JP2009300360A - Electromagnetic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic flowmeter providing a stable output signal, and not affected by an excitation circuit, by reselecting optimally an excitation switching control frequency, even when the excitation switching control frequency is shifted by a temperature change to fluctuate the output signal. <P>SOLUTION: This electromagnetic flowmeter provided with the switching control type excitation circuit is provided with an excitation switching frequency changing means for optimizing an excitation switching frequency in response to the temperature change. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electromagnetic flow meter, and more particularly to improvement of an excitation circuit of a switching control system.

  FIG. 4 is a configuration diagram of a switching control circuit for an excitation coil conventionally used in an electromagnetic flow meter. In FIG. 4, a capacitor 2 is connected in parallel to the DC power source 1. One end (positive side) of the DC power source 1 is connected to one end 31 of the exciting coil 3 via the switching element Q1, and this one end 31 is connected to the other end (negative side) of the DC power source 1 via the switching element Q3. ing. The other end 32 of the exciting coil is grounded and connected to one end of the exciting current detection resistor 4, and one end (positive side) of the DC power source 1 is connected to the other end 41 of the resistor 4 via the switching element Q2. The other end 41 is connected to the other end (negative side) of the DC power supply 1 via the switching element Q4. These switching elements Q1 to Q4 are composed of FETs. Parasitic capacitors D1 to D4 that are essentially formed in the package and cannot be removed in the manufacturing process are in a direction opposite to the current flowing from the DC power source. Connected in parallel.

  Timing signals T1 to T4 for controlling opening and closing of the switching elements Q1 to Q4 are respectively connected to control electrodes (FET gates) of the switching elements Q1 to Q4 via isolators P1 to P4 such as photocouplers and waveform shaping circuits B1 to B4. Is entered.

  In the exciting current detection resistor 4 connected in series with the exciting coil 3, the exciting current alternately flows in the reverse direction during the positive excitation period and the negative excitation period. Accordingly, between the grounded one end 32 and the other end 41 of the detection resistor 4, positive and negative voltages Vr proportional to the excitation current are generated corresponding to the positive excitation period and the negative excitation period.

  FIG. 5 is a configuration diagram of a circuit for generating timing signals T1 to T4 for controlling opening and closing of the switching elements Q1 to Q4. The excitation timing generation circuit 5 regulates the positive excitation period and the negative excitation period, generates a rectangular wave having a predetermined excitation basic frequency f1, and supplies a direct output to the switching element Q4 as a timing signal T4, and an inverter G3 The inverted output via is supplied to the switching element Q3 as the timing signal T3.

  The excitation control circuit 6 is configured to control excitation of the coil 3 by a pulse width modulation (PWM) method. Specifically, positive and negative voltages VREF proportional to the excitation current are converted into positive voltages via the absolute value circuit 7, and the difference between this voltage signal and the DC reference 8 (voltage Vs) is converted by the error amplifier 9. Amplified.

  The output voltage Ve of the error amplifier 9 is input to the positive input terminal of the comparator 12 having hysteresis characteristics due to the positive feedback resistors 10 and 11 via the resistor 10.

  The triangular wave signal oscillator 13 inputs a triangular wave signal Vp having an excitation switching control frequency f 2 higher than the excitation basic frequency f 1 to the negative side input terminal of the comparator 12.

  The output of the comparator 12 is guided to the AND gates G1 and G2, and fed back to the 12 positive input terminals via the voltage dividing circuit of the positive feedback resistors 11 and 10, thereby giving a predetermined hysteresis to the comparison operation. .

  When the triangular wave signal Vp rises and becomes equal to or higher than the output voltage Ve of the error amplifier 9, and further rises above the voltage corresponding to the hysteresis width determined by the positive feedback resistors 10 and 11, the comparator 12 inverts the output from positive to negative. Conversely, when the triangular wave signal Vp drops below the output voltage Ve of the error amplifier 9 and further falls below the voltage corresponding to the hysteresis width determined by the positive feedback resistors 10 and 11, the triangular wave signal indicates that the output is inverted from negative to positive. Repeat at each cycle. This comparison operation realizes pulse width modulation (PWM).

  The output of the comparator 12 and the timing signal T4 are input to the AND gate G1, and the timing signal T1 is output as a logical product of both. Similarly, the output of the comparator 12 and the timing signal T4 are input to the AND gate G2, and the timing signal T2 is output as a logical product of both.

