JP7501865B2 - LIQUID LEVEL POSITION ESTIMATION DEVICE AND LIQUID LEVEL POSITION ESTIMATION METHOD - Google Patents

Description

The present invention relates to a liquid level position estimation device that estimates the liquid level position of a liquid contained in a container.

Conventionally, the liquid level of a liquid contained in a container has been estimated by various methods. Examples of such estimation methods include the following.
- Visual estimation of liquid level position.
- Estimation of liquid level position based on the detection results of a float-type level sensor.
- Estimation of the liquid level by detecting changes in conductivity or heat dissipation at electrodes installed inside the container.
- Estimation of the liquid level position by irradiating the liquid surface with ultrasound or laser light.
- Estimation of the liquid level using a pressure sensor installed at the bottom of the container.

For visual inspection, it is necessary to provide a window in the container or a lid that can be opened and closed. In other words, in a sealed container without a window or lid, it is not possible to estimate the liquid level position by visual inspection. Furthermore, for inspections other than visual inspection, it is necessary to install components such as a float-type level sensor, electrodes, ultrasonic or laser light oscillators and detectors, or pressure sensors inside the container. Therefore, if these components break down, it is difficult to repair them. Furthermore, if the operation of these components is suspended, it is also difficult to install replacement components inside the container at a later date.

In response to this, by providing a sensor for estimating the liquid level position outside the container, it is possible to estimate the liquid level position without the difficulties described above. Patent Document 1 discloses an oil level detection device that is provided at a predetermined height on the outer surface of the compressor to detect the level of oil stored inside the compressor, and that includes an oil level detection sensor that detects the temperature at the installation location.

Patent No. 6184503

However, in order for the oil level detection device of Patent Document 1 to detect the oil level, it is necessary for the heat inside the compressor to be transferred to the outer surface of the compressor where the oil level detection sensor is installed. Therefore, for containers to which internal heat is not transferred or is transferred poorly, the oil level detection device of Patent Document 1 cannot detect the oil level with high accuracy.

One aspect of the present invention aims to realize a liquid level position estimation device that can accurately estimate the liquid level position without relying on visual inspection or installing an internal sensor for estimating the liquid level position of the liquid contained in a container.

In order to solve the above problems, a liquid level estimation device according to one aspect of the present invention is a liquid level estimation device that estimates the liquid level position of a liquid contained in a container, and includes a piezoelectric element provided on the outer surface of the container, a calculation unit that calculates the admittance or impedance of the piezoelectric element when the container is vibrated by the piezoelectric element, and an identification unit that identifies the liquid level position based on the calculation result of the calculation unit.

A liquid level estimation method according to one aspect of the present invention is a liquid level estimation method for estimating the liquid level of a liquid contained in a container, and includes a calculation step of calculating the admittance or impedance of a piezoelectric element provided on the outer surface of the container when the container is vibrated by the piezoelectric element, and a determination step of determining the liquid level position based on the calculation result in the calculation step.

According to one aspect of the present invention, the liquid level can be accurately estimated without relying on visual inspection or installing an internal sensor for estimating the liquid level of the liquid contained in the container.

1 is a diagram illustrating an example of the configuration of a liquid level position estimation system according to a first embodiment. FIG. 2 is a diagram illustrating an example of a measurement circuit according to the first embodiment. 3A to 3C are diagrams showing examples of waveforms of an input voltage, an output current, and an admittance measured or calculated by the liquid level position estimation device of the first embodiment. 4A to 4C are diagrams for explaining an example of linear interpolation processing executed by the liquid level position estimation device of the first embodiment. FIG. 4 is a diagram showing an example of a position where the piezoelectric element of the first embodiment is attached to a container; 3A to 3C are diagrams illustrating an example of a waveform of the real part of admittance, a baseline waveform, and a vibration characteristic spectrum acquired by the liquid level position estimation device of the first embodiment. 13A to 13C are diagrams showing another example of the waveform of the real part of admittance, the baseline waveform, and the vibration characteristic spectrum acquired by the liquid level position estimation device of embodiment 1. 13A to 13C are diagrams showing yet another example of the waveform of the real part of admittance, the baseline waveform, and the vibration characteristic spectrum acquired by the liquid level position estimation device of embodiment 1. 4 is a diagram showing an example of reference data acquired by the liquid level position estimation device of the first embodiment; FIG. 11 is a flowchart illustrating an example of advance preparation processing according to the first embodiment. 11 is a flowchart illustrating an example of a baseline acquisition process according to the first embodiment. 10 is a flowchart illustrating an example of a liquid level position specifying process and a determination process according to the first embodiment. 13A to 13C are diagrams for explaining an example of linear interpolation processing executed by the liquid level position estimating device of the second embodiment.

[Embodiment 1]
<Configuration of liquid level position estimation system 1>
1 is a diagram showing an example of the configuration of a liquid level position estimation system 1. As shown in FIG. 1, the liquid level position estimation system 1 includes a container 2, a liquid level position estimation device 3, and an output device 4.

Container 2 contains a predetermined amount of liquid. Container 2 is, for example, a sealed container whose interior cannot be seen with the naked eye. An example of container 2 is a shock absorber of a gas-insulated switchgear filled with shock absorber oil.

The liquid level position estimation device 3 estimates the liquid level position of the liquid contained in the container 2. The liquid level position estimation device 3 determines whether the estimated liquid level position is within an appropriate range and outputs the result to the output device 4.

