JP7680694B2 - Dielectric constant measurement method and short circuit standard - Google Patents

Description

The present invention relates to a dielectric constant measurement method and a short-circuit standard used for measuring dielectric constant.

Testing the concentration of elements such as blood glucose levels requires the sampling of blood, which places a significant burden on patients. For this reason, non-invasive element concentration measuring devices that do not require the sampling of blood have been put into practical use.

As a non-invasive element concentration measurement device, a method using electromagnetic waves, for example in the microwave to millimeter wave band, has been proposed. Compared to optical methods such as near-infrared light, this method has the advantage that there is less scattering inside the body and the energy of one photon is low.

As a method using electromagnetic waves in the microwave to millimeter wave band, a method using a resonant structure disclosed in Non-Patent Document 1 has been proposed. In Non-Patent Document 1, a measurement sample is brought into contact with a device with a high Q value, such as an antenna or resonator, and the frequency characteristics around the resonant frequency are measured. Since the resonant frequency is determined by the complex dielectric constant around the device, it is possible to estimate the component concentration based on the resonant frequency shift by predicting the correlation between the resonant frequency shift and the component concentration in advance.

Dielectric spectroscopy, disclosed in Patent Document 1, has been proposed as another method using microwave-millimeter wave electromagnetic waves. In dielectric spectroscopy, electromagnetic waves are irradiated into the skin of a human or animal, and the electromagnetic waves are absorbed in accordance with the interaction between the blood components being measured, such as glucose molecules and water, and the amplitude and phase of the electromagnetic waves are observed. A dielectric relaxation spectrum is calculated from the amplitude and phase of the observed electromagnetic waves relative to their frequency. The dielectric relaxation spectrum is generally expressed as a linear combination of relaxation curves based on the Cole-Cole equation, and the complex dielectric constant is calculated.

Complex dielectric constant correlates with the amount of blood components such as glucose and cholesterol contained in blood. A calibration model can be constructed by measuring in advance the correlation between the change in complex dielectric constant and the component concentration, and the component concentration can be calibrated based on the change in the measured dielectric relaxation spectrum. Regardless of which method is used, improved measurement sensitivity can be expected by selecting a frequency band that is highly correlated with the target component, so it is necessary to measure the change in dielectric constant in advance using broadband dielectric spectroscopy.

Among dielectric spectroscopy, the method using a coaxial probe (open-ended coaxial probe or open-ended coaxial line) as shown in Non-Patent Documents 2 and 3 and Patent Document 2 can use easily available samples such as water to calibrate the measuring instrument. In addition, it is possible to measure the dielectric constant of the sample by contacting the sample to be measured with the probe end without requiring special processing of the material. For this reason, it is suitable for measuring samples such as living organisms, fruits, and soil, whose electrical properties are to be evaluated without processing them.

JP 2013-32933 A Patent No. 6771372

M. Hofmann, G. Fischer, R. Weigel, and D. Kissinger, “Microwave-Based Noninvasive Concentration Measurements for Biomedical Applications”, IEEE Trans. Microwave Theory and Techniques, Vol.61, No.5, pp. 2195-2203, 2013 J P. Grant, R N. Clarke, G T. SYymm and N M. Spyrou, “A critical study of the open-ended coaxial line sensor technique for RF and microwave complex permittivity measurements”, J. Phys.E: Sci. Instrum,Vol.22, pp. 757-770,1989 T.P. Marsland, and S. Evans“Dielectric measurements with an open-ended coaxial probe”, IEE Proceedings, Vol. 134, No.4,1987

However, when measuring the dielectric constant using a coaxial probe, the measurement accuracy of the dielectric constant depends on the S11 parameter (reflection coefficient) of the calibration standard measured in advance and the short-circuit standard, which is a calibration standard that has conductivity. Therefore, if the measurement surface of the probe has an uneven shape, the measurement error of the short-circuit standard may become large, resulting in a problem of reduced measurement accuracy of the dielectric constant.

