Oil dielectric constant detection method based on resonant frequency deviation

Abstract

The invention discloses an oil dielectric constant detection method based on resonant frequency deviation, and belongs to the technical field of oil state monitoring and microwave sensing. The method comprises the following steps: constructing a lambda / 4 coaxial resonant cavity, and measuring a first resonant frequency of the cavity in an air state as a reference frequency; after to-be-measured oil liquid is injected into the resonant cavity induction area, the new resonant frequency of the resonant cavity induction area is measured; and calculating the relative dielectric constant of the oil according to a frequency deviation relation formula. According to the method, non-contact high-sensitivity detection is realized by utilizing resonant frequency change caused by medium disturbance, and the method is suitable for various industrial oil liquids such as lubricating oil, transformer oil, insulating oil and the like. The method is compact in structure, accurate in measurement and rapid in response, has good repeatability and anti-interference capability, and is suitable for online monitoring and batch deployment application.

Description

Technical Field

[0001] The present invention relates to the technical field of oil state monitoring and electromagnetic detection, and in particular to a method for detecting the dielectric constant of oil based on resonant frequency shift. Background Art

[0002] Oil, a widely used working medium in critical systems such as power equipment, industrial machinery, and vehicle engines, performs multiple functions, including lubrication, cooling, sealing, and insulation. Its performance is directly related to the safety and stability of equipment operation. Over long-term use, oil's chemical composition can change due to factors such as temperature rise, oxidation, moisture intrusion, and metal particle contamination, leading to degradation of parameters such as dielectric properties, viscosity, and conductivity. Therefore, accurately monitoring the oil's physical and electrical parameters, particularly the dielectric constant, which reflects its polarization ability, is crucial for assessing its aging, evaluating its contamination status, and determining when to replace it.

[0003] Existing methods for measuring the dielectric constant of oil primarily include the parallel plate capacitance method, the bridge method, infrared spectroscopy, and chemical titration. While these methods have certain applicability under laboratory conditions, they generally suffer from several shortcomings. The parallel plate capacitance method places high demands on the test environment and electrode structure, and is easily affected by factors such as the oil filling state, impurities, and electrode polarization, resulting in poor repeatability of test results. While the AC bridge method offers high accuracy, its test circuit is complex and requires high operator expertise, making it difficult to implement on-site. While infrared or Raman spectroscopy techniques can reflect changes in oil composition within specific wavelength bands, they are highly dependent on equipment price, sample processing, and analytical models, making them unsuitable for online monitoring and dynamic tracking.

[0004] With the development of radio frequency measurement and microwave sensing technologies, methods for testing the dielectric properties of materials based on the principle of electromagnetic resonance are gaining increasing attention. Among these methods, the coaxial resonant cavity structure, due to its small size, high quality factor, and strong electric field concentration, has become a promising method for measuring the dielectric parameters of liquids. By constructing a resonant cavity structure of specific dimensions, it resonates with electromagnetic waves in a specific mode, forming a standing wave distribution of the electric and magnetic fields within the cavity. When the medium within the cavity is altered, such as when filling it with a different type or state of oil, slight changes in its dielectric constant will cause a significant shift in the resonant frequency. Therefore, by measuring the change in resonant frequency, the relative dielectric constant of the oil can be inferred.

[0005] However, current research on oil dielectric property testing based on resonant frequency shift is mostly focused on theoretical modeling or high-purity sample experiments. There is still a lack of highly sensitive, repeatable, and easy-to-integrate detection methods and systems designed for actual oil types in industrial applications (such as transformer oil, lubricating oil, insulating oil, etc.).

