Microwave frequency determination method, level measurement device, and level measurement method

Abstract

To provide a method and the like that determine a proper frequency of a microwave in a level measurement device measuring the level of a bath face inside a furnace by using the microwave.SOLUTION: The present invention relates to a method that determines a proper frequency of a microwave in a level measurement device 100 comprising: an antenna 10; and a signal processing unit 20, and measuring a level of a bath face 3 by transmitting and receiving the microwave toward the bath face inside a furnace 1. The method is configured to calculate the reception signal intensity of the microwave represented by the amount of attenuation of the microwave by dust inside the furnace, and an antenna gain of the antenna as a function of the frequency of the microwave; calculate a noise level represented by thermal noise of the signal processing unit and radiation noise from the bath face as a function of the frequency of the microwave; and determine the frequency of the microwave so that a SN ratio to be determined by the reception signal intensity of the microwave and the noise level is equal to or more than a prescribed threshold.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for determining an appropriate microwave frequency in a level measurement device that uses microwaves to measure the level of a bath surface in a furnace, such as the slag surface of a converter, and to a level measurement device and a level measurement method using the same. [Background technology]

[0002] In the steelmaking process using a converter, molten pig iron is charged into the converter and blown into the molten pig iron through a lance to adjust the composition of the molten pig iron and produce molten steel. As the blowing process progresses, slag is produced on the surface of the molten material (molten pig iron or molten steel) in the converter.

[0003] In the above steelmaking process, in order to improve productivity, it is important to increase the flow rate of the gas (oxygen flow rate) blown from the lance and shorten the time required for blowing. However, if the gas flow rate is increased too much, slopping (the phenomenon in which foamed slag overflows from the furnace throat) and spitting (the phenomenon in which slag is scattered by the jet) will occur, which will not only reduce the yield but also cause problems such as the adhesion of ingots and slag to the furnace throat, exhaust hood, etc., hindering operation.

[0004] Therefore, in order to improve productivity, it is important to accurately measure the bath surface (slag surface) level in the converter in real time and to accurately grasp behavior such as slag foaming, which is a precursor to slopping, in real time.

[0005] For this reason, for example, as disclosed in Patent Documents 1 and 2, a radar-type level measurement device has been proposed that uses microwaves to measure the level of the slag surface of the molten material charged into a converter.

[0006] During blowing (especially during decarburization blowing), a lot of dust is generated from the bath surface inside the furnace, making it difficult to measure the bath surface level using a camera or laser. Even if microwaves with a longer wavelength than visible light are used, some attenuation of the microwaves due to dust is unavoidable, so it is desirable to use microwaves with as low a frequency as possible. From this perspective, the level measurement devices described in Patent Documents 1 and 2 use microwaves with frequencies below the X-band (approximately 10 GHz or less).

[0007] However, because there are restrictions on the size of the opening that can be installed in the exhaust hood of a converter, there is also an upper limit to the size of the antenna of the level measurement device that can be installed in this opening (for example, the total of the transmitting antenna and the receiving antenna must be 600 mm or less). When the antenna size is constant, the higher the microwave frequency, the greater the antenna gain and the stronger the received microwave signal. Conversely, when the antenna size is constant and the microwave frequency is lowered, the antenna gain becomes smaller and the strength of the received microwave signal becomes weaker.

[0008] In other words, there is a trade-off between the dust permeability of microwaves and the antenna gain of the antenna, and simply lowering the frequency to improve dust permeability does not allow accurate measurement of the bath surface level. Therefore, it is necessary to select an appropriate microwave frequency that achieves both dust permeability and antenna gain.

[0009] Non-Patent Document 1 describes the basic theory of radar-based remote sensing. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2016-180126 A [Patent Document 2] JP 2018-53347 A [Non-patent literature]

[0011] [Non-Patent Document 1] Shoichiro Fukao and Kyosuke Hamazu, "Radar Remote Sensing of Weather and Atmosphere", Kyoto University Press, 2005 Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide a method for determining an appropriate microwave frequency in a level measurement device that uses microwaves to measure the level of a bath surface in a furnace, such as the slag surface of a converter, and a level measurement device and a level measurement method that use the same. [Means for solving the problem]