The output device 4 outputs the determination result by the liquid level position estimation device 3. Examples of the output device 4 include a light emitting device, a display device, and a sound output device. For example, the sound output device outputs an alarm when the estimated liquid level position is not within an appropriate range. The output device 4 may also output the determination result to another terminal (e.g., a device or mobile terminal owned by the administrator) in a predetermined format (e.g., email format).

<Configuration of liquid level position estimation device 3>
As shown in FIG. 1, the liquid level position estimation device 3 includes a piezoelectric element 11, a signal generator 12, a control unit 13, and a storage unit 14.

The piezoelectric element 11 is provided on the outer surface of the container 2. The piezoelectric element 11 generates vibrations when a voltage is applied by the signal generator 12, and detects a voltage based on the pressure it receives due to the vibrations according to the liquid level position. The piezoelectric element 11 is attached, for example, within a range on the outer surface of the container 2 that determines whether the liquid level position is within an appropriate range. Since the appropriate range changes depending on the use of the container 2, the attachment position of the piezoelectric element 11 also changes depending on the use.

The signal generator 12 applies a voltage to the piezoelectric element 11. For example, the signal generator 12 applies a high-frequency AC voltage having a constant frequency to the piezoelectric element 11 while sweeping the frequency within a predetermined range. For example, the signal generator 12 applies an AC voltage having a sine wave voltage waveform and an amplitude of 30 Vp-p to the piezoelectric element 11 while increasing the frequency from 10 kHz to 100 kHz over a predetermined time (e.g., 6.5 seconds).

The specified time may be set to a time sufficient for the analysis unit 22 to acquire a vibration characteristic spectrum (described later) that can accurately identify the liquid level position. However, the specified time is set so that the time required to identify the liquid level position (the time required to determine whether the liquid level position is within an appropriate range) is not longer than necessary.

Figure 2 shows an example of a measurement circuit. As shown in Figure 2, the piezoelectric element 11 and the signal generator 12 are electrically connected via a current measurement resistor 15. The current measurement resistor 15 is a resistor that allows the measurement unit 21 to measure the output current of the piezoelectric element 11.

The control unit 13 comprehensively controls each component of the liquid level position estimation device 3. The control unit 13 controls, for example, the voltage supply from the signal generator 12 to the piezoelectric element 11. The memory unit 14 stores various data used by the control unit 13. The memory unit 14 stores, for example, reference data described below and a threshold value for determining whether the liquid level position is appropriate.

<Specific configuration of control unit 13>
In this embodiment, the control unit 13 mainly includes a measurement unit 21, an analysis unit 22, and a determination unit 23.

(Processing of the measurement unit 21)
The measuring unit 21 measures the input voltage (voltage) generated in the piezoelectric element 11 and the output current (current) flowing through the piezoelectric element 11 when the container 2 is vibrated for a predetermined time via the piezoelectric element 11. As shown in Fig. 2, the measuring unit 21 measures the voltage between two electrodes of the piezoelectric element 11 as the input voltage when the signal generator 12 applies an AC voltage to the piezoelectric element 11. The measuring unit 21 measures the current flowing through the current measuring resistor 15 as the output current of the piezoelectric element 11 when the signal generator 12 applies an AC voltage to the piezoelectric element 11. As described above, when the signal generator 12 applies a sine wave AC voltage to the piezoelectric element 11 while increasing the frequency from 10 kHz to 100 kHz for 6.5 seconds, the measuring unit 21 obtains waveforms (graphs, measurement results) as shown at 301 and 302 in Fig. 3.

301 in FIG. 3 is a diagram showing the waveform of the input voltage measured by the measurement unit 21. The vertical axis of 301 in FIG. 3 indicates the value of the input voltage, and the horizontal axis indicates the time. 302 in FIG. 3 is a diagram showing the waveform of the output current measured by the measurement unit 21. The vertical axis of 302 in FIG. 3 indicates the value of the output current, and the horizontal axis indicates the time. In FIG. 3, the frequency of the AC voltage at the start of application (time 0 seconds) is 10 kHz, and the frequency of the AC voltage immediately before the end of application (time 6.5 seconds) is 100 kHz.

The piezoelectric element 11 has the characteristic that the current value increases with the frequency of the applied AC voltage. As shown in 302 in FIG. 3, while the frequency of the AC voltage applied to the piezoelectric element 11 increases from 10 kHz to 100 kHz (while the time elapses from 0 seconds to 6.5 seconds), the value of the output current increases.

The measurement unit 21 stores the measured input voltage and output current in the memory unit 14. In the advance preparation process, the measurement unit 21 measures the input voltage and output voltage at each of a number of known liquid level positions and stores them in the memory unit 14. The advance preparation process is a process executed by the control unit 13 to create reference data (described below) for the analysis unit 22 to identify the liquid level position.

(Processing of analysis unit 22)
The analysis unit 22 reads out data indicating the input voltage and the output current from the storage unit 14, and analyzes the data to identify the liquid level position. The analysis unit 22 includes a calculation unit 221 and an identification unit 222.

The calculation unit 221 calculates the admittance of the piezoelectric element 11. Specifically, the calculation unit 221 obtains the waveforms of the input voltage and output current in the frequency domain by Fourier transforming the waveforms of the input voltage and output current (waveforms over time; waveforms in the time domain) measured by the measurement unit 21. The Fourier transform is an example of frequency analysis (frequency transformation) for transforming a waveform in the time domain into a waveform in the frequency domain. Hereinafter, the values and waveforms after this transformation are also referred to as values and waveforms after frequency transformation.

