JPH0216975B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a low frequency excitation type electromagnetic flowmeter that performs low frequency excitation for the purpose of improving the stability of the zero point and measures the flow rate of fluid using Faraday's law of electromagnetic induction. The present invention relates to a converter that can be driven by both external DC power sources and can constitute an electromagnetic flowmeter with high operational reliability. When a magnetic field is applied in a direction perpendicular to the flow velocity direction of the measured fluid, the electromotive force generated in a direction perpendicular to both the flow velocity direction and the magnetic field direction is detected according to Faraday's law of electromagnetic induction, and the measured fluid is In electromagnetic flowmeters that measure flow rates, commercial power is generally used as an external power source to drive the flowmeter, and a magnetic field that changes sinusoidally over time is created using this commercial power. However, in recent years, in order to improve the stability of the zero point of electromagnetic flowmeters, commercial power sources have been used as external power sources for driving the flowmeters, but rectangular waves or A so-called low-frequency excitation type electromagnetic flowmeter in which a magnetic field is formed using a trapezoidal wave-shaped excitation voltage or excitation current, and the steady-state value of the generated electromotive force is sampled for a period that is an integral multiple of the driving commercial power supply cycle. is widespread. Next, a conventional low frequency excitation type electromagnetic flowmeter using a commercial power source as an external drive power source will be described with reference to the drawings. FIG. 1 is a configuration diagram for explaining the operating principle of an electromagnetic flowmeter using Faraday's law of electromagnetic induction. In the figure, 1 is a measurement pipe through which a fluid to be measured 2 with an average flow velocity V [m/s] flows, and D [m]
is the inner diameter of the measuring tube 1, 3 is the iron core around which the exciting coil 4 through which the exciting current I [A] flows, and 5 is the exciting current I
The magnetic field created by B[T] is its magnetic flux density. As shown in the figure, when a magnetic field 5 with a magnetic velocity density B is applied perpendicular to the direction of the flow velocity V of the fluid to be measured 2, the measurement is performed in a direction perpendicular to both V and B according to Faraday's law of electromagnetic induction. Electrodes 6, 6 are fixedly installed on the inner wall of the tube 1 in contact with the fluid to be measured 2 and facing each other.
During this time, an electromotive force E=kBDV [V] is generated. k is a proportionality constant. The flow rate of the fluid to be measured 2 is Q [m ³ /s]
Then, Q=π/4×D ² V, so Q=π/4
×D/k×E/B Therefore, if D/k is known, the flow rate Q can be measured by detecting E/B. The above is the operating principle of the electromagnetic flowmeter, where 10 is a detector consisting of a measuring tube 1, an iron core 3, an excitation coil 4, and electrodes 6, 6, and 7 is a detector that receives an external power supply A for driving the electromagnetic flowmeter. outputs an excitation current I, calculates E/B using the electromotive force E and a signal corresponding to the magnetic flux density B obtained by means not shown as input, and obtains a signal P corresponding to the flow rate Q. A converter that outputs
8, 8 and 9, 9 are electric wires that respectively connect the electrodes 6, 6 and the excitation coil 4 to the converter 7; It consists of Since the electromagnetic flowmeter is based on the operating principle explained above, the exciting current can basically be either direct current or alternating current. However, the AC excitation method has many advantages over the DC excitation method, such as the voltage between the electrodes being unaffected by polarization in the electrodes, and it is also convenient in terms of the configuration of the excitation current. A commercial power source was used as an external power source for the device, and a commercial frequency excitation method was often adopted in which the commercial power source was used to form a magnetic field that varied sinusoidally over time at the commercial frequency. However, when such a commercial frequency excitation method is adopted, the electromotive force E input to the converter 7 is mixed with induction noise in phase and quadrature with the commercial power supply, causing the zero point of the signal P to fluctuate. Regarding the mechanism of this zero point fluctuation, for example, the following literature [Edamoto: "Measurement with an electromagnetic flowmeter" Measurement and Control, 5-
7, 51/57 (1966): Ito, Asada: “Progress of electromagnetic flowmeters and current status of standards” Measurement and Control, 14-11, 26/89
(1975): Sashima, Kuromori: “Recent trends in electromagnetic flowmeters”
Measurement and Control, 18-5, 29/33 (1979)]. For this reason, in recent electromagnetic flowmeters, it has become common to adopt a low frequency excitation method as described below to improve the stability of the zero point of the signal P. Fig. 2 is a block diagram showing an example of the configuration of a conventional commercial power supply driven low frequency excitation type electromagnetic flowmeter, and Fig. 3 is a time chart including signal or voltage waveforms of the main parts in Fig. 2. Parts having the same functions or meanings as in FIG. 1 are given the same reference numerals. t is the elapsed time. In both figures, Y is an external commercial power source as an external power source for driving the electromagnetic flowmeter, 71 is an AC/DC converter that converts the external commercial power source Y into a DC power supply with the required conditions in the converter 7, and 72 is an AC/DC converter The timing signal generation circuit outputs timing signals P2, P3, and P4 using an external commercial power source Y, where τ is the period of the external commercial power source Y, Ts is the sampling period, and N is an integer.
