and 10a. A non-inverting input terminal of the preamplifier 8a is connected to the measuring electrode 3a,
A signal voltage E _a is given. Its inverting input end is connected to tube potential electrode 7a, and its output end is connected to the bases of output transistors 9a and 10a. The collector of the output transistor 9a is connected to the positive power source _E1 , and the emitter is connected to the emitter of the output transistor 10a and at the same time to the output end 5a of the feed amplifier 4a. The collector of the output transistor 10a is a negative power supply
It is connected to E ₂ and has a complementary configuration with the output transistor 9a. Feed amplifier 4
The output end 5a of a is connected to the power supply electrode 6 as shown in FIG.
a and is connected to the ground electrode G via conductive conduit 1.
It is connected to the.

Assuming that the voltage at the output end 5a of the feed amplifier 4a, that is, the feed voltage of the feed electrode path 6a, is V _6a and the tube resistance between the feed electrode 6a and the ground terminal G is R _w , in a normal conductive conduit, R _w ≒ The supply voltage V _6a is the same as the tube potential V _7a and almost equal to the signal voltage Ea of the measuring electrode 3a. Although it depends on the flow rate, under normal usage conditions V _6a ≒ (0.1 to 1.0) mV. ...(1). At this time, the power P _w consumed in the conductive conduit 1 _is P _w = V ₆ a ² /R _w = (0.1 to 10) mW (2) I _a = V _6a /R _w = 1 to 10A...(3). on the other hand,
Since it is necessary to apply a power supply voltage of at least several volts (E ₁ , E ₂ =5 volts) between the collector and emitter of the output transistor 9a, the power consumption P _c of the output transistor 9a is equal to that of the output transistor 10a.
Letting the idle current of I _i be as follows.

P _c = (E ₁ − V _6a ) × (I _a + I _i ) = E ₁ × I _a (E ₁ ≫ V _6a , I _a ≫ I _i ) = 5 × (1 to 10) = 5 to 10 W ... (4) When the feed voltage V _6a is negative, power is consumed by the output transistor 10a, so that the power P _c is consumed inside the feed amplifier 4a. Therefore, the ratio between the power P c consumed inside the feed amplifier 4a and the power P _w consumed in the conductive conduit 1 is P _c / _{P w =} 5W/ even when I _a = 1A. 0.1 mW=50,000 (5), which means that 50,000 times more power is consumed inside the feed amplifier 4a than the power required to form a potential distribution in the conductive conduit 1.

This wastes energy resources and causes an increase in the cost of the feed amplifier 4a, particularly the cost required for its heat radiation.

<Purpose of the present invention> In view of the above-mentioned conventional technology, the present invention reduces the power consumed by an amplifier that supplies current to a conductive conduit to form a potential distribution, and suppresses a rise in temperature. The purpose of this is to improve reliability.

<Structure of the present invention> The present invention relates to an electromagnetic flowmeter in which the same potential distribution as the potential generated inside the conductive conduit through which the fluid to be measured flows is formed in the conductive conduit. an error amplifier that amplifies the difference between a signal voltage corresponding to the flow rate of fluid and a tube potential of the tube wall of the conductive conduit; a voltage/pulse conversion circuit that converts the output of the error amplifier into a pulse train; A switch driven by the output of the pulse conversion circuit, an inductance connected in series to the power supply and the switch via the tube wall, and connected in parallel to a series circuit consisting of the tube wall and the inductance. A flywheel diode is provided, and current is caused to flow from the power supply to the tube wall by opening and closing the switch to form the potential distribution.

<Example> Hereinafter, an example of the present invention will be described in detail based on the drawings. Components having the same functions as those shown in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant explanations will be omitted.