The calculation unit 221 obtains the admittance waveform (characteristic spectrum) by dividing the value of the output current after frequency conversion by the value of the input voltage after frequency conversion. In other words, the calculation unit 221 calculates the admittance after frequency conversion. However, the calculation unit 221 may calculate the admittance from the waveforms of the input voltage and output current in the time domain measured by the measurement unit 21, and perform a Fourier transform on the admittance to obtain the admittance waveform after frequency conversion.

In this way, the admittance of the piezoelectric element 11 calculated by the calculation unit 221 is calculated from the input voltage and output current of the piezoelectric element 11 measured when the container 2 is vibrated for a predetermined period of time, and has a waveform that is the result of frequency analysis.

The calculation unit 221 obtains, for example, waveforms as shown in 303 and 304 in FIG. 3 as the calculated admittance (admittance after frequency conversion). 303 and 304 in FIG. 3 are diagrams showing examples of admittance waveforms. 303 in FIG. 3 is a diagram showing the waveform of the real part of the admittance after frequency conversion calculated by the calculation unit 221. The vertical axis of 303 in FIG. 3 indicates the value of the real part of the admittance, and the horizontal axis indicates the frequency. 304 in FIG. 3 is a diagram showing the waveform of the imaginary part of the admittance after frequency conversion calculated by the calculation unit 221. The vertical axis of 304 in FIG. 3 indicates the value of the imaginary part of the admittance, and the horizontal axis indicates the frequency.

As a preparatory process, the calculation unit 221 calculates an admittance corresponding to each of a plurality of known liquid level positions based on the input voltage and output voltage at each of the liquid level positions measured by the measurement unit 21. The calculation unit 221 calculates the admittance after frequency conversion as each admittance.

The identification unit 222 identifies the liquid level position based on the calculation result of the calculation unit 221. Specifically, the identification unit 222 identifies the liquid level position by comparing the vibration feature spectrum obtained by the process described below with a reference feature spectrum, which is a feature spectrum acquired in advance. In order to identify the liquid level position, the identification unit 222 of this embodiment performs the following process.

In this embodiment, first, the identification unit 222 extracts the real part from the admittance (complex number) calculated by the calculation unit 221, and uses the waveform of the real part as a feature spectrum for preprocessing of the vibration feature spectrum that identifies the liquid level position.

In general, the waveform of the imaginary part of the admittance includes a waveform based on the electrical characteristics of the piezoelectric element 11. When identifying the liquid level position based on the vibration of the container 2 acquired by the piezoelectric element 11, it is preferable that the vibration feature spectrum does not include characteristics other than the vibration characteristics of the piezoelectric element 11 that reflect the vibration of the container 2 as much as possible. The vibration feature spectrum is a feature spectrum that has the vibration characteristics of the piezoelectric element 11. By using the waveform of the real part of the admittance as a feature spectrum for pre-processing of the vibration feature spectrum, the influence of the electrical characteristics that may be exerted on the vibration feature spectrum can be reduced.

Next, the identification unit 222 obtains a baseline by performing linear interpolation processing on multiple minimum points included in the waveform of the real part of the admittance. The identification unit 222 uses the feature spectrum obtained by subtracting the baseline from the waveform as the vibration feature spectrum. This makes it possible to reduce the influence of the surrounding environment of the container 2 or the adhesion of the piezoelectric element 11 to the container 2, which may affect the vibration feature spectrum. Here, a minimum point refers to a point that is smaller than the points before and after along the horizontal axis among multiple points (each value of the real part of the admittance) in the waveform of the real part of the admittance.

Figure 4 is a diagram for explaining an example of linear interpolation processing. 401 in Figure 4 is a diagram that shows a schematic diagram of the waveform of the real part of admittance. 402 in Figure 4 is a diagram that shows the result of linear interpolation processing performed by extracting multiple minimum points from the waveform shown in 401 in Figure 4. 403 in Figure 4 is a diagram that shows the result of linear interpolation processing performed by extracting multiple minimum points from the line obtained as a result of performing the linear interpolation processing in 402 in Figure 4.

As shown in 402 of FIG. 4, the identification unit 222 extracts multiple minimum points from the waveform of the real part of the admittance shown in 401 of FIG. 4. The identification unit 222 obtains a line connecting the multiple minimum points by performing linear interpolation processing on the multiple extracted minimum points. In 402 of FIG. 4, the extracted minimum points are circled, and the line obtained as a result of performing the linear interpolation processing is shown by a dashed line.

Furthermore, in this embodiment, the identification unit 222 extracts multiple minimum points from a line (waveform shown by a dashed line) that connects multiple minimum points obtained as a result of performing linear interpolation processing, as shown in 403 of FIG. 4. The identification unit 222 obtains a line that connects the multiple minimum points by performing linear interpolation processing on the extracted multiple minimum points. In 403 of FIG. 4, the extracted minimum points are surrounded by double circles, and the line obtained as a result of performing the linear interpolation processing is indicated by a dashed line. The identification unit 222 obtains the line (waveform) obtained as a result of the second linear interpolation processing as a baseline.

The waveform obtained as a result of the first linear interpolation process at 402 in FIG. 4 (waveform indicated by a dashed line) is compared with the waveform obtained as a result of the second linear interpolation process at 403 in FIG. 4 (waveform indicated by a dashed line). In this case, the waveform obtained the second time is smoother than the waveform obtained the first time. By obtaining a smooth baseline and subtracting this baseline from the waveform of the real part of the admittance, the vibration characteristics of the piezoelectric element 11 that reflect the vibration of the container 2 can be extracted with greater accuracy.