[0013] In order to solve the above problems, the present invention provides a microwave frequency determination method for determining a microwave frequency for use in a level measurement device that measures a bath surface level in a furnace, the method comprising the steps of: The level measuring device has an antenna that transmits microwaves toward the bath surface and receives the microwaves reflected from the bath surface, and a signal processing unit that calculates the level of the bath surface based on the microwaves received by the antenna, and the level measuring method includes a received signal strength calculation step of calculating the received signal strength of the microwaves, which is represented by the amount of microwave attenuation due to dust in the furnace and the antenna gain of the antenna, as a function of the microwave frequency, a noise level calculation step of calculating the noise level, which is represented by the thermal noise of the signal processing unit and the radiation noise from the bath surface, as a function of the microwave frequency, and a frequency determination step of determining the frequency of the microwaves so that the signal-to-noise ratio determined by the received signal strength of the microwaves calculated in the received signal strength calculation step and the noise level calculated in the noise level calculation step is equal to or greater than a predetermined threshold value.

[0014] According to the frequency determination method of the present invention, in the received signal strength calculation step, the microwave received signal strength, which is represented by the amount of microwave attenuation due to dust in the furnace and the antenna gain of the antenna, is calculated as a function of the microwave frequency. In other words, the microwave received signal strength is calculated taking into account the frequency dependence of both dust permeability and antenna gain. In addition, in the noise level calculation step, the noise level, which is represented by the thermal noise of the signal processing unit of the level measurement device and the radiation noise from the bath surface, is calculated as a function of the microwave frequency. That is, both the microwave received signal strength and noise level are calculated as functions of the microwave frequency by the received signal strength calculation step and the noise level calculation step. According to the frequency determination method of the present invention, since both the microwave reception signal strength and the noise level are expressed as functions of the microwave frequency, it is possible to determine the microwave frequency in the frequency determination step so that the S / N ratio determined by the microwave reception signal strength and the noise level is equal to or greater than a predetermined threshold value. In other words, it is possible to determine an appropriate microwave frequency that provides an S / N ratio equal to or greater than a predetermined threshold value.

[0015] Specifically, for example, in the received signal strength calculation step, the amount of attenuation of the microwaves due to dust in the furnace can be calculated based on the following formulas (1) to (5).

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[0016] In order to solve the above problems, the present invention provides a method for producing a liquid crystal display comprising the steps of: A level measurement device for measuring a bath surface level in a furnace, the level measurement device comprising: an antenna for transmitting microwaves toward the bath surface and receiving the microwaves reflected by the bath surface; and a signal processing unit for calculating the level of the bath surface based on the microwaves received by the antenna, the level measurement device executing a received signal strength calculation step for calculating a received signal strength of the microwaves represented by an amount of microwave attenuation due to dust in the furnace and an antenna gain of the antenna as a function of the frequency of the microwaves; a noise level calculation step for calculating a noise level represented by thermal noise of the signal processing unit and radiation noise from the bath surface as a function of the frequency of the microwaves; and a frequency determination step for determining a frequency of the microwaves such that an SN ratio determined by the received signal strength of the microwaves calculated in the received signal strength calculation step and the noise level calculated in the noise level calculation step is equal to or greater than a predetermined threshold value, and Using microwaves of the determined frequency and measure the level of the bath surface. It is also offered as a level measuring device. The present invention also provides a method for producing a semiconductor device comprising the steps of: A level measurement method for measuring a bath surface level in a furnace, comprising the steps of: By the above microwave frequency determination method, determining a frequency of microwaves to be used in the level measurement device; The level of the bath surface is measured using the level measuring device using microwaves of the determined frequency. and It is also provided as a level measurement method. According to the level measurement device and level measurement method of the present invention, it is possible to accurately measure the bath surface level using microwaves of an appropriate frequency determined by the frequency determination method of the present invention.

[0017] In the level measurement device and the level measurement method according to the present invention, it is considered that the level of the bath surface is measured using microwaves having a frequency of, for example, 15 to 19 GHz.

