JP2013253936A - Liquid level measuring apparatus and vco calibration method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid level measuring apparatus for acquiring an optimum F-T curve and performing highly accurate measurement, and to provide a VCO calibration method for the liquid level measuring apparatus.SOLUTION: An FMCW radar type liquid level measuring apparatus includes: a VCO 14 for generating a sweep frequency signal; a V-T table for sweeping an oscillation frequency of the VCO 14; a reflection member 26 located on a known distance position; a fixed sine wave signal generation part for transmitting the frequency sweep signal generated by the VCO 14 by an F-T curve whose linearity is held to the reflection member and obtaining a fixed sine wave signal from a beat waveform of a transmission signal and a reflection signal; a corrected V-T curve table obtained by analyzing an error of a beat waveform signal of a transmission frequency to a liquid surface and a reception frequency to the fixed sine wave signal; and a corrected V-T curve obtained by correcting the V-T table by the corrected V-T curve table. The VCO 14 is driven on the basis of the corrected V-T curve to obtain an F-T curve with linearity.

Description

  The present invention relates to a radar-type liquid level measuring apparatus that measures the liquid level in a tank that stores liquid such as liquefied gas and the like, and a VCO calibration method thereof, in particular, a configuration for increasing measurement accuracy. It is characterized by having.

One of the radar systems used in the liquid level measuring device is an FMCW (Frequency Modulated Continuous Wave) radar system. In this FMCW radar system, as shown in FIG. 8, in a predetermined fixed time (this time is referred to as a sweep time (T)), a predetermined frequency (F) is swept toward a measurement point. It transmits radio waves. As shown in FIG. 9, the transmission frequency is F during the round trip time t until the radio wave transmitted at the transmission point is reflected at the measurement point (the distance from the transmission point is L) and returns to the transmission point. -Swept by t / T (Hz). The swept frequency is the difference of the reception frequency F _R and the transmission frequency F _T of the moment of receiving the reflected wave (reception frequency F _R) (hereinafter referred to as "beat frequency F _B").

The round trip time t is t = (T / F) × F _B
Since it is, if it is possible to measure the beat frequency F _B, it can be radio waves from the transmission point to the measurement point measures the time t required for the round trip. Since the propagation speed of radio waves in free space is the same as the speed of light C, the distance L from the transmission point to the measurement point is
L = C × t / 2 = C × T × F _B / 2F (1)
It can be expressed as

  The principle of distance measurement by the FMCW radar method has been schematically described above. FIG. 7 shows a conventional example of an FMCW radar type distance measuring device. In FIG. 7, the FMCW radar type distance measuring device includes a DSP (Digital Signal Processor) 101, a digital / analog converter 103, a VCO (Voltage Controlled Oscillator) 104, a coupling circuit 105, a mixer 110, and an AGC (AGC). Automatic gain control circuit) 111 and analog / digital converter 112. The components including the digital / analog converter 103 and the VCO 104 constitute a transmission system, and the components from the mixer 110 to the analog / digital converter 112 constitute a reception system. The DSP 101 has a built-in memory 102. The relationship between the voltage applied to the VCO 104 that determines the oscillation frequency of the VCO 104 with respect to the sweep time T, that is, a voltage-time curve (hereinafter referred to as “VT curve”) is voltage-time. It is stored as a table (hereinafter referred to as “VT table”).

The DSP 101 reads the VT table from the memory 102, converts the voltage value that continuously changes with the passage of time into an analog signal by the digital / analog converter 103, and sets it as the control voltage of the VCO 104. The oscillation frequency of the VCO 104 changes continuously according to the control voltage. This oscillation signal is transmitted from the appropriate antenna to the measurement point (for example, the liquid level) via the coupling circuit 105. A system under measurement 120 is interposed between the antenna and the measurement point. The radio wave reflected at the measurement point returns to the measured system 120 and is captured by the antenna, and is guided to the receiving system via the coupling circuit 105. In the reception system, the oscillation signal upon reception the received signal in the mixer 110 is mixed, the difference between the reception frequency and the oscillation frequency of the moment, that is, the beat frequency F _B is taken out.

Beat frequency _{F B} signal, after being controlled to an appropriate amplitude value AGC111, is converted into a digital signal by an analog-to-digital converter 112 is input to the DSP 101. In the DSP 101, the distance L to the measurement point is obtained by performing arithmetic processing by applying the equation (1).