Further, in this case, Ts=τNs and Ns<N/2, where Ns is an integer. 73 is an excitation circuit which converts the DC voltage generated by the AC/DC converter 71 to excite the excitation coil 4 into a rectangular excitation voltage Ex having a period of τN in accordance with the timing signal P2. Output to excitation coil 4.
74 is a preamplifier circuit that amplifies the voltage between the electrodes 6 and 6 and outputs it as a signal S; 75 is a preamplifier circuit that amplifies the voltage between the electrodes 6 and 6 and outputs it as a signal S; 75 is a preamplifier circuit that amplifies the voltage between the electrodes 6 and 6 and outputs it as a signal S; Synchronization in which each portion (the shaded portion in FIG. 8) that has reached a nearly constant state on the negative side is synchronously rectified and sampled using timing signals P3 and P4, and output as signals S1 and S2, respectively. Rectifier circuit, 7
6 is a differential amplifier circuit that differentially amplifies the signals S1 and S2 and outputs a signal R corresponding to the electromotive force E=kBDV generated in the fluid under test 2; 77 is an excitation voltage Ex
Alternatively, the input signal R is obtained by using a signal corresponding to the magnetic flux density B obtained by means not shown, such as detecting the excitation current I or directly detecting the magnetic field created by the excitation coil 4. This is an output circuit that calculates R/B for the measured fluid 2 and outputs a signal P corresponding to the flow rate Q of the fluid 2 to be measured. In other words, since the conventional commercial power supply-driven low-frequency excitation type electromagnetic flowmeter is configured as explained above, the frequency of the excitation magnetic field is lower than the frequency of the external commercial power supply, and therefore the induced noise mixed into the signal S is reduced. In addition, the signal S is sampled and synchronously rectified only during a period of τNs when the excitation magnetic field has almost reached a constant value and immediately before the magnetic field changes in the opposite direction. S2
In this case, the influence of induction noise caused by the cycle of the external commercial power supply and differential noise generated by magnetic field fluctuations hardly appears, and as a result, the zero point fluctuation of the output signal P becomes extremely small. FIG. 4 is a schematic diagram showing the reason why the induced noise of the commercial power supply cycle mixed in the signal S is eliminated by sampling. As explained above, conventional low frequency excitation type electromagnetic flowmeters have been driven only by commercial power. However, recently in general industrial processes, operational stability,
In order to improve reliability, there is a growing demand for process instruments that use electricity to be uninterruptible. With the diversification of technology, there is a strong desire for process instruments that can be driven by either AC or DC power sources. Based on these demands, the present invention has developed a low-frequency excitation type electromagnetic device that can be driven by a DC power source and has a stable zero point even in environments where 50Hz or 60Hz induced noise sources exist independently or in a mixture of both. The flow meter can be configured,
Furthermore, it is an object of the present invention to obtain a converter that can be driven by either a DC power source or a commercial power source and can be used to configure a low frequency excitation type electromagnetic flowmeter that can use a driving power source depending on the power supply situation at the site where it is used. It is something to do. Next, a converter according to the present invention will be explained based on the drawings. FIG. 5a is a block diagram showing the configuration of an embodiment of a low frequency excitation type electromagnetic flowmeter converter that can be driven by a DC power supply according to the first invention of the present invention, and FIG.
A block diagram showing an example of the configuration of the timing signal generation circuit 80 in FIG.
FIG. 6b is a block diagram showing the configuration of an embodiment of the low frequency excitation type electromagnetic flow meter converter that can be driven by either a DC power source or a commercial power source according to the invention;
is the timing signal generation circuit 90 in FIG. 6a.
FIG. 5a is a block diagram showing an example of the configuration of the
In FIG. 6a, the detectors 10 are shown connected for convenience of explanation below. FIG. 7 is a time chart including signal and voltage waveforms of important parts in each of FIGS. 5 and 6. Fifth
In each figure of FIGS. 1 to 7, parts having the same function or meaning as each part shown in each of FIGS. 1 to 4 are given the same reference numerals. In each figure of FIG. 5a, b and FIG. 7, X is an external DC power source for driving the converter 7, and 78 is an external DC power source that converts the external DC power source X into a DC power source with the required conditions within the converter 7. A DC/DC converter 80 is a means for receiving power from the power source X, and a timing signal generating circuit 80 outputs timing signals P2, P3, and P4. Timing signal generation circuit 80
As shown in FIG.
