The present invention relates to a dielectric spectroscopic measurement apparatus and a method for measuring a complex permittivity of a slight amount of a liquid sample.
With the progression of aging, it has been a big issue of concern how to address lifestyle diseases. A test for blood sugar level or the like, which requires blood drawing, is a larger burden on patients. Accordingly, a non-invasive component concentration measurement apparatus not requiring blood drawing has attracted attention.
For a non-invasive component concentration measurement, a technology using a microwave-millimeterwave-band electromagnetic wave has been proposed. This technology has advantages including less in vivo scatter and a lower energy per photon as compared with optical measurement using a near-infrared light. Examples using a microwave-millimeterwave-band electromagnetic wave include a measurement technology using a resonance structure as described in Non-Patent Literature 1. In this technology, a measurement device with a high Q value, such as an antenna or a resonator, is brought into contact with a measurement sample, thereby measuring frequency characteristics in the vicinity of a resonance frequency. The resonance frequency is determined by a complex permittivity around the measurement device and, accordingly, a correlation between a shift amount of the resonance frequency and a component concentration is predicted, thereby estimating the component concentration from the shift amount of the resonance frequency.
As another measurement technology using a microwave-millimeterwave-band electromagnetic wave, a dielectric spectroscopy method as described in Patent Literature 1 has been proposed. In the dielectric spectroscopy method, an electromagnetic wave is applied into skin and the electromagnetic wave is absorbed in accordance with an interaction between blood components as measurement targets, such as glucose molecules and water, thereby observing an amplitude and a phase of the electromagnetic wave. A dielectric relaxation spectrum is calculated from the observed amplitude and phase with respect to a frequency of the electromagnetic wave.
The dielectric relaxation spectrum is typically expressed as a linear combination of relaxation curves on the basis of a Cole-Cole expression, based on which a complex permittivity is calculated. For a measurement of a biological component, the amount of a blood component, such as glucose or cholesterol, contained in the blood is correlated with the complex permittivity, and an electrical signal (with amplitude, phase) corresponding to a change therein is measured. A quantitative detection model is created by measuring in advance a correlation between a change in the complex permittivity and a component concentration, and quantitative detection is performed for determining the component concentration by comparing a change in the measured dielectric relaxation spectrum and the quantitative detection model. Irrespective of whether either of the measurement technologies is used, an improvement in measurement sensitivity can be expected by selecting a frequency band that has a strong correlation with a target component. Accordingly, it is important to measure a change in permittivity in advance by broadband dielectric spectroscopy in advance.
Among dielectric spectroscopy methods, a technology using a coaxial probe (an open-ended coaxial probe or an open-ended coaxial line) as described in Non-Patent Literature 2 is capable of using an easily available sample such as water for calibration of a measurement instrument. Further, this measurement technology eliminates the necessity of special machining of a material and makes it possible to measure a permittivity of a measurement sample by bringing the sample to measure into contact with a probe end surface. In view of the above, the measurement technology using the coaxial probe described in Non-Patent Literature 2 is suitable for measuring a sample difficult to machine, such as a living body or soil.
Patent Literature 1: Japanese Patent Laid-Open No. 2013-032933.
Non-Patent Literature 1: M. Hofmann et al., “Microwave-Based Noninvasive Concentration Measurements for Biomedical Applications”, IEEE Transactions on Microwave Theory and Techniques, vol. 61, no. 5, pp. 2195-2204, 2013.
Non-Patent Literature 2: J. P. Grant, “A critical study of the openended coaxial line sensor technique for RF and microwave complex permittivity measurements”, Journal of Physics E: Scientific Instruments, vol. 22, pp. 757-770, 1989.
However, a typical measurement using a coaxial probe, which is intended to accurately measure permittivity under conditions that a substance to measure has a sufficient thickness, is not capable of measuring permittivity in a case where a measurement target is thin unless a thickness of the measurement target is known. Further, in a case where a measurement target is multi-layered, a permittivity cannot be measured unless a permittivity or the like of a part of the measurement target is known.
