JPS58120118A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To improve the stability and response at a zero point, by sampling the voltages induced across electrodes for an exciting period and a rest period twice or more, and computing and compensating the noise components included in the induced voltages. CONSTITUTION:An electromagnetic flowmeter oscillator 2 is provided with an excitor coil 21, a pipe 22 wherein fluid flows, and the electrodes 23a and 23b. A constant current is supplied from a constant current power source 11 through switches 12a and 12b. The voltages ea induced across the electrodes 23a and 23b of the oscillator 2 are amplified 31, and its outputs eb are sampled 32. Its outputs eb are converted into digital signals 33. The voltages induced across the electrodes for the exciting period and for the rest period are sampled twice or more, respectively, and the noise components accompanied by the switching of the exciting current are computed. Then the noise components included in the voltages induced across the electrodes during the exciting period are compensated.

Description

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

Generally, an electromagnetic flowmeter is configured to apply a magnetic field perpendicular to the direction of fluid flow, simultaneously detect changes in electrical signals in the fluid flow path, and measure the fluid flow rate based on this. . Most 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 father-current excitation and DC excitation methods. It is being In a low-frequency excitation type electromagnetic flowmeter, the current supplied to the excitation coil is periodically switched between two steady-state values, and when the excitation current becomes constant, the induced voltage generated between the electrodes is sampled once each. After that, by taking the difference between adjacent sampling signals, the effects of electrochemical DC voltage 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,
If sampling is not performed after a sufficient period of time has passed after the excitation current reaches a certain value, the zero point will drift. This is due to the induced voltage generated between the electrodes, a signal component proportional to the fluid flow rate, an electrochemical DC voltage, an offset voltage caused by the circuit, and an electromagnetic component generated in the loop between the electrode and electrode lead when the excitation current is switched. Coupling noise and eddy current flowing in the fluid Electromagnetic coupling noise and eddy current noise between liquid resistance and electrodes reverse polarity every time the excitation current is switched, so they cannot be canceled even if the difference between adjacent sampling signals is taken, and the electromagnetic This is because although coupled noise becomes zero in a short time, eddy current noise does not become zero until a sufficient amount of time has elapsed. Therefore,
Considering the stability of the zero point, the lower the excitation frequency is, the more advantageous it is, and some electromagnetic flowmeters in practical use use +/32iC of the commercial power frequency. However, if the excitation frequency was too low, the response became slow.
Hunting occurs when a control loop is constructed. Furthermore, when the excitation frequency is lowered, electrochemical changes in DC voltage become a problem, and new means are required to compensate for these changes.

The present invention provides an excitation period in which an excitation current is passed through the excitation coil and a rest period in which no excitation current is passed, and the voltage induced between the electrodes is sampled two or more times each during the excitation period and the rest period. Calculate the noise component L,
By compensating for the noise component included in the voltage induced between the electrodes during the excitation period, a low-frequency excitation type electromagnetic flowmeter with excellent zero point stability and responsiveness has been realized.

FIG. 1 is a connection diagram showing an embodiment of the electromagnetic current vibration meter of the present invention. In the figure, 1 is an excitation circuit, and a switch 1 switches between a DC constant current source 11 and a constant current source from the constant current source 11.
2a and 12b. 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 23b. 3 is a signal processing circuit, which includes an AC amplifier 31 that amplifies the voltage ea induced between the electrodes 23 and 23b of the electromagnetic flowmeter transmitter 2, and an output e of the amplifier 31.
a switch 32 for sampling the amplifier output e sampled by the switch 32, an A/D converter 33 for converting the amplifier output e sampled by the switch 32 into a digital signal, and a desired digital operation based on the digital signal from the A/D converter 33. a D/A converter 35 that converts the output of the microprocessor 34 into an analog (3) signal;
It has a sample and hold circuit 36 that samples and holds the output of and generates an output voltage e0. The microprocessor 34 performs digital calculations and also generates a pulse Pla'' that drives the switches 12a and 12b of the excitation circuit 1.
lb' Generates pulses p21 to P3 that control the sampling switch 32 and sample hold circuit 36.

