JPH0450506Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an electromagnetic flowmeter, and particularly to an idea useful for use in a liningless electromagnetic flowmeter that does not have an insulating lining material in the conduit. <Prior art> Electromagnetic flowmeters generally coat the inner surface of the pipe with an insulating lining material to prevent the signal voltage generated in the pipe in response to the flow rate from being short-circuited by the conductive pipe. There is. However, recently, electromagnetic flowmeters without linings have been proposed from the viewpoint of preventing accidents due to deformation of the lining material and reducing costs. 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 feed amplifier 4
It is applied to the non-inverting input terminals of a and 4b and amplified.
The power supply voltage appearing at the output ends 5a, 5b is used to supply power to the power supply electrodes 6a, 6b fixed to the conductive conduit 1 near the measurement electrodes 3a, 3b via the power supply lines 7a, 7b. By passing a current through the ground electrode G, a potential distribution is formed in the conductive conduit 1. The potential near the measurement electrode of the potential distribution formed in this way is the measurement electrode 3a, 3b, the power supply electrode 6a,
6b, the tube potential is detected by the tube potential electrodes 8a, 8b fixed to the conductive conduit 1 between the feed amplifiers 4a, 4b.
It is fed back to the inverting input terminal of the circuit and stabilized in a balanced state. By the way, in general, electromagnetic flowmeters are often determined to generate a signal voltage of about 1 mV when the flow velocity of the fluid to be measured is 1 m/s due to the relationship with the signal-to-noise ratio. On the other hand, if a stainless steel pipe is used as the conductive conduit 1, the power supply electrodes 6a, 6b
The electrical resistance between the electrode G and the ground electrode G is about 0.1 mΩ or less. Therefore, a potential distribution of 1 mV is applied to conductive pipe 1.
In order to form a current of approximately 10A, the amplifier 4
It is necessary to pour it into the power supply electrodes 6a, 6b from a, 4b. The power required in this case is about 10 mW. In reality, since there is power loss within the feed amplifiers 4a, 4b and power loss in the feed lines 7a, 7b, the amount of power that should be fed by the feed amplifiers 4a, 4b increases. For example, in a buried type electromagnetic flowmeter, the main body of the transmitter including the conductive conduit 1 and the feed amplifier 4
It is necessary to configure the power supply section including the transmitter a and 4b separately, but in such a case, the distance between the main body of the transmitter and the power supply section is 50 m, and the power supply line 7a, 4b is connected using a cable with a nominal cross-sectional area of 2 ^mm2 . 7b, its resistance is 1Ω, so for a current of 10A
This results in a power loss of 100W. Therefore, this conventional technique has drawbacks such as a large power loss occurring in the feeder line relative to the required power, and the rated capacity of the feeder amplifier being dependent on the length of the feeder line. <Purpose of the invention> In view of the above-mentioned conventional technology, the purpose of the invention is to reduce power loss in the feed amplifier and the feed line connected to the output end of the feed amplifier, thereby saving power as a whole. . <Configuration of the present invention> The configuration of the present invention that achieves this objective is to apply the same potential distribution to the conductive conduit through which the measured fluid flows as the potential distribution generated in the measured fluid near the inner wall of the conductive conduit. In the liningless electromagnetic flowmeter to be formed, a potential difference detection means detects a potential difference between a measurement electrode that detects a signal voltage and a tube potential electrode that is provided near the measurement electrode and detects a tube potential of the conductive conduit. , a pulse conversion means for converting the output of the potential difference detection means into a predetermined pulse signal, a current conversion means for converting the pulse signal into a current, a rectification and smoothing means for rectifying and smoothing the output of the current conversion means, and the rectification. The output current of the smoothing means is supplied to the conductive conduit to form a potential distribution corresponding to the signal voltage in the conductive conduit, thereby reducing power loss in the power supply line. <Example> (First Example) Hereinafter, an example of the present invention will be described in detail based on the drawings. Note that parts having the same functions as those in the prior art are given the same numbers, and redundant explanations will be omitted. FIG. 2 is a block diagram showing a first embodiment of the present invention. The measurement electrode 3a and the non-inverting input terminal of the differential amplifier Q ₁ are connected, and the measurement electrode 3b and the inverting input terminal of the differential amplifier Q ₂ are connected, respectively. A trimmer R ₁ is connected between the output end of the differential amplifier Q ₁ and a common potential point C. On the other hand, tube potential electrode 8a
and the non-inverting input terminal of the differential amplifier Q ₂ and the tube potential electrode 8b and the inverting input terminal of the differential amplifier Q ₂ are connected, respectively. The trimmer R ₁ has a measurement electrode 3a,
This is for adjusting the ratio between the potential difference between the tube potential electrodes 3b and the potential difference between the tube potential electrodes 8a and 8b. A capacitor Ch is inserted between the sliding terminal of the trimmer _R1 and the non-inverting input terminal of the deviation amplifier _Q3 , and at the same time a resistor is connected between this non-inverting input terminal and the common potential point C.
