JPH04343021A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To obtain an electromagnetic flowmeter which car remove mixing noises and output a stable and correct flow rate signal even when the peripheral temperature is changed. CONSTITUTION:A high pass filter 1 which has a cutoff frequency in the vicinity of the frequency of a flow rate signal satisfactorily removes the electrochemical noises mixed in the flow rate signals obtained from electrodes A, B and having the 1/f frequency characteristic. This electromagnetic flowmeter is provided with the above high pass filter 1, a CPU 10, a PROM 11 which stores the temperature characteristic data of resistances and capacitors constituting the high pass filter 1, and a thermometer 12. When the flow rate signal output from the high pass filter 1 is input, it is corrected in accordance with the peripheral temperature and the result is outputted. In consequence, the noises can be remarkably reduced and the stable flow rate signal can be output even when the peripheral temperature is changed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter that measures the flow rate of a liquid flowing through a tube by applying an alternating magnetic field to the liquid.

0002

[Prior Art] Generally, an electromagnetic flowmeter applies an alternating magnetic field to a liquid flowing in a pipe and measures the flow rate from the potential difference generated between two electrodes arranged perpendicular to the direction of this magnetic field. , noise is generated between these two electrodes due to several factors other than the potential difference generated therebetween, that is, the flow rate signal. Among them, there is electrochemical noise caused by the interaction between the electrode and the fluid, and this noise is 1/f
Therefore, the lower the frequency, the higher the level. Conventionally, such electrochemical noise has been removed by appropriately selecting the circuit constants and components of the sample-and-hold circuit, as shown in FIG.

0003

[Problem to be Solved by the Invention] However, the receiving instrument operates by being supplied with power via a two-wire 4-20mA transmission line, and the detected flow rate signal is transmitted to the receiving instrument via this 4-20mA transmission line. In such electromagnetic flowmeters, the excitation current of the coil that generates the magnetic field must be 4 mA or less, so the electromotive force generated between the electrodes that indicates the flow rate signal is quite small compared to the noise level, and as a result, Since the level difference between the signal and the noise disappears, it becomes difficult to distinguish between the two, so it is necessary to perform even greater noise removal. The frequency band of electrochemical noise having this 1/f characteristic is generally below 5 Hz, while the frequency band of the flow rate signal is above 10 Hz. For this reason, conventionally, signal amplification has been performed while removing direct current components and low frequency noise using a high-pass filter. However, because the cutoff frequency is selected sufficiently small so that the signal frequency is not attenuated by the high-pass filter, the actual frequency is 0.5
Noise of about 5 Hz is hardly attenuated. Furthermore, if the cutoff frequency is selected to be 5Hz in order to reduce the noise in these frequency bands, the amount of attenuation will be as shown in Table 1, and at this time, the noise will be attenuated and the flow signal will also be attenuated. It becomes like this.

0004

[Table 1]

As shown in Table 1, there is no problem if the flow rate signal is about 7% (assuming the flow rate signal is 12.5Hz), but the resistors and capacitors that make up such a high-pass filter generally have temperature characteristics. However, since the amount of attenuation changes depending on the ambient temperature, there is a problem that a stable flow rate value cannot be obtained.

[0006]

[Means for Solving the Problems] In order to solve such problems, the present invention provides a plurality of signals having a cutoff frequency near the frequency of the flow rate signal and removing noise mixed in by inputting the flow rate signal. a filter, a memory that stores temperature characteristic data of elements constituting this filter, and reads out temperature characteristic data of the memory according to a detected ambient temperature when a flow rate signal output from the filter is inputted. It also includes a control means for correcting the input flow rate signal using the read temperature characteristic data.

