JP3111370B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter for measuring a flow rate of a measurement fluid from an electromotive force generated by applying a magnetic field to the measurement fluid, and particularly to a high-frequency excitation with low power consumption. To an improved electromagnetic flowmeter.

[0002]

2. Description of the Related Art There are various types of conventional electromagnetic flowmeters. Among them, there is an electromagnetic flowmeter having a configuration shown in FIG. The outline will be described below with reference to FIG.

[0003] Reference numeral 10 denotes an insulating cylindrical pipe through which the measurement fluid Q flows. An exciting current _If flows from the exciting circuit 12A to the exciting coils 11a and 11b, and the magnetic field B
Is generated and applied to the measurement fluid Q.

The exciting circuit 12A includes a timing circuit 1
Although the timing signal ST ₁ to switch the exciting current I _f from 3A for example, a rectangular wave is applied, the timing circuit 13A is controlled by the timing signal ST ₂ from the microprocessor (CPU) 14.

On the other hand, the voltage generated by the flow of the measurement fluid Q is detected by the detection electrodes 15a and 15b fixed to the pipe 10 and in contact with the measurement fluid Q. This detection electrode 1
The voltages e _S1 and e _S2 detected by 5a and 15b are applied to the input terminals of amplifiers Q1 and Q2 having high input impedance, respectively.

The outputs of the amplifiers Q1 and Q2 are applied to a differential amplifier Q3, and the difference between them is calculated and output as an output voltage e _S0 . These amplifiers Q1 and Q2,
The input circuit 16 is constituted by the differential amplifier Q3.

The output voltage e _S0 is output to the sample and hold circuit 17 as it is or via the inverting amplifier Q4. The sample and hold circuit 17 includes a switch SW ₁ ,
SW _{2 , a} hold capacitor C 1, an amplifier Q 5, and the like.

[0008] The output voltage e _S0 to one end of the switch SW _1,
Or are connected through an inverting amplifier Q4 to one end of the switch SW _2, the other end of the switch SW _1, SW ₂ is connected to the input of amplifier Q5 is connected to the common potential point COM through the capacitor C1 I have.

The switching of these switches SW ₁ and SW ₂ is controlled by a timing signal ST ₃ synchronized with a low-frequency excitation current _If output from the microprocessor 14 and is sampled.

Further, the output voltage of the amplifier Q5 is the low-frequency excitation current I _f output from the microprocessor 14.
The signal is converted into a digital signal by an analog / digital converter 18 whose conversion timing is controlled by a timing signal ST ₄ synchronized with the digital signal and output to the microprocessor 14.

The microprocessor 14 executes a flow rate calculation by using a built-in program using the input digital signal, and outputs the obtained flow rate signal to an output terminal 20 via an output circuit 19.

Next, the operation of the electromagnetic flow meter shown in FIG. 8 will be described with reference to the waveform diagram shown in FIG. When an exciting current _If having a rectangular waveform as shown in FIG. 9A flows through the exciting coils 11a and 11b, a signal voltage e _S0 proportional to the flow rate of the measurement fluid is output to the output terminal of the differential amplifier Q3. It is output as voltage e _S0 .

The output voltage e _S0 is sampled and analogized just before the excitation current _If is switched by a timing signal ST ₃ (FIG. 9B) synchronized with the excitation current _If output from the microprocessor 14. / Digital converter 18

The analog / digital converter 18 outputs a timing signal ST output from the microprocessor 14.
₄ (FIG. 9C), the timing signal ST ₃ (FIG.
At the same repetition period as in (b)), the signal is converted into a digital signal with the conversion time _Tc and output to the microprocessor 14.

[0015]

However, in the above-described electromagnetic flowmeter, since the exciting current is continuously supplied to the exciting coil, the power consumption increases, and the influence of the flow noise which increases on the low frequency side is reduced. In order to perform high-frequency excitation, power consumption further increases. Therefore, it becomes difficult to realize, for example, a two-wire electromagnetic flow meter that shares circuit power from the load side and transmits signals from the detector side to the load side by sharing the two transmission lines with high-frequency excitation. There is a problem.

