JPH085422A - Electromagnetic flow meter - Google Patents

Abstract

PURPOSE:To remove electrochemical noise and improve the measurement precision regarding a low energy consumption-type flow meter. CONSTITUTION:An electromagnetic flow meter is composed of a permanent magnet 1b and an excitation coil 1a installed together, a low-pass filter 7 which takes out the output induced between electrodes 3a, 3b when excitation is carried out only by the permanent magnet 1b, a high-pass filter 9 which takes out the output only by the excitation coil 1a when excitation is carried out by both the permanent magnet 1b and the excitation coil 1a, A/D converters 8a, 8b installed in the rear stages of the low-pass and the high-pass filters, and a computing means 10 to carry out sampling the outputs of these A/D converters 8a, 8b and take in the outputs. The computing means 10 computes an electrochemical output based on an output induced between the electrodes when excitation is carried out only by the permanent magnet 1b and also an output obtained when excitation is carried out by the excitation coil 1a, sends out an output after deducing the electrochemical noise from the output obtained when excitation is carried out by the permanent magnet 1b, and thus the current applied to the excitation coil 1a is made to be current which reverses a plurality of times to be positive and negative from zero level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter which converts a flow rate of a measuring fluid into an electric signal and outputs a flow rate signal corresponding to the electric flow rate, and particularly to improve the measurement accuracy in a low power consumption type. Electromagnetic flowmeter.

[0002]

2. Description of the Related Art An electromagnetic flowmeter applies a magnetic field by a coil in a direction perpendicular to the flow direction of a fluid to be measured flowing through a conduit, and the magnetic field causes a voltage generated in the fluid to be measured according to its flow rate. This is taken out by a pair of detection electrodes provided in a direction perpendicular to the magnetic field, and as a method of applying the magnetic field, a method of supplying an exciting current to an exciting coil is generally used. In that case, the power consumption of the electromagnetic flowmeter is almost determined by the excitation power. FIG. 4 is a block diagram showing a conventional example for reducing power consumption, and FIG. 5 is a timing chart thereof, which is described in JP-B-1-26491. This conventional example will be briefly described.

In FIG. 4, reference numeral 1 denotes an exciting unit, which is composed of a pipe 2 through which a fluid flows, electrodes 3a and 3b, a permanent magnet 1b and an exciting coil 1a. A magnetic circuit for vertically generating a magnetic field is formed in the pipe 2. Is forming. When the exciting current Iw flows through the exciting coil 1a, it is excited in a direction of demagnetizing (or increasing) the magnetic field of the permanent magnet 1b. An exciting current circuit 20 is composed of a constant current source 21 and a switch 22. When the switch 22 is turned on by the timing signal P ₁ from the timing signal generating circuit 35, the exciting current Iw is passed through the exciting coil 1a.

The signal processing circuit 3 includes electrodes 3a and 3b of the exciting unit 1.
The induced voltage e _a is applied during the excitation current Iw, and the induced voltage generated during the period t during which the exciting current Iw flows is used as noise due to the electrochemical potential contained in the induced voltage during the period T during which the exciting current Iw does not flow. And outputs the value V ₀ related to the fluid. In the figure, the amplifier 31 for amplifying the induced voltage e _a , the sampling switch S ₁ , the holding capacitor C _1, and the output e _{b of the} amplifier 31 are sampled. A circuit 32 for holding and a differential amplifier 33 for amplifying the difference between the output e _{b of the} amplifier 31 and the output of the sample hold circuit 32.
An output circuit 34 which includes a sampling switch S ₂ , a buffer amplifier using a holding capacitor C ₂ and an operational amplifier OP, and which outputs an output e _c of the differential amplifier 33 as V ₀ , a switch 22, and a sampling switch S Timing signals P ₁ , P ₂ , for controlling ₁ and S ₂ , respectively
An example including a timing signal generating circuit 35 for generating P ₃ is illustrated.