FIG. 3 is a block diagram showing a first embodiment of the present invention. However, since the configuration of the b side is the same as the a side, only the a side is shown for simplicity. 11a is the signal voltage
12a is a buffer amplifier for E _a , 12a is a buffer amplifier for tube potential V _7a , and 13a is an error amplifier for amplifying the difference between signal voltage E _a and tube potential V _7a . A signal voltage E _a is applied to the non-inverting input terminal of the buffer amplifier 11a, and its output terminal is connected to the non-inverting input terminal of the error amplifier 13a. The tube potential V _7a is applied to the non-inverting input terminal of the buffer amplifier 12a, and its output terminal is connected to the inverting input terminal of the error amplifier 13a. The output terminal of the error amplifier 13a is
It is connected to the input terminal of the voltage/pulse conversion circuit 14a, and outputs a pulse train signal _Ep having a duty cycle proportional to the output voltage of the error amplifier 13a.
One end of the inductance L is connected to the power supply electrode 6a, and the other end is connected to one end of the switch SW. The other end of the switch SW is connected to the positive electrode of the power supply _E3 , and the negative electrode is connected to the ground terminal G. A cathode of a flywheel diode D is connected to the other end of the inductance L, and its anode is connected to a ground terminal G. switch
SW is controlled by the pulse train signal E _p and the power supply
A current is supplied from E ₃ to the feeding electrode 6a, and the resulting tube potential V _7a of the tube potential electrode 7a is fed back to the buffer amplifier 12a.

FIG. 4 is a waveform diagram for explaining the operation of the circuit of FIG. 3, and the operation of FIG. 3 will be explained using this waveform diagram.
The signal voltage E _a with an on-off ratio of T ₁ /(T ₁ +T ₂ ) shown in FIG. 4a is buffered by a buffer amplifier 11a, and then amplified by an error amplifier 13a as a difference from the tube potential V _7a . Voltage/pulse conversion circuit 14
In a, the fourth
As shown in Figure b, a pulse train voltage _Ep whose duty cycle changes is output to turn on and off the switch SW. When the switch SW is on, the power supply E ₃
From switch SW, inductance L, pipe resistance
A current is supplied to R _w as shown by the solid line in FIG. When the switch SW is turned off, the tube wall current _I0 flowing through the inductance L flows through the flywheel diode D as shown by the broken line in FIG. 3, and is attenuated by the time constant L/ _Rw . In the case of period T ₁ in FIG. 4a during which the voltage/pulse conversion circuit 14a continuously outputs pulses, the switch SW is turned on again before the tube wall I ₀ attenuates to zero, and the power supply is turned on.
Current is supplied from E ₃ . Due to repeated on/off of the switch SW, the average value of the tube wall current I _p is the duty ratio D= which is the on/off ratio of the switch SW.
It can be controlled by changing t ₁ /(t ₁ +t ₂ ).

The duty ratio D is the pulse on time t ₁ while keeping the pulse repetition frequency 1/(t ₁ + t ₂ ) constant.
A pulse width modulation method that changes the pulse width, a frequency modulation method that changes the pulse repetition frequency 1/(t ₁ + t ₂ ) while keeping the pulse on time T ₁ constant, and a duty modulation method that changes t ₁ and t ₂ simultaneously. It can be changed by cycle modulation method.

On the other hand, during the period _T2 in FIG. 4a, the switch SW is not turned on, so the tube wall current _I0 attenuates to zero.
This change is shown in Figure 4c.

By changing the duty ratio of the output voltage E _p of the voltage/pulse conversion circuit 14a, the signal voltage E _a
A voltage having the same magnitude as the signal voltage is formed in the conductive pipe line 1 by passing a pipe wall current similar to the signal voltage through the pipe wall, detecting this with the tube potential electrode 7a, and feeding it back negatively to the error amplifier 13a. Functions as a liningless electromagnetic flowmeter.

FIG. 5 is a block diagram of an embodiment in which the signal voltage E _a takes three values. A bias addition amplifier 1 is provided between the buffer amplifier 11a and the error amplifier 13a.
5a is connected, and a DC bias voltage V _b is applied so that the value of the output (E _a +V _a ) of the bias addition amplifier 15 a is always positive or zero, and this voltage is converted into a pulse train by the voltage/pulse conversion circuit 14 a. This configuration changes the duty ratio.