[0018] The level measurement device and the level measurement method according to the present invention are suitably used when the furnace is a converter and the bath surface is a slag surface. Effect of the Invention

[0019] According to the present invention, in a level measurement device that uses microwaves to measure the level of a bath surface in a furnace, it is possible to determine an appropriate microwave frequency. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is an explanatory diagram for explaining a schematic configuration of a converter and a level measurement device according to an embodiment of the present invention. [Diagram 2] 2 is an explanatory diagram for explaining a model of the relationship between a transmission wave transmitted from the antenna shown in FIG. 1 and a reception wave received by the antenna. [Diagram 3] FIG. 2 is a flow diagram showing steps of a microwave frequency determination method according to one embodiment of the present invention. [Figure 4] FIG. 2 is a diagram showing particle number-based particle size distribution of dust particles used in the examples. [Diagram 5] In the examples, the received signal strength, noise level and signal to noise ratio are calculated as a function of frequency. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, with reference to the attached drawings as appropriate, one embodiment of the present invention will be described, taking as an example a case in which the furnace whose level is to be measured is a converter and the bath surface in the furnace whose level is to be measured is the slag surface. Fig. 1 is an explanatory diagram for explaining the schematic configuration of a converter and a level measurement device according to this embodiment, Fig. 1(a) is a diagram showing the overall configuration of the converter and the level measurement device, and Fig. 1(b) is a diagram showing the circuit configuration of the level measurement device.

[0022] As shown in FIG. 1(a), in a steelmaking process using a converter, molten pig iron 2 is charged into the converter 1, and a gas such as oxygen is blown into the molten pig iron 2 from a lance 4 to adjust the composition of the molten pig iron 2 and produce molten steel. As the blowing process progresses, slag is produced on the surface of the molten material (molten pig iron 2 or molten steel) in the converter 1. The level measurement device 100 according to this embodiment measures in real time the level of the surface (slag surface) 3 of the slag produced in the converter 1. The level of the slag surface 3 means the height of the slag surface 3 as viewed from the bottom of the converter 1 or a predetermined reference position.

[0023] Since steam, dust, and the like are generated in the converter 1 due to the blowing performed in the converter 1, an exhaust hood 5 is provided near the throat of the converter 1 to prevent the generated dust and the like from being released into the external environment. The exhaust hood 5 is formed with an opening 51 for inserting the lance 4 into the converter 1 and an opening 52 in which the antenna 10 provided in the level measurement device 100 is disposed.

[0024] 1, the level measurement device 100 includes an antenna 10 that transmits microwaves toward a slag surface 3, which is the bath surface in a furnace, and receives the microwaves reflected by the slag surface 3, and a signal processor 20 that is connected to the antenna 10 and calculates the level of the slag surface 3 based on the microwaves received by the antenna 10. The signal processor 20 of this embodiment also has a function of controlling the transmission of microwaves from the antenna 10. In this embodiment, the antenna 10 includes a microwave transmitting antenna 10a and a microwave receiving antenna 10b. The antennas 10a and 10b are conical horn antennas. As shown by the dashed arrow in FIG. 1(a), microwaves are transmitted from the transmitting antenna 10a toward the slag surface 3 in the converter 1, and the microwaves reflected by the slag surface 3 are received by the receiving antenna 10b. The level of the slag surface 3 is measured by calculation in the signal processing unit 20. However, the present invention is not limited to this, and it is also possible to use a single antenna 10 that serves both transmission and reception purposes.

[0025] The signal processing unit 20 of this embodiment is configured to measure the level of the slag surface 3 by a so-called FM-CW method, which uses a beat wave generated by mixing a microwave (hereinafter, appropriately referred to as a "transmitted wave") transmitted from the transmitting antenna 10a toward the slag surface 3 with a microwave (hereinafter, appropriately referred to as a "received wave") reflected by the slag surface 3 and received by the receiving antenna 10b. However, the present invention is not limited to this, and it is also possible to adopt a configuration for measuring the level of the slag surface 3 by other methods, such as using a phase difference of a microwave of a single frequency. As shown in FIG. 1(b), the signal processing unit 20 of this embodiment includes an oscillator 21, a power amplifier 22, a mixer 23, a low-noise amplifier 24, an IF amplifier 25, a low-pass filter 26, an AD converter 27, and a computer 28.