601 in FIG. 6 shows an example of the waveform of the real part of admittance, and an example of the baseline waveform obtained by performing two linear interpolation processes as described above using the waveform. The waveform of the real part of admittance is obtained when the AC voltage described above is applied to the piezoelectric element 11 for 6.5 seconds at a certain liquid level position. 602 in FIG. 6 shows an example of a vibration characteristic spectrum obtained by subtracting the baseline shown in 601 in FIG. 6 from the waveform of the real part of admittance shown in 601 in FIG. 6.

In the above, the identification unit 222 extracts multiple minimum points (double circles shown at 403 in FIG. 4) from a line (dashed line shown at 403 in FIG. 4) connecting multiple minimum points in the waveform of the real part of the admittance. The identification unit 222 may further extract multiple minimum points from a line (dash-dotted line shown at 403 in FIG. 4) connecting the extracted multiple minimum points. The identification unit 222 may perform this process of extracting minimum points until the number of minimum points falls within a predetermined range. In this case, the identification unit 222 performs linear interpolation processing on the multiple minimum points that fall within the predetermined range, and obtains the resulting waveform as a baseline.

The more times the process of extracting minimum points is performed, the fewer the number of minimum points extracted. Therefore, the waveform obtained as a result of the linear interpolation process becomes smoother. Therefore, by using this waveform as a baseline, the vibration characteristics of the piezoelectric element 11 that reflect the vibration of the container 2 can be extracted with even greater precision. The specified range may be set, through experiments or the like, to a range in which a baseline that allows the vibration characteristics to be extracted with high precision can be obtained.

The identification unit 222 may obtain multiple minimum values from the waveform of the real part of the admittance or a waveform that has been linearly interpolated, using a minimum value detection function or an envelope detection function of numerical analysis software or the like.

In the above-described process, linear interpolation is performed to obtain the baseline, but the present invention is not limited to this. The interpolation process to obtain the baseline may include, for example, spline interpolation or other interpolation processes. Furthermore, approximation may be performed instead of the interpolation process. The approximation process may include, for example, polynomial approximation, trigonometric function approximation, or other approximation processes.

(Preparation process in the identification unit 222)
The determination unit 222 extracts the real part of each admittance (complex number) calculated in the advance preparation process of the calculation unit 221. The determination unit 222 obtains a baseline for the real part of each admittance. The determination unit 222 obtains vibration feature spectra corresponding to each of the multiple liquid level positions by subtracting the baseline from the real part of each admittance.

The identification unit 222 calculates the degree of change of the vibration feature spectrum corresponding to each liquid level position relative to the vibration feature spectrum corresponding to a certain liquid level position. The identification unit 222 identifies the vibration feature spectrum corresponding to a certain liquid level position (reference condition) as the reference feature spectrum. In the following description, the "liquid level position" is also referred to as the "condition."

In this embodiment, the identification unit 222 calculates the deviation of the vibration feature spectrum corresponding to each liquid level position (each condition) from the selected reference feature spectrum. In this embodiment, the following formula 1 is used to calculate the RMSD (Root Mean Square Deviation) as the deviation.

Here, Re(Y _i,1 ) refers to a reference value in calculating RMSD. In this embodiment, the value of the reference feature spectrum is input to Re(Y _i,1 ). Re(Y _i,2 ) refers to a value to be compared with the reference value. In this embodiment, the value of the vibration feature spectrum corresponding to each condition to be compared with the reference feature spectrum is input to Re(Y _i,2 ). i refers to the number of values input. In this embodiment, i refers to the number of values that constitute the vibration feature spectrum. The greater the RMSD value, the greater the degree of change of the vibration feature spectrum relative to the reference feature spectrum.

Note that statistical processing other than RMSD may be used as the statistical processing for calculating the degree of change of the vibration feature spectrum relative to the reference feature spectrum. For example, RMS (Root Mean Square) or various error rates may be used. Examples of various error rates include RMSE (Root Mean Squared Error), MAE (Mean Absolute Error), and MSE (Mean Squared Error). Other examples of various error rates include RMSPE (Root Mean Square Percentage Error) and MAPE (Mean Absolute Percentage Error).

The identification unit 222 creates data linking the RMSD corresponding to each condition with the liquid level position for each condition as reference data that the identification unit 222 refers to in order to identify the liquid level position, and stores the data in the storage unit 14. FIG. 9 shows an example of the reference data created by the identification unit 222. In the example of FIG. 9, the RMSD corresponding to each condition is linked to a predetermined position and each position where the liquid level has dropped from the predetermined position. However, the RMSD corresponding to each condition may also be linked to the liquid level position (liquid level height) from the bottom of the container 2.

The identification unit 222 may also create reference data by performing an interpolation process or an approximation process on the RMSD for each condition. However, the identification unit 222 may also perform an interpolation process or an approximation process when referring to the reference data to identify the liquid level position.

(Liquid Level Position Identification Processing in Identification Unit 222)
When the identifying unit 222 identifies the liquid level position, the measuring unit 21 measures the input voltage and the output voltage of the piezoelectric element 11, and the calculating unit 221 calculates the admittance after frequency conversion. The identifying unit 222 extracts the real part from the admittance, and obtains the vibration characteristic spectrum by subtracting the baseline from the waveform of the real part.

The identification unit 222 calculates the degree of change of the acquired vibration feature spectrum relative to a reference feature spectrum (the same as the reference feature spectrum used in the advance preparation process). Specifically, the identification unit 222 calculates the RMSD of the acquired vibration feature spectrum. The identification unit 222 reads out reference data from the storage unit 14, and identifies the liquid level position by comparing the reference data with the calculated RMSD.

