JP2734162B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention relates to an electromagnetic flowmeter for detecting an abnormal state such as emptying of a measurement fluid in a conduit, and in particular, a noise voltage generated between electrodes. The present invention relates to an electromagnetic flowmeter improved so that detection can be performed with high sensitivity.

<Conventional technology> It is convenient for an electromagnetic flow meter to be able to self-detect when the inside of a conduit becomes empty.

FIG. 6 is a block diagram showing the configuration of such a conventional electromagnetic flow meter.

Reference numeral 10 denotes a conduit whose inner surface is insulated and through which the measurement fluid Q can flow, and a pair of detection electrodes 11a and 11b that contact the measurement fluid Q and a liquid contact electrode 12 that grounds the measurement fluid Q. 10 is insulated and fixed.

Detection electrodes 11a, 11b is connected to the cathode diodes D _1, D ₂ is connected to the input terminal of the preamplifier 13. The anodes of these diodes D ₁ and D ₂ are connected to a negative power supply −V
Connected to each other. The wetted electrode 12 has a common potential point CO
Connected to M.

An excitation coil 14 for applying a magnetic field to the measurement fluid Q
Is located close to this conduit 10.

The detector 15 is composed of the conduit 10 and the exciting coil 14 and the like.

Exciting current I _f through the resistor R ₁ from the exciting circuit 16 is flowed to the exciting coil 14. One end of the resistor R ₁ is common potential point
The other end is connected to the switch circuit 17.

The excitation circuit 16 is controlled by the timing signals S ₁ from the timing circuit 18, three steady-state, for example zero, positive,
Excitation current in the form of a rectangular wave that repeats zero and negative as one cycle
_If .

On the other hand, the output terminal of the preamplifier 13 to one end of the sampling switches SW ₁ of the switch circuit 17, the other end of the resistor R ₁ is connected to one end of the sampling switch SW _2.

The other ends of the sampling switches SW ₁ and SW ₂ are connected to the input terminals of the flow rate calculation circuit 20 and to the input terminals of the empty detection circuit 21, respectively.

Exciting current I _f sampled at a flow rate calculation circuit 20 in the sampling switch SW ₁ is zero, positive, zero, proportional to the sampled excitation current I _f in each sample voltage V _S1 and the sampling switch SW ₂ of each negative state The flow rate calculation for calculating the flow rate component using a predetermined calculation formula using the comparison voltage V _R is performed, and the flow rate signal _VQ is output to the output terminal via the output circuit 22. In this case, the comparison voltage V _R is used to divide the sample voltage V _S1 to eliminate the influence of the excitation current I _f.

On the other hand, empty detection circuit 21 the exciting current I _f is zero, positive, zero, and calculating the DC voltage component generated between the electrodes by using the negative of each sample voltage V _S1 of each state, and the DC voltage V _D measured The DC voltage V _D is compared with a set value SET corresponding to the value of DC noise generated between the detection electrodes 11a and 11b when the fluid Q becomes empty.
There is sent to the output circuit 22 a hold signal V _H1 when exceeding the value of the set value SET, forcibly fixes the flow rate signal V _Q in, for example, 0%.

This like switching circuit 17, the flow rate calculation circuit 20, synchronization of the operation in such an empty detection circuit 21, respectively, such as timing signals S _2, S ₃ and S ₄ are taken.

Next, the operation of the electromagnetic flow meter configured as described above for detecting the sky will be described.

Since the diodes D ₁ and D ₂ are connected to the negative power supply −V in opposite polarities, and the input impedance of the preamplifier 13 is high, the diodes D ₁ and D ₂ functioning as constant current elements are eventually turned off. Leak currents I _l1 and I _l2 are detected electrodes 11a and 1
It flows between 1b and the liquid contact electrode 12.

Therefore, the liquid contact resistance _Rfa ( _Rfb ) between the detection electrode 11a (11b) and the liquid contact electrode 12 by the measurement fluid Q and the leak current _I11 ( _I12 )
And a DC voltage V _da (V _db ) corresponding to the product of the detection electrodes 11a and 11b is generated.

However, when the measurement fluid Q is filled in the conduit 10, the difference between the generated DC voltages V _da and V _db is small because the liquid contact resistance R _fa (R _fb ) is small, and the sample voltage V _S1 Is also small.

However, when the measurement fluid Q becomes empty, the liquid contact resistance R
_{Since fa} (R _fb ) increases, the leakage current I
The product with _l1 (I _l2 ) also increases, and generally these differential voltages increase. Therefore, the DC voltage calculated using the sample voltage V _S1 also increases and exceeds the set value SET.