Or FMCW radar system distance described accurately measure the distance by measuring principle is to accurately measure the beat frequency F _B, To this end, always relationship sweep time T and sweep frequency F associated therewith It must be kept constant. In other words, the sweep frequency change ΔF in the unit sweep time ΔT is always constant, that is, the sweep frequency needs to be linear with respect to the sweep time on the horizontal axis.

  As in the example shown in FIG. 7, the FMCW radar transmitter generally uses the VCO 104. The VCO 104 oscillates by applying a control voltage, and can continuously change the oscillation frequency by continuously changing the control voltage corresponding to the sweep frequency in the sweep time T. In FIG. 8, the time from the start of sweep to the stop of sweep is represented by T, and the change width of the oscillation frequency from the start of sweep to the stop of sweep is represented by F. Hereinafter, a graph representing a change in the oscillation frequency of the VCO 104 during the sweep time T is referred to as a frequency-time characteristic. Hereinafter, the frequency-time characteristic is referred to as “FT characteristic”.

  The VCO is generally used in a PLL (Phase Locked Loop) circuit that can oscillate a signal having a stable frequency, and the use of sweeping the frequency like the VCO 104 is a special use. The linearity of the frequency-voltage characteristic (FV characteristic) output from the VCO 104 is insufficient to obtain the measurement accuracy required by the FMCW radar. Therefore, in the FMCW radar, in order to guarantee the linearity of the FT characteristic output from the VCO 104, a voltage-time table (hereinafter referred to as “VT table”) is generated, and this table is used. The oscillation voltage of the VCO 104 is controlled.

  Furthermore, since the FV curve of the VCO 104 changes slightly with changes in ambient temperature, only one fixed VT table can be used to perform measurements that meet the required accuracy in the temperature range required by the FMCW radar type distance measuring device. Can't do it. Therefore, conventionally, a plurality of VTs having different temperatures are stored in a memory as a table, and a VT table corresponding to the ambient temperature at the time of measurement is selected to correct the FT characteristic.

  However, in a distance measuring device that requires higher measurement accuracy, the required accuracy cannot be ensured by the above-described mechanism that is complemented by selecting from a plurality of VT tables having different temperatures. . Also, since the characteristics of the VCO itself change over time and the FV curve at each temperature changes, it is necessary to periodically measure the characteristics of the VCO to correct or update the VT table, which is troublesome. .

  There exists patent document 1 as what discloses the technique relevant to this invention. Patent Document 1 describes a frequency calibration structure including a VCO having two modes of a normal operation mode and a calibration (calibration) mode, which can be applied to an FMCW radar. However, the above VCO is incorporated in the PLL circuit in the calibration mode to replace the calibration VCO, and has a technical idea different from that of the present invention described below.

US Pat. No. 7,804,369

  It is an object of the present invention to obtain a liquid level measuring apparatus and its VCO calibration method capable of performing measurement with higher accuracy by making it possible to acquire an optimal FT characteristic in real time. .

The liquid level measuring device according to the present invention is:
By sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface, and the beat frequency which is the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is An FMCW radar type liquid level measuring device that measures the distance to the liquid level,
A VCO that generates a sweep frequency signal;
A voltage-time table (hereinafter referred to as “V-T table”) for determining an applied voltage for sweeping the frequency of a signal generated by the VCO according to frequency-time characteristics (hereinafter referred to as “FT characteristics”);
A reflective member disposed at a known distance position to correct the FT characteristic of the VCO;
A frequency sweep signal generated by the VCO is transmitted toward the reflecting member due to FT characteristics that maintain linearity, and a fixed sine wave signal is obtained from the beat waveform of the transmission signal and the reflected signal from the reflecting member. A fixed sine wave signal generator for obtaining
A correction voltage-time curve table (hereinafter referred to as “correction VT curve table”) generated by analyzing an error of the beat waveform signal of the transmission frequency to the liquid level and the reception frequency with respect to the fixed sine wave signal;
A corrected VT curve obtained by correcting the VT table with the corrected VT curve table,
The most important feature is to obtain linear FT characteristics by operating the VCO based on the corrected VT table.

  Even if the VT curve applied to the VCO has linearity, if the FT characteristic is non-linear due to the characteristics of the VCO, an error occurs in the frequency of the beat signal, and the liquid level measurement with high accuracy is possible. Can not. In that respect, according to the liquid level measuring apparatus according to the present invention, the VT table applied to the VCO can be corrected according to the VCO characteristic, and the FT characteristic having linearity can be obtained. This makes it possible to measure the liquid level with high accuracy without causing an error in the frequency of the beat signal.