[0007]

[Operation] Therefore, when a flow rate signal whose noise is removed by a filter and is output is input, the temperature characteristic data of the memory is read out according to the ambient temperature, and the above flow rate signal is converted into the read temperature characteristic data. is corrected and output. As a result, noise can be largely eliminated, and a stable and accurate flow rate signal can always be obtained even when the ambient temperature fluctuates.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. In the same figure, 1 indicates resistors R1, R2 and capacitor C.
1 and C2, a high-pass filter, 2 and 3 are buffers, 4 is a differential amplifier, 5 is resistors R3 and R4, capacitors CI and C2, steps S1 and S2, buffers 6 and 7
and a sample hold circuit composed of a differential amplifier 8, 9 an A/D converter that converts an analog signal into a digital signal, 10 a CPU, 11 a PROM, 12 a thermometer,
13 is a D/A converter that converts a digital signal into an analog signal, and 14 is a V/I converter that converts an analog voltage signal into an analog current signal.

When a magnetic field is applied to a tube through which a fluid such as a liquid flows, a potential difference is generated between the two electrodes A and B arranged in the tube, and the signal generated between the electrodes A and B is a high-pass signal. The filter 1 removes the noise components, and the signals are sent to the input terminals of the differential amplifier 4 via buffers 2 and 3, respectively. The differential amplifier 4 takes the difference between the signals from the electrodes A and B, uses this as a flow rate signal, and sends it to the sample hold circuit 5.
The sample and hold circuit 5 holds the flow rate signal. This held flow rate signal is then converted into a digital data signal by an A/D converter and sent to the CPU 10.

The CPU 10 to which the flow data signal is input detects the ambient temperature output from the thermometer 12, and also detects the temperatures of the capacitors and resistors constituting the high-pass filter 1, which are stored in advance in the PROM 11 in correspondence with this ambient temperature. The characteristic data is read out, and based on this data, the flow rate data signal is corrected according to the ambient temperature and sent to the D/A converter 13. As a result, the temperature-corrected flow rate data signal is converted into an analog signal by the D/A converter 13, and further converted into a current signal by the V/I converter to produce a 4-20 mA signal, which is composed of two wires. 20mA
It is sent out to a transmission line and received by a receiving instrument (not shown).

As described above, the high-pass filter 1 having a cutoff frequency near the signal frequency sufficiently attenuates electrochemical noise having a 1/f frequency characteristic, and is equipped with the CPU 10, PROM 11, and thermometer 12 to reduce noise. The system outputs a sufficiently attenuated flow rate signal with correction based on the ambient temperature. As a result, noise can be largely eliminated, and a stable and accurate flow rate signal can always be output even when the ambient temperature fluctuates.

[0012] In such an electromagnetic flowmeter, some components other than the high-pass filter 1 consisting of a resistor and a capacitor also have a temperature coefficient. PRO correction data in advance
If stored in M11, a more accurate flow rate signal can be obtained. In addition, the high-pass filter is not only one stage of high-pass filter 1, but also has a high-pass filter as shown in FIG.
A plurality of stages may be provided and the flow rate signal may be used while being amplified. With this configuration, a high-order filter can be formed. Further, as shown in FIG. 3, an active filter 30 may be used.

[0013]

As explained above, in the present invention, when a flow rate signal output from a filter is input, the temperature characteristic data of the memory corresponding to the detected ambient temperature is read out, and the flow rate signal is Since the correction is performed based on the read temperature characteristic data, noise is largely removed, and a stable and accurate flow rate signal can be obtained even when the ambient temperature fluctuates.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an electromagnetic flowmeter according to the present invention.

FIG. 2 is a block diagram showing another embodiment of the electromagnetic flowmeter.

FIG. 3 is a block diagram showing another embodiment of the electromagnetic flowmeter.

FIG. 4 is a block diagram of a conventional electromagnetic flowmeter.

[Explanation of symbols] 1, 20, 22 High pass filter

Claims (1)

[Claims] 1. In an electromagnetic flowmeter that applies an alternating magnetic field to a liquid flowing in a tube and measures the flow rate value from a flow rate signal generated between two electrodes arranged in the tube, A plurality of filters each having a cutoff frequency and inputting a flow rate signal to remove mixed noise, a memory storing temperature characteristic data of elements constituting the filter, and a memory that stores the flow rate signal output from the filters. An electromagnetic flow rate characterized by comprising a control means for reading out the temperature characteristic data of the memory according to the detected ambient temperature when inputted, and correcting the inputted flow rate signal by the read out temperature characteristic data. Total.