JPS5847215A - Electro-magnetic flow meter - Google Patents

Abstract

PURPOSE:To increase the stability of the zero point and the response by selecting exciting current to a signal generator among three steady values and superposing in the same direction and calculating the noise components in the voltages induced between the electrodes when the signal becomes one of the three steady values. CONSTITUTION:Currents Ia-Ic from the constant current sources 11a-11c of an exciting circuit 1 are changed over by means of switches 12a-12c and given to the exciting coil 21 of an oscillator 2. The voltage induced between electrodes 23a and 23b is given to a signal processing circuits 3 and also to a calculation circuit 33 via an A/C current amplifier and sample holding circuit 32a and 32b. A pulse generator 34 is synchronized with the commercial A/C power and control switches 12a-12c and Sa and Sb. With this arrangement the circuit 33 eliminates noise components that are included in the exciting voltage ea and the offset component due to the D/C voltage and the circuit. Accordingly the voltage Vo that is related only to the signal component that is proportional to the flow rate of a liquid flowing in the pipe 22 can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a low frequency excitation type electromagnetic flow needle.

Generally, an electromagnetic flow needle is configured to apply a magnetic field perpendicularly to the fluid flow direction, simultaneously detect changes in electrical signals in the fluid flow path, and measure the fluid flow rate based on this. Many modern electromagnetic flowmeters use low-frequency excitation methods, such as trapezoidal wave excitation and square wave excitation, which have superior zero point stability compared to AC excitation and DC excitation methods. It is being In a low-frequency excitation type electromagnetic flow needle, the current supplied to the excitation coil is periodically switched between two steady-state values, and the induced voltage generated between the electrodes when the excitation current becomes constant is sampled. By taking the difference between the matched sampling signals, the effects of electrochemical DC potential and circuit-based offset voltage are removed, and a signal corresponding to the fluid flow rate is obtained. Even in such a low-frequency excitation type electromagnetic flowmeter, the zero point will drift if sampling is not performed after a sufficient period of time has passed after the excitation current reaches a certain value. This is due to the induced voltage generated between the electrodes, in addition to a signal component proportional to the fluid flow rate and an offset component due to the electrochemical DC potential and circuit.
This includes electromagnetic coupling noise generated in the loop between the electrode and electrode lead when excitation current is switched, and first-order lag noise generated by the first-order lag circuit formed by the eddy current flowing in the fluid and the liquid resistance and the interfacial electric double layer capacitance of the electrode. The noise component associated with switching the excitation current is superimposed, and the polarity of this noise component reverses each time the excitation current is switched, so it cannot be eliminated even by taking the difference between adjacent sampling signals, and electromagnetic coupling noise can be eliminated in a short time. This is because, although it becomes zero, -th lag noise does not become zero until a sufficient amount of time has elapsed. Therefore, from the point of view of zero point stability, the lower the excitation frequency is, the more advantageous it is.Some of the electromagnetic flowmeters that have been put into practical use are selected to operate at the commercial power frequency, but the excitation frequency cannot be set too low. This can result in slow response or hunting when a control loop is configured. Further, when the excitation frequency is lowered, changes in electrochemical DC potential become a problem, and a new means is required to compensate for this change.

The present invention switches the excitation current supplied to the excitation coil of the electromagnetic flowmeter transmitter between at least three steady values, and when the excitation current reaches each steady value, induces the current between the electrodes of the electromagnetic flowmeter transmitter. If at least two of the voltages are superimposed in the same direction with noise components that occur due to switching of the excitation current, and the difference between these induced voltages is calculated to remove the noise components, the stability of the zero point This is a low-frequency excitation type electromagnetic flowmeter with excellent responsiveness.

FIG. 1 is a connection diagram showing one embodiment of the electromagnetic flow needle of the present invention. In FIG. 11, 1 is an excitation circuit, and a DC constant current source Ha
, jib, He and the current Ia of these constant current sources
, Ib, Ic switches 12a, 1
2b and j3c7, and the current value of the constant current source is selected to have the following relationship: (1)2-upper Ib''+Ic. 2 is an electromagnetic flowmeter transmitter, which includes an exciting coil 21. It includes a pipe 22 through which fluid flows and electrodes 23a and 25b. 3 is a signal processing circuit, which includes an AC amplifier 31 that amplifies the voltage ea induced between the electrodes 23m and 23b of the electromagnetic flowmeter transmitter 2, and two sample and hold circuits 32a and 52b that sample and hold the output eb of the amplifier 31.
and a pulse generator 54 and an arithmetic circuit 53 that calculates the difference between the hold values of the sample and hold circuits 32m and 52b.
A sample and hold circuit 32a, consisting of (
32b) is shown consisting of a sampling switch Sa (8b) and a hold capacitor 0B (Cb) (5). The pulse generator 34 is
A pulse P1 that is synchronized with a 50 Hz or 60 Hz commercial AC power source (not shown) and controls the switches 121 to 12c.
It generates sampling pulses P2@ and P2b that control sampling switches 8m and 8b.