R ₂ is connected. A high-pass filter is configured by capacitor Ch and resistor _R2 . This is to remove the DC potential between the measurement electrodes 3a and 3b. The inverting input terminal of the deviation amplifier _Q3 is connected to the output terminal of the differential amplifier _Q2 , and its non-inverting input terminal is connected to the output terminal 9 of the high-pass filter, so that the signal voltage and tube potential are connected to the output terminal. The deviation voltage associated with the difference in is obtained. The output terminal of the deviation amplifier _Q3 is V/
It is connected to the input end of the D converter 10, and its output end is connected to the input end of the current pulse output stage 11. The V/D converter 10 converts the deviation voltage into a pulse train signal. This pulse train signal is applied to the primary winding 13p of the current transformer 13 connected to the output end of the current pulse output stage 11 via the power supply line 12. The secondary winding 13s of the current transformer 13 is connected to the input end of the diode bridge 14, and its output end is connected to the power supply electrodes 6a, 6b via an inductor 15.
It is connected to the. Differential amplifiers Q ₁ and Q ₂ , deviation amplifier Q ₃ , V/
The D converter 10, current pulse output stage 11, etc. constitute the feed amplifier 16 as a whole, but among these, the differential amplifier Q ₁ , the trimmer R ₁ , the humpass filter Ch/R ₂ , etc. convert the signal voltage into a flow output. It can also be used as the first stage of the converter. Also, a current transformer 13, a diode bridge 14
The inductor 15 and the like are usually mounted on the transmitter or configured to be installed near it. The operation of the embodiment of FIG. 2 constructed as above will be explained using the waveform diagram of FIG. 3. In the embodiment _of FIG.
It is detected as the signal voltage shown in Figure a (the waveforms are shown when the signal voltage is _e1 and _e2 ). On the other hand, the potential difference between the tube potential electrodes 8a and 8b is detected as a tube potential by a differential amplifier _Q2 . The difference between this signal voltage and the tube potential is determined as a deviation voltage by the deviation amplifier _Q3 , and this is converted by the V/D converter 10 into a pulse train signal whose duty cycle is proportional to the signal voltage as shown in FIG. 3b. This pulse train signal is converted into a current pulse signal by the current pulse output stage 11 and is then converted to a current pulse signal by the power supply line 12.