The present invention has been made in consideration of the above circumstances, and its object is to provide a dielectric constant measurement method capable of improving the accuracy of dielectric constant measurement and a short-circuit standard body for use in dielectric constant measurement.

A dielectric constant measurement method according to one aspect of the present invention is a method for measuring the dielectric constant of an object to be measured, comprising the steps of: a dielectric spectroscopy system measuring the reflection coefficient and admittance of at least two calibration standards; a dielectric spectroscopy system measuring the reflection coefficient of a conductive short-circuit standard; a dielectric spectroscopy system measuring the reflection coefficient of the object to be measured; a dielectric spectroscopy system calculating the admittance of the object to be measured based on the reflection coefficients and admittance of each calibration standard, the reflection coefficient of the short-circuit standard, and the reflection coefficient of the object to be measured; and a dielectric spectroscopy system calculating the dielectric constant of the object to be measured based on the admittance of the object to be measured, wherein the short-circuit standard is a viscous gel.

One embodiment of the short-circuit standard of the present invention is a short-circuit standard used when measuring the dielectric constant of a measurement object using a dielectric spectroscopy system, and is in the form of a viscous gel with an admittance that is nearly infinite.

According to the present invention, it is possible to improve the measurement accuracy of the dielectric constant of the object to be measured.

FIG. 1 is a block diagram showing the configuration of a dielectric spectroscopic sensor and its peripheral devices in which a dielectric constant measuring method according to the present embodiment is adopted. FIG. 2 is an explanatory diagram showing the configuration of the measurement probe. FIG. 3 is a flowchart showing a process procedure of the dielectric constant measuring method according to this embodiment. FIG. 4A is an explanatory diagram showing a state in which the short-circuit grease is pressed by a measuring probe. FIG. 4B is an explanatory diagram showing a state in which the short-circuit grease is pressed by the measurement probe. FIG. 5A is an explanatory diagram showing a state in which a measurement probe is brought into contact with a metal plate that is a short-circuit standard, and shows a state in which there are no irregularities on the contact surface. FIG. 5B is an explanatory diagram showing the state in which the measurement probe is brought into contact with the metal plate that is the short-circuit standard, and shows a state in which there are projections and recesses on the contact surface. FIG. 6A is an explanatory diagram showing a state in which a measurement probe is brought into contact with short-circuit grease, which is a short-circuit standard body, and shows a state in which there are no irregularities on the contact surface. FIG. 6B is an explanatory diagram showing the state in which the measurement probe is brought into contact with short-circuit grease, which is a short-circuit standard body, and shows a state in which there are projections and recesses on the contact surface. FIG. 7 is a graph showing the change in frequency and phase of the reflected wave when the calibration standard is air and the short-circuit standard is short-circuit grease. FIG. 8 is a graph showing the change in frequency and phase of the reflected wave when the calibration standard is air and the short standard is a metal plate. FIG. 9 is a graph showing the change in dielectric constant of each sample with respect to the change in frequency when short-circuit grease is used as the short-circuit standard body. FIG. 10 is a graph showing the change in dielectric constant of each sample with respect to the change in frequency when a metal plate is used as the short-circuit standard. FIG. 11 is a block diagram showing the configuration of a dielectric spectroscopy sensor and its peripheral devices, in which a dielectric constant measuring method according to a modified example of the present embodiment is adopted.

[Description of the embodiment]
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a dielectric spectroscopic sensor and its peripheral devices to which a dielectric constant measurement method according to the present embodiment is applied. In the dielectric constant measurement method according to the present embodiment, two calibration standards P1 and P2 and a short-circuit grease Ps are prepared, and a dielectric spectroscopic sensor 100 measures a reflection coefficient ρ1 of the calibration standard P1, a reflection coefficient ρ2 of the calibration standard P2, and a reflection coefficient ρs of the short-circuit grease Ps. Furthermore, a dielectric spectroscopic system 200 measures a reflection coefficient ρm of a measurement target. The dielectric spectroscopic system 200 can be, for example, a general-purpose computer system including a CPU (Central Processing Unit, processor), a memory, a storage (HDD: Hard Disk Drive, SSD: Solid State Drive), a communication device, an input device, and an output device.