A fixed frequency oscillator 81 that outputs a rectangular wave signal P1 as a basic clock signal whose period Tp is equal to a time Td that is a common multiple of each period of the 60Hz power supply and whose period Tp is equal to 2Td.
, a frequency divider 82 which divides the signal P1 into half and outputs a timing signal P2 for operating the excitation circuit 73, and performs logical operations P1, P2 and 1,2 on the signals P1 and P2, respectively. A two-phase divider 83 outputs the signals P3 and P4 as timing signals P3 and P4 for operating the synchronous rectifier circuit 75.
[1],

[0] is one of the binary signals.
Therefore, if the excitation circuit 73 outputs the excitation voltage Ex in the positive direction due to the signal P2, the signal S will shift from the negative side to the positive side after some time has passed due to the time constant of the excitation coil 4, etc. A steady-state value on the positive side is reached. Then, when the excitation circuit 73 outputs the excitation voltage Ex in the negative direction by the signal P2, the signal S
reaches a steady-state value on the negative side. The positive side and negative side portions of the signal S are processed in the synchronous rectifier circuit 75 by the signals P3 and P4, respectively.
The signal is sampled and rectified only during the [1] state of the signal. That is, in this case, the sampling period Ts for the signal S is Ts=Tp-Tw=Td. In this case, the time from when the direction of the excitation voltage Ex is reversed until the sampling of the signal S starts is Tw, that is, Td, and the time Td is set so that the signal S becomes almost in a steady state within the time of Tw. has been done. Therefore, after the direction of the excitation voltage Ex is reversed, the signal S is sampled for a period of time Td, which is a common multiple of each period of the 50 Hz power supply and the 60 Hz power supply, in a state where it has reached a substantially steady value, and the signal S1 and It will be output as S2. Since the converter of this embodiment is configured as described above, when an electromagnetic flowmeter is configured in combination with the detector 10, a 50 Hz or 60 Hz commercial power source is present alone as an inductive noise source in the surrounding area. or even in an environment where both are mixed, the frequency of the excitation magnetic field is lower than the frequency of the noise source, the signal S has almost reached a steady value, and the Td just before the signal S changes in the opposite direction. Since the signal S is sampled only during this period, a stable output signal P with little zero point fluctuation can be obtained. Usually the frequency of commercial power supply is 50
Hz or 1Hz on the positive or negative side centered on 60Hz
It varies within the following range. In an environment where such an induced noise source exists, a slight zero point fluctuation occurs in the output signal P of the converter of this embodiment due to the frequency fluctuation described above. However, according to the inventor's experiments, the magnitude of this zero point fluctuation is only 0.2% of the full scale. Therefore, the zero point fluctuation of the converter of this embodiment does not pose a practical problem. Next, in each figure of FIG. 6a, b and FIG. 7, M and L are terminals for connecting an external DC power supply /DC converter 71 and the like constitute means for receiving external commercial power supply Y. The output of either the AC/DC converter 71 or the DC/DC converter 78 is the timing signal P.
2 and an excitation circuit 73, the excitation voltage Ex can be exchanged with the excitation voltage Ex. 90 is the power supply Y when the converter 7 is driven by the external commercial power supply Y.
or when the converter 7 is driven by an external DC power supply X, the timing signal generation circuit 8 of FIG.
Similarly to 0, the timing signals P2, P3, P4
This is a timing signal generation circuit that outputs . This timing signal generating circuit 90 is constructed as shown in FIG. 6b. That is, 81 is a fixed frequency oscillator that outputs a rectangular wave signal P1 as a first basic clock signal, 91 is an isolation transformer that isolates an external commercial power supply Y as an input from a sine wave signal as an output, and 92 93 is a zero-cross comparator that shapes the output signal of the isolation transformer 91 and outputs it as a rectangular wave signal, and 93 divides the output signal of the zero-cross comparator into a time Tc whose time width Tw is an integral multiple of the period of the external commercial power supply Y. This is a frequency divider that outputs a rectangular wave signal P5 as a pulse train signal synchronized with an external commercial power supply Y whose period Tp is equal to 2Tc. Means for generating a pulse train signal as a second basic clock signal synchronized with is configured. 9
Reference numeral 4 designates a signal switching device as a signal switching means configured to switch the rectangular wave signal P1 or P5 and output it to the frequency divider 82 and the two-phase divider 83. Therefore, when the signal P1 is inputted to the frequency divider 82 and the two-phase divider 88 by the switch 94, the output signal and P4 become 1.P2 and 1.2, respectively.