[0016]

According to the present invention, as a configuration for solving the above problems, a switching element and a capacitor whose opening and closing are controlled by a switching signal are connected in series to both ends of a DC excitation power supply. An excitation circuit for applying energy to the excitation coil from both ends of the capacitor via a reference resistor; a preamplifier for receiving a signal voltage generated by applying a magnetic field to the measurement fluid from the previous excitation coil; an LR mode and an LCR mode A switching signal generator for outputting a switching signal to switch between the two, a comparator for determining whether the voltage generated at both ends of the reference resistor is positive or negative, and a voltage in the LR mode among the outputs of the comparator to which the previous switching signal is input. Gate means for outputting a reference voltage in which noise is suppressed, and synchronous rectification for synchronously rectifying the output voltage of the preceding preamplifier using the preceding reference voltage and the preceding switching signal Road and, in which to output a flow rate signal corresponding to the flow rate of the synchronous rectifier circuit the previous measurement fluid by the signal processing using the output of.

[0017]

In the excitation circuit, a switch element and a capacitor whose opening and closing are controlled by a switching signal are connected in series to both ends of a DC excitation power supply, and energy is applied to the excitation coil from both ends of the capacitor via a reference resistor.

On the other hand, the preamplifier receives a signal voltage generated by applying a magnetic field to the measurement fluid from the excitation coil. Then, the switching signal generator outputs a switching signal for switching between the LR mode and the LCR mode.

The comparator determines whether the voltage generated across the reference resistor is positive or negative. The gate means receives the switching signal and outputs a reference voltage in which the LR mode voltage is suppressed from the output of the comparator.

The synchronous rectifier circuit synchronously rectifies the output voltage of the previous preamplifier using the previous reference voltage and the previous switching signal,
Thereafter, signal processing is performed using the output of the synchronous rectification circuit to output a flow rate signal corresponding to the flow rate of the measurement fluid.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 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. Parts having the same functions as those of the conventional electromagnetic flow meter shown in FIG. 8 are denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

Detection electrodes 15a and 15b in contact with the measurement fluid Q are fixed to the insulating cylindrical pipe 10 through which the measurement fluid Q flows. A magnetic field B is applied to the measurement fluid Q from the excitation coils 11a and 11b.

Excitation power supply E_bIs its power terminal T₁, T_TwoApplied to
And these power supply terminals T₁, T_TwoBetween the switching element S
W_ThreeAnd capacitor C_TwoAnd are connected in series. Soshi
And this capacitor C_TwoReference resistance R_rEncouraged through
Magnetic coils 11a and 11b are connected in series. reference
Resistance R_rIs connected to the exciting coil 11a at a common potential point C
Connected to OM. Then, the excitation circuit EXC
Magnetic power supply E_b, Switch element SW_Three, Capacitor C _Two, And
And reference resistance R_rIt is composed of

The switching element SW ₃ is a switching signal generator 21
Opening and closing is controlled by a switching signal S _T5 outputted from. The voltage V _r1 generated at both ends of the reference resistor R _r is output to the non-inverting input terminal (+) of the comparator 22 whose inverting input terminal (−) is connected to the common potential point COM.

The input terminal of the gate 23 has a comparator 2
2 output voltage V_r2And the switching signal S_T5With inverter 24
Inverted inversion switching signal <S_T5> Is applied and its output
End T _ThreeReference voltage V_r3Is output as

On the other hand, the signal voltages e _S1 and e _S2 generated at the detection electrodes 15a and 15b are output to amplifiers Q ₁ and Q ₂ , respectively, which are input to the differential amplifier Q ₃ and the signal voltage e Output as _S3 . These amplifiers Q ₁ ,
The preamplifier PAM is constituted by Q ₂ , the differential amplifier Q ₃ , and the like.

The multiplexer 25 includes a switch element SW
_4, SW _5, consists like SW _6, the switch element SW ₄
Common switch end by reference voltage V _r3, the switch SW of
The common switching terminal of No. ₅ uses the inverted reference voltage <V _r3 > obtained by inverting the reference voltage V _r3 by the inverter 26 to switch the switch element SW
₆ are switched respectively by switching signal S _T5.

The common switching terminal of each switching element is connected to the non-inverting input terminal (+) of the non-inverting amplifier 27 and to the switching terminal b of each of the switching elements SW ₄ , SW ₅ and SW _6. I have.

