JP3277747B2 - Capacitive electromagnetic flowmeter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a displacement type electromagnetic flowmeter, and more particularly to a displacement type electromagnetic flowmeter capable of detecting an empty state of a fluid in a conduit of an electromagnetic flowmeter transmitter.

[0002]

2. Description of the Related Art In general, in a case where a fluid is transported using a pump or the like to perform batch control, depending on the condition of the piping, when the pump is stopped, the piping may become empty. As a result, there is a problem that an error occurs in the output value of the electromagnetic flow meter. FIG. 9 is a block diagram showing a configuration of a conventional electromagnetic flow meter having an empty detection function. Reference numeral 10 denotes a conduit whose inner surface is insulated and through which the measurement fluid Q can flow. The conduit 10 has a pair of measurement electrodes 11a that come into contact with the measurement fluid Q.
11b is fixed insulated from the conduit 10, and the measuring fluid Q
Is connected to a ground point G.

[0003] An excitation coil 12 for applying a magnetic field to the measurement fluid Q is arranged close to the conduit 10, and an excitation current _If flows from an excitation circuit 13 to the excitation coil 12. And these conduits 10, excitation coil 1
2, etc., constitute the detector 14.

A preamplifier 15 is connected to the measuring electrodes 11a and 11b. The preamplifier 15 is connected to buffer amplifiers 15a and 15b whose input terminals are connected to the measuring electrodes 11a and 11b. The output terminal is composed of a differential amplifier 15c connected to the input terminal. Further, diodes D ₁ and D ₂ are connected to these measuring electrodes 11a and 11b.
Are connected. The anodes of these diodes D ₁ and D ₂ are connected to a negative power supply -V, respectively.
The constant current circuit 16 is provided by the diodes D ₁ and D ₂ and the negative power supply -V.
(16a, 16b) are formed.

A series circuit in which zener diodes D _Z1 and D _Z2 are connected in series with opposite polarities is connected between the output terminal of the buffer amplifier 15b and the common potential point COM. The output terminal of the differential amplifier 15c is connected to the input terminal of the signal processing circuit 17, the signal processing circuit 17 is output to the output terminal 18 calculates the flow rate signal V _Q using the measured voltage V _M1 appearing at the output terminal I do.

Further, the empty detection circuit 19 detects a difference voltage E _d1 corresponding to the difference between the DC voltages E _a and E _b appearing on the measurement electrodes 11 a and 11 b in the measurement voltage V _{M1 and} a first threshold value from the reference voltage source 20. The voltage V _R1 is compared with an empty detection signal V at its output terminal 21.
Outputs _e1 .

Next, the operation of the electromagnetic flow meter configured as described above will be described. From the excitation circuit 13, for example, a rectangular-wave-like excitation current _If is passed through the excitation coil 12, whereby a rectangular-wave-like magnetic field is applied to the measurement fluid Q. The measurement voltages generated at the measurement electrodes 11a and 11b are impedance-converted by the preamplifier 15 and output to the output terminal thereof as the measurement voltage _VM1 . Next stage of the signal processing circuit 17 outputs a flow rate signal V _Q to the output terminal 18 by the flow rate computation using the measured voltage V _M1.

On the other hand, the measuring electrodes 11a and 11b
Diode De is connected to the negative power source -V - diode by de D _1, D ₂ - reverse re de - constant current by the leak current circuit 1
Since 6 is formed, the detector 14 is wetted resistance R _a between the measuring electrodes 11a and 11b becomes empty, the DC voltage E _a of the measurement electrodes 11a and 11b R _b is large, E _b is larger Become.

[0009] The differential amplifier 15c DC voltage E _a of this or the like and calculates the difference E _b outputs a differential voltage E _d1 at its output. The empty detection circuit 19 determines that the difference voltage _Ed1 is equal to the threshold voltage _VR1.
Is exceeded, it is determined that the inside of the conduit 10 is empty, and its output terminal 2
For example, an empty detection signal V _e1 that is negatively output is output to the counter ₁ .

