JPH0541379Y2 - - Google Patents

Description

[Detailed description of the invention] <Industrial field of application> The present invention relates to an electromagnetic flowmeter that applies a magnetic field to a fluid to be measured and measures its flow rate, and particularly concerns its excitation method and accompanying signal processing method. This invention relates to an improved electromagnetic flowmeter.

<Prior Art> Industrial electromagnetic flowmeters have conventionally adopted a commercial frequency excitation method that uses a commercial power source to excite the flowmeter. The commercial frequency excitation method has (a) fast response speed and low cost. (b) It has the advantage of being less susceptible to the effects of low-frequency random noise (hereinafter referred to as flow noise) that increases with flow velocity that occurs in slurry fluids and low conductivity fluids, but However, it has the disadvantage that the zero point will fluctuate if left unattended for a long period of time, for example, about one day.

For this reason, a low frequency excitation method that excites at a low frequency of 1/2 of the commercial frequency or lower has been adopted. As is well known, the low frequency excitation method has the advantage of providing an electromagnetic flowmeter with a stable zero point. However, since the excitation frequency is low, it is close to the frequency of flow noise, and is therefore susceptible to the influence of flow noise, and this influence becomes particularly noticeable as the flow velocity increases. Furthermore, when damping is applied to reduce the influence of flow noise, the response becomes slow.

Therefore, as proposed in Japanese Patent Application No. 60-197168 (title of invention: electromagnetic flowmeter), a high-frequency excitation current component having a commercial frequency or higher frequency and a low-frequency excitation current component having a frequency lower than this are proposed. A two-frequency excitation method has been proposed in which a two-frequency magnetic field is formed by simultaneously causing a current component to flow through an excitation coil.

In addition, such an electromagnetic flowmeter generates a smaller electromotive force than a conventional commercial frequency electromagnetic flowmeter from the viewpoint of energy saving. For this reason, the amplification degree of the converter that processes the signal from the detector is increased compared to the conventional one.

<Problems to be solved by the invention> However, although this proposal has the advantage of having a stable zero point, strong resistance to flow noise, and good response, it is less prone to generation than conventional commercial frequency electromagnetic flowmeters. Even though the power is decreasing, the level of slurry noise remains the same, making it easy for the converter to saturate. When the converter is saturated, there is a problem in that the output fluctuates and becomes significantly different from the actual flow rate.

<Means for Solving the Problems> In order to solve the above problems, this invention provides excitation means for supplying magnetic fields having two different frequencies, a first frequency and a lower second frequency;
a first demodulating means for discriminating and outputting a signal voltage excited by the exciting means and generated corresponding to the flow rate based on the first frequency; and a high-pass filtering means for high-pass filtering the output of the first demodulating means. , a second demodulating means for discriminating and demodulating the signal voltage based on a second frequency, a low-pass filtering means for low-pass filtering the output of the second demodulating means, and each output of the high-pass filtering means and the low-pass filtering means. In an electromagnetic flowmeter, the electromagnetic flowmeter has a synthesis means that performs algebraic synthesis using Compensating amplification means for compensating for changes in amplification caused by changes in the amplification of the means, and noise detection means for detecting changes in noise included in the signal voltage, the variable amplification means and compensation are provided by the output of the noise detection means. The amplification degree of the means is controlled.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention.

10 is a conduit for a detector of an electromagnetic flowmeter, and an insulating lining is provided on the inner surface of the conduit. 11
a and 11b are electrodes for detecting a signal voltage. Reference numeral 12 denotes an excitation coil, and a magnetic field generated thereby is applied to the fluid to be measured. An excitation current I _f is supplied to the excitation coil 12 from an excitation circuit 13 .

The excitation circuit 13 is configured as follows. The reference voltage E ₁ is applied via the switch SW ₁ to the non-inverting input (+) of the amplifier Q ₁ , the output of which is connected to the base of the transistor Q ₂ . The emitter of transistor Q ₂ is connected to common via resistor R _f
It is connected to COM and to the inverting input terminal (-) of amplifier _Q1 . An excitation voltage E _s is applied between the common COM and the collector of the transistor Q ₂ through a series circuit of switches SW ₂ and SW ₃ and a series circuit of switches SW ₄ and SW ₅ connected in parallel to this. . The excitation coil 12 is connected to the connection point of the switches SW ₂ and SW ₃ and the connection point of the switches SW ₄ and SW ₅ , respectively. Timing signals S ₁ , S ₂ , and S ₃ control the opening and closing of switches SW ₁ , SW ₂ and SW ₅ , and SW ₃ and SW ₄ , respectively.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 14. preamplifier 1
4, the common mode voltage is removed and the impedance is converted, and the resultant signal is outputted to a coupling point 16 via a variable amplifier 15.