The operation of the present invention configured in this way will be explained below with reference to the waveform diagram of FIG. 2. First, the switches 12a and 12b are controlled by drive pulses p P as shown in FIGS.
During the interval T, the current flows from the constant current source 11 in the positive direction, and during the period T4 when Plb is on, the current I flows from the constant current source 11.
is switched in the opposite direction to flow through the excitation coil 21, and during the period T□+T3 when both pp are off, la'
lb No current is applied to the excitation coil 21. Therefore, excitation coil 2
1 has a rest period TT in which the steady-state value is zero, a positive period T and a negative period T4, as shown in FIG. 2 (c).
2. Exciting electric current is supplied. Note that each period Ti, T2゜(4) T3. T4 is selected to be an integer multiple of the commercial AC power supply period, and the excitation currents are selected as switches 12a and 12b.
When the current is switched, there is actually a delay in the rising and falling portions due to the time constant due to the inductance and resistance of the excitation coil 21, and then the steady value is reached, but this is not shown in the figure. As shown in FIG. 2, an induced voltage e is generated between the electrodes 23a and 23b of the electromagnetic field oscillator 2 in accordance with the excitation current. In addition to a signal component v8 proportional to the flow rate F of fluid flowing through the pipe 22, the induced voltage ea includes a noise component vn associated with switching of the excitation current, and an offset voltage component V due to electrochemical DC potential and circuits. , are superimposed.

The noise component V is the electromagnetic coupling noise generated in the loop between the electrode and the electrode lead when switching the excitation current, and the first-order lag caused by the eddy current flowing in the fluid formed by the liquid resistance R and the interfacial electric double layer capacitance C of the electrode. 1, which includes eddy current noise generated by the circuit, and as a result, the sampling voltages ea1 to e when sampling the induced voltage e twice in each period are as shown by the diagonal lines in Figure 2) during one cycle. If the fluid flow does not change slightly, 8, then each is given by the following equation.

When the excitation current is constant, it decreases almost exponentially (eoR). Therefore, if the sampling interval Δt of each period is constant, vnl, vn2. vn3. During vn4, even if the falling and rising waveforms due to excitation current switching are different, the following relationship holds true.

Δt Therefore, if the signal processing circuit 3 calculates the following equation, the noise component vnIIvn3. A compensation value enl 'f ``n3'eof'' corresponding to the offset components V and K can be calculated.

Therefore, the sampling voltage ea3"a4'"a7
' Calculate the following equation every time ea8 is sampled.9, the offset voltage component vof and the noise component vn3 due to switching of the excitation current. The influence of Kvn3 can be removed, and only the signal component V proportional to the fluid flow rate can be obtained (7). Hikoo offset voltage component V. , the compensation value e corresponding to . If f is not calculated in advance, the following calculation may be performed.

In the signal processing circuit 5 shown in the figure, after the induced voltage e from the electromagnetic flowmeter transmitter 2 is amplified by the amplifier 31, sampling pulses p2 are generated at regular intervals Δt as shown in FIG.
The sampling switch 32 driven by the second
Sampling pressure of e indicated by diagonal lines in the figure) ~ eI
The outputs eb□ to eb8 of the amplifier 31 corresponding to IL8 are sequentially applied to the A/D converter 33, converted into digital signals, and applied to the microprocessor 34. Using the digital signal given from the A/D converter 33, the microprocessor 34 first performs a digital operation corresponding to equation (5) to calculate a compensation value, and uses this compensation value to determine whether the excitation current flows. Each time a digital signal corresponding to the sampling voltage ea3' (8) ea4'``a7'' ea8 obtained during the period T2T T4 is input, correction calculations corresponding to equations (4) and (5) are performed by digital calculation. , the offset voltage component vof and the noise component vn are removed, and digital values corresponding to the signal component V proportional only to the fluid flow rate are sequentially output. ea1~ea8
The digital signals corresponding to are stored in dedicated registers, and their values are held until the next cycle's signal is input. The calculated compensation value is also stored in a dedicated register, and the value is stored in the pause period T1. T3
The sampling voltage ea1. '``a2'' a5
' ``Every time the digital signal corresponding to a6 is input, the calculation corresponding to equation (3) is performed and updated.In this case, if the moving average value from the past is used as the compensation value of en1, en3, and k, the calculation The output of the microprocessor 34 is converted into an analog signal by the D/A converter 35, and is sequentially applied to the sample and hold circuit 36 by the pulse P3 generated at the timing shown in FIG.