[0010]

In the above-mentioned conventional empty detecting means, since the electrodes are in contact with the liquid, the difference in electrode impedance between the empty state and the full state is greatly different. Therefore, the sky detection could be performed relatively easily by measuring the difference. However, if the above principle is used for a capacitive electromagnetic flowmeter in which the electrodes do not come into contact with the fluid, the capacitive electromagnetic flowmeter cannot detect empty even if it performs empty detection based on this because the electrode impedance is originally large. There's a problem. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a capacitive electromagnetic flow capable of detecting an empty space.

[0011]

According to the present invention, a magnetic field is applied by flowing a rectangular wave excitation current including a zero excitation section through a measurement fluid flowing through a pipeline.
In a capacitive electromagnetic flowmeter that detects a voltage corresponding to the flow rate of the measurement fluid through a capacitance formed between the measurement fluid and a pair of flow rate detection electrodes, when a fluid in the pipe is full of water, detecting the differential noise generated when rising and falling of the excitation current in the space-time, and at the full level
The apparatus further comprises an empty detection circuit for detecting an empty state based on a difference in differential noise at the time of the empty time .

[0012]

The difference in the magnitude of the differential noise occurs between the state where the pipe is full and the state where the pipe is empty. The average value of the differential noise peak in a full state for a predetermined time and the average value of the differential noise peak in an empty state are measured in advance, and a predetermined value for determining the empty state is set based on the difference between the average values. Then, in the fluid measurement state, the average value of the peak of the differential noise at a predetermined time is constantly compared with the stored predetermined value, and when the average value in the measurement state exceeds the predetermined value, it is determined that the fluid is empty. Further, if a synchronous rectifier circuit is used as the differential noise detecting means, empty detection can be performed in a state where the influence of flow noise when measuring a fluid having low conductivity is removed. Hereinafter, a detailed description will be given based on embodiments.

[0013]

FIG. 1 is a schematic diagram showing a schematic configuration of a displacement type electromagnetic flow meter according to the present invention. In FIG. 1, reference numeral 30 denotes a pipe (pipe) through which a measurement fluid Q flows, and this pipe 30 is made of an insulating material such as ceramics or vinyl chloride resin. Reference numerals 31 and 32 denote detection electrodes for detecting a signal voltage generated in the measurement fluid.
Numeral 2 is buried in the outer surface of the pipe 30 or in the pipe pipe wall while being insulated from the measurement fluid in a direct current manner.

On the outer surface of the pipe 30 in a direction perpendicular to the line connecting these detecting electrodes 31 and 32, a pair of exciting coils 33,
34 are arranged. Then, the exciting coils 33 and 34
Are provided with an electrostatic shield, and are connected to a ground electrode G which is a reference potential.

Guard electrodes 35 and 36 are arranged outside the detection electrodes 31 and 32 so as to cover the entirety of the detection electrodes 31 and 32 while maintaining insulation between the detection electrodes 31 and 32. . The pipe 30 is provided with a liquid contact electrode E, which determines a reference potential of a signal voltage generated by the measurement fluid, and is connected to a ground electrode G.

The detection electrodes 31 and 32 are connected to non-inverting input terminals (+) of preamplifiers 37 and 38 having high input impedance, respectively, and their inverting input terminals (-) are connected to guard electrodes 35 and 36, respectively. The output terminals of these preamplifiers 37 and 38 are connected. The output terminals of the preamplifiers 37 and 38 are connected to the input terminals of the differential amplifier 39, respectively. The excitation circuit 40 receives a control signal from a control circuit (not shown) and receives excitation signals 33 and 34.
Is supplied with an exciting current _If of a square wave which changes to positive and negative and has a steady value of two or more.