The operation of the present invention configured as described above will be explained below with reference to the time chart of FIG. First, switch 12a~
12c is controlled by drive pulses P, a, p, and c as shown in FIG. Ia during the period T2. when PIb is on. During T1, current Ib is passed from constant current source 11b to exciting coil 21, and during period TsK when Plc is on, current Ic is passed from constant current source 11c to exciting coil 21. Therefore, the excitation coil 21 is supplied with an excitation current Tw having five steady-state values Ia, Ib, and Ic as shown in FIG. 2). In addition, each period TI,
T2 T TM HT4 is the commercial AC power cycle T
When the excitation current Iw is switched by the switches 12a, 12h, and 12c, the excitation current Iw rises due to the time constant due to the inductance and resistance of the excitation coil, and is delayed during the fall portion (4). It reaches a steady value after , but it is not shown in the figure. There is a gap between the electrodes 21a and 23b of the transmitter 2 as shown in Fig. 2 (e).
), an induced voltage ea is generated according to the excitation current 1w. In addition to the signal components v1 to Wig proportional to the flow rate F of the fluid flowing through the pipe 22 due to the induced voltage ea, there is a noise component Vn associated with switching of the excitation current as shown by diagonal lines in FIG.
, ~vn4 and an offset voltage component Vno caused by an electrochemical DC potential or a circuit are superimposed.

Noise components Vnl to Vn4 are electromagnetic coupling noise generated in the loop between the electrode and electrode lead when switching the excitation current, and first-order lag caused by the eddy current flowing in the fluid formed by the liquid resistance and the interfacial electric double layer capacitance of the electrode. It includes first-order lag noise caused by the circuit, and is weighted in the same direction as the signal component during periods T2 and T3, and in the opposite direction to the signal component during periods T, T4. As a result, the value ea+ of the induced voltage during the period T1 when the steady-state value of the excitation current Iw is In is the sum of the signal component Vs+ proportional to the flow rate of the fluid, the offset component Vno, and the negative noise component -Vn+ (Va+ + Vno-Vn+) Then, the value e of the induced voltage during period T2 when the steady value of Tw is rb
a. is the sum of the signal component Va2 proportional to the fluid flow rate, the offset component Vno, and the positive noise component Vn2 (Vs2
+ vno + Vn2), and the steady value of Iw becomes Ic
The value of the induced voltage ea5 during the period TS becomes the sum of the signal component Vs5 proportional to the fluid flow rate, the offset component 0, and the positive noise component Vn5 (Va34vno+Vn5), and the induced voltage during the period T4 when the steady value of 1W becomes Tb. The value of ea
4 is the sum (Vs2
+Vno-Vn4). Note that the noise component vr11
~'v114 may occur with the opposite polarity to that described above depending on how the electrodes 25a and 23b are contaminated, and Vrll and Vn4 may be positive 1c and Vn2 and Vn5 may be negative.

Then, keep the noise component Vn 1 to Vn4 constant, the rise time and fall time of the excitation current, and set the period T1 to T4 as T1=
'r2 = 'r, = T4, and the time Th + ~'f@, for sampling ea1 to ea4 is Ts1
= Ta2 += TsS= Ta4 If K is selected, the sizes will be almost equal, Vn1= Vn2 =
It can be considered that Vn3 = Vn4 = Vn. Therefore, the induced voltage ea generated in the signal processing circuit 3 during the period T5
B and the induced voltage ea2 generated during period T2 (eaB
-ea2), we get (Vs5- Vs2)
Moreover, if the difference (ea4ea+) between the induced voltage ea4 generated in the period T4 and the induced voltage ea1 generated in the period T is found (VI2-Vat), the offset voltage Vno can be removed and the noise component Vn can also be removed. , a signal component Vn proportional to the fluid flow rate can be obtained.

In the signal processing circuit 3 shown in the figure, first, an amplifier 31 amplifies the induced voltage ea from the electromagnetic flowmeter oscillator 2 and supplies it to sample and hold circuits 52a and 32b. The sample bold circuit 32a receives period T,
The voltage eb3 obtained by amplifying the induced voltage ea3 of
2bK, the voltage eb2 obtained by amplifying the induced voltage em2 during the period T2 is held. Sample hold circuit 32
m, hold value eb5 of 32b. eb2 is added to the calculation circuit 35 and the calculation (eb!-eb2) is performed. Therefore, in the arithmetic circuit 53, the substantial K (eaB
-ea2) is not performed, the offset voltage Vno and the noise component Vn are removed, and the output terminal has a voltage V associated only with the signal component proportional to the fluid flow rate.