As a result, an output voltage e corresponding to a signal component V which is proportional only to the flow rate of the fluid is obtained at the output of the sample and hold circuit 36.

In this way, in the present invention, the cause of zero point drift and the noise component are effectively removed, so zero point stability can be obtained without lowering the excitation frequency, that is, without sacrificing response. .

In addition, in the above, Offsera)! Pressure component V. , is constant; however, due to fluctuations in the electrochemical DC potential, V
When the induced voltage e
The sampling voltages ea1 to ea8 are "al = v
However, if the signal processing circuit 3 essentially calculates the following equation, the noise component vn becomes "n3' offset voltage component V. 0. Change amount of offset voltage component Δ”of and K
Compensation value corresponding to "nl'en3'eof'J"
of” can be calculated.

Therefore, by calculating the following equation between these compensation values and the sampling values ea3'``a4''a7' ea8 of equation (6), the offset voltage component and noise component can be effectively removed, and the result is proportional to the fluid flow rate. signal component vs□~v
Only s4 can be obtained.

Further, in the above description, the case where the output eb of the amplifier 31 is supplied to the A/D converter 33 via the sampling switch 32 has been exemplified, but as shown in FIG. It may also be applied to the D converter 33. In this case, if the integration time T6 is selected to be an integral multiple of the commercial power supply cycle, the influence of power supply frequency noise can be removed. Furthermore, the third
In the figure, the integrator 37 includes a resistor RI, an operational amplifier OP, an integrating capacitor C□ connected to the feedback circuit of OP, a timing switch TS that controls the input integration time, and a An example is shown in which a reset switch R8 for resetting the integral value is removed. Furthermore, although sampling is performed twice in each of the excitation period and rest period in the above description, the same result can be achieved even if sampling is performed six times or more. In addition, in the above, the excitation waveform is set to zero, positive,
Although the case of zero/negative repetition 9 has been illustrated, it may be positive/negative/zero repetition or negative/positive/zero repetition. In addition, in the above, the sampling interval was exemplified as the fathom interval, but
They do not necessarily have to be equally spaced.

As explained above, in the present invention, an excitation period in which an excitation current is passed through the excitation coil and a rest period in which no excitation current is passed are provided, and the voltage induced between the electrodes 1c is sampled two or more times each during the excitation period and the rest period. The noise component included in the induced voltage is calculated and the noise component included in the voltage induced between the electrodes during the excitation period is compensated for. A flow meter is obtained.

[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 operating waveform diagram thereof, and FIG. 3 is a connection diagram showing another embodiment of the electromagnetic flowmeter of the present invention. 1... Excitation circuit, 2... Electromagnetic flowmeter transmitter, 21.
...Exciting coil, 23a, 23b...Electrode, 3...
- Signal processing circuit, 31... amplifier, S2... sampling switch, 33... A/D converter, 34...
Microprocessor, 35...D/A converter, 36.
...sample hold circuit, 37...integrator.

Claims (1)

[Claims] In an electromagnetic flowmeter using a low frequency excitation method, which has an excitation period in which a positive excitation current and a negative excitation current are passed through the excitation coil of the electromagnetic flowmeter transmitter, and a rest period in which no excitation current is passed, The voltage induced between the electrodes of the electromagnetic flowmeter transmitter is sampled two or more times each to calculate the noise component associated with switching the excitation current, and the noise component included in the voltage induced between the electrodes of the electromagnetic flowmeter transmitter during the excitation period is calculated. An electromagnetic flowmeter characterized by being equipped with a signal processing circuit that performs calculations to compensate for noise components associated with switching of excitation current.