The output of the differential amplifier 39 is an A / D converter 50
The output of the A / D converter 50 is connected to the CPU 51.
Is connected to the flow rate calculation circuit 51a. Reference numeral 52 denotes an absolute value circuit connected to the output terminal of the differential amplifier 39, and the output of the absolute value circuit is input to a sample hold circuit 53. Then, the output of the sample-and-hold circuit is converted into a digital signal by the A / D converter 54, and then input to the null operation circuit 51b in the CPU. The sample hold 53
The switch 53c provided therein resets the voltage of the capacitor 53d at a predetermined timing from the empty operation circuit 51. 53e is a diode.

Next, the operation of the embodiment configured as described above will be described. An excitation circuit 4 is supplied to the measurement fluid Q by a control signal output from a control signal generation circuit (not shown).
0, the DC current contained in the excitation coil 33 is switched.
A rectangular wave exciting current _If that changes between positive and negative flows through 34, and a magnetic field having a similar waveform is applied. here,
When the measurement fluid Q flows into the pipe 30, the insulating pipe 30
A signal voltage corresponding to the flow rate of the measurement fluid Q is generated on the inner surface of. This signal voltage is applied to the measurement fluid Q and each detection electrode 31,
32, each of the detection electrodes 3
1, 32 are detected.

The detected signal voltage is supplied to a preamplifier 37,
The output signal is amplified by a differential amplifier 39 via an output A
The signal is converted into a digital signal by the / D converter 50, input to the flow rate calculation circuit 51a incorporated in the CPU, and output as an electric signal corresponding to the flow rate. On the other hand, the absolute value circuit 52 outputs the output of the differential amplifier 39 that changes to positive or negative as a continuous positive electrical signal. This signal is input to the sample-and-hold circuit 53, where the pre-amplifier 53a and the diode 53
b, through the preamplifier 53c and the A / D converter 54, to the null operation circuit 51b.

Here, the calculation procedure of the null calculation circuit 51b will be described with reference to the flowchart of FIG. Prior to the flow rate measurement, the pipe 30 is filled with the fluid Q, and the above-described rectangular wave is applied while the flow velocity is zero. 3 (a), (b),
(C) shows an example in which an output signal of the differential amplifier 39 is viewed on an oscillograph when tap water is used as a measurement fluid and a rectangular wave (a) including a zero excitation section is applied as an excitation current. (B) diagram shows the differential noise during rise and fall of the exciting current in a state where a full water pipe, the peak value h ₁ of the illustrated differential noise is assumed to be 2.5mV for example. In the state where the fluid is flowing, the portion A indicated by the dotted line fluctuates up and down in accordance with the flow rate.

FIG. 3C shows the differential noise when the exciting current rises and falls when the pipe is empty. In the illustrated case, it is assumed that the peak value h ₂ of the differential noise is, for example, 10 mV. As can be seen from FIGS. 8B and 8C, the peak value of the differential noise is significantly different between the full state and the empty state.

FIGS. 4A, 4B, and 4C show that the peak value of the differential noise shown in FIG. 3 is converted into a positive and continuous electric signal by an absolute value circuit (see FIG. 1), which is converted into a sample and hold circuit. This shows the state of output from 53 to the A / D converter 54. Here, for example, six peaks are detected per 100 ms, and the highest peak voltage is output. That is, the capacitor 53d is charged at the first peak of 2.2 mV, and its voltage value rises to 2.5 mV by the next signal, and in the next 2.1 mV, the previous minute is maintained, and the next 2.7 mV is maintained.
Signal rises to 2.7 mV. In the next and the next 2.2 and 2.5 mV, the previous 2.7 mV is maintained, and 2.7 mV which is the maximum voltage value of these six is set at a predetermined timing until the next time differential noise is detected. / D converter 5
4 and at the same time, the switch 53c is momentarily turned on to make the charge in the capacitor zero.