For example, air or water can be used as the calibration standards P1 and P2. In addition, organic solvents such as alcohols can be used as the calibration standards P1 and P2. When using an organic solvent, it is advisable to select one whose dielectric constant frequency characteristics are known.

As the short-circuit grease Ps, a liquid metal type grease made of a material having conductivity close to that of a metal and properties close to that of a liquid at room temperature can be used. As the short-circuit grease Ps, for example, a liquid metal type grease containing at least one of Hg, Ga, In, Sn, Cu, and Al can be used. As the short-circuit grease Ps, other than a liquid, metal particles having a diameter of about several μm to several tens of μm may be placed in a container to give it fluidity. The short-circuit grease Ps has an admittance that is almost infinite. In other words, the short-circuit grease Ps has an impedance of almost 0 Ω. The short-circuit grease Ps is an example of a short-circuit standard body. The short-circuit standard body may be a material other than the short-circuit grease Ps as long as it has an admittance that is almost infinite and is a gel-like material with viscosity.

Figure 2 is an explanatory diagram showing an example in which a coaxial measuring probe 10 is used as a dielectric spectroscopy sensor 100 used when measuring the reflection coefficient. Figure 2 shows a plan view and a cross-sectional view of the measuring probe 10. As shown in Figure 2, the measuring probe 10 is concentric in plan view. Note that in this embodiment, an example in which the measuring probe 10 is used as the dielectric spectroscopy sensor 100 is described, but it is also possible to use a substrate-type sensor as the dielectric spectroscopy sensor 100.

2, the measurement probe 10 has a conductive internal conductor 11 in the center, and a dielectric 12 is provided around the internal conductor 11 so as to be concentric with the internal conductor 11. A conductive external conductor 13 is provided around the dielectric 12 so as to be concentric with the dielectric 12. A fringe 14 is provided on the lower end surface of the measurement probe 10. The upper end surface of the measurement probe 10 is connected to the dielectric spectroscopy system 200 shown in FIG. 1.

The fringe 14 is made of metal. The fringe 14 ensures contact with samples such as the calibration standards P1, P2 and the short circuit grease Ps. The fringe 14 serves to disperse pressure applied to the samples.

In the dielectric constant measurement method according to this embodiment, the admittance ym of the object to be measured is calculated according to the procedure shown below. Furthermore, the dielectric constant εm of the object to be measured is calculated based on the calculated admittance ym. The admittance ym and the dielectric constant εm are numerical values represented by complex numbers consisting of a real part and an imaginary part. The procedure for calculating the dielectric constant εm will be described below with reference to the flowchart shown in FIG. 3.

First, in step ST1 of Fig. 3, the dielectric spectroscopy system 200 applies a voltage of a predetermined frequency between the internal conductor 11 and the external conductor 13 shown in Fig. 2 with the fringe surface of the measurement probe 10 in contact with the calibration standard P1 described above, and measures the reflection coefficient ρ1. Next, with the fringe surface in contact with the calibration standard P2, a voltage of a predetermined frequency is applied between the internal conductor 11 and the external conductor 13, and measures the reflection coefficient ρ2.

For example, when the calibration standard P1 is air, a voltage of a predetermined frequency is applied between the inner conductor 11 and the outer conductor 13 with the fringe surface placed in the air. The reflected wave at this time is transmitted to the dielectric spectroscopy system 200. The dielectric spectroscopy system 200 measures the reflection coefficient ρ1 of the calibration standard P1 based on the detected reflected wave. The reflection coefficient ρ2 of the calibration standard P2 can also be measured in a similar manner.

In step ST2, the dielectric spectroscopy system 200 applies a voltage of a predetermined frequency between the inner conductor 11 and the outer conductor 13 while bringing the fringe surface of the measurement probe 10 into contact with the short-circuit grease Ps, and measures the reflection coefficient ρs.

