JPH0726660Y2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to an electromagnetic flowmeter for measuring the flow rate of a measurement fluid by a signal voltage generated by generating a rectangular wave magnetic field and applying the magnetic field to the measurement fluid. In particular, the present invention relates to an electromagnetic flow meter improved so as to shorten the rise / fall time when switching the magnetic field.

<Prior Art> FIG. 4 is a block diagram showing an outline of the configuration of a conventional electromagnetic flowmeter, which is a base for improvement.

Reference numeral 10 denotes a conduit having an inner surface insulated and a non-magnetic pipe arranged on the outer side thereof, and a pair of electrodes 11 and 12 insulated from the pipe and in contact with the measurement fluid Q are fixed to the conduit.

An exciting coil 13, which is connected to each other in series with the conduit 10,
A magnetic field B is applied from 14, and one end of this exciting coil 13 is connected from one end of exciting power supplies 15 and 16 connected in series to each other via switches SW ₁ and SW ₂ and a detection resistor 17, respectively. One end of the exciting coil 14 is connected to a common connection point C ₁ at the other ends of the exciting power supplies 15 and 16.

Further, one end of the detection resistor 17 is connected to the common potential point COM of the circuit, and the voltage V _fo generated at the other end is input to the excitation control circuit 18. The excitation control circuit 18 applies control signals S ₁ and S ₂ for controlling the opening and closing of the switches to the switches SW ₁ and SW ₂ , respectively, based on this voltage V _fo .

Further, between the common potential point COM and the common connection point C ₁ , a series circuit of a diode D ₁ and a switch SW ₃ whose cathode is connected to the common potential point COM side, and a cathode are connected to the common connection point C ₁ side. The connected diode D ₂ and the series circuit of switch SW ₄ are connected to each other, and the control signal from the excitation control circuit 18
Opening and closing of the switches SW ₃ and SW ₄ are controlled by S ₃ and S ₄ .

Further, the signal voltage V _s1 generated at the electrodes 11 and 12 is output to the signal processing circuit 19 and subjected to predetermined signal processing to output its output terminal.
It is output to 20 as a flow rate signal V _Q.

Next, the operation of the electromagnetic flow meter configured as described above will be described with reference to the waveform charts shown in FIGS.

Voltage V _fo in the exciting current I _fo detection resistor 17 shown in FIG. 5 (a)
Is input to the excitation control circuit 18, and the control signal S ₁ having a waveform as shown in FIG. 5 (b) and the control signal S ₂ having a waveform as shown in FIG. 9 (c) are respectively switched. SW ₁ and S
By outputting to W ₂ , these switches SW ₁ and SW ₂ are opened and closed as shown in FIGS. 5 (f) and 5 (g). During the transient period T _{1 when the} exciting current I _fo changes from a negative level to a positive level, for example, the switch SW ₂ remains off, but the switch SW ₁ continues to be on for a long time and quickly excited. The current I _fo is controlled so as to reach the steady value + I _c . After reaching the steady value + I _c , the switch SW ₁ is repeatedly turned on and off to control the steady value + I _{c to} be constant.

On the contrary, during the transient period T ₂ when the exciting current I _fo changes from a positive level to a negative level, the switch SW ₁ remains off, but the switch SW ₂ remains on for a long time. The exciting current I _fo is controlled so as to reach the steady value −I _c quickly. After reaching the steady value −I _c , switch SW
₂ repeats on / off and controls so that this steady value −I _c becomes constant.

As a result of the control by the excitation control circuit 18 as described above, the magnetic field B follows a change almost similar to the excitation current, and the magnetic field B is shown in FIG.
It changes as shown in. Therefore, if the flow rate is constant, a signal voltage V _s1 having a waveform shown in FIG. 5 (k) is generated at both ends of the electrodes 11 and 12 as in the magnetic field B, and this signal voltage V _s1 is applied to the signal processing circuit 19. Is output. The both ends of the electrodes 11 and 12 when the differential shaped noise voltage obtained by differentiating the magnetic field B e _N is generated as shown in FIG. 5 (1), which is superimposed on the signal voltage V _s1 signal processing circuit 19 Is output to.