(7) is obtained. In addition, sample hold circuits 52a, 52
bK By sampling pulses P, α, and P2b as shown by dotted lines in FIG. The offset voltage Vno and the noise component Vn can be similarly removed by holding them and performing the calculation (eb4-eb,) in the performance circuit 53.

In this way, in the present invention, since the noise component that causes the drift of the zero point is effectively removed, the stability of the zero point can be obtained without lowering the excitation frequency and without making K, that is, the response chronic.

The excitation circuit 1, as shown in FIG. voltage wire and triangular wave generation circuit 14
The comparator 15 compares the output & of
By on/off controlling the current supplied from B, the steady value of the excitation current is kept at a constant value determined by the value of the set voltage Er, and the switch (8 ) Drive 12a to 12b and change the value of the set voltage Er from Er1 to E.
r2→E "3→Br2→Erl" and the steady value of the excitation current Iw is changed to Ia→Ib −+ Ic −+ T
It is also possible to sequentially switch λ from b to Ia. In this case, there is an advantage that the power loss of the excitation circuit can be reduced and the efficiency can be improved. Note that the diode D is for releasing the magnetic energy accumulated in the excitation coil 21 when the switch 12 is off, so that the excitation current continues to flow. Also, bile difference amplifier 13
In place of the triangular wave generating circuit 14 and the comparator 15, the fifth
As shown in the figure, a comparator 15 having hysteresis characteristics may be used to form a self-oscillation type to simplify the configuration.

In addition, in the signal processing circuit 3, as shown in FIG. 3, a sample and hold circuit 32c consisting of a 1-sampling switch 8c, a hold capacitor Oc, and an operational amplifier O20 is provided on the output side of the arithmetic circuit 33. As shown in the chart sample ho)'circuit s2a
K pulse P2. eb of the output eb of the amplifier 31 by
5 and eb4 are held alternately, and the sample and hold circuit 32b receives pulses P2bK) to output eb2 and eb,
is held alternately, and the sample hold circuit 32CK
(eb3-eb2) and (eb4-eb, ) of the output ec of the arithmetic circuit 35 are alternately sampled and held by the pulse P2cVC, and a signal related to a signal component proportional to the fluid flow I is output to the output terminal. Output voltage V
You may also extract Q, in this case (ebg
-eb2) and (ey -et+l) are output alternately, which has the advantage of doubling the response compared to the embodiment shown in Fig. 1. Instead of the sandal hold circuits 32a and 32b,
An integrating circuit may be used for each. Further, as shown in FIG. 5, an integrating circuit 35, an inverting amplifier 36, and a changeover switch 37 may be used instead of the sample hold circuit 32m, 521) and the arithmetic circuit 35. The integrating circuit 35 includes a resistor RI, an operational amplifier OP2. It has an integrating capacitor connected to the feedback circuit of OP2, a timing switch '1'S that controls the input integration time, and a reset switch R8 that resets the integrated value. When the selector switch 67 is on the a side, the output eb of the amplifier 51 is sent to the integrating circuit 35.
is added directly, and when on the b side, the output eb of the amplifier 61 is added via the inverting amplifier 36. The changeover switch 37 is connected to the a side during the 'r, , 'r2 periods and to the b side during the T5°T4 period by the pulse PS generated at the timing shown in FIG. 6, and the timing switch T8 is connected to the sixth side.
Each time the sampling pulse P2 'l)' shown in the figure is generated, it is turned on for a certain period of time im, and the output ebt of the amplifier 31 or the output -eb of the inverting amplifier 36 is given to the integrating circuit 55 for a certain period of time. The reset switch R8 is turned on and off by the reset pulse P4 shown in FIG.
Reset 5.

Therefore, as shown in FIG. 6, the integrating circuit 55 calculates the difference between the integral value of eb and the integral value of eb2, and the difference between the integral value of eb4 and the integral value of eb,
K alternately outputs the difference between the integral values of
(em3-em2) and (em4-ea+) are calculated alternately to remove noise components and offset components.

Further, the signal processing circuit 3 may be configured using a fibroprocessor 38 as shown in FIG. In FIG. 7, the output ec of the integrating circuit 35 is converted into a digital signal by the A/D converter 39m and given to the microprocessor 3B, and after the desired processing is performed by the microprocessor 58, the analog output voltage VD is Necessary (11) If necessary, it is outputted via the D/A converter 59b.