For example, assuming a non-invasive biological component application such as analysis of a sugar content in a fruit or estimation of an in vivo glucose concentration, in many cases, a site where a coaxial probe is to be brought into contact is supposed to have a two-layer structure including at least a barrier layer for retaining water and an in vivo layer containing a lot of water. In such a case, it is desirable that an influence of the barrier layer be reduced so that a component concentration estimated from an in vivo permittivity and permittivity information can be calculated. However, it is difficult to measure an in vivo material permittivity and a thickness of the barrier layer in advance and, accordingly, a typical technology using a coaxial probe is not capable of an accurate measurement.
Embodiments of the present invention can solve a problem as described above and an object thereof is to enable a multilayer measurement target to be accurately measured by a dielectric spectroscopy method using a coaxial probe.
A dielectric spectroscopic measurement apparatus according to embodiments of the present invention includes: a first probe including a coaxial line and having an opened end as a detection end; a second probe including a coaxial line and having an opened end as a detection end, the second probe having a longer penetration length than the first probe; and a measurement instrument configured to determine, from a result of a measurement of a measurement object using the first probe and a result of a measurement of the measurement object using the second probe, a permittivity of a second medium of the measurement object in which a first medium on an outer-layer side that is thinner than a penetration length of the first probe and the second medium on a deep-layer side relative to the first medium are stacked on each other.
A dielectric spectroscopic measurement method according to embodiments of the present invention of determining, by a dielectric spectroscopy method using a first probe including a coaxial line and having an opened end as a detection end and a second probe including a coaxial line and having an opened end as a detection end, the second probe having a longer penetration length than the first probe, a permittivity εs of a second medium of a measurement object in which a first medium on an outer-layer side that is thinner than a penetration length of the first probe and the second medium on a deep-layer side relative to the first medium are stacked on each other, the dielectric spectroscopic measurement method includes: a first step of determining an actual measured value of permittivity of the first medium by a measurement of the measurement object using the first probe and determining an actual measured value Ymeasured of admittance at the detection end of the second probe by a measurement of the measurement object using the second probe; a second step of determining, with use of a model of admittance at the detection end of the second probe with a permittivity ε1 of the first medium and the permittivity εs, a model value Ymodel of admittance at the detection end of the second probe with an assumption that the permittivity ε1 is defined as the actual measured value of permittivity and the permittivity εs is defined as a variable; and a third step of determining the permittivity εs at which the actual measured value Ymeasured and the model value Ymodel become equal.
As described hereinbefore, according to embodiments of the present invention, the use of the first probe and the second probe, which has a longer penetration length than the first probe, enables a multilayer measurement target to be accurately measured by a dielectric spectroscopy method using a coaxial probe.
Description will be made below on a dielectric spectroscopic measurement apparatus according to an embodiment of the present invention with reference to
The first probe 101 includes a coaxial line and has an opened end as a detection end 101a. The second probe 102 includes a coaxial line and has an opened end as a detection end 102a. Further, the second probe 102 has a longer penetration length than the first probe 101. With use of these probes, a permittivity of a measurement object 150 is to be measured as an electrical signal.
The measurement instrument 103 determines, from a result of a measurement of the measurement object using the first probe 101 and a result of a measurement of the measurement object using the second probe 102, a permittivity of a second medium 152 of the measurement object 150 in which a first medium 151 on an outer-layer side that is thinner than the penetration length of the first probe 101 and the second medium 152 on a deep-layer side relative to the first medium 151 are stacked on each other.
The first probe 101 includes the coaxial line including an outer conductor 111 and an inner conductor 112 with a space between the outer conductor 111 and the inner conductor 112 filled with a dielectric layer 113 including a fluorine resin or the like. Electrical properties such as impedance and admittance of the measurement object 150 can be measured by the first probe 101 with use of a leakage electromagnetic field occurring between the outer conductor 111 and the inner conductor 112, which are brought into contact with the measurement object 150 at the detection end 101a.