The signal processing circuit 19 is a signal in the period T _s of the influence of the noise voltage e _N lost by the sampling signal S _P output from excitation control circuit 18 in order to avoid the influence of the noise voltage e _N (FIG. 5 (m)) The voltage V _s1 is sampled, subjected to signal processing, and output to the output end 20 as the flow rate signal V _Q.

In the above control of the exciting current I _fo , the exciting control circuit 18 outputs the control signals S ₃ and S ₄ shown in FIGS. 5 (d) and 5 (e) to the switches SW ₃ and SW ₄ , respectively, to control the opening / closing thereof. . When switch SW ₁ is repeatedly turned on and off, switch S
W ₃ is kept on, and when the switch SW ₁ is off, the energy stored in the exciting coils 13 and 14 is released through the switch SW ₃ and the diode D ₁ . In this case, switch SW
₂ remains off, but switch SW ₄ also remains off. On the contrary, when the switch SW ₂ is repeatedly turned on / off, the switch SW ₄ is kept on,
When the switch SW ₂ is off, the energy stored in the exciting coils 13 and 14 is released through the switch SW ₄ and the diode D ₂ . In this case, the switch SW ₁ maintains the off state, but the switch SW ₃ also maintains the off state.

<Problems to be Solved by the Invention> However, the conventional electromagnetic flowmeter having the rectangular excitation waveform as described above has the following problems.

When the excitation frequency is low, the differential noise voltage e _N is small, and the signal voltage V _s1 can be sampled after the differential noise voltage e _N disappears. Although there is no problem, on the other hand, when the excitation frequency is lowered, flow noise, which is random noise having a large frequency spectrum at a low frequency generated by the flow of fluid, occurs.

On the contrary, if the excitation frequency is simply raised to avoid the influence of this flow noise, the influence of the noise voltage e _N becomes large.

Therefore, paying attention to the fact that the noise voltage e _N generated by increasing the excitation frequency is caused by the long transition periods T ₁ and T ₂ of the change in the excitation current, and shortening this period reduces the rectangular wave shape. The purpose of the present invention is to enable excitation at a high excitation frequency and eliminate the influence of flow noise together with the influence of differential noise voltage.

<Means for Solving the Problems> In order to achieve the above object, the present invention provides an excitation coil for generating a magnetic field and a signal detection for detecting a signal voltage generated in response to the measured flow rate when the magnetic field is applied. Means and an exciting means for supplying an exciting current through a changeover switch for switching the polarity of the exciting current flowing in the exciting coil, and an exciting signal related to the exciting current is inputted to control opening / closing of the changeover switch based on the exciting signal. Control the exciting current to a predetermined value, and also detects the period from the start of changing the polarity of the exciting current to the time when the exciting current becomes zero and controls the changeover switch to off, and the changeover switch is turned off. In the meantime, a resistor having a resistance value sufficiently larger than the resistance value of this exciting coil is connected to the exciting coil.

<Operation> The control means controls the opening / closing of the changeover switch based on the excitation signal to control the excitation current to a predetermined value and also detects the period from the start of changing the polarity of the excitation current to the time when the excitation current becomes zero. Control the changeover switch to off, and while this changeover switch is off, connect a resistor with a resistance value that is sufficiently larger than the resistance value of this exciting coil to the exciting coil, and change the time constant of the exciting circuit during this period. Make it smaller.
As a result, the exciting current quickly reaches the steady value.

<Embodiment> An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. The parts having the same functions as those of the conventional configuration shown in FIG. 4 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

One end of the detection resistor 17 is connected to the common potential point COM of the circuit,
The voltage V _f1 generated at the other end is input to the excitation control circuit 21. The excitation control circuit 21 sends control signals S ₅ and S ₆ for controlling the opening and closing of the switch based on this voltage V _f1 to the switch SW, respectively.
₁ , Apply to SW ₂ .