In the above structure, the exciting part 1 is normally excited only by the permanent magnet 1b, and the fluid is given a magnetic field having a magnetic flux density B _DC . On the other hand, when the switch 22 of the exciting current circuit 21 is turned on by the timing signal P ₁ as shown in FIG. 5A from the timing signal generating circuit 35 and the exciting current Iw is supplied to the exciting coil 1a for a certain time t, the permanent magnet is generated. Since the magnetic field due to 1b is excited in the direction of demagnetizing (or increasing) the magnetic flux density B _{AC in the} fluid.
Is given.

As a result, the induced voltage generated between the electrodes 3a and 3b is as shown in FIG. 5B, which changes when the exciting current Iw is passed (the solid line shows the case of demagnetization and the dotted line shows the case of increased magnetization). Here, the flow velocity of the fluid is V, and the diameter of the pipe 11 is D
And V _err is the noise due to the electrochemical potential,
Induced voltage V generated during period T when exciting current Iw is not flowing
_The induced voltage V _DC1 generated during the period t in which _DC and the exciting current Iw are flowing is given by the following equation. V _DC = k _DC B _DC VD + V _err (1) V _DC1 = k _DC1 B _DC1 VD + V _err (2) k _DC = k _AC ; shape factor

The induced voltage e _a from the oscillator 1 is the amplifier 31
One input terminal (+) of the differential amplifier 33 after being amplified by
Is added to The output e _{b of the} amplifier 31 is also given to the sample hold circuit 32. When the switch S ₁ is turned on by the sampling pulse P ₂ generated at the timing shown in FIG. 5C during the period t during which the exciting current I _W is flowing, the sample-hold circuit 32 amplifies the voltage e _b2 obtained by amplifying e _a2. Hold as. The output e _b2 of the sample hold circuit 32 is applied to the other input terminal (−) of the differential amplifier 33. As a result, the output e of the differential amplifier 33
_c is a period in which only the permanent magnet is excited (period in which no exciting current flows), _ec = k ₂ (e _b1 −e _b2 ) = k ₁ k ₂ (B _DC −B _AC ) VD ... ( 3) k ₁ ; gain of the amplifier 31 k ₂ ; gain of the differential amplifier 33, and the noise component V _err including the electrochemical potential is removed and becomes a value proportional to only the flow rate of the fluid.

The output e _C of the differential amplifier 33 meets the output regenerator 34 and is sent to the output terminal OUT as the output voltage V ₀ . The output furnace 34 is driven by a pulse P ₃ generated at the timing shown in FIG. 2D, and is turned on by a switch S ₂ which is turned off during a period t during which the exciting current I _W is flowing. Output e of T differential amplifier 33
_C is sent out.

[0009]

By the way, the electrochemical noise changes with time, but in the above-mentioned conventional example, one output signal within a certain time (t) is judged as the electrochemical noise. However, determining the electrochemical noise with only one signal has a problem in accuracy. It is an object of the present invention to provide an electromagnetic flow meter that can be highly accurate by accurately calculating the electrochemical noise in this fixed time.

[0010]

According to the structure of the present invention for solving the above-mentioned problems, a permanent magnet and an exciting coil are provided as an exciting means of an electromagnetic flowmeter, and normally, only the permanent magnet is used for excitation. The excitation coil is provided with a means for causing a current to flow for a certain period of time as required to perform excitation by both the permanent magnet and the excitation coil, and is included in the voltage induced between the electrodes when excitation is performed only by the permanent magnet. In an electromagnetic flow meter in which noise caused by an electrochemical potential is removed by using a voltage induced between electrodes when excitation is performed by both a permanent magnet and an excitation coil, excitation is performed only by the permanent magnet. A low-pass filter that extracts the output induced between the electrodes during the excitation and an excitation coil during the excitation by both the permanent magnet and the excitation coil. A high pass filter to extract only the output by Le, the low pass, the A / D converter disposed downstream of the high-pass filter, calculation means for capturing by sampling the output of A / D converter (CP
U), the calculating means obtains an electrochemical output from an output induced between the electrodes when being excited only by the permanent magnet and an output obtained when being excited by the exciting coil, Electrochemical noise is subtracted from the output when excited by a permanent magnet and output, and the current flowing through the exciting coil is a current that is inverted several times between positive and negative at a zero level. .