Further, the detection end 102a of the first probe 101 can be provided with, for example, a fringe 114. The disc-shaped fringe 114 can be provided at an end portion of the columnar first probe 101. The fringe 114 is provided on the outer conductor 111. A surface of the fringe 114 in a direction perpendicular to a waveguide direction of the coaxial line is, for example, wider than a region where the electric field strength of the leakage electric field from the detection end 101a becomes 1% or less of a maximum value.
The second probe 102 includes the coaxial line including an outer conductor 121 and an inner conductor 122 with a space between the outer conductor 121 and the inner conductor 122 filled with a dielectric layer 123 including a fluorine resin or the like. An outer diameter of the inner conductor 122 is larger than an outer diameter of the inner conductor 112. Electrical properties such as impedance and admittance of the measurement object 150 can be measured by the second probe 102 with use of a leakage electromagnetic field occurring between the outer conductor 121 and the inner conductor 122, which are in contact with the measurement object 150 at the detection end 102a.
Further, the detection end 102a of the second probe 102 can be provided with, for example, a fringe 124. The disc-shaped fringe 124 can be provided at an end portion of the columnar second probe 102. The fringe 124 is provided on the outer conductor 121. A surface of the fringe 124 in a direction perpendicular to a waveguide direction of the coaxial line is, for example, wider than a region where the electric field strength of the leakage electric field from the detection end 102a becomes 1% or less of a maximum value.
Further, as illustrated in
The measurement instrument 103 includes a first process unit 131, a second process unit 132, a third process unit 133, a high-frequency measurement unit 134, and a display unit 135. The high-frequency measurement unit 134 sweeps a frequency within a predetermined range to generate an electromagnetic wave and supplies the electromagnetic wave to the first probe 101 and the second probe 102. In addition, with the electromagnetic wave absorbed in the measurement object 150 at each of the first probe 101 and the second probe 102, the high-frequency measurement unit 134 measures (observes) an amplitude and a phase of the electromagnetic wave.
It should be noted that the high-frequency measurement unit 134 is, for example, a vector network analyzer. Alternatively, a commercially available impedance analyzer, LCR meter, or the like is usable as the high-frequency measurement unit 134.
The first process unit 131 first determines an actual measured value of permittivity of the first medium 151 from a measurement result measured by the high-frequency measurement unit 134 through the measurement of the measurement object 150 using the first probe 101. The first process unit 131 also determines an actual measured value Ymeasured of admittance at the detection end 102a of the second probe 102 from a measurement result measured by the high-frequency measurement unit 134 through the measurement of the measurement object 150 using the second probe 102.
With use of a model of admittance at the detection end 102a of the second probe 102 with a permittivity ε1 of the first medium 151 and a permittivity εs of the second medium 152, the second process unit 132 determines a model value Ymodel of admittance at the detection end 102a of the second probe 102 with an assumption that the permittivity ε1 is the actual measured value of permittivity and the permittivity εs is a variable.
The third process unit 133 determines the permittivity εs at which the actual measured value Ymeasured and the model value Ymodel become equal. The display unit 135 displays a result determined by the third process unit 133.
Next, the dielectric spectroscopic measurement apparatus according to the embodiment will be described in more detail.
A characteristic impedance of the coaxial line is represented by Expression (1) below. In Expression (1), Zo is a characteristic impedance (Ω) of the coaxial line, εr is a parameter indicating a relative permittivity of a dielectric layer in the coaxial line, a is a radius of an outer diameter of an inner conductor, and b is a radius of an inner diameter of an outer conductor. Further, a cutoff frequency of the coaxial line is represented by Expression (2) below. In Expression (2), fc is the cutoff frequency and v is the speed of light. Expressions (1) and (2):
For example, the high-frequency measurement unit in the measurement instrument 103 is typically designed such that the characteristic impedance becomes 50 Ω or 75 Ω. Accordingly, the parameters a, b, and εr are designed such that an upper limit of a measurement frequency does not become the cutoff frequency fc or less and the characteristic impedance satisfies the above. For example, in a case where the upper limit of the measurement frequency is 50 GHz, the characteristic impedance is 50 Ω, and the dielectric layer between the outer conductor and the inner conductor is a fluorine resin (εr ≈ 2.2), a is 0.175 mm, and b is 0.8 mm.