[0026] FIG. 2 is an explanatory diagram that illustrates a schematic relationship between a transmission wave transmitted from the antenna 10 and a reception wave received by the antenna 10. As shown in FIG. The oscillator 21 of the signal processing unit 20 generates a frequency modulated waveform, i.e., a waveform whose frequency changes over time, and transmits this waveform to the power amplifier 22 and the mixer 23. The power amplifier 22 amplifies this waveform and then transmits it to the transmitting antenna 10a. As a result, a transmission wave whose frequency f changes linearly for each sweep period is transmitted from the transmitting antenna 10a as shown in Fig. 2. In the example shown in Fig. 2, the frequency modulation width of the transmission wave is F [Hz], and the transmission wave whose sweep period is ST [s] is transmitted.

[0027] On the other hand, as shown in Fig. 2, the received wave received by the receiving antenna 10b has a frequency modulation width of F [Hz] and a sweep period of ST [s], similar to the transmitted wave. However, the received wave has a time delay Δt [s] relative to the transmitted wave, which is proportional to the distance R to the slag surface 3 to be measured. This time delay Δt [s] is proportional to the frequency difference Δf [Hz] between the transmitted wave and the received wave at the same time. This received wave is amplified by the low-noise amplifier 24 and then transmitted to the mixer 23. In the mixer 23, the transmitted wave and the received wave are mixed (more precisely, the output waveform of the oscillator 21 and the output waveform of the low-noise amplifier 24 are mixed) to generate a beat wave having a frequency Δf.

[0028] The beat wave generated by the mixer 23 is amplified by the IF amplifier 25, and after unnecessary high frequency components are removed by the low pass filter 26, it is converted into an analog-to-digital signal by the AD converter 27 and input to the computer 28. A program for performing FFT (Fast Fourier Transform) processing and the like is installed in the computer 28, and the computer 28 performs FFT processing on the beat wave after the AD conversion to generate a power spectrum with frequency as the horizontal axis. The frequency Δf can be identified by detecting the peak position of this power spectrum, and the time delay Δt and thus the distance R to the slag surface 3 can be calculated from the identified frequency Δf. It is also possible to generate a power spectrum with the distance corresponding to each frequency on the horizontal axis instead of the power spectrum with the frequency on the horizontal axis, and directly calculate the distance R to the slag surface 3 by detecting the peak position of this power spectrum.

[0029] The level measurement device 100 according to this embodiment can measure the distance R to the slag surface 3 and thus the level of the slag surface 3 as described above, but is characterized by the method of determining the frequency f of the microwaves used (corresponding to the frequency f of the waveform generated by the oscillator 21). This point will be described below.

[0030] Fig. 3 is a flow diagram showing steps of the microwave frequency determination method according to this embodiment. As shown in Fig. 3, the frequency determination method according to this embodiment includes a received signal strength calculation step ST1, a noise level calculation step ST2, and a frequency determination step ST3. Each of the steps ST1 to ST3 will be described below.

[0031] <Received signal strength calculation step ST1> In the received signal strength calculation process ST1, the received signal strength of the microwave (specifically, the strength of the received wave received by the receiving antenna 10b), which is represented by the amount of microwave attenuation due to dust in the converter 1 and the antenna gain of the antenna 10, is calculated as a function of the microwave frequency f.

[0032] [Microwave attenuation due to dust] The amount of microwave attenuation by dust can be quantified by a model of microwave scattering by dust particles flying above the molten material in the converter 1. The amount of attenuation A per unit length of the microwave (electromagnetic wave) transmitted from the antenna 10 is expressed by the following formula (1), where the unit is [dB / m] and the attenuating side is a positive value, and the microwave frequency is f [Hz].

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[0033] In addition, K in the above formula (2) α is expressed by the following formula (4), and K in the above formula (3) β is expressed by the following equation (5).

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[0034] The above-described formula (1) indicates that the higher the microwave frequency f, the greater the amount of microwave attenuation by dust particles (attenuation A per unit length of microwaves).

[0035] [Antenna gain of antenna 10] The antenna gain G of the antenna 10, which determines the performance of the level measurement device 100, is expressed by the following equation (6).

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[0036] Since c=fλ holds between the speed of light c, frequency f, and wavelength λ, the above equation (6) can be rewritten as the following equation (7).

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[0037] [Calculating received signal strength] In the reception signal strength calculation step ST1, the microwave reception signal strength P is calculated based on the following formula (9) using the attenuation A per unit length of the microwave and the antenna gain G of the antenna 10 described above. r Calculate.