(Processing of Determination Unit 23)
The determination unit 23 determines whether the liquid level position identified by the identification unit 222 is within an appropriate range, which is a range suitable for the liquid level position in the container 2.

The appropriate range is set in advance according to the use of the container 2. For example, if the lower limit and upper limit of the appropriate range are set in advance as threshold values, the determination unit 23 determines that the liquid level position is not appropriate when the liquid level position identified by the identification unit 222 falls below the lower limit or exceeds the upper limit.

The appropriate range may be set in advance as a range from a predetermined position of the container 2 (e.g., the liquid level position when a specified amount is contained) to a position that is a predetermined distance lower. In this case, the position may be set in advance as a threshold value. The determination unit 23 determines that the liquid level position is not appropriate when the liquid level position identified by the identification unit 222 falls below the threshold value. On the other hand, the appropriate range may be set in advance as a range from a predetermined position of the container 2 to a position that is a predetermined distance higher. In this case, the position may be set in advance as a threshold value. The determination unit 23 determines that the liquid level position is not appropriate when the liquid level position identified by the identification unit 222 exceeds the threshold value.

In other words, the determination unit 23 may determine whether the liquid level position (liquid volume) has fallen beyond an appropriate range, or may determine whether the liquid level position (liquid volume) has risen beyond an appropriate range. How the determination is made may be determined according to the application of the container 2.

The appropriate range may also be set based on the degree of change of the vibration feature spectrum relative to the reference feature spectrum. In this case, the determination unit 23 determines whether the degree of change (e.g., RMSD) calculated by the identification unit 222 is within the appropriate range. If it is not within the appropriate range, the determination unit 23 determines that the liquid level position is not appropriate.

<Example>
An experiment was conducted in which water was contained in a metal container serving as the container 2, and a piezoelectric element 11 was attached to the outside of the container 2, to obtain reference data.

(Experimental conditions)
The signal generator 12 applied an AC voltage having a sine wave voltage waveform and an amplitude of 30 Vp-p to the piezoelectric element 11. The signal generator 12 set the frequency of the AC voltage at the start of application to the piezoelectric element 11 (time 0 seconds) to 10 kHz, and monotonically increased the frequency of the AC voltage so that the frequency of the AC voltage immediately before the end of application (time 6.5 seconds) became 100 kHz.

The container 2 was an iron container with a height of 140 mm. The piezoelectric element 11 had a rectangular surface shape and dimensions of 30 mm wide x 20 mm long x 0.5 mm thick.

Figure 5 shows an example of the attachment position of the piezoelectric element 11 on the container 2. As shown in Figure 5, the piezoelectric element 11 is attached to the side of the container 2 at a height of 90 mm to 110 mm from the container 2.

(Experiment details)
The amount of liquid when water was contained in the container 2 up to a height of 130 mm was defined as the specified amount (standard liquid amount, regular liquid amount). In Fig. 5, the liquid level position at this time is indicated as P1.

The liquid level position estimation device 3 obtained a vibration feature spectrum corresponding to the liquid level position P1 when a specified amount was contained, and vibration feature spectra corresponding to each liquid level position when water was contained up to the other five heights. In this experiment, the liquid level position P1 at a height of 130 mm was set as the specified position (reference condition). In other words, the vibration feature spectrum at a height of 130 mm was set as the reference feature spectrum. In addition, the liquid level positions for the five conditions other than the reference condition were set to heights of 120 mm, 110 mm, 90 mm, 70 mm, and 0 mm (when the container 2 is empty). The liquid level position estimation device 3 calculated the RMSD of the vibration feature spectra corresponding to heights of 120 mm, 110 mm, 90 mm, 70 mm, and 0 mm, respectively, relative to the reference feature spectrum.

The liquid level positions at heights of 120 mm, 110 mm, 90 mm, 70 mm, and 0 mm can be said to be the positions when the liquid level drops by 10 mm, 20 mm, 40 mm, 60 mm, and 130 mm, respectively, from the height of 130 mm. In this case, the liquid level position P1 at the height of 130 mm can be expressed as 0 mm. Hereafter, each liquid level position when the height of 130 mm is taken as 0 mm will be expressed as a liquid level drop position.

The liquid level position estimation device 3 created reference data by linking each RMSD to the liquid level drop position. In this experiment, the input voltage and output current were measured three times under each condition to obtain three vibration feature spectra corresponding to each measurement. The average of the three vibration feature spectra under the reference condition (liquid level position at a height of 130 mm (liquid level drop position: 0 mm)) was used as the reference feature spectrum. The RMSD for each condition was calculated as follows. Under each condition, the RMSD for each of the three vibration feature spectra was calculated using the above reference feature spectrum. The average of the three RMSDs calculated under each condition was used as the RMSD for each condition.

(Experimental result)
Fig. 6 is a diagram showing the experimental results when the liquid level is at a height of 130 mm (lower liquid level position: 0 mm), Fig. 7 is a diagram showing the experimental results when the liquid level is at a height of 120 mm (lower liquid level position: 10 mm), and Fig. 8 is a diagram showing the experimental results when the liquid level is at a height of 70 mm (lower liquid level position: 60 mm).