While the characteristic impedances of the first probe 101 and the second probe 102 are designed to be the same in value, the outer diameter of the inner conductor 122 is designed to be larger than the outer diameter of the inner conductor 112. It means that the first probe 101 and the second probe 102 have a structure that satisfies Expression (3). It should be noted that in Expression (3), numbers of the variables denote the first probe 101 and the second probe 102. Expression (1):
For example, in a case where the upper limit of the measurement frequency is 50 GHz, the characteristic impedance is 50 Ω, and the material of the dielectric layer is a fluorine resin (εr ≈ 2.2), a1, b1, a2, and b2 are 0.175 mm, 0.8 mm, 0.33 mm, and 1.5 mm, respectively. It should be noted that in the embodiment, a1 < a2 and b1 < b2. and the second probe 102 has a wide opening and is low in cutoff frequency. At this time, a decay rate of an electric field strength of each of the probes in a direction toward the measurement object 150 is as in
Here, the first process unit 131 calculates the permittivity of the measurement object 150 from impedance, admittance, reflection coefficient, etc. measured by the high-frequency measurement unit 134. For example, with use of a first reference substance, a second reference substance, and a third reference substance, permittivities of which are known in advance, the permittivity of the measurement object 150 is calculated by Expression (4) and Expression (5) below, or the like. Expressions (4) and (5):
Here, ρ1 is a reflection coefficient determined as a result of a measurement of the first reference substance, ρ2 is a reflection coefficient determined as a result of a measurement of the second reference substance, and ρ3 is a reflection coefficient determined as a result of a measurement of the third reference substance. Further, ρ4 is a reflection coefficient determined as a result of a measurement of a target substance.
Further, y1 is a linear mapping of admittance determined as a result of a measurement of the first reference substance having a permittivity of ε1, y2 is a linear mapping of admittance determined as a result of a measurement of the first reference substance having a permittivity of ε2, and y3 is a linear mapping of admittance determined as a result of a measurement of the first reference substance having a permittivity of ε3. Further, y4 is a linear mapping of admittance determined as a result of a measurement of the measurement object 150 having a permittivity of ε4. Go denotes a characteristic impedance of a portion of each of the probes projecting outside with respect to the detection end.
The permittivity of the measurement object 150 is calculated by using the first reference substance, the second reference substance, and the third reference substance, each of which has a known permittivity, as calibration standards. Air, solid, liquid metal, water, or an organic solvent such as alcohol is usable as the calibration standards.
Here, the dielectric spectroscopic measurement apparatus (the second process unit 132) according to the embodiment provides an effective permittivity model for the object 150 as a material including a dielectric body 151a and a dielectric body 152a as illustrated in
A model of admittance for measuring a two-layer medium including the above-described two types of dielectric bodies can be represented by, for example, Expression (6) below (see Reference Literature 1). Expression (6):
In Expression (6), εc is a permittivity of an insulation body of the coaxial line, ko is a wave number of a measurement frequency, ε1 and γ1 are a permittivity and a propagation constant of the outer-layer-side dielectric body, εs and γs are a permittivity and a propagation constant of the deep-layer-side dielectric body, Jo(x) is a o-order Bessel function, and ζ is a variable with Hankel transform. Further, the penetration depth dp1 of the first probe 101 is designed to be larger than a thickness of the first medium 151. This causes an influence of the permittivity and thickness of the outer layer, or first medium 151, to be encompassed in the permittivity ε1 determined by measurement using the first probe 101, which makes it possible to treat the dielectric body 152a in the effective permittivity model illustrated in
It should be noted that a model of admittance for measuring a two-layer medium including the above-described two types of dielectric bodies can also be represented by, for example, Expression (7) below (see Reference Literature 1). It should be noted that in Expression (7), M is a decay rate of a strength of the coaxial probe. Further, Expression (8) is used for an evaluation function. εmeas is a measured effective permittivity. Expressions (7) and (8):
Next, description will be made on a dielectric spectroscopic measurement method according to an embodiment of the present invention with reference to
First, in Step S101, a measurement surface, or outer surface, of the measurement object 150 (the first medium 151) is subjected to calibration so that the outer surface serves as a boundary surface between the probe and the measurement target. As a material having a known permittivity, air, metal, or pure water is used as a standard sample, thereby obtaining data for calibration. In a case where metal is not used as the standard sample, two types of organic solvents such as alcohol may be used instead.