When the signal P5 is input to the frequency divider 82 and the two-phase divider 83 by the switch 94, the output signals P3 and P4 become 5·P2 and 5·P4, respectively. The signals correspond to the results of each logical operation of 2. Therefore, the signal P1 or P1 is now applied to the switch 94.
5 is input and the excitation circuit 73 is driven by the signal P2, the signal S is input to the synchronous rectifier circuit 75 during the [1] state of the signals P3 and P4 in the same manner as explained in FIG. sampled only. The sampling period Ts in this case is
Td or Tc, and the time Tw from when the direction of the excitation voltage Ex is reversed until the sampling of the signal S starts is also Td or Tc, and this time Td or Tc is the time when the direction of the excitation voltage Ex is reversed. It is set so that the signal S reaches a steady state within this time after. Therefore, when the signal P1 is input to the switch 94, the signal S is sampled for a period of time Td, which is a common multiple of each period of the 50 Hz power supply and the 60 Hz power supply, while the signal S is at a substantially steady value, and the signal S is input to the switch 94. When the signal P9 is input to the signal P9, the signal S is sampled for a period of time Tc, which is an integral multiple of the period of the external commercial power supply Y, while the signal S has almost a constant value.
The signals are output from the synchronous rectifier circuit 75 as signals S1 and S2. Since the converter of this embodiment is configured as described above, when it is combined with the detector 10 to configure an electromagnetic flowmeter, when the flowmeter is driven by an external DC power source X, the power source X is connected to the terminal. By connecting the signal P1 to the frequency divider 82 and the two-phase divider 83 by the signal switch 94, or by connecting the power supply Y to the terminal L when the electromagnetic flowmeter is driven by an external commercial power supply Y. By connecting the signal P5 to the frequency divider 82 and the two-phase divider 83 by the signal switch 94, an excitation magnetic field is formed at a frequency lower than that of the commercial power supply in both cases, and the signal S is approximately In the state where the steady value is reached and just before the signal S changes in the opposite direction, if the drive is from an external DC power supply X, the time
Since the signal S is sampled only during the period Td, and when driven by an external commercial power supply Y, the signal S is sampled for a period Tc which is an integral multiple of the cycle of the power supply Y, so a stable output signal P with little zero point fluctuation can be obtained. Become. As explained above, in the first aspect of the present invention, in a low frequency excitation type electromagnetic flowmeter that measures the flow rate of a fluid to be measured based on Faraday's law of electromagnetic induction, the converter of the flowmeter is first connected to an external power source. and a fixed frequency oscillator, and the output signal of the fixed frequency oscillator is used as a basic clock signal to output the exciting current and the positive steady value of the electromotive force generated in the fluid to be measured. Since each of the negative side steady values is sampled to be synchronously rectified for a period that is a common multiple of each cycle of a plurality of commercial power supplies having different frequencies in response to fluctuations in the magnetic field caused by the excitation current, such conversion is possible. Since the meter can be driven by a DC power supply, it is possible to make the electromagnetic flowmeter uninterruptible by connecting it to a DC uninterruptible power supply that performs battery backup. It is possible to obtain an electromagnetic flowmeter that outputs a flow rate signal with good zero point stability even in an environment where each power source exists individually or in a mixed manner. Therefore, such a converter has the advantage of being able to constitute a highly reliable low frequency excitation type electromagnetic flowmeter. Furthermore, in a second invention of the present invention, in addition to the configuration of the first invention in which the converter of the low frequency excitation type electromagnetic flowmeter can be driven by an external DC power supply, a means for receiving an external commercial power supply, and a means for receiving an external commercial power supply are provided. Means for generating a pulse train signal as a second basic clock signal in synchronization with the commercial power supply, and means for switching both the pulse train signal and the output signal of the fixed frequency oscillator to output the excitation current and sample the electromotive force. and a signal switching means configured to control the excitation current by using the output of the fixed frequency oscillator as the first basic clock signal when the converter is driven by an external DC power supply. When the converter is driven by an external commercial power supply, the second basic clock signal is switched to the second basic clock signal by the signal switching means. This type of converter can be driven by both DC power and commercial power because it is configured to output the excitation current using a pulse train signal and perform sampling only during a period that is an integer multiple of the period of the external commercial power supply. Therefore, it is possible to construct a low-frequency excitation type electromagnetic flowmeter that can quickly respond to the power supply situation at the site where it is used and has a stable zero point even in an environment where an induced noise source of the commercial power frequency exists.