[0006] Therefore, there is an urgent need for a method to detect the dielectric constant of oil based on resonant frequency shift. Summary of the Invention

[0007] In view of the above-mentioned drawbacks, the technical problem to be solved by the present invention is to provide a method for detecting the dielectric constant of oil based on resonant frequency shift, comprising the following steps: (1) Constructing the resonant cavity structure: The coaxial resonant cavity with a / 4 structure consists of an inner conductor and an outer conductor, forming a standing wave boundary condition with one end short-circuited and the other end open-circuited. The cavity length is set to , and set the middle sensing area for injecting the oil to be tested; (2) Measure the cavity resonant frequency: In the cavity state, the first resonant frequency is measured using a vector network analyzer. ,This frequency corresponds to the resonance point of the resonant mode in the air medium; (3) Injecting the oil sample to be tested: slowly inject the oil sample to be tested into the sensing area, maintaining a constant temperature during the injection process and avoiding interference from bubbles and impurities to ensure the stability of the electromagnetic response; (4) Measure the resonant frequency after oil filling: After the liquid filling is stable, measure the new resonant frequency again using the vector network analyzer , compared to There is an offset; (5) Calculation of the oil dielectric constant: Based on the frequency values measured in steps (2) and (4), the relative dielectric constant of the oil is calculated using the following formula: :

[0008] in: : relative dielectric constant of the oil to be tested (dimensionless); : Resonant frequency when not filled with oil (unit: Hz); : Resonant frequency after oil injection.

[0009] As a preferred technical solution of the present invention, the length of the coaxial resonant cavity is and electromagnetic wave wavelength The following quarter-wavelength relationship is satisfied:

[0010] in: is the speed of light in vacuum, approximately m / s; is the resonant frequency; is the relative permittivity of the current medium.

[0011] As a preferred technical solution of the present invention, the inner conductor radius of the coaxial resonant cavity is 0.5mm to 2.5mm, outer conductor radius 3mm to 10mm, both meet the ball

[0012] Its corresponding characteristic impedance It can be expressed by the following formula:

[0013] in: : Characteristic impedance (unit: ; Natural logarithm function.

[0014] As a preferred technical solution of the present invention, the resonant frequency is measured by a vector network analyzer, using the reflection coefficient The minimum value or transmission coefficient The maximum value of the resonance point is detected, the scanning frequency range is 500MHz to 2.5GHz, and the frequency resolution is not less than 1 .

[0015] As a preferred technical solution of the present invention, the offset of the resonant frequency is At least 1MHz, the system sets a minimum effective recognition threshold. When the offset is less than the threshold, an error prompt or repeated measurement operation is triggered to improve detection accuracy and effectiveness. As a preferred technical solution of the present invention, the method further includes calibrating the system using a standard dielectric constant oil sample to obtain a correction factor , and used to modify the calculation formula:

[0016] in: Corrected oil dielectric constant, : System correction factor obtained through experiments (dimensionless).

[0017] As a preferred technical solution of the present invention, the resonant cavity is placed in a temperature control device, and the ambient temperature is controlled at Within the specified range, a thermistor and a closed-loop PID algorithm are used to control the temperature to stabilize the measurement, so as to avoid the influence of temperature changes on electromagnetic field disturbance and measurement frequency.

[0018] As a preferred technical solution of the present invention, the steps (2) to (5) are repeated three or more times, and the dielectric constant values obtained each time are 、 etc. are used to calculate the mean:

[0019] in , in order to improve the repeatability and stability of the test results.

[0020] As a preferred technical solution of the present invention, the oil to be tested is a mixed liquid selected from any one or more of the following: transformer oil, synthetic lubricating oil, hydraulic oil, fuel oil or insulating oil, with a dielectric constant range of 2.0 to 4.5. The oil type identification or aging status assessment is performed by comparing the test results with the standard range of the database.

[0021] As a preferred technical solution of the present invention, the outside of the resonant cavity is provided with a copper-plated metal shielding shell and a ferrite absorbing material lining. The absorbing material has a thickness of 2mm to 5mm and is used to shield environmental electromagnetic interference, reduce background noise, and improve the signal-to-noise ratio and resolution of the resonant frequency measurement.

[0022] As can be seen from the above technical solution, the present invention provides a method for detecting the dielectric constant of oil based on resonant frequency shift. Compared with the prior art, the present invention has the following beneficial effects: This invention leverages the high sensitivity of resonant frequency to changes in the dielectric constant within a cavity to achieve non-contact, precise detection of minute fluctuations in the oil's dielectric parameters. By constructing a λ / 4 coaxial resonant cavity structure, the dielectric constant of the oil can be inferred from the measured frequency change, with a detection accuracy better than ±0.01. This makes it suitable for identifying subtle changes in oil during early contamination, aging, or compositional degradation, significantly outperforming traditional capacitive or chemical detection techniques.

