JPH06103207B2 - Electromagnetic flow meter - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electromagnetic flow meter for applying a magnetic field to a fluid to be measured and measuring its flow rate, and particularly to its excitation method and its accompanying signal processing method. The present invention relates to an improved electromagnetic flow meter.

<Prior Art> An industrial electromagnetic flow meter has conventionally adopted a commercial frequency excitation method in which a commercial power source is used for excitation. The commercial frequency excitation method (a) has a fast response speed and can be manufactured at low cost.
(B) It has the advantage that it is not easily affected by low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated by a slurry fluid or low-conductivity fluid, but it is relatively In addition, the zero point fluctuates if left unattended for a long period of time, for example, about one day.

For this reason, the low frequency excitation method, which excites at a low frequency of 1/2 or less than the commercial frequency, has been adopted. As is well known, the low frequency excitation method has an advantage that a stable electromagnetic flowmeter having a zero point can be obtained. However, since the excitation frequency is low, it is close to the frequency of the flow noise, so that it is easily affected by the flow noise, and this influence becomes remarkable especially when the flow velocity becomes large.

Therefore, it is possible to apply damping with a large time constant in order to remove the influence of this flow noise.

<Problems to be Solved by the Invention> However, when damping is applied with a large time constant, there occurs a problem that the response to a change in the flow rate becomes slow.

<Means for Solving the Problems> In order to solve the above problems, the present invention performs a composite excitation in which an exciting current having a high frequency and a frequency lower than the high frequency excites, and Response detection / amplification means for detecting the deviation between the signal related to either the low frequency demodulation output or the flow rate output and the high frequency signal of the signal voltage, and the response correction of the low frequency output by the output of this response detection / amplification means. And a response correcting means for performing the operation.

<Examples> Examples of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention.

Reference numeral 10 is a conduit of a transmitter of the electromagnetic flow meter, which has an insulating lining on its inner surface. 11a and 11b are electrodes for detecting a signal voltage. Reference numeral 12 is an exciting coil, and the magnetic field generated by this is applied to the fluid to be measured. A constant current of commercial frequency is supplied from the commercial constant current source 14 to the exciting coil 12 via the resistor 13, and at the same time, to the exciting coil 12 from the low frequency constant current source 16 via the resistor 15, for example, about 50/8 Hz. The low-frequency constant current of is superposed. As a result, the magnetic field having two different frequencies of the commercial frequency and 1/8 of the commercial frequency is applied to the fluid to be measured.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b, and the preamplifier
Output to 17. The preamplifier 17 removes the common mode voltage and converts the impedance, and outputs the result to the coupling point 19 via the output terminal 18.

At the connection point 19, the deviation between the output of the preamplifier 17 and the output of the multiplier 20 is taken, amplified by the amplifier 21, and synchronously rectified or sample-held by the demodulator 22 including a low-pass filter having a small time constant. The smoothed DC output is voltage /
It is converted into a pulse frequency signal having a constant pulse width by the frequency converter 23, is fed back to the multiplier 20, and is output to the low-pass filter (low-pass filter) 24 having a large time constant to be smoothed and its output V _L Is output to the response correction point 25. The multiplier 20 is composed of, for example, a switch. A low-frequency comparison voltage generated across the resistor 15 is applied to one end of the switch, the switch is opened / closed by the output pulse of the voltage / frequency converter 23, and the voltage generated at the other end is connected to the connection point 19
Output to. Further, the demodulator 22 is applied with a low-frequency comparison voltage from the resistor 15.

The amplifier 21, the demodulator 22, the voltage / frequency converter 23, the low-pass filter 24, and the multiplier 20 constitute a low-frequency signal processing system 26 for processing a low-frequency signal voltage, and the flow rate of the fluid to be measured. The signal corresponding to the low frequency excitation is processed and output to the response correction point 25 as the output V _L. This low frequency signal processing system 26
The time constant at is a large time constant of the low-pass filter 24,
Keep the response slow.

On the other hand, at the output terminal 18 of the preamplifier 17, the low frequency signal processing system 26
A commercial frequency signal processing system 27 is connected in parallel with the.

The deviation between the output voltage of the output terminal 18 of the preamplifier 17 and the output voltage of the multiplier 28 is taken at a connection point 29 and amplified by an amplifier 30.
The output of the amplifier 30 is synchronously rectified using the commercial frequency comparison voltage generated in the resistor 13 in the demodulator 31 as a reference voltage, or is sampled and held to be a smoothed DC voltage. This DC voltage is converted into a pulse frequency signal having a constant pulse width by the voltage / frequency converter 32 and fed back to the multiplier 28. The output voltage of the voltage / frequency converter 32 is smoothed by the low-pass filter 33 having a small time constant to obtain the output V _H as a DC voltage, which is applied to one end of the input of the response detection amplifier 34.
Further, the output V _L is applied to the other end of the input of the response detection amplifier 34, the deviation between these outputs V _L and V _H is taken, and the response correction point is passed through the high-pass filter (high-pass filter) 35. It outputs to 25 as a response compensation signal V _c . At the response correction point 25, the response compensation signal V _c is subtracted from the output V _L to perform response correction, and the combined output V _c is output to the output end 36.