It outputs to the current transformer 13 via. The current transformer 13 performs current transformation (amplification) of this current pulse signal, and a third
The alternating current signal shown in FIG. 3c is obtained and rectified by the rectifier diode 14 to obtain the waveform shown in FIG. 3d. This is smoothed by the inductor 15 to obtain the current shown in FIG. 3e in the tube wall of the conductive conduit 1. This current generates a voltage on the wall of the conductive conduit 1 having the same magnitude as the signal voltage of the measurement electrodes 3a and 3b, thereby stabilizing the system. Incidentally, the flow rate signal is obtained by extracting a signal voltage from the output terminal 9 and the common potential point C in FIG. 2, and usually converting this into a current signal and outputting it. Although FIG. 3 shows the waveform of the embodiment when the signal voltage due to positive zero binary excitation is converted into a pulse train signal by the V/D converter 10, FIG. 2 shows waveforms in an embodiment when the V/D converter 10 converts the signal into a pulse train signal. In this case, after adding a constant DC bias voltage to the signal voltage shown in Figure 4a and converting it into a pulsating voltage that changes only in one direction as shown in Figure 4b, c
It is sufficient to convert it into the pulse train signal shown in the figure. FIG. 5 is a waveform diagram of an embodiment when a signal voltage by three-value excitation is converted into a pulse train signal by the V/D converter 10. The signal voltage shown in Figure 5a is amplified by a deviation amplifier and becomes the output waveform shown in Figure b, but when a constant DC bias voltage is added to this for pulse conversion, a pulsation that changes only in one direction as shown in Figure c is generated. Convert to voltage. When this is converted into a pulse train signal with a duty cycle of zero at the lowest level, the waveform shown in Figure d is obtained. As described above, in this embodiment, a signal voltage can be converted into a pulse train signal even when excitation that fluctuates in positive and negative directions is performed. (Second Embodiment) FIG. 6 is a block diagram showing a second embodiment of the present invention. In the first embodiment shown in FIG. 2, feedback is applied to the conductive conduit 1 for the difference voltage between the measuring electrodes 3a and 3b, but in the embodiment shown in FIG. The configuration is such that feedback is applied individually to the conductive conduit 1 for each generated signal voltage. That is, the measurement electrodes 3a,
Signal voltage detected at 3b and tube potential electrodes 8a, 8b
The tube potentials detected by the feed amplifiers 16a and 16a ,
16b, and the current pulse signal is sent from the output to the current transformer 13 via the power supply lines 12a and 12b.
a, 13b, and further diode bridge 1
4a, 14b and inductors 15a, 15b, power is supplied to the power supply electrodes so that the voltage difference between the signal voltage and the tube potential becomes zero. The flow rate signals are taken out from terminals 9a and 9b, respectively. (Third Embodiment) FIG. 7 is a block diagram showing a third embodiment of the present invention. In the first and second embodiments, the duty cycle modulation type current pulse feedback method was used, but in this embodiment, the voltage difference between the signal voltage and the tube potential is converted into an amplitude-modulated pulse, and the current The pulse is transformed (amplified) by a current transformer mounted on the oscillator, and a feedback current is supplied to the conductive conduit. The following description will be made based on FIG. 7 and using the waveform diagram in FIG. 8. In the same manner as in the first embodiment, the deviation voltage between the signal voltage (FIG. 8a) and the tube potential is obtained at the output terminal of the deviation amplifier _Q3 . This deviation voltage is applied to the current pulse output stage 11.
A switch SW is installed on the output side of the current pulse output stage 11.