It is a block diagram which shows the Example of the liquid level measuring apparatus which concerns on this invention. It is a graph which shows the process of obtaining the FT characteristic before correction | amendment in the said Example. It is a graph which shows the process of obtaining the FT characteristic after correction | amendment in the said Example. (A) shows the frequency sweep when the linearity of the FT characteristic is maintained, (b) shows the frequency sweep when the linearity of the FT characteristic is not maintained (c) ) Is a graph showing the difference in beat waveform depending on the presence or absence of linearity of the FT characteristic. It is a flowchart which shows the calibration process in the said Example in order. It is a schematic diagram which shows notionally the mode of measurement by the liquid level measuring apparatus which concerns on this invention. It is a block diagram which shows the example of the conventional liquid level measuring apparatus. It is a graph which shows the example of the frequency sweep by FMCW radar. It is a graph which shows the distance measurement principle by the frequency sweep of FMCW radar.

  Embodiments of a liquid level measuring apparatus and a VCO calibration method according to the present invention will be described below with reference to the drawings.

  FIG. 1 shows an embodiment of an FMCW radar type liquid level measuring device according to the present invention. In FIG. 1, the liquid level measuring device includes a DSP 11, a digital / analog converter 13, a VCO 14, a coupling circuit 15, a switch 16, a mixer 20, an AGC 21, and an analog / digital converter 22. The components including the digital / analog converter 13 and the VCO 14 constitute a transmission system, and the components from the mixer 20 to the analog / digital converter 22 constitute a reception system. The DSP 11 has a built-in memory 12, and the memory 12 stores the relationship of the voltage applied to the VCO 14 that determines the oscillation frequency of the VCO 14 with respect to the sweep time T, that is, the VT curve as a VT table.

  The parts described so far are the same as those of the conventional radar-type distance measuring apparatus shown in FIG. 7 except that the switch 16 is added. Therefore, the operation at the time of measurement is also the same as that in the conventional example. That is, the DSP 11 reads from the VT table stored in the memory 12 a voltage that controls the operation of the VCO 14 in synchronization with the clock frequency determined by the sweep time and the number of sampling points. The read control voltage value is converted into an analog signal by the digital / analog converter 13 and applied to the VCO 14 as a control voltage. The VCO 14 outputs a signal having a frequency corresponding to the control voltage, that is, a high-frequency signal whose frequency continuously changes with time. This high frequency signal is transmitted from the appropriate antenna to the system under test 25 via the coupling circuit 15 and the switch 16.

  The system under measurement 25 has a liquid level of the liquid 41 that is a measurement point, and the high-frequency signal is reflected by the liquid level. The reflected high-frequency signal returns to the system to be measured 25 and is captured by the antenna, and is guided to the receiving system through the switch 16 and the coupling circuit 15. In the mixer 20 of the reception system, the reception signal and the oscillation signal at the time of reception are mixed, and a difference signal between the reception frequency and the oscillation frequency at that moment, that is, a beat waveform signal is extracted. The beat waveform signal is adjusted to an appropriate amplitude by the AGC 21, converted to a digital signal by the analog / digital converter 22, and input to the DSP 11.

In the DSP 11, during the sweep time T, the VT table is read and the beat waveform signal is fetched, and the beat waveform signal data group on the fetched time axis is filtered to remove unnecessary noise. Furthermore, converted into data of the frequency axis the beat waveform signal data group and FFT (Fast Fourier Transform) processing in DSP 11, extracts the beat frequency _{F B} from the result. Obtaining the distance L to the measurement point by processing the beat frequency F _B (1) Enter the formula. Thus, DSP 11 is a filter for the noise removal, and a FFT processing unit as an extraction portion of the beat frequency _{F B.}

  An installation example of the liquid level measuring apparatus described above is schematically shown in FIG. In FIG. 6, reference numeral 10 denotes a radar unit, and reference numeral 40 denotes a tank in which the liquid level measuring device 10 is installed. The tank 10 contains a liquid 41. The radar unit 10 is an FMCW radar type liquid level measuring device configured by each block shown in FIG. 1, and the converter 32 outputs a signal output from the unit 10. It is an antenna that converts radio waves into the liquid surface and radiates radio waves.

F _T is the frequency of the radio waves radiated to the waveguide 30 toward the liquid surface from the radar unit 10, F _R is back towards the radar unit 10 of the radio wave liquid level is reflected by the waveguide 30 Indicates the frequency of radio waves.