The output of the low-frequency signal processing system 26 is stable at the zero point, but due to the large time constant of the low-pass filter 24, it is not affected by the flow noise but has a slow response. On the contrary,
The output of the commercial frequency signal processing system 27 shows a fast response although the zero point is unstable.

Therefore, the output of the response detection amplifier 34, which is the difference between the output of the low-pass filter 24 and the output of the low-pass filter 33, contains the DC component, but this is obtained through the high-pass filter 35. Response compensation signal
Since V _c does not include a DC component and thus does not impair zero stability and exhibits a fast response, the output of the response correction point 25 compensates for the slow response of the low-pass filter 24 with the response compensation signal V _c , resulting in a fast response. The output is stable even at the zero point and is not affected by flow noise.

FIG. 2 is a block diagram showing another embodiment of the present invention.
Portions having the same functions as those of the embodiment shown in FIG. 1 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

The output of the voltage / frequency converter 23 is applied to the multiplier 37,
The multiplier 37 modulates this with the commercial frequency comparison voltage obtained by the resistor 13 and outputs the modulated output to one end of the input of the response detection amplifier 38. The response detection amplifier 38 calculates and outputs the deviation between this modulation output applied to one end of the input and the output of the preamplifier 17 applied to the other end. The output of the response detection amplifier 38 is demodulated by the demodulator 31 by the commercial frequency comparison voltage obtained by the resistor 13 and the response compensation signal V _c ′ corresponding to the commercial frequency component of the signal voltage is output to the response correction point 39. .

At the response correction point 39, the low-frequency output from the low-pass filter 24 is applied, and this output is output to the voltage / frequency converter 23 with its response corrected by the response compensation signal V _c ′. The output is converted into a DC voltage by the voltage / frequency converter 40 and output to the output end 36.

As described above, when the response compensation signal V _c ′ is applied to the inside of the low frequency signal processing system 26, the amplification degree of the response detection circuit including the response detection amplifier 38, the demodulator 31, the multiplier 62, etc. Even if it is not stable, it does not cause an error.

<Effects of the Invention> According to the present invention as specifically described above with reference to the embodiments, according to the present invention, the zero point on the low frequency side is stable, and the output is not affected by flow noise but has a slow response. Since the fast response is detected by the response detecting means and the slow response on the low frequency side is corrected by this output, the zero point is stable and is not affected by the flow noise, and an output with a fast response can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention, and FIG.
The figure is a block diagram showing a second embodiment of the present invention. 10 ... Conduit, 12 ... Excitation coil, 14 ... Commercial constant current source, 16 ... Low frequency constant current source, 19 ... Coupling point, 20, 28, 37 ... Multiplier, 23 ...
Voltage / frequency converter, 24 ... Low-pass filter, 25,69 ... Response correction point, 38,60 ... Response detection amplifier.

Claims (2)

[Claims] 1. A first frequency and a second frequency lower than the first frequency.
Exciting means for supplying magnetic fields having three different frequencies, second demodulating means for discriminating and outputting the signal voltage excited by the exciting means and corresponding to the flow rate based on the second frequency, and the second demodulating means. Low-pass filtering means for low-pass filtering the output of the demodulating means, first demodulating means for discriminating the signal voltage based on the first frequency and demodulating, output of the first demodulating means and the low-pass filter. Response detecting / amplifying means for detecting a deviation from the output, and response correcting means for calculating a difference between a response compensating signal obtained from the output of the response amplifying means via a high-pass filter and the output of the low-pass filter. An electromagnetic flowmeter, comprising: 2. A first frequency and a second frequency lower than the first frequency.
Exciting means for supplying magnetic fields having three different frequencies, second demodulating means for discriminating and outputting the signal voltage excited by the exciting means and corresponding to the flow rate based on the second frequency, and the second demodulating means. Low-pass filtering means for low-pass filtering the output of the demodulation means, response detecting / amplifying means for amplifying the deviation between the flow rate output and the signal voltage, and output of the response detecting / amplifying means for demodulating based on the first frequency. A response compensating means for compensating the response using the response compensating signal thus obtained and the output of the low pass filter, and outputting a signal related to the output of the response compensating means as the flow rate signal. Electromagnetic flow meter.