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[0038] The radar cross section σ of the slag surface 3 to be measured, and the complex relative dielectric constant ε of the dust particles r , the complex relative permeability μ of the dust particle r , the particle number-based particle size distribution of dust particles q D The dust particle's true specific gravity ρ and dust concentration d are calculated in advance, the maximum distance to the slag surface 3 is determined as the distance R used in equation (9), and equations (1) and (8) are substituted for A and G in equation (9), respectively, so that equation (9) can be expressed as a function of the microwave frequency f. Therefore, the microwave reception signal strength P r can be calculated. The radar cross section σ of the slag surface 3 can be calculated, for example, by using electromagnetic field analysis using the finite element method or by measuring the received signal strength of microwaves from the slag surface 3 in advance. D can be measured, for example, by collecting dust particles passing through the exhaust hood 5 and using an optical particle size distribution meter for the collected dust particles. Also, the dust concentration d can be measured, for example, by attaching an optical dust concentration meter to the exhaust hood 5.

[0039] <Noise level calculation process ST2> In the noise level calculation step ST2, a noise level represented by thermal noise of the signal processing unit 20 (specifically, the AD converter 27) and radiation noise from the slag surface 3 is calculated as a function of the microwave frequency f.

[0040] [Thermal noise of AD converter 27] The main noise generated in the level measurement device 100 is the thermal noise of the signal processing unit 20, and in particular, the thermal noise of the AD converter 27. t is expressed by the following equation (10).

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[0041] [Radiation noise from slag surface 3] In this embodiment, since the molten material in the converter 1 is at a high temperature of 1000°C or more, the influence of electromagnetic waves in the microwave band emitted from the slag surface 3 cannot be ignored. Since the electromagnetic waves emitted from the slag surface 3 are incoherent, they are detected as white noise by the receiving antenna 10b. A signal corresponding to this white noise is mixed with the transmission wave (output waveform of the oscillator 21) in the mixer 23, amplified by the IF amplifier 25, passed through the low-pass filter 26, AD converted by the AD converter 27, and input to the computer 28. Therefore, the radiation energy in the bandwidth determined by the cutoff frequency of the low-pass filter 26 is considered to be the magnitude of the radiation noise. Therefore, the radiation noise N r In calculating the spectral radiance L(λ,T t ) is calculated.

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[0042] [Noise level calculation] The thermal noise N t and radiation noise N r The sum of these and the noise figure NF [dB] of the circuitry constituting the signal processing unit 20 is the noise input to the AD converter 27. This noise can be reduced by signal processing performed by the computer 28. As the signal processing for reducing noise, for example, three types of processing are considered: FFT processing, oversampling, and coherent integration. The noise reduction amount N s The number of FFT points is N FFT , the sampling frequency of the AD converter 27 is f s If the number of coherent integrations is n and the bandwidth of the intermediate frequency signal after passing through the low-pass filter 26 (the bandwidth of frequencies that the beat wave can take) is BW, then it is expressed by the following equation (13).

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[0043] Finally, in the noise level calculation step ST2, the thermal noise N t and the radiation noise N r The noise factor NF of the circuit constituting the signal processing unit 20 and the noise reduction amount N s Using these, the noise level N is calculated based on the following equation (14).

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[0044] <Frequency determination process ST3> In the frequency determination step ST3, the microwave reception signal strength P calculated in the reception signal strength calculation step ST1 is rand the noise level N calculated in the noise level calculation step ST2. r The signal-to-noise ratio is calculated as the ratio (difference when expressed in dB) of the received microwave signal strength P r Since both the signal level N and the noise level N are expressed as a function of the microwave frequency f, the SNR is also expressed as a function of the microwave frequency f. Therefore, in the frequency determining step ST3, the frequency f (range of frequency f) of the microwaves can be determined so that the calculated S / N ratio is equal to or greater than a predetermined threshold value Th (for example, 10 dB).