FIG. 6 is a waveform diagram showing the operation of the embodiment shown in FIG. Corresponding to the three values of the signal voltage E _a (Figure 6a), E _a1 (positive), E _a2 (zero) and E _a3 (negative), the voltage/pulse conversion circuit is constructed as shown in Figure 6 b. 14
By switching the duty ratio of the output voltage E _p of a into three stages, the tube wall current is changed as shown in Fig. 6c.
The average current of I ₀ is changed in three stages, and the positive signal voltage,
Tube wall currents corresponding to zero, and three negative values can be supplied to the conductive conduit.

FIG. 7 is a block diagram showing a third embodiment of the present invention. The signal voltages E _a and E _b respectively generated at the measurement electrodes 3 a and 3 b are detected by the differential amplifier 16 , and the signal voltages generated at the tube potential electrodes 7 a and 7 b are respectively detected by the differential amplifier 17 . is input to the error amplifier 18, and its output is converted into a pulse train signal E _p ' by the voltage/pulse conversion circuit 19, which is then used as a switch.
The configuration is such that the SW is controlled. According to this configuration, the number of each of the error amplifier 18, voltage/pulse conversion circuit 19, switch SW, diode D, and inductance L can be reduced to one.

<Effects of the invention> In this invention, by controlling the on/off of the switch to control the current from the power supply, the same potential distribution as the signal voltage distribution is formed in the conductive conduit. No power is consumed inside the amplifier, and only the power necessary to form a potential distribution can be supplied to the conductive conduit. Note that the semiconductor switch added for this purpose consumes almost no power when it is on or off, and only consumes power during the transient state of on/off switching, and the power consumption in the voltage/pulse conversion circuit is also small.

Therefore, the overall power consumption can be significantly reduced compared to the conventional technology, which contributes to improving reliability.

Furthermore, when the amplifier component that supplies current to the conductive conduit is integrated with the conductive conduit, etc., the surrounding environment for use is poor, and in the conventional case, heat generation in the feed amplifier is added to this, resulting in reliability problems. However, in the case of the present invention, sufficient reliability can be ensured since essentially little heat is generated.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a conventional electromagnetic flowmeter, FIG. 2 is a block diagram showing a specific configuration of the feed amplifier in FIG. 1, and FIG. 4 is a waveform diagram explaining the operation of the circuit in FIG. 3, FIG. 5 is a block diagram showing the main part of the second embodiment of the present invention, and FIG. 6 is an implementation of the circuit in FIG. FIG. 7 is a waveform diagram showing the operation of the example, and FIG. 7 is a block diagram showing the third embodiment of the present invention. 1... Conductive conduit, 3a, 3b... Measurement electrode,
4a, 4b...Feeding amplifier, 13a...Error amplifier, 14a...Voltage/pulse conversion circuit, E _a ...
Signal voltage, E _p ... Pulse train signal, E ₃ ... Power supply, D ... Flywheel diode, L ... Inductance, SW ... Switch, R _w ... Tube resistance, V _6a ... Power supply voltage.

Claims (1)

[Scope of utility model registration request] In an electromagnetic flowmeter that forms a potential distribution in the conductive conduit that is the same as the potential generated inside the conductive conduit through which the fluid to be measured flows, a signal voltage corresponding to the flow rate of the fluid to be measured and the conductive an error amplifier that amplifies the difference between the tube wall potential of the sexual canal wall and a tube potential; a voltage/pulse conversion circuit that converts the output of the error amplifier into a pulse train; and a switch driven by the output of the voltage/pulse conversion circuit. , an inductance connected in series to a power supply and the switch via the tube wall, and a flywheel diode connected in parallel to a series circuit constituted by the tube wall and the inductance, An electromagnetic flowmeter characterized in that a current is caused to flow from the power supply to the tube wall by opening and closing a switch to form the potential distribution.