Specifically, as shown in FIG. 4A, the short-circuit grease Ps is placed on the upper surface of the installation stand 20, and the fringe surface 14a of the measurement probe 10 is brought into contact with the short-circuit grease Ps. Furthermore, the fringe surface 14a presses the short-circuit grease Ps from above. As a result, as shown in FIG. 4B, the short-circuit grease Ps is deformed into a flat shape, and the short-circuit grease Ps comes into contact with the entire lower end surface of the measurement probe 10.

In this state, a voltage of a predetermined frequency is applied between the internal conductor 11 and the external conductor 13 shown in Figure 2. The reflected wave at this time is transmitted to the dielectric spectroscopy system 200. The dielectric spectroscopy system 200 calculates the reflection coefficient ρs of the short circuit grease Ps based on the detected reflected wave.

In step ST3, the dielectric spectroscopy system 200 applies a voltage of a predetermined frequency between the inner conductor 11 and the outer conductor 13 while the fringe surface of the measurement probe 10 is in contact with the surface of the object to be measured, and measures the reflection coefficient ρm. Note that hereinafter, the reflection coefficients ρ1, ρ2, ρs, and ρm may be referred to as the "S11 parameters."

In step ST4, the dielectric spectroscopy system 200 acquires the admittances y1 and y2 of the calibration standards P1 and P2. Each admittance y1 and y2 can be known from the material of each calibration standard P1 and P2. Since the short circuit grease Ps is a conductor, the admittance ys is almost infinite.

If the admittance of the object to be measured is ym, the following equations (1) and (2) hold.

In equation (1), "y1, y2, ys, ym" are linear maps of admittances, and are represented by the same symbols as the admittances y1, y2, ys, and ym. In equation (2), "G0" is the conductance of the measurement probe 10 in a vacuum, and "C0" is the capacitance of the measurement probe 10 in a vacuum.

In step ST5, the dielectric spectroscopy system 200 substitutes the numerical values measured or obtained in the processes ST1 to ST4 described above into the above equation (1).

In step ST6, the dielectric spectroscopy system 200 calculates the admittance ym of the measurement object based on equations (1) and (2). That is, the admittance ys of the short circuit grease Ps can be set to "ys = ∞", so the following equations (3), (4), and (5) can be obtained from the above equations (1) and (2).

By substituting equation (4) into equation (5), we obtain the following equation (6).

In step ST7, the dielectric spectroscopy system 200 calculates the dielectric constant εm of the object to be measured using the above formula (6). As described above, the dielectric constant εm is a complex number.

As is clear from the above formulas (1) to (6), the dielectric constant εm of the measurement object is calculated based on the S11 parameters (reflection coefficients ρ1, ρ2, ρs) of the calibration standards P1 and P2 and the short circuit grease Ps. Therefore, if the S11 parameter cannot be measured accurately, the error in the dielectric constant εm increases, and high-precision measurement is not possible.

For this reason, when a metal with no viscosity, such as a conductive metal plate or metal piece, is used as the short-circuit standard, the S11 parameter is measured with the fringe surface in contact with the surface of the short-circuit standard 35 as shown in Figure 5A. However, due to the metal processing accuracy and the shrinkage of the dielectric 12 provided in the measurement probe 10, unevenness may exist on the bottom end surface of the measurement probe 10.

For example, as shown in Figure 5B, a gap 36 occurs between the underside of the fringe 14 and the underside of the measurement probe 10. In such a case, the presence of the gap 36 makes it impossible to measure the reflection coefficient of the short-circuit standard with high accuracy.

In other words, in the above equation (1), the measurement accuracy of the reflection coefficient ρs decreases, and ultimately the measurement accuracy of the dielectric constant εm of the object to be measured decreases.

In contrast, the dielectric constant measurement method according to the present embodiment uses a viscous gel-like short-circuit grease Ps as the short-circuit standard. Therefore, when the bottom end surface of the measurement probe 10 is flat, the bottom end surface adheres closely to the short-circuit grease Ps without any gaps, as shown in FIG. 6A.