[0045] In the level measurement device 100 according to this embodiment, the oscillator 21 of the signal processor 20 is set to generate a waveform of a frequency f determined by executing the received signal strength calculation step ST1, the noise level calculation step ST2, and the frequency determination step ST3 described above. Specifically, in this embodiment, in order to measure the level of the slag surface 3 by the FM-CW method, the frequency range determined by the frequency modulation width F (see FIG. 2) of the waveform generated by the oscillator 21 is set to be included in the range of the determined frequency f. When the oscillator 21 generates a waveform of a single frequency (when the level of the slag surface 3 is measured using a microwave of a single frequency), it is sufficient to set this single frequency to be included in the range of the determined frequency f.

[0046] In this embodiment, when calculating the noise level N in the noise level calculation step ST2, the noise reduction amount N s However, if signal processing to reduce noise is not performed, the noise reduction amount N s The noise level N can be calculated without taking into account

[0047] In addition, in this embodiment, the furnace to be measured for level is a converter 1, and the bath surface in the furnace for level measurement is the slag surface 3. However, the present invention is not limited to this, and can also be applied to, for example, measuring the level of the charge in a blast furnace, the bath surface level in the lower part of a blast furnace, or the bath surface level of a copper refining furnace. In addition, in this embodiment, an example has been described in which the antenna 10 has a configuration separated into a transmitting antenna 10a and a receiving antenna 10b, but the present invention is not limited to this, and it is also possible to use a single integrated transmitting and receiving antenna that serves both transmitting and receiving purposes.

[0048] Hereinafter, an example in which an appropriate microwave frequency f is determined for the level measurement device 100 according to this embodiment will be described. In this embodiment, the received signal strength P r In calculating this, the distance R to the slag surface 3 is set to 25 [m], and the true specific gravity ρ of the dust particles is set to 4490.5 [kg / m 3 ], and the complex relative dielectric constant of the dust particle, ε r is 62.4 + 14.1j, and the complex relative permeability μ r is set to 1.24 + 0.59j, and the dust concentration d is set to 50 [g / m 3 ], 100 [g / m 3 ] and 200 [g / m 3 ], the radar cross section σ of the slug surface 3 is -43 [dBsm], and the microwave transmission signal strength Pt is 10 [dBm]. D The particle size distribution shown in Figure 4 was used as the particle size distribution. The vertical axis of Figure 4 shows the ratio of the number of dust particles having each particle size shown on the horizontal axis to the total number of particles. In addition, when calculating the noise level N, the temperature T t is 1700[℃], the temperature T of the AD converter 27 is 40[℃], the noise figure NF is 21.5[dB], and the number of FFT points N FFT was set to 2048 points, the sampling frequency B of the AD converter 27 was set to 1.2 [MHz], and the number of coherent integrations n was set to 1. Under the above-mentioned conditions, the received signal strength calculation step ST1, the noise level calculation step ST2, and the frequency determination step ST3 are performed, and the received signal strength P r , the noise level N and the signal-to-noise ratio were calculated as a function of frequency f.

[0049] FIG. 5 shows the received signal strength P r The noise level N and the SNR calculated as a function of frequency f are shown in Fig. 5(a). 3 ], and Fig. 5(b) shows the results when the dust concentration d is 100 [g / m 3 ], and Fig. 5(c) shows the results when the dust concentration d is 200 [g / m 3 ] is shown. As shown in Fig. 5, as the frequency f increases, the noise level N increases monotonically, but the received signal strength P r Regarding the microwave attenuation caused by dust is large, but the antenna gain of the antenna 10 is also large, so the received signal strength P r fluctuates so as to have a maximum value. As a result, it is found that there exists an optimum frequency f at which the SNR has a maximum value.

[0050] For example, if the S / N ratio is 10 dB or more, it is considered that the distance R to the slag surface 3 can be measured. If the threshold value Th is set to 10 dB, as shown in FIG. 5(a), the distance R to the slag surface 3 can be measured if the dust concentration d is 50 [g / m 3 5(b), when the dust concentration d is 100 [g / m], the S / N ratio is equal to or greater than the threshold value Th when the frequency f is in the range of 8 to 72 [GHz], as indicated by the arrow in the figure (the S / N ratio reaches a maximum value near 25 GHz), and therefore this range can be determined as an appropriate frequency f range. 3 5(c), the S / N ratio exceeds the threshold value Th when the frequency f is in the range of 8 to 32 [GHz] as indicated by the arrow in the figure (the S / N ratio reaches a maximum value near 18 GHz). Therefore, for example, this range can be determined as the appropriate frequency f range. However, when the dust concentration d is 200 [g / m 3], there is no frequency f at which the SNR exceeds the threshold value Th of 10 dB. In this case, it is necessary to review the threshold value Th, increase the microwave transmission signal strength, reduce the noise level N, etc. Although not shown in the figure, the upper limit of the dust concentration d at which the frequency f at which the S / N ratio is 10 dB or more exists is 130 [g / m 3 ], and the frequency f at which the S / N ratio reaches its maximum value is 16.5 [GHz]. When the dust concentration d is 130 [g / m 3 In this case, if the frequency modulation width F required for distance measurement based on the FM-CW method is 3 [GHz], then the appropriate frequency f range may be determined to be, for example, 15 to 19 [GHz]. [Explanation of symbols]