The signal voltage at node 16 is applied via switch SW ₇ or via a series circuit of inverting amplifier Q ₃ and switch SW ₈ to a low-pass filter 17, each with a small time constant.

Also, the signal voltage at the connection point 16 is
via SW ₉ or with inverting amplifier Q ₄
The signals are applied via a series circuit of SW ₁₀ to low-pass filters 18 each having a small time constant. The switches SW ₇ , SW ₈ , SW ₉ , and SW ₁₀ receive timing signals S ₇ , S ₈ , S 9 , and S ₉ from the timing circuit 19, respectively.
Opens and closes in S ₁₀ . The low-pass filter 17 is passed through a low-pass filter 20 with a large time constant, and the output of the low-pass filter 18 is passed through a series circuit of a variable gain amplifier _Q5 and a high-pass filter 21 with a large time constant to an adder 22. is added. Adder 2
The output of 2 is the compensation amplifier 23 and the low pass filter 2.
4 to the output terminal 25 as a signal voltage V ₀ .

In addition, the variable amplifier Q ₅ is a low pass filter 20
This is to adjust the magnitude of the output voltage V _L of the high-pass filter 21 and the output voltage V _H of the high-pass filter 21 to be equal in magnitude.

On the other hand, the output of the preamplifier 14 is detected by a noise detection circuit 26, and the output of the noise detection circuit 26 is used to control the variable amplifier 15.
and the corresponding amplification degree of the compensation amplifier 23 are controlled to be constant.

The variable amplifier 15 is configured as follows. Preamplifier 1 is connected to the inverting input (-) of amplifier Q ₆ , whose non-inverting input (+) is connected to the common potential point COM.
The output end of 4 is connected via a resistor R _i , and between the output end there are a series circuit of a resistor R ₁ and a switch SW ₁₂ , a series circuit of a resistor R ₂ and a switch SW ₁₃ , and a series circuit of a resistor R ₃ and a switch SW 12. The series circuits of switch SW ₁₄ are connected in parallel. These switch SW ₁₂ ,
SW ₁₃ and SW ₁₄ are switched by control signals S ₁₂ , S ₁₃ and S ₁₄ from the noise detection circuit 26, respectively.

The noise detection circuit 26 is configured as follows.
The absolute value of the output of the preamplifier 14 is calculated in an absolute value circuit 27 and outputted to a low-pass filter 28 .
The output of the low pass filter 28 is connected to the reference voltage at one end of the input.
The voltage is applied to the other end of the comparator Q ₇ to which E ₅ is applied, and to the other end of the comparator Q ₈ to which the reference voltage E ₆ is applied to one input end, and its magnitude is determined. The outputs of the comparators _Q7 and _Q8 are applied to each input terminal of the NOR gate _Q9 , and the NOR of these is calculated and the control signal _S14 is outputted to the output terminal. Also, Noah Gate
The outputs of the comparator _Q7 and the output of the comparator _Q8 inverted by the inverter _Q11 are applied to each input terminal of Q10, and the NOR of _these is calculated by the NOR gate _Q10 and its output is It outputs a control signal _S13 at the end. Furthermore, the control signal S ₁₂ is obtained as the output of the comparator Q ₇ . These control signals detect the magnitude of noise generated at the output end of the preamplifier 14, and change the amplification degree of the variable amplifier 15 by switching each switch according to the magnitude.

Further, the compensation amplifier 23 is configured as follows in the same way as the variable amplifier 15. Amplifier with non-inverting input terminal (+) connected to common potential point COM
The output terminal of the preamplifier 14 is connected to the inverting input terminal (-) of Q ₆ ′ via a resistor R _i ′, and a series connection of a resistor R ₁ ′ and a switch SW _{12 ′ is connected to the inverting input terminal (−) of Q 6} ′. A series circuit of a resistor R ₂ ′ and a switch SW ₁₃ ′, and a series circuit of a resistor R ₃ ′ and a switch SW ₁₄ ′ are connected in parallel. , these switches
SW ₁₂ ′, SW ₁₃ ′, and SW ₁₄ ′ are switched by control signals S ₁₂ , S ₁₃ , and S ₁₄ from the noise detection circuit, respectively.
By these switches, the compensation amplifier 23 is varied so as to compensate for the amplification degree of the variable amplifier 15,
The constants of the elements are selected so that the overall amplification is kept constant. In this way, the degree of amplification is changed according to the magnitude of the noise to prevent saturation of the circuit due to noise.

