JPH0749978B2 - Electromagnetic flow meter - Google Patents

Description

The present invention relates to an electromagnetic flow meter, and more particularly to an electromagnetic flow meter including an A / D converter.

(B) Conventional Technology As a conventional electromagnetic flowmeter, there is one shown in FIG. This electromagnetic flowmeter is exposed on the inner wall surface of the fluid pipe 1 and has a pair of electrodes.
While 2a and 2b are provided facing each other, an exciting coil 3 is arranged so that a magnetic field is applied in a direction perpendicular to both the axial direction of the fluid tube 1 and the direction connecting the electrodes 2a and 2b. A low-frequency pulsed exciting current is supplied to the exciting coil 3 from the exciting circuit 4 via the resistor R. That is, the switches S ₁ and S ₂ and the switches S ₃ and S ₄ of the excitation circuit ₄ are alternately turned on / off by the switch control signal from the control calculation unit (CPU) 8, and the current from the constant current source 13 is supplied. It is inverted in polarity and flows into the exciting coil 3 (see a, exciting current in FIG. 4). When a conductive liquid such as water, sewage, chemicals, etc. is flowing in the fluid pipe 1, an induction voltage corresponding to the flow rate is generated by the electromagnetic induction action between the electrodes 2a and 2b (b in FIG. 4, electrode). Refer to the voltage generated between). The induced voltage is differentially amplified by the differential amplifier 6 through the buffer amplifiers 5a and 5b, converted into a digital signal by the A / D converter 7, and taken into the control calculation unit 8. The exciting current is converted into a voltage signal by the resistor R, passed through the amplifier 9, converted into a digital signal by the A / D converter 10, and taken into the control calculation unit 8. A / D converter 7, 1
The sampling operation of 0 is performed at the timing when the exciting current stabilizes as shown by c in FIG. Control calculation unit 8
Calculates the flow rate based on these inputs. The calculation result, that is, the flow rate is converted into an analog signal by the D / A converter 11 and converted into a signal of 4 to 20 mA by the output circuit 12 and output.

Now, for example, the voltage generated in the electrodes 2a and 2b when excited in the positive direction is E ₁ , the excitation current is I _1, and the generated voltage and the excitation current when excited in the negative direction are E ₂ and I ₂ . . When the conversion coefficient of the A / D converter 7 is K ₁ and the conversion coefficient of the A / D converter 10 is K ₂ , the flow rate Q is Q = (K ₁ * E ₁ −K ₁ * E ₂ ) / _{(K 2 * I 1 -K 2} * I 2) = (K 1 / K 2) * a _{(E 1 -E 2) / (} I 1 -I 2). Normally, K ₁ = K _2, so that Q = (E ₁ −E ₂ ) / (I ₁ −I ₂ ), and this flow rate Q is calculated by the arithmetic control unit 8. The influence of the drift of the constant current source 13 can be removed by dividing the generated voltage by the value of the exciting current measured at the same time. Also, since the difference is taken as K ₁ * E ₁ −K ₁ * E ₂ , the A / D
The effect of the zero point of the converter can be eliminated.

(C) Problems to be Solved by the Invention In the above conventional electromagnetic flowmeter, when attention is paid to the A / D converter, the span performance of the A / D converter is measured as a whole as shown in the above equation. Directly affects the accuracy of. For example, considering the temperature change, the temperature characteristic of the span of these A / D converters is directly related to the performance. Therefore, conventionally, in order to ensure accuracy, it is unavoidable to use an A / D converter with excellent temperature characteristics, and since two A / D converters 1 are required, the cost is extremely high. Was becoming. Another method is to prepare a reference voltage with excellent temperature characteristics and periodically apply this voltage to the A / D converter to compensate for temperature changes in the A / D converter. The voltage source was also expensive, and there was a problem because of the high cost.

The present invention has been made in view of the above problems, even if using an inexpensive A / D converter, excellent temperature characteristics, etc., to provide an electromagnetic flow meter to obtain high accuracy I am trying.