Therefore, the empty detection circuit 21 outputs the holding signal V _H1 to its output terminal.
Output to a fixed flow rate signal V _Q of the output circuit 22 for example to 0%. In this case, as the set value SET, the detection electrodes 11a, 11
Considering the polarization voltage imbalance occurring in b
It is set to a value equivalent to about 0.7V.

<Problems to be Solved by the Invention> However, the conventional electromagnetic flowmeter having such an empty detection function is a constant current circuit in which the input terminal of the preamplifier 13 requiring a high input impedance is formed of a diode. Causes a short circuit, which leads to a decrease in input impedance as a whole and an increase in cost due to the addition of hardware.Furthermore, the direct current of the detection electrode causes a change in the surface condition of the detection electrode, which is a factor of accuracy deterioration. There is a problem such as performing.

<Means for Solving the Problems> In order to solve the above problems, the present invention has two different frequencies of a first frequency and a second frequency lower than the first frequency, and has three frequencies of positive excitation, negative excitation, and non-excitation. A magnetic field having two excitation states is applied to the measurement fluid, and a high-frequency flow signal having a first frequency component and a low-frequency flow signal having a second frequency component generated by the flow rate of the measurement fluid are sampled, and these are sampled. In an electromagnetic flowmeter that synthesizes and outputs a flow signal, a first analog / digital converter that samples a high-frequency signal and outputs a first digital signal, and a second that samples a low-frequency signal and outputs a second digital signal. (2) an analog / digital conversion means, which is outside the sampling period of the low-frequency flow signal and is half the period of the high-frequency flow signal.
Sampling means for sampling a noise signal in a non-excitation period of 1/4 period, and comparing the noise signal with a preset noise level, and determining that the measured fluid is empty when the noise level is exceeded. Means.

<Operation> The first analog / digital converter samples a high-frequency signal and outputs a first digital signal, and the second analog / digital converter samples a low-frequency signal and outputs a second digital signal.

Then, the sampling means is outside the sampling period of the low-frequency flow signal and / 4 of a half cycle of the high-frequency flow signal.
The noise signal is sampled during the non-excitation period of the period, and the noise signal is compared with a preset noise level by an empty determination unit. When the noise level is exceeded, it is determined that the measurement fluid is empty.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings.
FIG. 1 is a flowchart for explaining the configuration of the embodiment of the present invention, FIG. 2 is a configuration diagram showing the hardware configuration of the embodiment of the present invention, and FIG. 3 explains the operation of the embodiment shown in FIG. FIG. 4 is an operation diagram showing the contents of the operation shown in FIG. 1, and FIG. 5 is an enlarged view showing a part of the waveform diagram shown in FIG.

Hereinafter, embodiments of the present invention will be described with reference to these drawings.

An excitation current _If is supplied from an excitation circuit 27 to the excitation coil 14.

The excitation circuit 27 is configured as follows. Reference voltage E ₁
Is applied to the non-inverting input of amplifier Q ₁ (+), the output end thereof is connected to the base of the transistor Q _2. Inverting input of amplifier Q ₁ together with the emitter of the transistor Q ₂ is connected to the common COM via a resistor R _f (-) to which is connected. It is applied through a series circuit of the excitation voltage switch SW ₄ E _S is connected in parallel to a series circuit of the switch SW ₂ and SW ₃ and SW ₅ between the collector of the common COM and the transistor Q ₂ . The excitation coil 14 is connected to a connection point between the switches SW ₂ and SW _{3 and} a connection point between the switches SW ₄ and SW ₅ . The timing signals S ₂ , S ₃ , S ₄ , S ₅ are connected to switches SW ₂ , S
Controls the opening and closing of W ₃ , SW ₄ and SW ₅ .

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 13. The common mode voltage is removed and the impedance is converted by the preamplifier 13, and the result is output to the output terminal 28.

The output of the preamplifier 13 at the output terminal 28 is converted into a digital signal by an analog / digital converter (A / D _L ) 29 and an analog / digital converter (A / D _H ) 30, respectively.
And stored in a random access memory (RAM) 32 via The read-only memory (ROM) 33 stores a predetermined calculation program and initial data. The calculation is performed according to the calculation procedure stored in the ROM 33 under the control of the processor (CPU) 34, and the result is stored in the RAM 32. Is done.

35 is a clock generator, wherein the generated clock is supplied to the divider 36 by CPU34 and analog / digital converter 30 as a frequency-divided system clock S _h to 1 / n.

The CPU 34 operates on a bus according to the arithmetic program stored in the ROM 33.
The timing for determining the waveform of the exciting current _If is output to the timing signal output port (TO) 37 via the interface 31. The timing signal output port 37 outputs timing signals S ₂ , S ₃ , S ₄ , S ₅ for switching the exciting current according to this timing.