Further, a resistor R ₁ having a sufficiently large resistance value with respect to the resistances of the exciting coils 14 and 15 is connected to both ends of the switch SW _{1 and} a polarity opposite to the direction of the exciting current flowing from the exciting power supply 15. A series circuit with a diode D ₃ is connected. In addition, a resistor R ₂ having a sufficiently large resistance value with respect to the resistances of the exciting coils 14 and 15 and an exciting power source are provided at both ends of the switch SW _2.
A series circuit with a diode D ₄ connected so as to have a reverse polarity with respect to the direction of the exciting current flowing from 15 is connected.

A cathode is connected to the common connection point C ₁ side, and a series circuit of a diode D ₁ and a switch SW ₃ whose cathode is connected to the common potential point COM side between the common potential point COM and the common connection point C ₁ . The point that the diode D ₂ and the series circuit of switch SW ₄ are respectively connected is the same as the conventional one, but the excitation control circuit
Control signals S ₇ and S ₈ from 21 switch SW ₃ and switch S
The structure is different from the conventional one in that the opening and closing of W ₄ is controlled.

Next, the operation of the embodiment shown in FIG. 1 configured as described above will be described with reference to the waveform chart shown in FIG.

First, the case where the exciting current has a constant value −I _c (t <t ₀ ) in the waveform of the exciting current I _f1 shown in FIG. 2A will be described.

In this case, it is basically the same as the case shown in FIG. The excitation control circuit 21 receives the voltage V _f1 generated by the excitation current I _f1 (Fig. 2 (a)) across the resistor 17, and based on this, the control signal S ₅ (Fig. 2 (b)) and the control signal are generated. S ₆ (Second
Figure (c)) is output to the switches SW ₁ and SW ₂ (Figure 2 (e)). As a result, the switch SW ₁ is maintained in the off state as shown in FIG. 2 (d), and the switch SW ₂ is repeatedly turned on / off as shown in FIG. 2 (e) to keep a constant current value −I _c. Are controlled to become. In this case, the switch SW ₄ is turned always ON by the control signal S ₈ shown in FIG. 2 during the switch SW ₂ is repeatedly turned on / off (g), the switch SW ₂ is repeatedly turned on / off The flyback currents from the exciting coils 13 and 14 are passed through the diode D ₂ when it is off. In this case,
The switch SW ₃ controls the control signal S as shown in FIG. 2 (h).
₇ (FIG. 2 (f)) keeps the off state.
While the switch SW ₂ is off, the exciting current is in the exciting coils 13, 1
Although it decreases with the time constant of 4, the ON / OFF cycle of the switch SW ₂ is set sufficiently shorter than this time constant, so this decrease can be ignored, and the value of the exciting current I _f1 is It is held at a constant value -I _c .

It should be noted that while the exciting current I _f1 is held at a constant value −I _c , the diodes D ₁ and D ₂ are connected to the resistors R ₁ and R ₂ respectively,
Since it is reverse-biased with respect to 15 and 16, the exciting current does not flow, and while the switch SW ₂ is repeatedly turned on / off, the switches SW ₁ and SW ₃ are kept off (Fig. 2 ( d), (h)).

Next, in the waveform of the exciting current shown in FIG. 2A, during the period (t ₀ <t <t ₁ ) from the starting time t _{0 of the} polarity change of the exciting current to the time t ₁ when the exciting current becomes zero. The operation will be described.

During this transient period (t ₀ <t <t ₁ ), FIG.
As shown in (e), (h), and (i), the switches SW ₁ to SW ₄ are all kept off by the control signals S ₅ , S ₆ , S ₇ , and S ₈ output from the excitation control circuit 21. To be done. Therefore, the counter electromotive force generated in the exciting coils 13 and 14 is the exciting power source 15
, The diode D ₃ is forward biased and turned on as shown in FIG. 2 (j), and the exciting current flows through the resistor R ₁ . Since this resistance R ₁ is selected to be sufficiently larger than the sum of the resistance R _{f of the} exciting coil and the resistance R _d of the detecting resistor 17, if the inductance of the exciting coils 13 and 14 is L, the exciting current I _f1 The time constant of (L / (R ₁ + R _f + R _d ) becomes very small, and the exciting current I _f1 sharply decreases to zero.