(D) Means and Actions for Solving the Problems An electromagnetic flowmeter of the present invention is a pair of electrodes that are exposed on the inner wall of a fluid pipe and face each other, and are orthogonal to the axial direction and the electrode direction of the fluid pipe. An exciting coil for applying a magnetic field in the direction of excitation, an exciting circuit for supplying a low-frequency periodic current to the exciting coil, an exciting current detecting circuit for detecting an exciting current flowing in the exciting coil, and an induction between the pair of electrodes. A flow rate signal derivation circuit that outputs a signal voltage according to the flow rate, a first A / D converter that converts the flow rate signal from the flow rate signal derivation circuit into a digital signal, and the output of the excitation current detection circuit is a digital signal. A flow rate signal deriving circuit and a second A / D converter for converting into a flow rate and a calculation means for calculating a flow rate by receiving signals from the first and second A / D converters. Excitation current detection circuit output Is the first A /
A switching circuit for switching and connecting to the D converter and the second A / D converter is provided, and the arithmetic means has the output of the flow rate signal deriving circuit connected to the first A / D converter, When the output of the current detection circuit is connected to the second A / D converter, and when the output of the flow rate signal derivation circuit is connected to the second A / D converter, the output of the excitation current detection circuit is A unit for calculating the flow rate when connected to one A / D converter, a product calculating unit for calculating the product of both calculated flow rates, and a square root of the calculated product, and the square root result is the flow rate. It is configured to include a square root calculating means for outputting as a signal.

When measuring the flow rate with this electromagnetic flow meter, the output of the flow rate signal derivation circuit is first connected to the first A / D converter by the switching circuit, and the output of the excitation current detection circuit is connected to the second A / D converter. Connect to the converter. In this connected state, the flow rate is calculated by the calculation means based on the outputs of the first and second A / D converters. Next, operate the switching circuit, connect the output of the flow rate signal derivation circuit to the second A / D converter, connect the output of the excitation current detection circuit to the first A / D converter, and connect this In the state, the voltage signal according to the flow rate output from the second A / D converter and the first A / D
The flow rate is calculated by the calculation means based on the voltage signal corresponding to the exciting current output from the converter. Then, the product of the respective calculated flow rates is obtained, and the product is squared to obtain the flow rate signal. In the above process, the conversion factors K ₁ and K ₂ of the first and second A / D converters are removed from the calculation formula of the flow rate Q, so that the flow rate Q is the conversion coefficient K ₁ of the A / D converter. , K ₂ independent,
Not affected by temperature changes.

(E) Examples Hereinafter, the present invention will be described in more detail with reference to Examples.

FIG. 1 is a block diagram of an electromagnetic flowmeter showing an embodiment of the present invention. The electromagnetic flowmeter of this embodiment is provided with a pair of electrodes 2a and 2b facing each other, which is exposed on the inner wall surface of the fluid pipe 1 like the one shown in FIG. Further, the exciting coil 3 is arranged so that the magnetic field is applied in a direction in which both the axial direction of the fluid pipe 1 and the direction connecting the electrodes 2a and 2b are vertical. Further, an exciting circuit 4 for supplying an exciting current to the exciting coil 3, buffer amplifiers 5a, 5b for amplifying a voltage generated between the electrodes 2a, 2b and a differential amplifier 6, a resistor R for detecting an exciting current, and both ends of this resistor R. The amplifier 9 that amplifies the voltage corresponding to the excitation current, the two A / D converters 7 and 10, the arithmetic control unit (composed of a microcomputer) 8 that calculates the flow rate, the calculated flow rate To an analog value D /
The output of A converter 11 and this D / A converter 11 is 4 to 20 mA.
The output circuit 12 converts the current into a current and outputs the current.

The electromagnetic flowmeter of this embodiment is characterized in that the output of the differential amplifier 6 and the output of the amplifier 9 are switched by the switching circuit 14 so that they are alternately connected to the A / D converter 7 and the A / D converter 10. That is what I did.