[0011]

[Operation] The output of the low-pass filter contains the flow rate signal and electrochemical noise when excitation is performed only by the permanent magnet. The output of the high-pass filter does not include electrochemical noise when excited by the exciting coil. Therefore, by subtracting the electrochemical noise calculated based on the output of the high-pass filter from the output of the low-pass filter, the electrochemical noise when excited by the permanent magnet can be compensated.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention.
The same components as those of the conventional example shown in FIG. 3 are designated by the same reference numerals, and duplicate description will be omitted. Reference numerals 4 and 5 are high-impedance buffer amplifiers, and their input terminals are electrodes 3
It is connected to a and 3b. The output of the buffer amplifier 4 is connected to the non-inverting terminal of the differential amplifier 6, and the output of the buffer amplifier 5 is connected to the inverting terminal of the differential amplifier 6.

The output of the differential amplifier 6 is input to an A / D converter 8a through a low pass filter 7 composed of a resistor 18, a capacitor C ₁ , an amplifier 7, etc., and its output is a CPU.
Input to (calculation means) 10. The output of the differential amplifier 6 is input to the A / D converter 8b through the high pass filter 9 including the capacitor C ₂ and the amplifier 9, and the output is C
Input to PU10. Reference numeral 11 denotes an AC excitation circuit that outputs, for example, a sine wave (or a triangular wave including a triangular wave and a trapezoidal wave) current of 100 Hz or more. Reference numeral 12 denotes an exciting current on / off switch.

In the above structure, the exciting part 1 is usually excited only by the permanent magnet 1b, and the fluid is given a magnetic field having a magnetic flux density B _DC . The signals of the detection electrodes 3a and 3b at this time include electrochemical noise.
The voltage is as shown in (a). This voltage signal passes through the low pass filter 7 and is converted into a digital signal by the A / D converter 8a. The converted signal is taken into the CPU 10 at a predetermined timing by a timing signal Pa signal from a timing signal generating circuit (not shown) included in the CPU 10 and stored in the memory.

Next, when a timing signal Pb is generated from a timing signal generating circuit (not shown) included in the CPU 10, the switch 12 of the exciting current circuit 11 is turned on for a fixed time, and the exciting coil 1a is turned on for a fixed time t. Excitation current Iw _AC is supplied. As a result, the magnetic field B _DC due to the permanent magnet 1b and the magnetic field B _AC due to the exciting current are superimposed, and the signal of the detection electrode becomes as shown in FIG. 2 (b). The high-pass filter 9 removes the direct current component shown in FIG. 2 (c) and only the alternating current component shown in FIG. 2 (d) is converted into the A / D converter 8b.
Output to.

FIG. 3A is an enlarged view of portion A of FIG. 2D. When the exciting current is a rectangular wave, differential noise as shown by B in the figure occurs. The CPU 10 is adapted to sample and take in the signal in the constant current region of this signal at the timing shown in FIG.

Here, the flow rate signal (V) when the current flowing in the exciting coil is a current which is inverted a plurality of times with the zero level as a boundary
_AC ) is expressed by the following equation. V _AC = k _AC B _AC VD (4) When V _err is _calculated from this flow signal based on the flow signal V _DC when excited by only the permanent magnet sampled immediately before, V _err = V _DC − {(k _DC B _DC ) / (k _AC B _AC )} V _AC (5), and electrochemical noise can be detected. C
The PU 10 calculates V _DC -V _err and outputs a signal that compensates (removes) electrochemical noise.

Next, considering the noise propagation for each parameter of the equation (4), the propagation error ΔV _err is ΔV _err = │∂V _err / ∂V _DC │ + │∂V _err / ∂B _DC │ + │∂ V _err / ∂B _AC | + | ∂V _err / ∂V _AC | ... (6) In equation (5), since k _DC and k _AC are geometrical elements, it is assumed that no error is caused by this.