Even if the bottom end surface of the measurement probe 10 has irregularities as shown in Fig. 6B, the viscous gel-like short-circuit grease Ps fills the gaps created by the irregularities, so the bottom end surface of the measurement probe 10 can be tightly attached to the short-circuit grease Ps without any gaps. As a result, the measurement accuracy of the reflection coefficient ρs can be improved, and the dielectric constant εm of the measurement object can be measured with high accuracy.

Figure 7 is a graph showing the relationship between the frequency of the voltage generated between the internal conductor 11 and external conductor 13 of the measurement probe 10 and the phase of the reflected wave generated by the application of the voltage, where curve Q1 is a graph when air is used as the calibration standard, and curve Q12 is a graph when the short-circuit grease Ps adopted in this embodiment is used as the short-circuit standard.

Figure 8 is a graph showing the relationship between the frequency of the voltage generated between the internal conductor 11 and the external conductor 13 of the measurement probe 10 and the phase of the reflected wave, where curve Q11 is a graph when air is used as the calibration standard, and curve Q12 is a graph when a conventionally used short-circuiting metal plate is used as the short-circuiting standard instead of short-circuiting grease Ps.

As can be seen from curves Q11 and Q12 in Figure 8, with the conventional method, almost no phase difference occurs between the calibration standard and the short-circuit standard, and the characteristics of air, which is close to free single reflection, and a short circuit, which is close to fixed single reflection, cannot be accurately measured.

In contrast, as can be seen from curves Q1 and Q2 in Figure 7, when short-circuit grease Ps is used as the short-circuit standard body, the phase is inverted by approximately 180 degrees between curves Q1 and Q2, indicating that good measurement results are obtained.

This is because even if the underside of the measurement probe 10 is not flat but has projections and recesses, the short-circuit grease Ps is present to fill the spaces of the projections and recesses as shown in Figure 6B.

Figure 9 is a graph showing the results of measuring the dielectric constant of water and a glucose aqueous solution with a concentration of 5 g/dL using short-circuit grease Ps as a short-circuit standard. Figure 10 is a graph showing the results of measuring the dielectric constant of water and a glucose aqueous solution with a concentration of 5 g/dL using a conductive metal plate as a short-circuit standard.

Curves q1 and q11 shown in Figures 9 and 10 show the theoretical value of the dielectric constant of water, curves q2 and q12 show the measured value of the dielectric constant of water, and curves q3 and q13 show the measured value of the dielectric constant of an aqueous glucose solution.

In the conventional method shown in Figure 10, the dielectric constant q12 of water and the dielectric constant q13 of the glucose aqueous solution are almost the same, with almost no difference being obtained. Also, there is a deviation between the theoretical value q11 of the dielectric constant of water and the measured value q12. In contrast, when the method of this embodiment shown in Figure 9 is adopted, the measured value q2 of the dielectric constant of water is almost the same as the theoretical value q1. Also, in this embodiment, as shown by symbol q3, the change in the dielectric constant of the glucose aqueous solution with respect to the change in frequency can be confirmed.

In other words, in the dielectric constant measurement method of this embodiment, by using a viscous, gel-like short-circuit grease Ps as the short-circuit standard body, dielectric spectroscopy measurements can be performed with high accuracy.

Thus, the dielectric constant measurement method according to this embodiment is a method for measuring the dielectric constant of an object to be measured, and includes the steps of: a dielectric spectroscopy system 200 measuring the reflection coefficients ρ1, ρ2 and admittances y1, y2 of at least two calibration standards P1, P2; a dielectric spectroscopy system 200 measuring the reflection coefficient ρs of a conductive short-circuit standard; a dielectric spectroscopy system 200 measuring the reflection coefficient ρm of the object to be measured; a dielectric spectroscopy system 200 calculating the admittance ym of the object to be measured based on the reflection coefficients ρ1, ρ2 and admittances y1, y2 of each calibration standard, the reflection coefficient ρs of the short-circuit standard, and the reflection coefficient ρm of the object to be measured; and a dielectric spectroscopy system 200 calculating the dielectric constant εm of the object to be measured based on the admittance ym of the object to be measured, and the short-circuit standard is a viscous gel.