[0023] Then, the sky detection circuit 51b takes a time of 100 ms.
The peak value 2.7 mV during the storage area (RA
M). Repeat the same for 3 seconds, for example.
And the average value of the peak at that time (V_A→ y '…
The general formula, step 1) in FIG. 2, is stored.Next, drain the tap water from the pipe to make it empty,
Apply the same exciting current. And even in this empty state
For example, the same is repeated for 3 seconds,
Average value (V_B... Step 2) of FIG. 2 is stored.

Here, in tap water, when the pipe is empty, the average of the maximum value of the differential noise is 1 for example.
0 mV, and the average of the maximum value in the full state is 2.7 mV
Assume that In this case, the voltage difference is 7.3 mV, but this difference is not always stable and some fluctuation occurs.

Therefore, the operator is required to calculate the difference (10
-2.7 = 7.3 mV), and the average value (2.
For example, about 1/2 (3.5 mV) of the difference is added to 7 mV) to obtain 6.2 mV (2.7 + 3.5), and this value is set as a predetermined value (V _C ) for determining the empty state. That is, when the voltage when the fluid is measured in the full state exceeds a predetermined value of 6.2 mV, the value (in the RAM (not shown)) built in the empty operation circuit 51b is determined so as to determine that the state is empty. A predetermined value V _C (V _A <V _C <V _B )-Step 3 in FIG. 2 is input.

Next, the normal measurement state (Step 4 in FIG. 2)
The empty calculation circuit 51b calculates the average of the peak value of the differential noise every three seconds, for example, and compares the calculated value with the predetermined value (V _C ) stored in the RAM (step 5 in FIG. 2). ). When the difference exceeds V _C (6.2 mV), it is determined that the vehicle is empty (step 6 in FIG. 2). In the present embodiment, the measurement liquid is described as tap water.However, the difference between the peak values of the differential noise in the full / empty state changes when the excitation current, the measurement liquid, the shape of the electrode, and the like change, and therefore, prior to the measurement. it is necessary to previously enter the predetermined value V _C Te.

In the above-described embodiment, the difference (10−2.7 = 7.3 mV) is calculated by calculating the average value of the peak value of the differential noise between the time when the water is full and the time when the water is empty, and calculates the difference. An average value (2.7 mV) is about half of the difference (3.
5 mV) and a predetermined value (6.
2mV)), which is, for example, about 差 of the difference.
5 mV is input to the RAM as a predetermined value (V _C1 ), and the average value of the peak of the differential noise in the normal measurement state is subtracted from the average value of the peak of the differential noise in the space-time obtained in advance, which is １ ／ of the difference. May be determined to be empty when exceeds the predetermined value (V _C1 ).

The above-described measuring apparatus is effective for determining empty when the fluid to be measured has a relatively high electrical conductivity. However, when the fluid to be measured has a relatively low conductivity, the determination is impossible because the fluid is affected by flow noise. It is considered that the flow noise is caused by a change in the potential at the interface on the inner surface of the pipe (pipe) due to the fluctuation of ions carried by the fluid. FIG. 5 shows an experimental example in which the relationship between the frequency characteristic of the power spectral density of the flow noise and the noise voltage was measured by setting the diameter of the tube to 25 mm, the ion-exchanged water as the fluid, and the flow rate to 1 m per second.

According to the figure, the flow noise has a characteristic dependent on the fluid conductivity, and becomes remarkable especially at 5 μS / cm or less. In addition, it has a frequency-dependent characteristic, and has a corner frequency at about 10 Hz, but exhibits a characteristic close to 1 / f at frequencies higher than 10 Hz. That is, when a fluid having a low conductivity is flowed, the output of differential noise is, for example, about 0.5 to 3 mV when the flow velocity is zero, and the differential noise when empty is 5 to 5 mV.
It is about 10 mV. On the other hand, when the flow velocity increases, the output due to the flow noise becomes, for example, 5 to 20 mV or more. Therefore, when the flow velocity of the low-conductivity fluid increases, the value exceeds the predetermined value Vc, so that a warning of sky detection is issued, and the function of sky detection cannot be performed.