  The embodiment of the liquid level measuring device according to the present invention shown in FIG. 1 is different from the conventional radar type distance measuring device in that a near-end reflecting member 26 is provided. The near-end reflecting member 26 is made of a member that can reflect radio waves radiated from the liquid level measuring device, that is, the radar unit 10, and is installed at a known length position from the reference plane position. The near-end reflecting member 26 is a member necessary for performing the VCO calibration which is a feature of the present invention. Next, the necessity of VCO calibration and the VCO calibration itself will be described.

  The relationship of the oscillation frequency to the control voltage in the VCO does not necessarily have linearity. In general, the characteristics change due to temperature changes or secular changes. FIG. 2A shows an example of a general FV characteristic of the VCO, and the linearity is broken. FIG. 2B shows a VT table before correction stored in the memory 12, and the control voltage changes linearly with respect to the time axis. Assume that the DSP 11 reads the VT table from the memory 12, converts a control voltage value that changes over time into an analog signal by the digital / analog converter 13, and applies it to the VCO 14. Even if the VT curve is a straight line, if the linearity of the FV characteristic of the VCO 14 is broken as described above, the FT characteristic representing the relationship of the oscillation frequency with respect to the time axis output from the VCO 14 is: As shown in FIG. 2C, the line is non-linear. Therefore, even if the liquid level is measured using this FT characteristic, it is not possible to perform measurement with good accuracy.

  Therefore, the VT curve stored in the memory 12 is corrected corresponding to the FV characteristic of the VCO 14 so that the FT characteristic of the signal output from the VCO 14 becomes a straight line. FIG. 3 shows this correction, and FIG. 3 (a) shows the same non-linear FV characteristics of the VCO 14 as in FIG. 2 (a). FIG. 3B shows a VT curve based on the corrected VT table stored in the memory 12. The corrected VT curve has a non-linearity opposite to the non-linear FV characteristic of the VCO 14. When the corrected VT curve table is read, and the control voltage value that changes over time is converted into an analog signal by the digital-analog converter 13 and applied to the VCO 14, a straight line as shown in FIG. Thus, it is possible to obtain a corrected FT characteristic in which the property is ensured. When the liquid level is measured using the corrected FT characteristic, it is possible to perform measurement with good accuracy.

  The process of generating a corrected VT table corresponding to the FV characteristic of the VCO 14 as shown in FIG. 3B is referred to as VCO calibration (calibration) here. By performing VCO calibration, it is possible to obtain FT characteristics that maintain linearity, and to perform liquid level measurement with high accuracy.

Next, a specific configuration for performing VCO calibration and a VCO calibration method will be described. VCO calibration, as shown in FIG. 9, FMCW radar, when the linearity of the F-T characteristics are ensured through the sweep time, the beat frequency F _B signal caused by the reflection of radio waves from a known distance, Take advantage of having the same frequency throughout the sweep frequency band.

  In the embodiment shown in FIG. 1, the VCO calibration is performed by the DSP 11 switching the switch 16 from the system under measurement 25 to the near-end reflecting member 26 side through the control signal line CAL. As described above, the near-end reflecting member 26 is installed at a known length position from the reference plane position. Therefore, if the linearity of the FT characteristic is ensured, the reflected signal from the near-end reflecting member 26 is obtained. The beat signal waveform obtained by receiving the signal becomes a fixed sine wave signal. On the other hand, in the case of a non-linear curve like the FT characteristic before correction shown in FIG. 2C, the beat frequency fluctuates within the sweep time T, so the beat signal waveform is accurate. Not a sine wave signal. FIG. 4 shows a difference in operation between the case where the linearity of the FT characteristic is maintained and the case where it is not maintained.

Figure 4 (a) shows a case where in the sweep time T beat frequency F _B does not change, because the sweep frequency of the transmitted and received signals in the sweep time are both kept linearity, transmission signals and reception frequency F _B of the signal of the beat signal is constant everywhere within a sweep time. Accordingly, the waveform of the beat signal at this time is as indicated by the waveform A in FIG. 4 (c), the beat frequency F _B are accurate sine wave has no collapse of waveforms at a constant.

FIG contrast. 4 (b), the beat frequency F _B in the sweep time T is varied, F-T characteristic shows a case made of a non-linearity. As the waveform of the beat signal at this time is shown in waveform B of FIG. 4 (c), becomes a waveform collapse from the waveform A is an accurate sine wave, the beat frequency F _B is deviated from the original beat frequency. Therefore, even seek distance L of the beat frequency F _B signals to the measuring point by applying the formula (1), can not be obtained a highly accurate distance L, causing errors in the measurement results.