In this case, the microprocessor 58 uses the integrating circuit 35 to calculate (eb3-eb2) and (eb, -el) as in FIG.
g) may be converted into a digital signal by the A/D converter 39a and provided as a digital signal, but as shown in the time chart of FIG.
The values obtained by integrating s and eb4 individually by the integrating circuit 35 are converted into digital signals by the A/D converter 39a and provided, and the microprocessor 38 essentially converts them into digital signals (eb, eb4, etc.).
-eb2) and (eb4-ebl) may be alternately digitally operated to remove the offset voltage and noise component and extract a signal component proportional to the fluid flow rate. e
b5+eb, -eb2-ebl) may be performed. At this time, the reference voltage V'
Pulse P! As shown in FIG. 8, the value integrated over a certain period of time Ii is also sent to the microprocessor 38 via the A/D converter 39a through the switch Td driven by the switch Td, which calculates a signal component proportional to the flow rate of the fluid. If the obtained result is divided, the characteristics of the integrating circuit 55 will not be affected. In FIG. 7 (12), a voltage/frequency converter may be used instead of the integrating circuit 350, a counter may be used instead of the A/D converter 391I, and a voltage/pulse converter may be used instead of the integrating circuit 35. A width conversion circuit may be used, and a reference clock and a counter may be used in place of the A/D converter 39a.
When a double-integrating A/D converter is used, the integrating circuit 35 can be omitted. Further, in FIG. 7, a case is illustrated in which the set voltage E' of the excitation circuit 1 is applied from the microprocessor 38 via the D/person converter 17 as shown in FIG. 8(A).

In the above explanation, the case where the steady-state value of the excitation current Iw is sequentially switched from Ia to 1+IC-+Xb to Ia was exemplified, but when only (e113-em2) is obtained, the period T
4 may be omitted and K may be used as shown in Figure 9 (a) (, (
If only emjear) is to be obtained, the period T2 may be omitted as shown in FIG. 9(b). In this case, there is an advantage that the excitation period T can be shortened. Further, power consumption may be reduced by setting any one of the three steady values In, Ib, and Ic of the excitation current Iw to zero to provide a period in which no excitation current flows. For example, in the waveform of the excitation current shown in FIG. 10 (-f), Ta is set to zero, Te=2Ib is selected,
-ea2) or (eli-eal), the exciting current waveform in Fig. 10 (b) sets Ib to zero,
And when Ic=2Ib is selected and (eaB-eli) is obtained, the waveform of the excitation current in Figure 10 (c) is when Tc is set to zero and Ib=21a is selected to obtain (eli-eag). . The waveform of the excitation current (in Fig. 10) is
■ Set 11 to zero and also set period T4 to zero (eaB
This is a case where L7 and power consumption are further reduced so as to obtain -ea2). In the above description, the case where the excitation current 1w has five steady values, Ia, Tb, and re, has been described, but there may be three or more, and the steady value may be negative. Furthermore, although the case of a rectangular wave is illustrated as the excitation current Iw, various waveforms can be used as necessary, such as a trapezoidal wave or a waveform obtained by converting the frequency of a commercial AC power source and then rectifying it. .

As explained above, in the present invention, the excitation current supplied to the excitation coil of the electromagnetic flowmeter transmitter is switched between at least three steady values, and when the excitation current reaches each steady value, the electromagnetic flowmeter transmitter The noise component caused by switching the excitation current is superimposed on at least two of the voltages induced between the electrodes in the same direction, and the difference between the induced voltages is substantially calculated to eliminate the noise. -7 so as to remove the components, so a low frequency excitation type electromagnetic flowmeter with excellent zero point stability and responsiveness can be obtained. Therefore, if a control loop is configured using the electromagnetic flowmeter of the present invention, the control loop will be stabilized without hunting.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing one embodiment of the electromagnetic flowmeter of the present invention, Fig. 2 is an explanatory diagram of its operation, Fig. 3 is a connection diagram showing another embodiment of the electromagnetic flowmeter of the present invention, and Fig. 4 is a connection diagram showing an embodiment of the electromagnetic flowmeter of the present invention. Diagram explaining its operation, 5th
Figure 6 is a connection diagram showing another embodiment of the electromagnetic flowmeter of the present invention.
7 is a connection diagram of still another embodiment of the electromagnetic flowmeter of the present invention, FIG. 8 is an explanatory diagram of its operation, and FIG.
and [1VFi] is an explanatory diagram of another waveform of the excitation current at the electromagnetic flow rate of 1ttK of the present invention. 1... Excitation circuit, 2... Electromagnetic flow meter transmitter, 5...
Signal processing circuit, 32a, 32b, S2c... Sample hold circuit (15) circuit, 33... Arithmetic circuit, 35... Integrating circuit, 58...
...Microprocessor, 39a...A/D converter. (17) -125-(16) (Ino indignation /L q chant Cron fX fu'- (Ino 嬶 (Ron (ni)

Claims (1)

[Claims] means for switching the excitation current supplied to the excitation coil of the electromagnetic flowmeter transmitter between at least three steady values; An electromagnetic flow needle characterized by having means for detecting at least two induced voltages in which noise components generated due to switching of excitation current are superimposed in the same direction, and substantially calculating the difference between the two induced voltages.