FIG. 6 shows a processing flow for removing the influence of the flow noise. In this case, in addition to the average value of the peaks of the differential noise when the water is full and when the air is empty, the variance (ΔV _A) in each state is shown. → Sy ² ... general formula) is calculated.

That is, here, the average value (VA) of the peak of the differential noise when the water is full is obtained by the same procedure as described above (step 1). Next, the variance (ΔV _A ) of the differential noise when the water is full is calculated using the above equation (step 2). Similarly, the average value (VB) of the peak of the differential noise in the empty time is obtained (step 3).

Similarly, the variance (ΔV _B ) of the differential noise at the time of space is calculated using the equation (step 4). V
A predetermined value (VC) for determining the empty state is set from the values of A and VB (step 5). Then, set the dispersion value [Delta] V _C for determining the presence or absence of flow velocity (ΔV _C> ΔV _A, Δ
V _B ). That is, if the fluid is flowing, ΔV _C always increases due to the influence of the flow noise, so that it is set to a predetermined value larger than ΔV _A and ΔV _B (step 6).

[0033] performs the normal flow rate measurement after setting the VC, the [Delta] V _C (step 7). On the other hand, the empty detection circuit 51b calculates the variance (ΔV _f ) of the differential noise in the state where the fluid is flowing by the same method as in Steps 2 and 3, and Step 6
Is compared with the variance value ΔV _C set in the above. This ΔV _f is ΔV _C
If it is larger, it is found that the conductivity of the fluid is low and the fluid is flowing, so the determination is YES and the flow rate measurement is continued (step 8). If NO (ΔV _f is smaller than ΔV _C ), the conductivity is high and the water is full. It can be considered empty regardless of the high or low conductivity.

Therefore, in the case of NO, the flow rate signal ( _Vf ) is compared with a predetermined value (VC) for determining the empty state (step 9). Here, V _f is to continue the measurement is determined that is smaller than VC is high conductivity fluid is filled with water, V _f is VC
If it is larger, it can be seen that it is empty regardless of whether the conductivity is high or low. If the calculation is performed according to the above sequence, the sky detection can be performed without depending on the fluid conductivity.

FIG. 7 shows an embodiment according to claim 2 of the present invention. The means for taking the output of the differential amplifier 39 into the flow rate calculation circuit 51a via the A / D converter 50 and performing the flow rate calculation is as follows. As before. In the present invention, the output of the differential amplifier 39 is introduced into a synchronous rectification circuit, and the output of this circuit is taken into the flow rate calculation circuit 51a via the A / D converter 54.

Here, the synchronous rectifier circuit 55 comprises an inverting amplifier 55a for receiving the output of the differential amplifier, an analog switch 55b for receiving the output of the amplifier, and a low-pass filter 55c for receiving the output of the switch. The analog switch 55b is composed of three switches of X, Y, and Z. The X switch is connected to the output terminal of the differential amplifier 39 in the on state (1), and is connected to the ground terminal in the off state. .

The switch Y is connected to the output terminal of the inverting amplifier 55a in the ON state (1), and is connected to the ground terminal in the OFF state. The Z terminal is connected to the ground terminal in the on (1) state, and is connected to the low-pass filter 55c in the off state.

Next, the operation of the synchronous rectifier circuit 55 will be described with reference to FIG.
This will be described with reference to FIG. The analog switch is switched in synchronization with the rising and falling timings of the excitation of the excitation circuit. 8A shows an excitation waveform, FIG. 8B shows differential noise generated when the excitation waveform rises and falls, and FIG. 8C shows a switch X that turns on and off in synchronization with the rise and fall of the excitation waveform. The timings of Y and Z are shown.