  FIG. 6 is an explanatory diagram of the switching state and switching control of the switching elements Q1 to Q4 in the positive excitation period and the negative excitation period in such a configuration. First, according to the excitation timing signals T3 and T4, the switching element Q3 is turned off and Q4 is turned on in the positive excitation period, and the switching element Q3 is turned on and Q4 is turned off in the negative excitation period.

  Further, during the positive excitation period, the switching element Q2 is off and switching control is executed by Q1. During the negative excitation period, the switching element Q1 is off and switching control is executed by Q2. With such control of each switching element, during the positive excitation period, a current indicated by i1 in FIG. 4 flows through the switching element Q1, the exciting coil 3, the detection resistor 4, and the switching element Q4.

The current indicated by i2 in FIG. 4 indicates the current flowing through the parasitic diode D3 connected in parallel to the switching element element Q3 due to the back electromotive force of the exciting coil 3 when the switching element Q1 is off. In the negative excitation period, a current similar to i1 flows through the switching element Q2, the detection resistor 4, the excitation coil 3, and the switching element Q3, and constant current control is executed.
JP 2002-202165 A

  The switching control type excitation circuit configured as described above has great advantages such as low power consumption, but the excitation current essentially has a ripple of the excitation switching control frequency component as shown in FIG.

  As shown in FIG. 7B, the current ripple of the excitation switching control frequency component is a one-turn noise peculiar to the electromagnetic flow meter that is directly superimposed on the measurement signal of the electromagnetic flow meter during the period in which the excitation current is controlled at a constant current. Included in the signal.

  As shown in FIG. 7C, the signal of (B) is sampled and fetched as a flow rate signal in an interval at an appropriate timing. When the excitation switching control frequency becomes (2n + 1) times the excitation fundamental frequency, this sampling is performed. The excitation switching control frequency component noise included in the signal in the period does not become zero but appears as output fluctuation. When (2n) / T is sampled, it is integrated and becomes 0. However, if the frequency stability of the oscillator 13 is poor and the frequency deviates even slightly, a beat (swell) may occur at a very low frequency. is there. In particular, it is conceivable that the electrical characteristics of the oscillator 13 change due to temperature changes and the frequency shifts.

  The present invention solves these problems. The purpose of the present invention is to optimally select the excitation switching control frequency even when there is a fluctuation in the excitation switching control frequency due to a temperature change and the output signal fluctuates. In other words, an electromagnetic flow meter that is not affected by the excitation circuit that can obtain a stable output signal can be realized.

  In order to achieve such a problem, the invention according to claim 1 of the present invention is an excitation switching that optimizes an excitation switching frequency in accordance with a temperature change in an electromagnetic flowmeter provided with an excitation circuit of a switching control system. A frequency changing means is provided.

  According to a second aspect of the present invention, in the electromagnetic flowmeter according to the first aspect, the excitation switching frequency changing means obtains a beat frequency having a maximum amplitude from a frequency analysis result of the output signal, and is excited based on the beat frequency having the maximum amplitude. The switching frequency is changed.

  According to a third aspect of the present invention, in the electromagnetic flowmeter according to the first aspect, the excitation switching frequency changing means obtains a beat frequency having a maximum amplitude from a frequency analysis result of the output signal, and the maximum amplitude at the beat frequency is set to a predetermined value. The excitation switching frequency is changed based on the comparison result with the threshold value.

  According to the present invention, a stable output signal can be obtained by optimally reselecting the excitation switching control frequency even when there is a deviation in the excitation switching control frequency due to a temperature change and the output signal fluctuates. it can.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, an excitation circuit 20 is configured in the same manner as the circuit configuration of the prior art shown in FIGS. The output signal detected by one electrode 21a of the detector 21 is input to one input terminal of the differential amplifier 23 via the buffer 22a, and the output signal detected by the other electrode 21b is differenced via the buffer 22b. The signal is input to the other input terminal of the dynamic amplifier 23. The output signal S of the differential amplifier 23 is converted into a digital signal by the A / D converter 24 and input to the CPU 25. The CPU 25 performs frequency analysis by FFT to obtain the oscillation frequency included in the output signal, and outputs a control signal for changing the excitation switching frequency to the excitation circuit 20 according to the calculated oscillation frequency.

  Further, the CPU 25 outputs a measurement result based on the output signals of the electrodes 21 a and 21 b of the detector 21 to the output circuit 26 and displays it on the display 27.