601 in FIG. 6 is a diagram showing the waveform of the real part of the admittance extracted from the admittance calculated by the calculation unit 221, and the baseline waveform obtained by performing two linear interpolation processes as described above using the waveform obtained by the identification unit 222. The same is true for 701 in FIG. 7 and 801 in FIG. 8. 602 in FIG. 6 is a diagram showing a vibration characteristic spectrum obtained by subtracting the baseline shown in 601 in FIG. 6 from the waveform of the real part of the admittance shown in 601 in FIG. 6. The same is true for 702 in FIG. 7 and 802 in FIG. 8. In each graph in FIG. 6 to FIG. 8, the vertical axis indicates the value of the real part of the admittance, and the horizontal axis indicates the frequency.

Comparing Figures 6, 7, and 8, it can be seen that the waveform of the real part of the admittance differs depending on the liquid level position. In addition, it can be seen that the vibration characteristic spectrum for each liquid level position is also different.

Figure 9 is a diagram showing an example of reference data showing the RMSD value corresponding to each liquid level drop position. The vertical axis of Figure 9 shows the RMSD value, and the horizontal axis shows the liquid level drop position. From Figure 9, it can be seen that the RMSD increases as the liquid level position drops. In other words, differences in the liquid level position are reflected in the admittance and vibration characteristic spectrum, and it can be seen that it is effective to use the admittance and vibration characteristic spectrum as evaluation indices for identifying the liquid level position.

Therefore, as shown in Figure 9, by acquiring reference data that links the RMSD of each liquid level position with the liquid level position in advance, the liquid level position estimation device 3 can estimate the liquid level position from the measurement results of the input voltage and output current. For example, if you want the liquid level estimation device 3 to determine that there is a shortage of liquid when the liquid level falls below the liquid level position P2 at a height of 70 mm, you can set the height of 70 mm or the RMSD at that time as the threshold value.

<Processing in the liquid level position estimation device 3>
(Preparation process)
Fig. 10 is a flowchart showing an example of advance preparation processing in the liquid level position estimation device 3. As shown in Fig. 10, the control unit 13 performs processing to acquire vibration feature spectra corresponding to each of a plurality of known liquid level positions (each condition).

The calculation unit 221 acquires the waveforms of the input voltage and output current in the time domain (time domain waveforms of the input voltage and output current) measured by the measurement unit 21 for certain conditions (S1). The calculation unit 221 performs a Fourier transform on each of the input voltage and output current waveforms (S2). The calculation unit 221 calculates the admittance (complex number) by dividing each value constituting the output current waveform after the Fourier transform by each value constituting the input voltage waveform after the Fourier transform (S3; calculation step). The calculation unit 221 may perform a Fourier transform after dividing the output current value in the time domain by the input voltage value in the time domain.

The identification unit 222 extracts the real part from the calculated admittance (S4). The identification unit 222 obtains a baseline from the waveform of the extracted real part of the admittance (S5). The processing method of S5 will be described later. The identification unit 222 obtains a vibration characteristic spectrum by subtracting each value constituting the obtained baseline from each value constituting the waveform of the extracted real part of the admittance (S6).

The control unit 13 repeatedly executes the processes of S1 to S6 until all of the vibration feature spectra corresponding to each condition are acquired. When all of the vibration feature spectra have been acquired, the control unit 13 proceeds to the process of S7.

The identification unit 222 selects a reference condition from all conditions (S7). For example, the liquid level position when a specified amount of liquid is contained is selected as the reference condition. However, any liquid level position may be selected as the reference condition. The identification unit 222 calculates the RMSD of the vibration feature spectrum corresponding to each condition with respect to the vibration feature spectrum (reference feature spectrum) corresponding to the selected reference condition (S8). The identification unit 222 creates data linking the calculated vibration feature spectrum for each condition with each liquid level position as reference data (S9). The identification unit 222 stores the created reference data in the storage unit 14.

(Baseline acquisition process)
FIG. 11 is a flowchart showing an example of a baseline acquisition process in the liquid level position estimating device 3.

The identification unit 222 extracts multiple minimum points from the waveform of the real part of the admittance extracted in S4 of FIG. 10 (S21). The identification unit 222 performs linear interpolation processing on the multiple extracted minimum points (S22). The identification unit 222 further extracts multiple minimum points from the waveform after the linear interpolation processing (S23). The identification unit 222 performs linear interpolation processing on the multiple minimum points extracted in S23 (S24). The identification unit 222 acquires the waveform after the linear interpolation processing in S24 as a baseline (S25).

As mentioned above, it is possible to extract minimum points until the number of minimum points falls within a predetermined range. In this case, the determination unit 222 acquires the waveform after the final linear interpolation process as the baseline.

(Liquid Level Position Identification and Determination Processing)
12 is a flowchart showing an example of the liquid level position identification process (liquid level position estimation method) and the determination process in the liquid level position estimation device 3. Note that the processes in S31 to S36 are the same as the processes in S1 to S6 except that they are processes based on the input voltage and output current measured when identifying the liquid level position, and therefore a description thereof will be omitted.

In S37, the identification unit 222 calculates the RMSD of the vibration feature spectrum acquired in S36 relative to the reference feature spectrum selected in the advance preparation process. The identification unit 222 identifies the liquid level position by comparing the calculated RMSD with the reference data created in the advance preparation process (S38; identification step).

The determination unit 23 determines whether the liquid level position identified by the identification unit 222 is within an appropriate range (S39). The determination unit 23 outputs the determination result to the output device 4 (S40).

<Modification>
In this embodiment, the determination unit 222 obtains the vibration feature spectrum by subtracting the baseline from the waveform of the real part of the admittance in order to calculate the degree of change (e.g., RMSD), but is not limited to this. For example, if the determination unit 222 does not take into account the various influences described above,
(1) The real part of the admittance itself,
(2) The admittance (complex number) itself, or the imaginary part of the admittance itself, or
(3) (2) above with the baseline subtracted;
may be used as the vibration characteristic spectrum. In other words, the determination unit 222 may create the reference data and determine the liquid level position by using any one of (1) to (3) above.