[0023] This device utilizes a standard coaxial structure, resulting in a highly miniaturized cavity and no complex external electrodes or electrode processing steps. It can be easily integrated into existing online oil monitoring systems, transformer condition monitoring units, or intelligent lubricant management and control platforms. Its structural design is compatible with a variety of industrial environments and offers excellent vibration and interference resistance, making it suitable for deployment in multiple scenarios.

[0024] The method described in this paper is independent of specific oil type and is applicable to a wide range of industrial oils, including transformer oil, lubricating oil, hydraulic oil, fuel oil, and insulating oil. It supports repeated measurements, automatic correction, and temperature control, resulting in high system stability and consistent measurement results. Calibration with standard oil samples allows for standardized results and widespread adoption, demonstrating promising industrial application prospects and value. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces and describes the drawings required for use in the embodiments of the present invention or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention, and those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0026] Figure 1 Flowchart of the oil dielectric constant detection method based on resonant frequency shift. DETAILED DESCRIPTION

[0027] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] To further illustrate the technical content of the present invention, the "oil dielectric constant detection method based on resonant frequency shift" of the present invention is described in detail below in combination with the specific structure and detection process.

[0029] The present invention utilizes the resonance phenomenon of electromagnetic standing waves in a λ / 4 coaxial resonant cavity. When the dielectric properties of the medium inside the cavity (i.e., the oil to be measured) change, it will cause a slight shift in the resonant frequency. By precisely measuring this shift, the relative dielectric constant of the oil can be inferred.

[0030] 1. System Construction and Pre-conditions: This embodiment uses a closed coaxial resonant cavity as the core detection device. The resonant cavity includes an inner conductor (center needle) and an outer conductor (metal shell), which are coaxially arranged. The inner conductor radius mm, inner diameter of outer conductor mm, with short-circuit plates and microwave transition ports encapsulated at both ends, forming Boundary conditions, namely:

[0031] in: : The resonant cavity length is set to 28.5mm; m / s: speed of light in vacuum; Resonant frequency, in Hz; : Relative dielectric constant of the cavity medium.

[0032] In the air state (i.e. no liquid filling), the first resonance frequency of the cavity The measured value is 1.200GHz. This state is used as the system reference frequency. A vector network analyzer (VNA) is used to perform a frequency sweep with a sweep range of 1.00-1.30GHz and a resolution better than 1MHz, and the reflection coefficient is recorded. The minimum point is taken as the resonance point. 2. Sample injection and frequency change detection: The sample to be tested is a certain type of synthetic transformer oil. Stable for 30 minutes in a constant temperature environment to avoid temperature interference. Slowly inject the oil into the resonant cavity sensing area through the injection pump to fill the center of the cavity where the electric field distribution is the strongest. After injection, re-scan the spectrum and measure the new resonant frequency. GHz. Calculate its relative dielectric constant from the frequency offset:

[0033] To improve accuracy, the operation was repeated three times, and the dielectric constants were 1.21, 1.22, and 1.23 respectively, and the average value was taken:

[0034] 3. System correction and calibration process: In order to correct the equipment error, this system introduces a standard oil sample as a calibration reference. For example, if the dielectric constant is known to be The frequency offset of a certain reference oil sample was measured to be 2.04, so the correction factor was introduced:

[0035] Use this for actual measurement sample correction:

[0036] This improves the adaptability and repeatability of the detection system to various oils.

[0037] IV. Structural auxiliary devices and environmental control: To reduce measurement errors and environmental noise interference, the following auxiliary structures are provided outside the resonant cavity: Shielding layer: The outer shell is made of copper-plated aluminum alloy to effectively isolate external radio frequency interference; Absorbing material: The inside of the cavity is lined with a 3mm thick ferrite absorbing sheet to improve the purity of the resonant spectrum; Temperature control system: The cavity temperature is stabilized at , ensuring that the sample is measured under a constant thermophysical state.

[0038] V. Scope of application and technical effect: This method is applicable to the following liquid types: Power transformer oil (such as 25# oil, 45# oil); high-performance synthetic lubricants (such as polyethers, esters); aviation hydraulic oil, hydraulic transmission oil; biodiesel, light fuel oil; special insulating oil, mineral oil.