The switching terminal a of the switching element SW ₄ receives the signal voltage e _S3 , the switching terminal a of the switching element SW ₅ receives the inverted signal voltage <e _S3 > via the inverting amplifier 28, and the switching element SW ₆ Are applied with the potential of the common potential point COM.

The signal obtained at the output of the non-inverting amplifier 27
Voltage e_S4Is the resistance R_L1And capacitor C _L1Low frequency composed of
The signal is filtered by the filter 29, and is analyzed through the non-inverting amplifier 30.
Output to the log / digital converter 31. These circles
A multiplexer 25, a non-inverting amplifier 27, an inverting amplifier 28,
Synchronous rectification circuit with low-pass filter 29, non-inverting amplifier 30, etc.
Constituting the SRF.

[0031] is output to the output terminal 33 as the analog / digital converter 31 is converted into a digital signal corresponding to the signal voltage e _S4 in the signal processing in the following signal processing circuit 32 is made flow signal Q _S.

Next, the operation of the embodiment constructed as described above will be described with reference to the waveform diagrams of the respective parts shown in FIG. 3, the equivalent circuit diagrams of the respective modes shown in FIG. 2, and the characteristic diagrams of the respective modes shown in FIG. Will be explained.

From the switching signal generator 21 to the switch element SW
₃ switching signal S _T5 is sent to, thereby the switch element SW ₃ as shown in FIG. 3 (a) will only ON period T _LR. This period _TLR is called the LR mode, and its equivalent circuit is shown in FIG. 2A, and the exciting current _Iex flows from the exciting power supply _{Eb as} shown in FIG. 3C according to this equivalent circuit.

The excitation current I _ex flowing through this time is _{I ex = E b [1-} exp (-R f t / L f)] / R f (1). R _f and L _f are excitation coils 11a and 11b, respectively.
Shows the resistance and inductance of.

Here, the time t ₀ at which the exciting current I _ex becomes I _ex = I ₀ is t ₀ = − (L _f / R _f ) ln [1- (I _ex R _f / E _b )] (2) ).

This time t ₀ is -ln [1- (I _ex R _f / E _b )] times the circuit time constant T _C = L _f / R _f .
By increasing the excitation power supply E _b can be reduced time t _0. This is illustrated in FIG. 4A. The horizontal axis I _ex R _f / E _b, the vertical axis -ln [1-
(I _ex R _f / E _b )].

[0037] In the period of the LR mode, the switch elements of the synchronous rectifier circuit SFR by switching signal S _T5 SW ₆
Is fixed at the common potential point COM, so that the output terminal of the non-inverting amplifier 27 has a signal voltage e _S4 (FIG. 3 (f)).
Does not appear.

Next, the switching element S is _generated by the switching signal ST5.
W ₃ is switched off, the period T _LCR shown in FIG. 3 (a)
Comes to the LCR mode, the equivalent circuit is as shown in FIG. 2 (b), the exciting coil 11a, the exciting current I _ex to 11b flows by the charge stored in the capacitor C _2.

The exciting current I _ex attenuates while oscillating as shown in FIG. This decay time constant T _d is T _d =
2L _f / R _f, which is twice the time constant of the LR mode.
In addition, the amplitude value becomes 63.2% in the time when the attenuation time constant _Td is (1-1 / e) times. This attenuation is shown in FIG.
This is as shown in FIG.

As can be seen from FIG. 4B, the attenuation rate is 3T.
if _d, in 95% damping factor, if the ratio time constant of the LR mode R _ate, R _ate = (time constant of the constant / LR mode when the LCR _{mode) = (3T d / T C} ) = 6, and the time constant of the LCR mode has a time constant that is six times that of the LR mode.

In the LCR mode, the excitation coils 11a, 1
The signal voltages e _S1 and e _S2 generated by the magnetic field generated by the exciting current I _ex flowing through 1b are input to the preamplifier PAM, and output to the output terminal thereof as the signal voltage e _S3 (FIG. 3 (e)). .

On the other hand, the reference resistor R by the exciting current I _ex
The voltage V _r1 generated at both ends of _r is output to the comparator 22 and only the portion corresponding to the positive waveform in the oscillation waveform is output in a rectangular wave shape. Further, the gate 23 outputs the inversion switching signal <S shown in FIG. _The period corresponding to the LR mode in the positive waveform is suppressed by _T5 >, and a reference voltage _Vr3 having a waveform as shown in FIG.