The switching circuit 14 includes switches S ₅ , S ₆ , S ₇ , and S _8. The output terminal of the differential amplifier 6 is connected to one end of the switches S ₅ and S ₇ , and the output terminal of the amplifier 9 is connected to the switch S ₆ , Each of the switches S ₅ and S ₆ is connected to one end of S ₈ , the other ends of the switches S ₅ and S ₆ are connected to the input end of the A / D converter ₇ , and the other ends of the switches S ₇ and S ₈ are connected to the A / D converter 10 respectively.
Is connected to the input end of.

The arithmetic control unit 8 has a function of switching the switches S _{1 to} S ₄ of the exciting circuit 4 to generate an exciting current, and a function of supplying the sampling signals A and B to the A / D converters 7 and 10, and , function of supplying a switching signal to turn on / off the switch S ₅ to S ₈ to the switching circuit 14, also, by the switching of the switching circuit 14, and a flow rate signal input through the a / D converter 7, a / D
A function of calculating the flow rate by the exciting current input through the converter 10, and conversely, a flow rate signal input through the A / D converter 10 and the flow rate signal input through the A / D converter 7. It has a function of calculating the flow rate by the exciting current, a function of calculating the product of the above two flow rates, and a function of calculating the flow rate by square rooting the calculated product.

Next, the overall operation of the electromagnetic flowmeter of this embodiment will be described.

First, the arithmetic control unit 8 causes the switches S ₁ and S _{2 of the} excitation circuit 4 to operate.
And S ₃ and S ₄ are alternately switched, and the exciting current shown in FIG. 2 a is flown from the exciting circuit 4 to the series circuit of the exciting coil 3 and the resistor R. An alternating magnetic field is applied to the fluid pipe 1 by this exciting current, whereby a voltage corresponding to the flow rate of the liquid flowing in the fluid pipe 1 is generated between the electrodes 2a and 2b by electromagnetic induction (waveform in FIG. 2b). reference). This generated voltage is output via the buffer amplifiers 5a and 5b and the differential amplifier 6. on the other hand,
A voltage corresponding to the current flowing through the exciting coil 3 is derived from both ends of the resistor R and is output via the amplifier 9. The switches S ₅ and S ₈ of the switching circuit 14 are turned on by the switch switching signal (see e in FIG. 2) from the arithmetic control unit 8, whereby the output of the differential amplifier 6 is input to the A / D converter 7. Connected to the end. The output terminal of the amplifier 9 is connected to the input terminal of the A / D converter 10. In synchronization with the switch-on signal of the switch S ₅ and S ₈ from the operation control unit 8, the A / D converter 7 and the A / D converter 10 to each sampling signal A (see c in FIG. 2) is given , The output of the differential amplifier 6 and the output of the amplifier 9, respectively,
That is, the flow rate signal and the exciting current are taken into the arithmetic control unit 8 via the A / D converters 7 and 10. The arithmetic control unit 8 calculates the flow rate from the input flow rate signal and the exciting current.

Next, subsequently, the arithmetic control unit 8 controls the switch S _{6 of the} switching circuit 14.
And S ₇ are supplied with switching signals (see f in FIG. 2) for turning on these switches, and at the same time, A / D converters 7 and 10 are connected.
Also, the sampling signal B (see d in FIG. 2) is output. When the switches S ₆ and S ₇ are turned on, the output terminal of the differential amplifier 6 is connected to the A / D converter 10 this time. The output end of the amplifier 9 is connected to the input end of the A / D converter 7. As a result, the flow rate signal is taken in through the A / D converter 10 and the exciting current is taken in through the A / D converter 7 in the arithmetic control unit 8. The flow rate is similarly calculated from the flow rate signal and the exciting current. Then, the arithmetic control unit 8 obtains the product obtained twice and performs square root calculation, and the square root value is used as the flow rate Q of the final result value and converted into an analog signal by the D / A converter 11, Output through the output circuit 12.

Regarding the operations of the switching circuit 14, the A / D converter 7, the A / D converter 10 and the operation control unit 8, the operation is performed by the A / D converter 7 at the first timing when the first sampling signal A is input. The signal input to the control unit 8 is K ₁ * E ₁ , and the signal fetched from the A / D converter 10 is K ₂ * I ₂ . On the other hand, the signal taken into the arithmetic control unit 8 at the second timing when the sampling signal B is input is K ₂ * E from the A / D converter 7.
_{2 and} K ₁ * I ₁ from A / D converter 10.