[0051] 1. Converter (furnace) 3. Slag surface (bath surface) 4. Lance 5. Exhaust hood 10, 10a, 10b... Antenna 20 Signal processing section 23 Mixer 26 Low-pass filter 100 Level measuring device ST1: Received signal strength calculation process ST2: Noise level calculation process ST3: Frequency determination process

Claims

1. A method for determining a microwave frequency for use in a level measurement device for measuring a bath surface level in a furnace, comprising: The level measuring device includes an antenna that transmits microwaves toward the bath surface and receives the microwaves reflected from the bath surface, and a signal processing unit that calculates the level of the bath surface based on the microwaves received by the antenna, a receiving signal strength calculation step of calculating a receiving signal strength of the microwave, which is represented by an attenuation amount of the microwave due to the dust in the furnace and an antenna gain of the antenna, as a function of a frequency of the microwave; a noise level calculation step of calculating a noise level represented by thermal noise of the signal processing unit and radiation noise from the bath surface as a function of the frequency of the microwave; a frequency determination step of determining a frequency of the microwave such that an S / N ratio determined by the received signal strength of the microwave calculated in the received signal strength calculation step and the noise level calculated in the noise level calculation step is equal to or greater than a predetermined threshold value; A method for determining a microwave frequency comprising:

2. 2. The microwave frequency determination method according to claim 1, wherein in the received signal strength calculation step, an amount of attenuation of the microwave due to dust in the furnace is calculated based on the following formulas (1) to (5). ##EQU00021## In the above formula (1), A [dB / m] is the attenuation per unit length of the microwave transmitted from the antenna, f [Hz] is the frequency of the microwave. In the above formulas (2) and (3), π is the constant of the circumference of a circle, c [m / s] is the speed of light, d [kg / m ３ ] is the dust concentration, ρ [kg / m ３ ] is the true specific gravity of the dust particle, D [m] is the particle diameter of the dust particle, V Ｄ [m ３ ] is the volume of a single dust particle of diameter D, q Ｄ represents the particle number-based particle size distribution of dust particles. In the above formulas (4) and (5), ε ｒ is the complex dielectric constant of the dust particle, μ ｒ denotes the complex relative permeability of the dust particle.

3. A level measuring device for measuring the level of a bath surface in a furnace, comprising: An antenna that transmits microwaves toward the bath surface and receives the microwaves reflected from the bath surface, and a signal processing unit that calculates the level of the bath surface based on the microwaves received by the antenna, The level measuring device includes: a receiving signal strength calculation step of calculating a receiving signal strength of the microwave, which is represented by an attenuation amount of the microwave due to the dust in the furnace and an antenna gain of the antenna, as a function of a frequency of the microwave; a noise level calculation step of calculating a noise level represented by thermal noise of the signal processing unit and radiation noise from the bath surface as a function of the frequency of the microwave; a frequency determination step of determining a frequency of the microwave such that an S / N ratio determined by the received signal strength of the microwave calculated in the received signal strength calculation step and the noise level calculated in the noise level calculation step is equal to or greater than a predetermined threshold value; A level measuring device that measures the level of the bath surface using microwaves having the frequency determined in the frequency determination process.

4. A level measurement method for measuring the level of a bath surface in a furnace, comprising: A step of determining a frequency of a microwave used in the level measurement device by the microwave frequency determination method according to claim 1 or 2; measuring the level of the bath surface using the level measuring device using microwaves of the determined frequency; A level measurement method comprising the steps of:

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