is inserted, and this switch SW is turned on and off at the oscillation frequency of the oscillator 17 (FIG. 8a). Therefore, the current pulse input from the current pulse output stage 11 to the current transformer 13 via the feed line 12 becomes a signal current whose amplitude is modulated at a constant frequency, as shown in FIG. 8c. This current pulse is transformed into a current by a current transformer 13, rectified 14 and smoothed 15, and then supplied to the power supply electrodes 6a and 6b. As a result, the same potential distribution as the signal voltage is formed in the conductive conduit 1, thereby achieving the purpose of no lining. In the duty cycle modulation method in the embodiments of FIGS. 2 and 6, the repetition frequency of the pulse changes depending on the magnitude of the current signal, so the frequency component of the current pulse changes depending on the flow velocity. Therefore, when the induced noise from the current pulse is superimposed on the signal voltage, a beat between the signal and the frequency of the induced noise may occur. Particularly when the flow rate is low, the frequency of the current pulse becomes low and components close to the signal frequency increase, resulting in poor stability near the zero point. However, according to the embodiment shown in FIG. 7, even when the flow rate is low, the frequency of the current pulse remains constant and only the amplitude decreases, so the stability of the output near the zero point does not deteriorate. FIG. 9 shows an oscillator circuit for completely eliminating the induced noise from the current pulse to the signal voltage and the beat of the signal in the embodiment of FIG. 7. In FIG. 9, 18 is a phase detector, and one end of its input is supplied with a timing signal (frequency:
s) is given, and its output is passed through the low-pass filter 1
given to 9. The output of the low-pass filter 19 is input to a voltage controlled oscillator (VCO) 20,
The output (frequency) is input to the 1/n frequency divider circuit 21. The output frequency of the voltage controlled oscillator 21 is divided by 1/n by the 1/n frequency divider circuit 21, and the output is input to the other end of the phase detector 18, and is controlled as a whole so that the frequency difference with the timing signal becomes zero. be done. By using the output frequency of the voltage controlled oscillator 20 created in this way as a signal for switching the switch SW, the frequency of the current pulse is synchronized to n times the signal frequency, completely eliminating the beat between the induced noise and the signal. be able to. <Effects of the invention> As specifically explained in conjunction with the embodiments above, according to the invention, a pulse signal is sent to the feed line and the current is amplified (transformed) by the current transformer, so the distance between the feed amplifier and the transmitter is reduced. There is no need to run a large current through the power supply line even when the power supply line is far away, and power loss in the power supply line can be significantly reduced. In addition, since current amplification is performed by a current transformer, there is no need for a large-capacity power amplifier to control large currents in the feed amplifier, and since the current pulse output element only operates on and off, small elements with low loss can be used. . Furthermore, by using a high frequency pulse train signal, a small current transformer can be used even when applied to a low frequency excitation type electromagnetic flowmeter. There are various effects such as

[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 first embodiment of the present invention, and Fig. 3 is a waveform explaining the operation of the embodiment of Fig. 2. 4 is a waveform diagram explaining the operation of the embodiment of FIG. 2 in the case of binary excitation, and FIG. 5 is a waveform diagram explaining the operation of the embodiment of FIG. 2 in the case of three-level excitation. FIG. 6 is a block diagram showing a second embodiment of the present invention, FIG. 7 is a block diagram showing a third embodiment of the present invention, and FIG. 8 is a waveform explaining the operation of the embodiment of FIG. 7. 9 are block diagrams showing a modified embodiment of the oscillator in FIG. 7. 1... Conductive conduit, 3a, 3b... Measurement electrode,
6a, 6b...Power supply electrode, 8a, 8b...Tube potential electrode, 10...V/D converter, 12a, 12
b...Feeding line, 13a, 13b...Current transformer,
16a, 16b ... feeding amplifier, 17... oscillator.

Claims (1)

[Scope of utility model registration request] In a liningless electromagnetic flowmeter that forms a potential distribution in the conductive conduit that is the same as a potential distribution generated in the measured fluid near the inner wall of the conductive conduit through which the measured fluid flows, a measurement electrode that detects a signal voltage; potential difference detection means for detecting a potential difference between a tube potential electrode provided near the measurement electrode and detecting the tube potential of the conductive conduit; and a pulse for converting the output of the potential difference detection means into a predetermined pulse signal. a converting means, a current converting means for converting the pulse signal into a current, a rectifying and smoothing means for rectifying and smoothing the output of the current converting means, and supplying the output current of the rectifying and smoothing means to the conductive conduit to generate the conductive pipe. A liningless electromagnetic flowmeter that is characterized by forming a potential distribution corresponding to the signal voltage in the sexual duct.