Switch element SW ₄ of synchronous rectifier circuit SRF
And the SW _5, although the reference voltage V _r3 (FIG. 3 (d)) and the inverted reference voltage <V _r3> are applied respectively, since these are the switching elements operate in a complementary manner, the signal voltage e _S3 (FIG. 3
(E)) and its inverted signal voltage <e _S3 > are synchronously rectified by these reference voltages to form a non-inverting amplifier 27.
_Is output as a signal voltage e _S4 (FIG. 3 (f)).

The low-pass filter 29 filters the signal voltage e _S4 and outputs it to the analog / digital converter 31 via the non-inverting amplifier 30. The low-pass filter 29 converts the signal voltage e _S4 into a digital signal. processing is outputted to the output terminal 33 as a flow rate signal Q _S is made.

[0045] In LCR mode attenuates vibration exciting current I _ex, but condition at this time is ^{_{R f 2 <4L f / C}} 2.
Since the signal voltage generated at this time is synchronously rectified by the synchronous rectifier circuit SFR, 90 ° components such as eddy current and one-turn noise flowing in the measurement fluid are canceled and the output terminal 3
3 does not appear.

[0046] The vibration frequency f in the LCR mode, f = [(1 / L f C 2) - (R f / 2L f) 2] is represented by ^1/2 / 2 [pi, increasing this frequency value To excite at a high frequency (for example, 75 Hz or more),
Thereby, the influence of the flow noise can be reduced.

Next, power consumption will be described. First,
LR mode of time -ln of _{(L f / R f) [} 1- (I ex
R _f / E _b )] times. For example, if L _f / R _f is set to 0.1, the time is 1/10 of L _f / R _f and the exciting current I _ex
Can be launched.

On the other hand, the time of the LCR mode is 6 (L _f /
R _f ). The time difference between these modes is 60 times. Furthermore, when _{(I ex R f / E b} ) a reduced, it is possible to increase this ratio, it becomes possible to contribute to reduction in power consumption.

In other words, since there is no power to be supplied to the exciting coil at the time of signal processing, the time in the LCR mode is increased, and the time in the LR mode is further reduced, so that the electromagnetic flow rate which operates with low power consumption is reduced. Meter can be realized.

The power consumption P at this time is represented by P ≒ E _{b Im} T _LR / [2 (T _LCR + T _LR )]. However, I _m is the maximum value of the exciting current I _ex.

FIG. 5 shows the configuration of a modification of the excitation circuit shown in FIG. This exciting circuit supplies positive and negative exciting currents to the exciting coil so that positive and negative symmetric signal voltages are generated, so that it is hardly affected by offset drift of a predetermined polarity generated in the detection electrodes 15a and 15b. Things.

[0052] The both ends of the capacitor C _2, the positive excitation power supply +
And E _b1, a diode D _1, a positive and energizing current + I _ex 'direction of flow of the switching element SW ₇ which diode D ₃ in the opposite polarity is constituted by field effect transistors connected in is connected in parallel .

[0053] Further, both ends of the capacitor C _2, a negative excitation power supply -E _b1, a diode D _2, a negative excitation current -
The direction of flow of I _ex 'and the switching element SW ₈ composed of a field effect transistor the diode D ₄ in reverse polarity is connected are connected in parallel.

[0054] And these switch elements SW₇And su
Switch element SW₈Means switching signal generator 21
Switching signal S sent from_T6, S_T7With its opening and closing controlled
And the exciting current ± I_exParticipation caused by ´
Lighting voltage V_r4Is connected to the non-inverting input terminal (+) of the comparator 22.
Is output.

Next, the operation of the circuit shown in FIG. 5 will be described with reference to FIG. First, the (series of 11a and 11b) the exciting coil 11 between the switch element SW ₇ as ON period of the switching signal S _T6 its LR mode shown in FIG. 6 (a), a positive excitation current + I _ex ' It flowed, and off between switch elements SW ₇ for a period of LCR mode.

Thus, as shown in FIG. 6C, the capacitor C ₂ is charged in the LR mode, and the capacitor C ₂ oscillates attenuated by the charge charged in the capacitor C ₂ and the inductance and resistance of the exciting coil 11 in the LCR mode.