In the analog switch 55b, when the X switch is on and the Y and Z switches are off (A), the output of the differential amplifier 33 is output to the low-pass filter 55c, At the timing when the switch of Y is turned off and the switch of Z is turned on (b), each switch is in the ground state, and the output to the low-pass filter side is zero. Also, the switch of X is off,
When the Y and Z switches are turned on (C), the output of the inverting amplifier 55a is output to the low-pass filter side.

(F) is a state in which the switches X, Y, and Z are driven at the timings (a), (b), and (c) synchronized with the above-described excitation waveform to extract differential noise as a positive continuous electric signal. The effect of the flow noise is removed from this signal by synchronous rectification. The low-pass filter 55c takes in this signal, smoothes it, and outputs it as a DC voltage to the A / D converter 54.

The output of the A / D converter 54 is an empty detection circuit 51
, But the subsequent processing is performed in the same procedure as that described with reference to FIG. By using the synchronous rectification circuit 55, the influence of the flow noise can be eliminated, and the empty detection can be performed by the procedure of the flowchart shown in FIG. 2 regardless of the level of the conductivity.

[0042]

As described above, the present invention detects the differential noise generated at the rise and fall of the exciting current when the fluid in the pipeline is full and empty, and detects the differential noise when the fluid is full and empty. Since the empty state is detected based on the difference between the differential noises, the empty state detection by the capacitive electromagnetic flow meter can be realized with a relatively simple configuration. Also,
In the present invention, the predetermined value serving as the reference for the sky detection is determined by the average value of the peak of the differential noise, so that the sky state can be detected more accurately. Regardless, an empty state can be detected.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a main part configuration of an embodiment of the present invention.

FIG. 2 is a flowchart showing a procedure of detecting an empty state of the electromagnetic flow meter according to the present invention.

FIG. 3 is an explanatory diagram showing differential noises in a full state and an empty state that occur with application of an exciting current.

FIG. 4 is an explanatory diagram showing a variation of differential noise and a state of a voltage charged in a capacitor.

FIG. 5 is a diagram illustrating a relationship between a frequency characteristic of power spectral density of flow noise and a noise voltage.

FIG. 6 is a flowchart illustrating a procedure of sky detection in which a measure against low conductivity is taken.

FIG. 7 is a configuration diagram showing another embodiment of the sky detecting means taking a low conductivity measure.

FIG. 8 is an operation explanatory diagram showing the operation of FIG. 7;

FIG. 9 is an explanatory view of a main part configuration of an electromagnetic flow meter having an empty detecting means generally used conventionally.

[Explanation of symbols]

 Reference Signs List 30 pipe (pipe) 31, 32 detection electrode 33, 34 excitation coil 35, 36 guard electrode 37, 38, 53a, 53b preamplifier 39 differential amplifier 40 excitation circuit 50, 54 A / D converter 51 CPU 51a flow calculation circuit 51b Empty operation circuit 55 Synchronous rectifier circuit 55a Inverting amplifier 55b Analog switch 55c Low-pass filter

Claims (3)

(57) [Claims] 1. A magnetic field is applied by flowing a rectangular wave excitation current including a zero excitation section through a measurement fluid flowing through a pipeline.
In a capacitive electromagnetic flowmeter that detects a voltage corresponding to the flow rate of the measurement fluid through a capacitance formed between the measurement fluid and a pair of flow rate detection electrodes, when the fluid in the pipe is full of water, An empty detection circuit that detects differential noise generated at the time of rising and falling of the exciting current in empty time and detects an empty state based on a difference between the differential noise between the full level and the empty time. Capacitive electromagnetic flow meter. 2. A differential rectifier circuit detects differential noise generated when the exciting current rises and falls when the fluid in the pipeline is full and when the fluid is empty, and based on the difference between the differential noises, detects the differential noise. 2. The displacement type electromagnetic flow meter according to claim 1, further comprising an empty detection circuit for detecting a state. 3. The capacitive electromagnetic flow meter according to claim 1, wherein the differential noise for detecting the full or empty state is an average value of peaks output within a predetermined time.