Third, the period from time t ₁ to the excitation current is zero in the waveform of the exciting current shown in FIG. 2 (a) to time t ₂ to reach a steady state value + I _c of (t ₁ <t <t ₂₎ The operation will be described.

During this transient period, the control signals S ₅ and S ₇ (Fig. 2 (b),
From (f), the switches SW ₁ and SW ₃ are always on as shown in FIGS. 2 (d) and 2 (h). In this state, the counter electromotive force of the exciting coils 13 and 14 is lower than the exciting voltage of the exciting power source 15, so that the diode D ₃ is reverse biased again and turned off. Therefore, no exciting current flows through the resistor R ₁ . Also, since the switch SW ₁ is always turned on during this period, the excitation coil 13 and 147 have the switch S 1
An exciting voltage is applied from the exciting power supply 15 via W ₁ and the detection resistor 17. Therefore, the exciting current increases to a constant value + I _c with a time constant (L / (R _d + R _f )).

Fourth, in the waveform of the exciting current shown in FIG. 2 (a), the period after the time t ₂ when the exciting current reaches the steady value + I _c (t>
The operation of t ₂ ) is basically the same as that in the case of the period t <t ₀ . In this case, + I _c corresponds to the constant value −I _c , and operation of the diode D ₄ corresponds to operation of the diode D ₃ .

FIG. 3 is a block diagram showing the configuration of the second embodiment of the present invention.

In this configuration, resistors R ₁ and R ₂ are inserted in series in the exciting coils 13 and 14 by using switches SW ₅ and SW ₆ instead of the diodes D ₃ and D ₄ in FIG. . Diodes D ₃ and D _{4 are} generated by the control signals S ₉ and S ₁₀ from the excitation control circuit 22.
These switches SW ₅ and SW ₆ are turned on at the same timing as when they are turned on. Even in this case, the operation is similar to that of the embodiment shown in FIG.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the changeover switch is controlled to be turned off during the period from the start of changing the polarity of the exciting current to the time when the exciting current becomes zero. Since a resistor having a resistance value sufficiently larger than the resistance value of this exciting coil was connected to this exciting coil during this OFF, the time constant of the exciting circuit became extremely small during this period, and the exciting current was quickly changed. A steady value can be reached, and even if the excitation frequency of the rectangular wave excitation current is increased, it can be prevented from being affected by differential noise, and at the same time, the effect of flow noise can be eliminated. .

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention, FIG. 2 is a waveform diagram explaining the operation of the embodiment shown in FIG. 1, and FIG. 3 is a configuration of another embodiment of the present invention. FIG. 4 is a block diagram showing the structure of a conventional electromagnetic flow meter, and FIG.
The figure is a waveform diagram for explaining the operation of the electromagnetic flow meter shown in FIG. 11, 12 ... Electrodes, 13, 14 ... Excitation coils, 15, 16 ... Excitation power supply, 17 ... Detection resistors, 18, 21, 22 ... Excitation control circuit, 19 ... Signal processing circuit.

Claims (1)

[Scope of utility model registration request] 1. An exciting coil for generating a magnetic field, a signal detecting means for detecting a signal voltage applied in response to a measured flow rate when the magnetic field is applied, and a changeover switch for switching the polarity of an exciting current flowing through the exciting coil. An exciting means for supplying the exciting current via the exciting means and an exciting signal related to the exciting current are input, and based on the exciting signal, the opening / closing of the changeover switch is controlled to control the exciting current to a predetermined value. Control means for detecting the period from the start of changing the polarity of the exciting current to the time when the exciting current becomes zero and controlling the changeover switch to OFF, and the exciting coil to the exciting coil while the changeover switch is OFF. An electromagnetic flowmeter characterized in that a resistance having a resistance value sufficiently larger than the resistance value of the coil is connected.