  FIG. 5 illustrates a VCO calibration procedure for correcting the VT table so that the FT characteristic of the output signal of the VCO 14 is a straight line in the embodiment shown in FIG. The processing steps are shown in the vertical direction in the center of FIG. Each processing step is provided with a reference numeral such as S1, S2,.

  As a premise for performing the VCO calibration, as described above, the switch 16 is switched to the near-end reflecting member 16 side while the linearity of the FT characteristic is ensured, and the reflected signal from the near-end reflecting member 26 is changed. Receive. The near-end reflecting member 26 is a fixed-length reflecting point whose distance from the measuring device unit is accurately measured in advance. Since the distance to the near-end reflecting member 26 is determined in advance with high accuracy, a beat signal waveform of a fixed sine wave signal can be obtained by ensuring the linearity of the FT characteristic. The beat signal waveform of the fixed sine wave signal is referred to as a “fixed sine wave”, and the portion that generates the fixed sine wave signal is referred to as a “fixed sine wave signal generation unit”. The fixed sine wave signal generation unit exists in the DSP 11 as a part having one function. The FT characteristic in which linearity is ensured is preset so that the sweep frequency changes linearly with respect to the sweep time, and is stored in the memory 12 of the DSP 11 as a VT table. .

Next, as shown in step S1 in FIG. 5 performs sweep initial V-T table or the uncorrected V-T table, the frequency F _T and the radio wave of the radio wave radiated is reflected back by the liquid surface obtaining a beat waveform of a radio wave frequency _{F R} (S2). The sinusoidal waveform in the upper right of FIG. 5 shows an example of the beat waveform acquired as described above. Next, a shift of the beat waveform with respect to the fixed sine wave is analyzed while comparing the beat waveform acquired in step S2 with the fixed sine wave on the time axis (S3). The waveform diagram in the second stage on the right side of FIG. 5 shows an image of the beat waveform analysis, and it can be seen that the acquired beat waveform deviates on the time axis with respect to the fixed sine wave.

  In the beat waveform analysis step, a waveform error function: e (x) is generated from the result of analyzing the error of the beat waveform with respect to the fixed sine wave on the sweep time axis (S4). The waveform diagram in the third stage on the right side of FIG. 5 shows an example of the waveform error function: e (x), and on the time axis, the waveform error continuously occurs from the top to the bottom with respect to the reference value. I can see the situation.

  Next, in order to prevent the waveform error from occurring, a corrected VT curve table for correcting the VT table is generated based on the waveform error function: e (x) (S5). The waveform diagram at the bottom right side of FIG. 5 shows a corrected VT curve for correcting the VT curve, which is a waveform corresponding to the waveform error function: e (x). A portion for generating a corrected VT curve table for correcting the VT curve exists as a portion that functions in the DSP 11.

  Next, a corrected VT table is generated by applying the corrected VT curve table (S6). Of the two graphs shown in the lower left of FIG. 5, the left graph shows a VT curve, the curve a1 shows a VT curve before correction, and the curve a2 shows a VT curve after correction. . Of the two graphs, the right graph shows the FT characteristic output from the VCO 11, the curve b1 shows the FT characteristic before correction, and the curve b2 shows the FT characteristic after correction. . The corrected VT curve indicated by a2 indicates an example of the corrected VT table generated in step S6. By applying this corrected VT table to the VCO 11, the Can obtain the FT characteristic after correction while maintaining the linearity shown by b2. The corrected VT curve table and the part that generates the VT curve table exist in the DSP 11 as a part that performs one function.

As described above, the calibration of the VCO 11 is performed, and a signal having an FT characteristic whose frequency linearly changes with respect to the sweep time can be output from the VCO 11 after the calibration. The signals linearly varying F-T characteristics emitted toward the liquid surface to be measured, the beat signal F _B obtained by receiving the reflected signal from the liquid surface is no distortion in the waveform, the liquid The distance to the surface can be accurately measured.

  Since the calibration of the VCO 11 can be performed in real time on a fixed sine wave signal, it can quickly respond to changes in the environmental conditions in which the liquid level measurement device is installed, changes in the characteristics of the VCO over time, etc. The liquid level can always be measured with high accuracy.

  The measurement object according to the present invention may be anything as long as it is a liquid, but is particularly suitable for an application for measuring a highly accurate liquid level.