The timing signal output port 37 to sample the output of the preamplifier 13 output a timing signal S _l to the analog / digital converter 29 in accordance with the timing for instructing the CPU 34.

On the other hand, the arithmetic program stored in the ROM 33 causes the RAM 32
A predetermined operation is executed by the CPU 34 using the data stored in the CPU 32. The result of the operation is stored in the RAM 32 and output as a flow rate output to the output terminal 39 via the digital / analog converter 38 via the bus 31. Is done.

Next, the operation of the embodiment shown in FIGS. 1A and 1B will be described with reference to FIGS. FIG. 1 (A) mainly shows a flow rate calculation procedure, and FIG. 1 (B) shows a noise detection procedure.

System clock S _h obtained at the output of the divider 36 shown in FIG. 2 is a waveform shown in FIG. 3 (a), this is CPU34
Is supplied to

In Step 1 of FIG. 1, CPU 34 bus 3 by a predetermined calculation program stored in the ROM33 in synchronism with the interrupt timing of the system clock S _h (FIG. 3 (g))
A timing signal indicating the switching timing of the excitation waveform is output to the timing signal output port 39 via 1.

In step 2, the timing signal output port 37 receives this switching timing, and receives the timing signals S ₅ (FIG. 3 (b)), S ₄ (FIG. 3 (c)), S ₃ (FIG. 3 (d)). ,
S ₂ (FIG. 3 (e)) is connected to the switch SW of the excitation circuit 27, respectively.
₅ , SW ₄ , SW ₃ and SW ₂ are output. Or the timing signal S _4
May be simultaneously output to the switches SW ₃ and SW ₄ , and the timing signal S ₂ may be simultaneously output to the switches SW ₂ and SW ₅ . The excitation circuit 27 receives these timing signals and outputs an excitation current _If having a waveform shown in FIG. As shown in FIGS. 3 (h) and (i), this excitation waveform is a waveform which forms one cycle with timing numbers i of 0 to 15 and repeats this cycle. In FIG. Is shown. This excitation waveform is a multiplication waveform obtained by multiplying a low-frequency waveform and a high-frequency waveform.

Next, the process proceeds to step 3. Steps 3 to 6 show the procedure for reading data from the analog / digital converters 29 and 30.

In step 3, the system clock S _h (FIG. 3 (a))
Analog / digital converter 29 in each cycle in synchronization with
As shown in FIG. 3 (j), data input from
A predetermined data area of the RAM 32 under the control of the CPU 34 through the 31
And stores it in the H _i.

Next, the process proceeds to step 4, where it is determined whether or not the read timing number i is 0. If the read timing number i is not 0, the process proceeds to step 6, and if it is 0, the process proceeds to step 8.

In step 6, it is determined whether or not the read timing number i is 8, and if not, the process proceeds to step 8, and if it is 8, the process proceeds to step 7.

In step 7, the data input from the analog / digital converter 29 is shown in FIG. 3 (l) according to the sample timing based on the timing signal _Sl (FIG. 3 (k)) output from the timing signal output port 37. As bus 31
, L ₀ (n−1), L ₀ (n), L ₀ (n + 1),... In a predetermined data area of the RAM 32 under the control of the CPU 34 via
And the process proceeds to step 5.

Next, in step 6, the timing signal output port 37
The sample timing by the output timing signal S _l (FIG. 3 (k)) from the data inputted from the analog / digital converter 29 of FIG. 3 CPU34 through a bus 31 as shown in (l) Data,..., L ₁ (n-1), L ₁ (n), L ₁ (n +
1),..., And proceed to step 8.

In step 8, it is determined whether or not the timing number i is an odd number. If it is an odd number, the process proceeds to step 9, and if not, the process proceeds to step 11.

Step 9 performs a high-frequency demodulation operation. In demodulation operations, using the data H _i stored in the RAM 32, in Figure 3 column of the high frequency demodulation operation e _Hi shown in FIG. 4 which is stored in the ROM33 in the control of CPU34 at the timing shown in (m) The operation is performed using the operation expression shown below, and the result is stored in the RAM 32. In addition,
In FIG. 4, the constant A is A = Tc, where Tc is a differentiation or integration constant, and ΔTc is a calculation cycle shown in FIG. 3 (f).
It is indicated by / (Tc + ΔTc).

Next, the process proceeds to step 10. Here, the high-frequency side high-frequency wave operation F _Hi is executed.