[0019] _{ΔV err = 1 + (k DC} / V AC B AC) V AC · ΔB DC + (k DC B DC / k AC B AC 2) V AC · ΔB AC + (k DC B DC / k AC B AC ) ΔV _AC (7) Since the direct-current excitation by the permanent magnet depends on the inherent magnetic force, it can be regarded as ΔB _DC ≈0, and the alternating-current excitation by the high frequency excitation can be made ΔB _AC ≈0 by the magnetic field control by the constant current control. is there. Therefore, the expression (7) can be expressed as ΔV _err = 1 + (k _DC B _DC / k _AC B _AC ) ΔV _AC (8)

In order to reduce ΔV _err in the equation (8), the AC magnetic field (B _AC ) is increased relative to the DC magnetic field (B _DC ) or the fluctuation noise (ΔV _AC ) of the _AC detection signal (V _AC ). It turns out that it is better to make _AC smaller. here,
Since the object of the present invention is low power consumption, increasing the alternating magnetic field (B _AC ) increases the exciting current, which is contrary to the object. Therefore, paying attention to the fluctuation noise (ΔV _AC ) of the _AC detection signal (V _AC ), the exciting current I _W in the above conventional example.
Sampling is performed only once during the period (t) during which the current is flowing. Therefore, the fluctuation noise (ΔV _AC ) remains Δ
It becomes the main element of V _err .

However, in the present invention, sampling is performed a plurality of times (six times in the figure) during the period (t) in which the exciting current I _W is flowing, so ΔV _AC · 6 ^−1/2 can be obtained. , If sampling is performed n times, ΔV _AC · n ^-1/2
Becomes Therefore, ΔV _err in the equation (8) can be reduced, accurate compensation can be performed, and a low power consumption type electromagnetic flow meter with improved measurement accuracy can be realized. Note that more accurate compensation can be performed by using an alternating current of 100 Hz or more as the exciting current.

[0022]

As described above in detail with reference to the embodiments, according to the present invention, a low-pass filter for extracting an output induced between electrodes when only permanent magnets are excited and the permanent magnets are provided. A high-pass filter that extracts only the output from the exciting coil when the excitation is performed by both the exciting coil, an A / D converter provided in the latter stage of the low-pass and high-pass filters, and outputs of these A / D converters. Is sampled and taken in. The computing means is electrochemical based on the output induced between the electrodes when excited by only the permanent magnet and the output obtained when excited by the exciting coil. Obtain the output, subtract the electrochemical noise from the output when excited by the permanent magnet, and output the current. Since the current inverted several times to positive and negative bordering the Roreberu can be realized accurate compensation possible becomes low power consumption type electromagnetic flowmeter with improved measurement accuracy.

[Brief description of drawings]

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

FIG. 2 is an operation explanatory diagram of FIG. 1;

FIG. 3 is an enlarged view of a portion A of FIG. 2 and a diagram showing sampling timing.

FIG. 4 is a configuration diagram showing a conventional example.

FIG. 5 is an operation explanatory diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Excitation part 1a Excitation coil 1b Permanent magnet 2 Pipe lines 3a, 3b Electrodes 4,5 Buffer amplifier 6 Differential amplifier 7 Low pass filter 8a, 8b A / D converter 9 High pass filter 10 CPU 11 AC excitation circuit 12 On-off switch

Claims (1)

[Claims] 1. A permanent magnet and an exciting coil are provided together as an exciting means of an electromagnetic flow meter, and normally, only the permanent magnet is used for excitation, and a current is passed through the exciting coil for a certain period of time as necessary to excite the permanent magnet and the exciting magnet. A means for performing excitation by both the coil and the coil is provided, and noise due to the electrochemical potential contained in the voltage induced between the electrodes when excitation is performed only by the permanent magnet is caused by both the permanent magnet and the exciting coil. In an electromagnetic flowmeter configured to remove the voltage induced between the electrodes when the excitation is performed, a low-pass filter that extracts the output induced between the electrodes when the excitation is performed only by the permanent magnet, A high-pass filter for extracting only the output from the exciting coil while exciting with both the permanent magnet and the exciting coil; When the A / D converters provided in the subsequent stages of the pass and high pass filters and the calculating means for sampling and outputting the outputs of these A / D converters, the calculating means excites only by the permanent magnets. The electrochemical output is obtained from the output induced between the electrodes of the electrode and the output obtained while being excited by the exciting coil, and the electrochemical noise is subtracted from the output when being excited by the permanent magnet. The electromagnetic flowmeter, wherein the current passed through the exciting coil is a current that is inverted positively and negatively a plurality of times with a zero level as a boundary.