[Brief explanation of drawings]

Fig. 1 is a block diagram for explaining the operating principle of an electromagnetic flowmeter, Fig. 2 is a block diagram showing an example of the structure of a conventional commercial power supply-driven low frequency excitation type electromagnetic flowmeter, and Fig. 3 is a block diagram for explaining the operating principle of an electromagnetic flowmeter. Fig. 4 is a schematic diagram showing the reason why the induced noise of the commercial power supply cycle is eliminated, and Fig. 5a is the external DC power supply according to the first invention of the present invention. FIG. 5b is a block diagram of an example of the configuration of the timing signal generation circuit in FIG. 5a, and FIG. 6a is a block diagram of an example of the configuration of the timing signal generation circuit in FIG. A block diagram of an embodiment of a converter for driving dual external commercial power sources, Fig. 6b
6 is a block diagram of an example of the structure of the timing signal generation circuit shown in FIG. 6a, and FIG. 7 is a time chart including signal and voltage waveforms of the main parts in FIGS. 5 and 6. In the figure, 2...Fluid to be measured, 5...Magnetic field, 7
...Converter, 10...Detector, 71...AC/DC converter as means for receiving external commercial power supply, 75
...Synchronous rectifier circuit, 78...DC/DC converter as external DC power receiving means, 81...Fixed frequency oscillator, 91, 92, 93...Isolation transformer and zero cross comparator each forming means for generating a pulse train signal , frequency divider, 94...
Signal switching device as signal switching means, V...flow velocity,
B... Magnetic flux density, E... Electromotive force, I... Exciting current, A... External power supply, X... External DC power supply as external power supply, Y... External commercial power supply as external power supply, P... Output Flow rate signal, τ...cycle of commercial power supply, Ts...sampling period, L...terminal as means for receiving external commercial power supply, P1...square wave signal as first basic clock signal, P5...second
Pulse train signal as the basic clock signal.

Claims (1)

[Claims] 1. A magnetic field that varies over time with a waveform close to a rectangular shape including a trapezoid is applied to the fluid to be measured at a frequency lower than the commercial power frequency, and the fluid is subjected to a magnetic field based on Faraday's law of electromagnetic induction. A low frequency excitation type electromagnetic flowmeter that detects the generated electromotive force and measures the flow rate of the fluid receives an external power supply, outputs an excitation current for forming the magnetic field, and samples the electromotive force, A converter that outputs a signal corresponding to the flow rate using signals corresponding to the sampling result and the magnetic flux density of the magnetic field, including means for receiving an external DC power source as the external power source and outputting a basic clock signal. and a fixed frequency oscillator that uses the basic clock signal to generate a plurality of commercial power sources with different frequencies for signals corresponding to the output of the excitation current and the steady-state positive value and steady-state negative value of the electromotive force, respectively. A converter for a low frequency excitation type electromagnetic flowmeter, characterized in that the sampling is performed only during a period that is a common multiple of each period. 2 Applying a temporally varying magnetic field with a waveform close to a rectangular shape including a trapezoid at a frequency lower than the commercial power frequency to the fluid to be measured, and detecting the electromotive force generated in the fluid based on Faraday's law of electromagnetic induction. A low frequency excitation type electromagnetic flowmeter that measures the flow rate of the fluid receives an external power source, outputs an excitation current for forming the magnetic field, samples the electromotive force, and compares the sampling result and the magnetic field. The converter outputs a signal corresponding to the flow rate using signals corresponding to the magnetic flux densities, respectively, the converter includes means for individually receiving an external DC power source and an external commercial power source as external power sources, and a first basic clock. a fixed frequency oscillator for outputting a signal, means for generating a pulse train signal synchronized with the external commercial power supply as a second basic clock signal, and the first basic clock signal and the second basic clock signal. and signal switching means for switching, and when receiving the external DC power supply, the signal switching means uses the first basic clock signal to control the output of the excitation current and the positive steady value and negative steady value of the electromotive force. The sampling is performed for a period that is a common multiple of each period of a plurality of commercial power sources having different frequencies for signals corresponding to respective values, and when receiving the external commercial power source, the signal switching means selects the second signal.
The output of the excitation current and the sampling for a period that is an integer multiple of the period of the external commercial power supply for signals corresponding to the positive steady value and negative steady value of the electromotive force using a basic clock signal of A converter for a low frequency excitation type electromagnetic flowmeter.