The calculation unit 221 may also calculate impedance (=input voltage/output current) instead of admittance. Since admittance and impedance have an inverse relationship, the determination unit 222 can obtain substantially the same determination results even when using impedance as when using admittance. When using impedance, the same processing as that for admittance (e.g., processing to extract the real part and processing to obtain the baseline) may be performed.

＜Effects＞
The liquid level position estimation device 3 includes a piezoelectric element 11, a calculation unit 221 that calculates the admittance or impedance of the piezoelectric element 11 when the container 2 is vibrated by the piezoelectric element 11, and an identification unit 222 that identifies the liquid level position based on the calculation result of the calculation unit 221. The piezoelectric element 11 is provided on the outer surface of the container 2.

With the above configuration, the liquid level position estimation device 3 can accurately estimate the liquid level position without visually inspecting the inside of the container 2 or installing a sensor inside to estimate the liquid level position.

In addition, with the liquid level position estimation device 3, there is no need to detect the temperature inside the container 2 on its outer surface, as with the oil level detection device of Patent Document 1. Therefore, the liquid level position estimation device 3 can accurately estimate the liquid level position even if the container 2 is a container to which internal heat is not transferred or is transferred poorly.

Furthermore, in the oil level detection device of Patent Document 1, the oil level detection sensor cannot detect the temperature inside the compressor until the heat inside the compressor is transferred to its outer surface. Therefore, it may take time to detect the oil level. The liquid level position estimation device 3 estimates the liquid level position based on the vibration of the container 2 caused by the piezoelectric element 11 vibrating the container 2. Therefore, it takes less time to estimate the liquid level position than the oil level detection device of Patent Document 1.

The liquid level position estimation device 3 may also identify the liquid level position by comparing the characteristic spectrum of the admittance or impedance after the Fourier transform with a reference characteristic spectrum acquired in advance.

The admittance or impedance obtained by vibrating the piezoelectric element 11 for a predetermined period of time changes in value depending on the liquid level position. Therefore, the liquid level position estimation device 3 can determine the degree of change relative to the liquid level position when the reference feature spectrum was obtained by the above comparison. Therefore, the liquid level position estimation device 3 can determine the liquid level position based on the degree of change.

The identification unit 222 may also use the characteristic spectrum of the real part of the admittance or impedance of the piezoelectric element 11 calculated by the calculation unit 221 as a characteristic spectrum for identifying the liquid level position.

With the above configuration, the identification unit 222 can reduce the influence of the electrical characteristics of the piezoelectric element 11 and perform the above comparison using a feature spectrum that more faithfully represents the vibration characteristics of the piezoelectric element 11 that reflect the vibration of the container 2. Therefore, the identification unit 222 can identify the liquid level position with higher accuracy.

The identification unit 222 also obtains a baseline by performing an interpolation process or an approximation process on multiple minimum points included in the characteristic spectrum of the admittance or impedance (or the real part of these). The identification unit 222 may use the characteristic spectrum obtained by subtracting the baseline from the characteristic spectrum as the characteristic spectrum for identifying the liquid level position.

With the above configuration, the identification unit 222 can reduce the influence of the environment around the container 2 or the adhesion of the piezoelectric element 11 to the container 2, and can perform the above comparison using a characteristic spectrum that more faithfully represents the vibration characteristics of the piezoelectric element 11 that reflects the vibration of the container 2. Therefore, the identification unit 222 can identify the liquid level position with higher accuracy.

The identification unit 222 may also extract multiple minimum points on a line connecting the multiple minimum points, and perform an interpolation process or an approximation process on the multiple extracted minimum points. In this case, the number of extracted minimum points can be reduced, making it possible to obtain a smooth baseline.

The identification unit 222 also extracts a number of minimum points from a line connecting the multiple minimum points, and executes the process of extracting a number of further minimum points from the line connecting the extracted multiple minimum points, until the number of minimum points falls within a predetermined range. The identification unit 222 may execute an interpolation process or an approximation process on the multiple minimum points that fall within the predetermined range. In this case, the number of extracted minimum points can be reduced, making it possible to obtain a smoother baseline.

The determination unit 23 may also determine whether the liquid level position identified by the identification unit 222 is within an appropriate range. In this case, the liquid level position estimation device 3 can determine whether the liquid level position is appropriate. When the liquid level position estimation device 3 outputs the determination result to an external device (output device 4), it can notify the user using the container 2 of the determination result of the liquid level position.

[Embodiment 2]
Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals are given to members having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

Figure 13 is a diagram for explaining an example of linear interpolation processing in this embodiment. 1301 in Figure 13 is a diagram that shows a schematic waveform of the real part of admittance. 1302 in Figure 13 is a diagram that shows the result of extracting multiple minimum points in the waveform shown in 1301 in Figure 13 and performing linear interpolation processing.

In this embodiment, the identification unit 222 divides the waveform shown in 1301 of FIG. 13 into multiple regions as shown in 1302 of FIG. 13. For a region that includes multiple minimum points among the multiple divided regions, the identification unit 222 selects a predetermined number of minimum points from the multiple minimum points. The identification unit 222 performs linear interpolation processing on the predetermined number of minimum points selected in each of the multiple regions. This allows the identification unit 222 to obtain a smooth baseline. The width for dividing the multiple regions, the minimum points to be selected, and the number of minimum points may be set to a width that allows a smooth baseline to be obtained through experiments, etc.