[0039] The typical measurement range of dielectric constant is 2.0-4.5. Through database comparison, it can further identify the type of oil, infer the contamination level or aging degree.

[0040] As used herein, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0041] The present invention is not limited to the above-mentioned optimal implementation mode. Anyone should be aware that any structural changes made under the inspiration of the present invention, and any technical solutions that are the same or similar to the present invention, fall within the scope of protection of the present invention.

Claims

1. A method for detecting the dielectric constant of oil based on resonant frequency shift, characterized in that: The steps include: (1) Constructing the resonant cavity structure: The coaxial resonant cavity with a / 4 structure consists of an inner conductor and an outer conductor, forming a standing wave boundary condition with one end short-circuited and the other end open-circuited. The cavity length is set to , and set the middle sensing area for injecting the oil to be tested; (2) Measure the cavity resonant frequency: In the cavity state, the first resonant frequency is measured using a vector network analyzer. ,This frequency corresponds to the resonance point of the resonant mode in the air medium; (3) Injecting the oil sample to be tested: slowly inject the oil sample to be tested into the sensing area, maintaining a constant temperature during the injection process and avoiding interference from bubbles and impurities to ensure the stability of the electromagnetic response; (4) Measure the resonant frequency after oil filling: After the liquid filling is stable, measure the new resonant frequency again using the vector network analyzer , compared to There is an offset; (5) Calculation of the oil dielectric constant: Based on the frequency values measured in steps (2) and (4), the relative dielectric constant of the oil is calculated using the following formula: : ； in: : relative dielectric constant of the oil to be tested; : Resonant frequency when not filled with oil; : Resonant frequency after oil injection.

2. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The length of the coaxial resonant cavity and electromagnetic wave wavelength The following quarter-wavelength relationship is satisfied: ； in: is the speed of light in vacuum, approximately m / s; is the resonant frequency; is the relative permittivity of the current medium.

3. The oil dielectric constant detection method based on resonant frequency shift according to claim 1, characterized in that The inner conductor radius of the coaxial resonant cavity 0.5mm to 2.5mm, outer conductor radius 3mm to 10mm, both meet the following requirements: ； Its corresponding characteristic impedance It can be expressed by the following formula: ； in: :characteristic impedance; Natural logarithm function.

4. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The resonant frequency is measured by a vector network analyzer using the reflection coefficient The minimum value or transmission coefficient The maximum value of the resonance point is detected, the scanning frequency range is 500MHz to 2.5GHz, and the frequency resolution is not less than 1 .

5. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The shift of the resonant frequency At least 1MHz, the system sets a minimum effective recognition threshold. When the offset is less than the threshold, an error prompt is triggered or the measurement operation is repeated to improve detection accuracy and effectiveness.

6. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The method further includes calibrating the system using a standard dielectric constant oil sample to obtain a correction factor , and used to modify the calculation formula: ； in: Corrected oil dielectric constant, : System correction factor obtained through experiments.

7. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The resonant cavity is placed in a temperature control device, and the ambient temperature is controlled at Within the specified range, a thermistor and a closed-loop PID algorithm are used to control the temperature to stabilize the measurement, so as to avoid the influence of temperature changes on electromagnetic field disturbance and measurement frequency.

8. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The steps (2) to (5) are repeated three times or more, and the dielectric constant values obtained each time are 、 For calculating the mean: ； in , in order to improve the repeatability and stability of the test results.

9. The method for detecting the dielectric constant of oil based on resonant frequency shift according to claim 1, characterized in that The oil to be tested is a mixed liquid selected from any one or more of the following: transformer oil, synthetic lubricating oil, hydraulic oil, fuel oil or insulating oil, with a dielectric constant range of 2.0 to 4.

5. The oil type identification or aging status assessment is performed by comparing the test results with the standard range in the database.

10. The oil dielectric constant detection method based on resonant frequency shift according to claim 1, characterized in that The resonant cavity is provided with a copper-plated metal shielding shell and a ferrite absorbing material lining. The absorbing material has a thickness of 2mm to 5mm and is used to shield environmental electromagnetic interference, reduce background noise, and improve the signal-to-noise ratio and resolution of the resonant frequency measurement.

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