Next, flow of negative exciting current -I _ex 'to the exciting coil 11 as an on between the switch elements SW ₈ periods of the switching signal S _T7 its LR mode shown in FIG. 6 (b), LC
Turning off the period of R mode switch elements SW _8.

As a result, as shown in FIG. 6C, the capacitor C ₂ is charged with the opposite polarity in the LR mode, and the charge charged in the capacitor C ₂ and the exciting coil 1 in the LCR mode.
Due to the inductance and resistance of No. 1, a damped oscillation having a polarity opposite to that of the positive excitation occurs.

As described above, by alternately repeating the positive excitation and the negative excitation, it is possible to cancel the influence of the offset voltage appearing in the detection electrodes 15a and 15b with a constant polarity in a long term.

The switching signals S _T6 and S _T7 transmitted from the switching signal generator 21 shown in FIG. 1 are separately transmitted to the switching elements SW ₇ and SW ₈ , respectively.
These are added to the inverter 24 and sent out.

In the configuration shown in FIG. 1, the core as the return path of the magnetic flux is not explicitly shown for simplicity.
In practice, a core is used. As the material of the core, a soft magnetic material which has a good frequency response, does not saturate, and has no magnetic circuit delay is used.

[0062] Further, although in the configuration shown in FIG. 1 has been described as an excitation circuit having an excitation waveform to a sinusoidal vibration, the use of the magnetic material the magnetic flux density in a low coercive force H _c is saturated as the core, a rectangular wave excitation You can also

For example, if the time of the LCR mode is 3T ', the excitation current value is attenuated to 95% at the time of 3T'. If the magnetic circuit is saturated when the exciting current value of 0.05 × I _ex is given, the obtained magnetic flux becomes a square wave. In this case, if the saturation magnetic flux has high sensitivity to temperature, a Hall element or the like may be arranged near the exciting coil to correct the temperature of the magnetic flux density.

[0064]

As described above, according to the present invention, the LR for supplying the voltage from the excitation power supply is described.
Mode and LCR mode without supply
Since the signal is detected by performing high-frequency excitation while maintaining the resonance frequency in the mode high, problems such as flow noise generated by low-frequency excitation can be solved with low power consumption. It is possible to realize a two-wire type electromagnetic flow meter that requires electric power.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of one embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of each mode in the embodiment shown in FIG.

FIG. 3 is a waveform chart showing waveforms at various parts in the embodiment shown in FIG. 1;

FIG. 4 is a characteristic diagram of each mode of the embodiment shown in FIG.

FIG. 5 is a configuration diagram of a modified embodiment obtained by improving the embodiment shown in FIG. 1;

FIG. 6 is a waveform chart for explaining the operation of the modified embodiment shown in FIG. 5;

FIG. 7 is a characteristic diagram showing characteristics of the core of the embodiment shown in FIG. 1;

FIG. 8 is a block diagram showing a configuration of a conventional electromagnetic flow meter.

FIG. 9 is a waveform diagram illustrating the operation of the electromagnetic flow meter shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Pipe 11, 11a, 11b Excitation coil 12A Excitation circuit 13A Timing circuit 14 Microprocessor 17 Sample and hold circuit 21 Switching signal generator 22 Comparator 25 Multiplexer 27 Non-inverting amplifier 28 Inverting amplifier 29 Low-pass filter 32 Signal processing circuit EXC Excitation Circuit PAM Preamplifier SRF Synchronous rectifier circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01F 1/58-1/60

Claims (1)

(57) [Claims] An exciting circuit for applying energy to an exciting coil from both ends of a capacitor via a reference resistor and a switch element whose opening and closing are controlled by a switching signal is connected in series to both ends of a DC exciting power supply. A preamplifier that receives a signal voltage generated by applying a magnetic field to the measurement fluid from the excitation coil, and a switching signal generator that outputs the switching signal for switching between LR mode and LCR mode;
A comparator for determining whether the voltage generated at both ends of the reference resistor is positive or negative; a gate for receiving the switching signal and outputting a reference voltage in which an LR mode voltage is suppressed among outputs of the comparator; A synchronous rectifier circuit for synchronously rectifying the output voltage of the preamplifier using a switching signal; anda signal processing using the output of the synchronous rectifier circuit to output a flow rate signal corresponding to the flow rate of the measurement fluid. Characteristic electromagnetic flow meter.