11 DSP
12 Memory 13 Digital to Analog Converter 14 VCO
DESCRIPTION OF SYMBOLS 15 Coupling circuit 16 Switch 20 Mixer 21 AGC circuit 25 System to be measured 26 Near end reflection member

The liquid level measuring device according to the present invention is:
By sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface, and the beat frequency which is the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is An FMCW radar type liquid level measuring device that measures the distance to the liquid level,
A VCO that generates a sweep frequency signal;
A voltage-time table (hereinafter referred to as “V-T table”) for determining an applied voltage for sweeping the frequency of a signal generated by the VCO according to frequency-time characteristics (hereinafter referred to as “FT characteristics”);
A reflective member disposed at a known distance position to correct the FT characteristic of the VCO;
A frequency sweep signal generated by the VCO is transmitted toward the reflecting member due to FT characteristics that maintain linearity, and a fixed sine wave signal is obtained from the beat waveform of the transmission signal and the reflected signal from the reflecting member. A fixed sine wave signal generator for obtaining
A correction voltage-time curve table (hereinafter referred to as “correction VT curve table”) generated by analyzing an error of the beat waveform signal of the transmission frequency to the liquid level and the reception frequency with respect to the fixed sine wave signal;
And a corrected V-T table obtained by correcting the V-T table above correction V-T curve table,
The most important feature is to obtain linear FT characteristics by operating the VCO based on the corrected VT table.

Claims (6)

By sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface, and the beat frequency which is the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is An FMCW radar type liquid level measuring device that measures the distance to the liquid level,
A VCO that generates a sweep frequency signal;
A voltage-time table (hereinafter referred to as “V-T table”) for determining an applied voltage for sweeping the frequency of a signal generated by the VCO according to frequency-time characteristics (hereinafter referred to as “FT characteristics”);
A reflective member disposed at a known distance position to correct the FT characteristic of the VCO;
A frequency sweep signal generated by the VCO is transmitted toward the reflecting member due to FT characteristics that maintain linearity, and a fixed sine wave signal is obtained from the beat waveform of the transmission signal and the reflected signal from the reflecting member. A fixed sine wave signal generator for obtaining
A correction voltage-time curve table (hereinafter referred to as “correction VT curve table”) generated by analyzing an error of the beat waveform signal of the transmission frequency to the liquid level and the reception frequency with respect to the fixed sine wave signal;
A corrected VT curve obtained by correcting the VT table with the corrected VT curve table,
A liquid level measuring device that obtains linear FT characteristics by operating the VCO based on the corrected VT table.   The corrected VT curve table analyzes the deviation of the beat waveform signal from the fixed sine wave signal while comparing the beat waveform signal and the fixed sine wave signal on the time axis, and generates a waveform error based on the result. The liquid level measuring device according to claim 1 obtained according to a function.   3. The liquid level measuring device according to claim 1, further comprising an antenna or a conversion device for guiding a sweep frequency signal generated by the VCO toward the liquid level. By sweeping a predetermined frequency during the sweep time, the radio wave is transmitted toward the liquid surface, and the beat frequency that is the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is used. A VCO calibration method for an FMCW radar type liquid level measuring device that measures the distance to the liquid level,
Generating a sweep frequency signal with a VCO;
Generating a frequency sweep signal based on a VT table in the VCO;
A frequency sweep signal generated by the VCO is transmitted to a reflecting member arranged at a known distance position by an FT characteristic that maintains linearity, and the transmission signal and a reflected signal from the reflecting member are transmitted. Obtaining a fixed sine wave signal from the beat waveform of
Analyzing the error of the beat waveform signal of the transmission frequency to the liquid level and the reception frequency with respect to the fixed sine wave signal to generate a corrected VT curve table;
Obtaining the corrected VT table by correcting the VT table with the corrected VT curve table,
A VCO calibration method for a liquid level measuring device that obtains linear FT characteristics by operating the VCO based on the corrected VT table.   The corrected VT curve table analyzes the deviation of the beat waveform signal with respect to the fixed sine wave signal while comparing the beat waveform signal and the fixed sine wave signal on the time axis, and generates a waveform error resulting therefrom. 5. The VCO calibration method for a liquid level measuring device according to claim 4, wherein the VCO calibration method is generated according to a function. 6. The VCO calibration method for a liquid level measuring apparatus according to claim 4, wherein a sweep frequency signal generated by the VCO is guided to the liquid level by an antenna or a conversion device.