Here, the flow rate Q _{1 at} the first timing is And the second flow rate Q ₂ is Becomes

The control calculation unit 8 obtains the product of these Q ₁ and Q ₂ , As is clear from this equation, the coefficients K ₁ and K ₂ of the A / D converters 7 and 10 are eliminated, and as a result, the output flow rate Q is the same as that of the A / D converters 7 and 10. It is possible to perform measurement with extremely good characteristics that is not affected by the conversion coefficients K ₁ and K ₂ and is not affected by temperature changes and the like. That is, the temperature changes,
Even if the conversion coefficient changes from K ₁ to K ₁ ′ or from K ₂ to K ₂ ′, the calculation result Q is not affected and is not affected by the change. Also, of course, K ₁ * E ₁ −K ₁
Since each difference is obtained in the form of * K ₂ , the effect of the zero point can be eliminated as in the conventional example.

In the above embodiment, the case where the exciting current is alternately repeated positively and negatively is shown as an example.
It is of course applicable to the case of repeating negative / pause / positive / ..., or the case of repeating positive / pause / positive / pause / ..., or negative / pause / negative / pause / ....

(F) Effect of the Invention According to the present invention, the output of the flow rate signal derivation circuit and the output of the excitation current detection circuit are connected to the first and second A / D converters by switching them respectively. The flow rate signal and exciting current are switched and extracted from the A / D converter of each, the flow rate is calculated based on these signals, and the calculated product of the flow rate is squared to obtain the flow rate. The conversion factor of the / D converter will be removed from the flow rate result and therefore A /
Since the flow rate can be detected without being affected by the conversion coefficient of the D converter, it is possible to measure the flow rate with high accuracy without being affected by temperature changes and the like. In addition, since it is possible to use an inexpensive A / D converter, it is possible to realize an inexpensive electromagnetic flow meter as a whole.

[Brief description of drawings]

FIG. 1 is a block diagram of an electromagnetic flowmeter showing an embodiment of the present invention, FIG. 2 is a waveform time chart for explaining the operation of the electromagnetic flowmeter, and FIG. 3 is a conventional electromagnetic flowmeter. The block diagram and FIG. 4 are waveform time charts for explaining the operation of the same electromagnetic flow rate. 1: Fluid tube, 2a ・ 2b: Electrode, 3: Excitation coil, 4: Excitation circuit, 5
a ・ 5b: Buffer amplifier, 6: Differential amplifier, 7 ・ 10: A / D converter, 8: Arithmetic control unit, 9: Amplifier, R: Excitation current detection resistor,
14: Switching circuit.

Claims (1)

[Claims] 1. A pair of electrodes exposed on the inner wall of a fluid pipe and facing each other, an exciting coil for applying a magnetic field in a direction orthogonal to the axial direction and the electrode direction of the fluid pipe, and the exciting coil. An exciting circuit for flowing a low-frequency periodic current, an exciting current detecting circuit for detecting an exciting current flowing through the exciting coil, and a flow rate signal deriving circuit for outputting a signal voltage according to a flow rate induced between the pair of electrodes, The first A / D that converts the flow rate signal from this flow rate signal derivation circuit into a digital signal
A converter, a second A / D converter for converting the output of the exciting current detection circuit into a digital signal, and the first and second A / D converters.
In an electromagnetic flowmeter including a calculation means for calculating a flow rate by receiving a signal from the / D converter, the outputs of the flow rate signal derivation circuit and the exciting current detection circuit are supplied to the first A / D converter and the A switching circuit for switching and connecting to two A / D converters is provided, and the arithmetic means is
The output of the flow rate signal derivation circuit is connected to the first A / D converter, the output of the exciting current detection circuit is connected to the second A / D converter, and the output of the flow rate signal derivation circuit Is connected to the second A / D converter, and the output of the exciting current detection circuit is connected to the first A / D converter. An electromagnetic flowmeter comprising: a product calculating means for calculating a product; and a square root calculating means for square rooting the calculated product and outputting the square root result as a flow rate signal.