Next, in Step S102, measurement using the first probe 101 and measurement using the second probe 102 are performed.
Next, in Step S103, the first process unit 131 determines the actual measured value of permittivity of the first medium 151 from a measurement result measured by the high-frequency measurement unit 134 through the measurement of the measurement object 150 using the first probe 101. Further, the first process unit 131 determines the actual measured value Ymeasured of admittance at the detection end 102a of the second probe 102 from a measurement result measured by the high-frequency measurement unit 134 through the measurement of the measurement object 150 using the second probe 102 (a first step).
Next, in Step S104, with use of a model of admittance at the detection end 102a of the second probe 102 with a permittivity ε1 of the first medium 151 and a permittivity εs of the second medium 152, the second process unit 132 determines a model value Ymodel of admittance at the detection end 102a of the second probe 102 with an assumption that the permittivity ε1 is the actual measured value of permittivity and the permittivity εs is a variable (a second step). The model of admittance can be a model represented by, for example, Expression (6).
Next, in Step S105, the permittivity εs of the second medium 152 is determined by an inverse problem analysis where the actual measured value Ymeasured and the model value Ymodel become equal (a third step).
A dielectric spectroscopic spectrum can be obtained by repeatedly performing the above-described Step S101 to Step S105 for a number of times corresponding to predetermined frequency points.
It should be noted that a measurement instrument in a dielectric spectroscopic measurement apparatus according to the above-described embodiment may be provided by computer equipment including a CPU (Central Processing Unit), a main storage, an external storage, a network connection apparatus, etc. so that the CPU is caused to work in accordance with a program developed in the main storage (run the program), thereby implementing the above-described functions (the dielectric spectroscopic measurement method). The above-described program is a program for a computer to perform the dielectric spectroscopic measurement method described in the above-described embodiment. Further, the functions may be distributed among a plurality of pieces of computer equipment.
Further, the measurement instrument in the dielectric spectroscopic measurement apparatus according to the above-described embodiment may include a programmable logic device (PLD) such as an FPGA (field-programmable gate array). For example, logic elements of the FPGA may be provided with a first process unit, a second process unit, a third process unit, and a fourth process unit as individual circuits, thereby being able to function as a measurement instrument. The first process unit, the second process unit, the third process unit, and the fourth process unit can each be written in the FPGA with a predetermined writing apparatus connected. Further, the writing apparatus connected to the FPGA enables the above-described circuits written in the FPGA to be seen.
As described hereinbefore, according to embodiments of the present invention, the use of the first probe and the second probe, which has a longer penetration length than the first probe, enables a multilayer measurement target to be accurately measured by a dielectric spectroscopy method using a coaxial probe.
It should be noted that embodiments of the present invention are not limited to the exemplary embodiments described hereinbefore and it is obvious that a lot of modifications and combinations are achievable by a person having ordinary skill in the art within the technical scope of the present invention.
Reference Literature 1: Kok Yeow You, “RF Coaxial Slot Radiators: Modeling, Measurements, and Applications”, ISBN: 9781608078226.
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This patent application is a national phase filing under section 371 of PCT application no. PCT/JP2020/015498, filed on Apr. 6, 2020, which is incorporated herein by reference in its entirety.
Filing Document | Filing Date | Country | Kind |
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PCT/JP2020/015498 | 4/6/2020 | WO |