At the time of the wave operation, the data e _Hi stored in the RAM 32 and the previous wave operation result are used, and the ROM 33 is controlled under the control of the CPU 34.
The calculation is performed using the calculation formula indicating the high-band wave calculation F _Hi column shown in FIG. 4 and stored in the RAM 32.

Next, the process proceeds to step 11. In step 11, it is determined whether or not the timing number i is 0 or 8, and if it is 0 or 8, the process proceeds to step 12, and if it is not 0 or 8, step 14 is performed.
Make a decision to move to

In step 12, a low frequency demodulation operation is performed. In the demodulation operation, the data stored in the RAM 32,..., L ₀ (n−
1), L ₀ (n), L ₀ (n + 1),... L ₁ (n−1), L
_{Using 1} (n), L ₁ (n + 1),..., At the timing shown in FIG. 3 (n), the low frequency demodulation operation e _Li shown in FIG. The operation is performed using the operation expression shown below, and the result is stored in the RAM 32. In FIG. 4, the constant B is represented by B = ΔT / (ΔT + Tc).

In step 13, the low-frequency wave calculation F _Li on the low frequency side is executed.

At the time of the wave operation, the data e _LO stored in the RAM 32,
e Using _LB and the previous wave operation result,
The operation is performed by the operation expression shown in the column of the low-band wave operation F _Li shown in FIG. 4 and stored in the ROM 33, and the result is stored in the RAM 32.

In step 14, it is determined whether or not the timing number i is an odd number. If the timing number i is an odd number, the process proceeds to step 15, and if not, the process proceeds to the relay point A.

In step 15, an addition operation is performed. High-frequency operation result F _Hi and low-frequency operation result F _Li stored in RAM 32
DOO used, the process proceeds to Figure 4 to show the addition operation e The results are stored in the RAM32 relay point and the calculation by the arithmetic expression shown in the section of _A A stored in the ROM33 in the control of the CPU 34.

As described above, the signal voltage including two frequencies of the low frequency and the high frequency detected by the electrodes 11a and 11b is divided into the low frequency side and the high frequency side by using the microcomputer, and is read. The output is demodulated at low frequency through the low pass filter, and the high frequency side is demodulated at high frequency and the output is output via the high pass filter. Is added and synthesized, and the zero point is stable, and it is strong and responsive to flow noise, which is low frequency random noise that increases with increasing flow velocity in slurry fluids and low conductivity fluids. Good flow output can be obtained.

Thereafter, the flow shifts to the noise detection flow shown in FIG. The relay point A is connected to the relay point A shown in FIG.

Figure 5 is read timing T ₀ by the timing signal S _l shown in FIG. 3 (k) (n), T 1 (n), T 0 (n +
1) is a waveform diagram in which each waveform in the vicinity of... Is enlarged, and noise detection will be described together with this enlarged diagram.

In such a two-frequency-excited electromagnetic flowmeter, when the inside of the conduit 10 becomes empty, as shown in FIG. 5 (a), the exciting current _If changes from, for example, a non-excited state to a positively excited state. Is expanded, the excitation current changes at a constant rate. Accordingly, as shown in FIG. 5 (b), the excitation voltage _Vf generates a square wave-like high voltage as a differential waveform in this transition process.

For this reason, as shown in FIGS. 5 (c) and 5 (d), the transition portion of the rising / falling of the exciting current is caused by the stray capacitance or resistance formed between the electrodes 11a and 11b and the exciting coil 14. induced noise N _C, N _R is generated in the electrode in. These induced noises N _C and N _R have a magnitude that also saturates the preamplifier 13 and causes drift. When such a phenomenon occurs, it takes a considerable time to return to the correct output even if the inside of the conduit 10 becomes full after that.

Therefore, sampling the noise voltage in this in order to detect the empty state Figure 5 timing point of a non-excited state of the timing signal S _l, as shown in (f) T ₀ (n). In this period, as shown in FIG. 5 (e), a small amount of the component of the flow signal V _S remains, which is, for example, a value of about ± 10 cm / s in terms of flow velocity. The noise detection shown in (B) is performed.

In FIG. 1 (B), step 16 is a step of judging the timing of sampling the noise signal.
Read timing number i whether the CPU34 is determined 1, noise from i = 1 if A / D _L 29 is a timing number corresponding to a point in time T ₀ (n) shown in FIG. 3 (k) The data is read (step 17), and the noise data N
₀ (n) is stored in a predetermined work area NSUM in the random access memory 32 (FIG. 3 (l)).

Next, proceeding to step 18, the CPU 34 updates the contents of the work area by executing an addition operation of adding the current noise data N ₀ (n) to the noise data stored in the previous work area NSUM. Move to step 19.