FIG. 2 is a flowchart showing an example of a process flow for changing the excitation switching frequency Fsw in the CPU 25.
1) First, the output signal S is read from the A / D converter 24 (step SP1).
2) Subsequently, the frequency analysis of the output signal S is performed using FFT (step SP2).
3) Next, the frequency Fbeat of the beat having the largest amplitude is obtained from the result of the frequency analysis (step SP3).

4) The beat frequency Fbeat is compared. If the beat frequency Fbeat is not 0, the excitation switching frequency Fsw is changed (step SP4). Fbeat is obtained by the equation (1), and is a relative value from n * Fex (information on whether the beat frequency Fbeat is + or − is necessary). Of the beat frequencies Fbeat determined by the equation (1), the lowest frequency (fundamental wave) has the largest amplitude.
Fbeat = Fsw−n * Fex (1)
Here, Fsw represents an excitation switching frequency, Fex represents an excitation basic frequency, and n represents an integer.
5) Change the excitation switching frequency Fsw.
The excitation switching frequency Fsw is changed based on the equation (2).
Fsw = Fsw−Fbeat (2)

  Thus, by optimizing the excitation switching frequency Fsw according to the temperature change, an electromagnetic flow meter that can obtain a stable output that is not affected by the excitation circuit can be realized.

  In the example of FIG. 2, the beat frequency Fbeat is used as the evaluation criterion. However, as shown in FIG. 3, the beat amplitude Abeat obtained as a result of the frequency analysis may be used as the evaluation criterion. A certain threshold Ath is prepared, and if the beat amplitude Abeat included in the output signal is larger than the threshold Ath, the excitation switching frequency Fsw is changed so that the beat amplitude becomes smaller.

FIG. 3 is a flowchart showing another example of the flow of processing for changing the excitation switching frequency Fsw in the CPU 25.
1) First, the output signal S is read from the A / D converter 24 (step SP1).
2) Subsequently, the frequency analysis of the output signal S is performed using FFT (step SP2).
3) Next, the amplitude Abeat of the frequency Fbeat of the beat having the largest amplitude is obtained from the result of the frequency analysis (step SP3).

4) The beat frequency Fbeat is compared. If the beat frequency Fbeat is not 0, the amplitude Abeat is compared (step SP4).
5) The amplitude Abeat is compared with the threshold value Ath. If the amplitude Abeat is sufficiently small with respect to the threshold value Ath, the influence can be ignored. Therefore, the excitation switching frequency Fsw is not changed. If the amplitude Abeat is not sufficiently small with respect to the threshold value Ath, the influence cannot be ignored. change.
6) Change the excitation switching frequency Fsw.
The excitation switching frequency Fsw is changed based on the above-described equation (2) as in FIG.

  As described above, according to the present invention, there is provided an electromagnetic flow meter that can obtain a stable output signal by optimally reselecting the excitation switching control frequency even if the output signal fluctuates due to a temperature change. realizable.

It is a block diagram which shows one Example of this invention. It is a flowchart explaining the flow of operation | movement of FIG. It is a flowchart explaining the flow of other operation | movement of FIG. It is an example of a structure of the switching control circuit of the exciting coil used with the conventional electromagnetic flowmeter. FIG. 5 is a configuration diagram of a circuit for generating timing signals T1 to T4 for controlling opening and closing of switching elements Q1 to Q4 in FIG. 4. It is explanatory drawing of the switching state and switching control of each switching element Q1-Q4 of FIG. FIG. 5 is an operation waveform example diagram of each part in the excitation circuit of FIG. 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 DC power supply 2 Capacitor 3 Excitation coil 4 Excitation current detection resistance Q1-Q4 Switching element (FET)
D1 to D4 Parasitic capacitor P1 to P4 Isolator B1 to B4 Waveform shaping circuit T1 to T4 Timing signal

Claims (3)

In an electromagnetic flow meter with a switching control type excitation circuit,
An electromagnetic flowmeter comprising an excitation switching frequency changing means for optimizing an excitation switching frequency according to a temperature change.   2. The electromagnetic flowmeter according to claim 1, wherein the excitation switching frequency changing means obtains a beat frequency having a maximum amplitude from a frequency analysis result of an output signal, and changes the excitation switching frequency based on the beat frequency having the maximum amplitude. .   The excitation switching frequency changing means obtains a beat frequency having the maximum amplitude from the frequency analysis result of the output signal, and changes the excitation switching frequency based on a comparison result between the maximum amplitude at the beat frequency and a predetermined threshold value. The electromagnetic flow meter according to claim 1.

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