Next, the operation of the electromagnetic flowmeter shown in FIG. 1 will be explained with reference to the waveform diagram shown in FIG. 2.

The timing signal S ₁ repeats on/off as shown in FIG. 2A, thereby controlling the application of the reference voltage E ₁ to the non-inverting input terminal (+) of the amplifier Q ₁ . On the other hand, timing signals S ₂ (Figure 2 B) and S ₃
(Figure 2 C) switches SW ₂ and SW ₅ at low frequency,
And switches SW ₃ and SW ₄ are turned on alternately, so the low frequency (period: 2t) as shown in Figure 2 D is generated.
and high frequency (period: 2T) are combined excitation current I _f
flows.

The signal voltage at the node 16 obtained via the variable amplifier 15 is sampled by the timing signals _S7 and _S8 shown in FIG. 2E and F, so that the voltage shown in _FIG . Get on the side. A voltage obtained by smoothing this with a low-pass filter 17 is obtained on the output side of the low-pass filter 20.

Furthermore, the signal voltage at the coupling point 16 is changed to the timing signal S ₉ , at the timing shown in FIG.
Since it is sampled by S ₁₀ , the switch
A signal voltage shown _{in FIG} .

Each signal voltage added at the addition point 22 is smoothed by a low-pass filter 20 via a compensation amplifier 23 and output to the addition point 22.

On the other hand, the output of the preamplifier 14 contains, for example, slurry noise, and the switch is activated according to the logic in which the magnitude of this slurry noise N is determined by the relationship between the magnitudes of the reference voltages E ₅ and E ₆ (E ₅ ≠ E ₆ ). SW ₁₂ ,
The amplification degree of the variable amplifier 15 is changed by switching SW ₁₃ and SW ₁₄ . At the same time, the amplification degree of the compensation amplifier 23 is changed to have a reciprocal relationship with this, thereby ensuring the same amplification degree as a whole.

In this way, when the noise is large, the amplification degree is changed according to the noise level even if some accuracy is sacrificed, and the saturation of the circuit due to noise is prevented, and when the noise is small, the normal amplification degree is ensured and the overall Ensure stable output.

Although the above configuration is based on a discrete circuit, the configuration is not limited to this and may be configured using a microcomputer, for example.

<Effects of the invention> As explained above in detail with the embodiments, according to the invention, the magnitude of noise included in the signal voltage is detected and the distribution of the overall amplification degree is changed according to this magnitude. However, since this prevents the circuit from saturating, it is possible to ensure a stable output against noise. In particular, in the two-frequency excitation method, a circuit with a large time constant is inserted, and when the circuit is saturated, it takes a considerable amount of time to return to normal, so the adoption of the present invention is highly effective.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention.
FIG. 2 is a waveform diagram illustrating the operation of the embodiment shown in FIG. 1. 10... Conduit, 12... Excitation coil, 13...
Excitation circuit, 14... Preamplifier, 15... Variable amplifier, 19... Timing circuit, 20... Low pass filter, 21... High pass filter, 23...
...Compensation amplifier, 26...Noise detection circuit, 27...
...Absolute value circuit.

Claims (1)

[Scope of utility model registration request] excitation means for supplying magnetic fields having two different frequencies, a first frequency and a second frequency lower than the first frequency; a first demodulating means for separately outputting the output; a high-pass filtering means for high-pass filtering the output of the first demodulating means; a second demodulating means for discriminating and demodulating the signal voltage based on the second frequency; In an electromagnetic flowmeter, the electromagnetic flowmeter has a low-pass filtering means for low-pass filtering the output of the two demodulating means, and a synthesizing means for algebraically synthesizing the outputs of the high-pass filtering means and the low-pass filtering means,
variable amplification means for variably amplifying the signal voltage and outputting it to the first and second demodulation means; and compensation provided at a subsequent stage of the combining means to compensate for changes in amplification caused by changes in amplification of the variable amplification means. It is characterized by comprising an amplifying means and a noise detecting means for detecting a change in noise contained in the signal voltage, and controlling the amplification degree of the variable amplifying means and the compensating means by the output of the noise detecting means. Electromagnetic flowmeter.