In this embodiment, even if there are irregularities on the lower end surface of the measurement probe 10, by using a viscous gel-like short-circuit standard body, it is possible to fill the short-circuit standard body into the gap created by the irregularities, and the reflection coefficient ρs of the short-circuit standard body can be measured with high accuracy.

Therefore, the dielectric constant εm of the object to be measured can be calculated with high accuracy using the above-mentioned equations (3) to (6), thereby improving the measurement accuracy of the dielectric constant εm.

By using short-circuit grease Ps, which is a liquid metal type grease, as the short-circuit standard body, the admittance can be made almost infinite, thereby improving the measurement accuracy of the dielectric constant εm of the object to be measured.

By using a liquid metal type grease containing at least one of Hg, Ga, In, Sn, Cu, and Al as the short-circuit standard, the electrical conductivity can be increased, and the measurement accuracy of the dielectric constant εm of the object to be measured can be further improved.

By using air, water, or an organic solvent as the calibration standards P1 and P2, the reflection coefficients ρ1 and ρ2 of the calibration standards P1 and P2 can be easily measured. This reduces the effort required to measure the dielectric constant of the object being measured.

[Description of Modifications]
Next, a modified example of the above-described embodiment will be described. Fig. 11 is a block diagram showing the configuration of a dielectric spectroscopy sensor and its peripheral devices in which a dielectric constant measurement method according to the modified example is adopted. In the dielectric constant measurement method according to the modified example, three calibration standards P1, P2, and P3 and short-circuit grease Ps are prepared, and the reflection coefficient ρ1 of the calibration standard P1, the reflection coefficient ρ2 of the calibration standard P2, and the reflection coefficient ρs of the short-circuit grease Ps are measured by the dielectric spectroscopy sensor 100. Furthermore, the reflection coefficient ρm of the measurement object is measured by the dielectric spectroscopy system 200.

By calculating the reflection coefficient using the three calibration standards P1 to P3, even if the conductance G0 and capacitance C0 shown in equation (2) are unknown values, the conductance G0 and capacitance C0 can be calculated based on the reflection coefficient of each calibration standard P1 to P3.

In other words, in the dielectric constant measurement method of the modified example, it is possible to measure the dielectric constant εm of the object to be measured even if the conductance G0 and capacitance C0 of the measurement probe 10 are unknown.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention.

REFERENCE SIGNS LIST 10 Measurement probe 11 Inner conductor 12 Dielectric 13 Outer conductor 14 Fringe 20 Installation stand 35 Short circuit standard 36 Air gap 100 Dielectric spectroscopy sensor 200 Dielectric spectroscopy system P1, P2, P3 Calibration standard Ps Short circuit grease

Claims (4)

A method for measuring a dielectric constant of a measurement object, comprising the steps of:
a dielectric spectroscopy system measuring a reflection coefficient and an admittance of at least two calibration standards;
the dielectric spectroscopy system measuring the reflection coefficient of a short standard having a nearly infinite conductive admittance;
measuring a reflection coefficient of the measurement object by the dielectric spectroscopy system;
the dielectric spectroscopy system calculating an admittance of the measurement object based on the reflection coefficient and admittance of each calibration standard, the reflection coefficient of the short standard, and the reflection coefficient of the measurement object;
The dielectric spectroscopy system calculates a dielectric constant of the measurement object based on an admittance of the measurement object,
The dielectric constant measuring method, wherein the short standard body is in a viscous gel state. The dielectric constant measuring method according to claim 1 , wherein the short-circuit standard body is a liquid metal type grease. 3. The dielectric constant measuring method according to claim 2, wherein the short standard body is a liquid metal type grease containing at least one of the group consisting of Hg, Ga, In, Sn, Cu, and Al. The dielectric constant measuring method according to any one of claims 1 to 3, wherein the calibration standard is any one of air, water, and an organic solvent.

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