Figure 13 shows an example of selecting one minimum point from a divided region. Of the divided first region Ar1, second region Ar2, and third region Ar3, the first region Ar1 and the third region Ar3 contain two minimum points. Therefore, in the first region Ar1 and the third region Ar3, one of the two minimum points (the minimum point circled in the figure) is selected. The identification unit 222 performs linear interpolation processing on the minimum points selected (extracted) one by one from the first region Ar1, the second region Ar2, and the third region Ar3.

As described in the first embodiment, the determination unit 222 may divide the waveform obtained by performing linear interpolation processing on a number of minimum points after the number of minimum points falls within a predetermined range into a number of regions. In this case, it may be possible to obtain a smoother baseline.

Furthermore, as in the first embodiment, instead of the linear interpolation process, another interpolation process or various approximation processes may be executed. Furthermore, as the waveform to be divided, instead of the waveform of the real part of the admittance, the waveform of the admittance (complex number) or the waveform of the imaginary part extracted from the admittance (complex number) may be used.

[Software implementation example]
The control block (particularly the control unit 13) of the liquid level position estimation device 3 may be realized by a logic circuit (hardware) formed on an integrated circuit (IC chip) or the like, or may be realized by software.

In the latter case, the liquid level position estimation device 3 includes a computer that executes the instructions of a program, which is software that realizes each function. This computer includes, for example, one or more processors, and a computer-readable recording medium that stores the program. The object of the present invention is achieved by the processor reading the program from the recording medium and executing it in the computer. The processor can be, for example, a CPU (Central Processing Unit). The recording medium can be a "non-transient tangible medium," such as a semiconductor memory (e.g., ROM (Read Only Memory)), as well as a tape, a disk, a card, a programmable logic circuit, or the like. The device may also include a RAM (Random Access Memory) that expands the program. The program may be supplied to the computer via any transmission medium (such as a communication network or broadcast waves) that can transmit the program. One aspect of the present invention can also be realized in the form of a data signal embedded in a carrier wave, in which the program is embodied by electronic transmission.

In other words, as one aspect of the present invention, the control unit 13 may function as a liquid level position estimation device that executes a program including at least the following calculation process and identification process. The calculation process is a process for calculating the admittance or impedance of the piezoelectric element 11 when the piezoelectric element 11 applies vibration to the container 2. The identification process is a process for identifying the liquid level position based on the calculation result of the calculation process.

[Additional Notes]
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

2 container 3 liquid level position estimation device 11 piezoelectric element 23 determination unit 221 calculation unit 222 identification unit

Claims (8)

A liquid level position estimation device that estimates a liquid level position of a liquid contained in a container, comprising:
A piezoelectric element provided on an outer surface of the container;
a calculation unit that calculates a characteristic spectrum of admittance or impedance of the piezoelectric element by performing a frequency analysis based on a voltage and a current of the piezoelectric element measured when the piezoelectric element vibrates the container for a predetermined time ;
a determination unit that calculates a deviation or error rate of the feature spectrum calculated by the calculation unit relative to a reference feature spectrum, which is a feature spectrum obtained in advance, and determines the liquid level position by comparing the calculated deviation or error rate with reference data that links the deviation or error rate obtained in advance to the liquid level position . The liquid level estimation device according to claim 1 , wherein the specifying unit uses a feature spectrum of a real part of the admittance or impedance of the piezoelectric element calculated by the calculating unit as a feature spectrum for specifying the liquid level. The identification unit is
performing an interpolation process or an approximation process on a plurality of minimum points included in the characteristic spectrum to obtain a baseline;
3. The liquid level estimation device according to claim 1 , wherein a characteristic spectrum obtained by subtracting the baseline from the characteristic spectrum is used as the characteristic spectrum for identifying the liquid level position. The liquid level position estimation device according to claim 3 , wherein the specifying unit extracts a plurality of minimum points on a line connecting the plurality of minimum points, and performs the interpolation process or the approximation process on the extracted minimum points. The identification unit is
extracting a plurality of minimum points from a line connecting the plurality of minimum points, and extracting a further plurality of minimum points from the line connecting the extracted plurality of minimum points until the number of minimum points falls within a predetermined range;
The liquid level position estimating device according to claim 3 , wherein the interpolation process or the approximation process is performed on a plurality of minimum points that fall within the predetermined range. The identification unit is
Dividing the characteristic spectrum into a plurality of regions;
For a region including a plurality of minimum points among the plurality of regions, a predetermined number of minimum points are selected from the plurality of minimum points;
The liquid level position estimating device according to claim 3 , wherein the interpolation process or the approximation process is performed on a predetermined number of minimum points selected in each of the plurality of regions. The liquid level position estimation device according to claim 1 , further comprising a determination unit that determines whether the liquid level position identified by the identification unit is within an appropriate range. A liquid level position estimation method for estimating a liquid level position of a liquid contained in a container, comprising:
a calculation step of calculating a characteristic spectrum of admittance or impedance of the piezoelectric element by performing a frequency analysis based on a voltage and a current of the piezoelectric element that are measured when the piezoelectric element is provided on the outer surface of the container and vibrates the container for a predetermined period of time;
a determination step of determining a liquid level position of the liquid by calculating a deviation or error rate of the feature spectrum calculated in the calculation step relative to a reference feature spectrum, which is a feature spectrum obtained in advance, and comparing the calculated deviation or error rate with reference data that links the deviation or error rate obtained in advance to the liquid level position.

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