If the timing number i is not 1 in step 16, the process proceeds to step 20. In step 20, the CPU 34 determines whether the read timing number i is 9 or not, and FIG.
If i = 9, which is the timing number corresponding to the time point of T ₁ (n) shown in (1), the noise data is read from the A / D _L 29 (step 27), and this noise data N ₁ (n) is stored in the random access memory 32. (FIG. 3 (l)).

Next, proceeding to step 22, the CPU 34 executes an addition operation of adding the current noise data −N ₁ (n) to the noise data stored in the previous work area NSUM to update the contents of the work area. , The timer K is incremented by 1 (step 23), and the routine goes to step 24.

Here, in step 22, as noise data, -N
₁ (n) is added. This is because, as shown in FIG. 3 (f), when i = 1 and i = 9, the direction of change of the exciting current _If is opposite, so that the noise voltage generated is also large. The addition polarity is reversed in consideration of the opposite.

In step 23, the timer K is updated in order to take the average because one time data is affected by slurry noise and the like.

Step 24 determines whether or not the value of the timer K has reached a predetermined maximum value KMAX. About 1 to 10 seconds are adopted as KMAX. When the timer K has not reached the maximum value KMAX, the process proceeds to step 19 to collect more data. Step 25 when the maximum value KMAX is reached
And reset the timer K to prepare for the next time
Move to 26.

Step 26 is where the CPU 34 determines whether the inside of the conduit 10 is empty or full. For example, by using a predetermined calculation program stored in the ROM 33, the data stored in the work area NSUM and KMAX are used to calculate the ratio between them to calculate the average noise data, and the average noise data and the reference are calculated. Compare with data NCMP. As the NCMP, a value corresponding to 15 to 20 m / s, which is larger than a normal flow signal, is selected. As a result, when the reference data NCMP is smaller, it is determined that the water is full, and the process proceeds to step 19;

In step 19, the process waits until the next interrupt timing, and when the next interrupt timing comes, the flow from step 1 is executed again.

As described above, the timing numbers i = 1 and 9 are the periods during which the flow signal on the low frequency side is not sampled and the period corresponding to 1/4 of the high frequency half cycle corresponding to the rising / falling period of the exciting current. Can be performed to determine whether the interior of the conduit 10 is empty.

If it is determined to be empty, normal demodulation is promoted by, for example, reducing the value of the time constant Tc.

When a gain switching circuit is attached to the preamplifier 13, the gain is gradually lowered according to the measured value of the noise so that the gain does not saturate with the noise. In this way, it is possible to prevent a shift between high-frequency / low-frequency input voltages.

<Effects of the Invention> As described above in detail with the embodiment, the present invention calculates a noise value by using a microcomputer in consideration of a calculation timing, calculates the noise value, and compares the calculated noise value with a reference value. This makes it possible to determine whether the inside of the conduit is empty, so that it does not lower the input impedance and does not prepare hardware especially for sky detection, realizing sky detection at low cost can do.

[Brief description of the drawings]

FIG. 1 is a flowchart for explaining the configuration of the embodiment of the present invention, FIG. 2 is a configuration diagram showing the hardware configuration of the embodiment of the present invention, and FIG. 3 explains the operation of the embodiment shown in FIG. FIG. 4 is an operation diagram showing the contents of the operation shown in FIG. 1, FIG. 5 is an enlarged view showing a part of the waveform diagram shown in FIG. 3, and FIG. FIG. 2 is a block diagram illustrating a configuration of an electromagnetic flowmeter. 10 ... conduit, 11a, 11b ... conduit, 13 ... preamplifier, 14
…… excitation coil, 20 …… flow calculation circuit, 21 …… empty detection circuit, 27 …… excitation circuit, 29, 30 …… analog / digital converter, 32 …… random access memory, 33 …… read only memory, 34 ... Processor.

Claims (1)

(57) [Claims] A first frequency and a second frequency lower than the first frequency;
With three different frequencies, positive excitation, negative excitation, and non-excitation
A magnetic field having two excitation states is applied to the measurement fluid, and a high-frequency flow signal having the first frequency component and a low-frequency flow signal having the second frequency component generated by the flow rate of the measurement fluid are sampled. A first analog / digital converter for sampling the high-frequency signal and outputting a first digital signal, and a second digital signal for sampling the low-frequency signal. And a second analog / digital conversion means for outputting the low-frequency flow signal and a quarter of the half cycle of the high-frequency flow signal outside the sampling period of the low-frequency flow signal.
Sampling means for sampling a noise signal in the non-excitation period of the period; empty determination means for comparing the noise signal with a preset noise level and determining that the measured fluid is empty when the noise level is exceeded An electromagnetic flowmeter comprising: