Wind power precision bearing backlash measurement system and method

Abstract

The invention belongs to the technical field of wind power equipment maintenance, and discloses a wind power precision bearing backlash measurement system, which comprises a laser ranging module, a data processing module and an auxiliary assembly, and is characterized in that the laser ranging module enhances a laser triangulation method. The method has the following advantages: the measurement efficiency is improved, the single measurement time is shortened from 2 hours to 3 minutes, the downtime is greatly reduced, and the usability of the fan is improved; the data reliability is improved, an encrypted report containing a timestamp, a GPS coordinate and an operator ID is automatically generated, human errors are reduced, and data traceability is ensured; the maintenance cost is reduced, the bearing replacement period is prolonged from 5 years to 7 years, the annual maintenance cost of a single fan is reduced by 120,000 yuan, and sudden faults are avoided through preventive maintenance; the safety performance is improved, the risk that workers enter the gearbox in a traditional method is avoided, the injury probability of operators is reduced, and the industrial safety standard is met.

Description

Technical Field

[0001] This invention belongs to the field of wind power equipment maintenance technology, specifically a wind power precision bearing backlash measurement system and method. Background Technology

[0002] Wind turbine bearings are key transmission components in wind turbine generator sets. They are specifically designed for wind turbines and are responsible for supporting rotating parts (such as the main shaft, gearbox, and generator rotor) and ensuring low-friction operation. They bear complex alternating loads from wind impacts, including radial force, axial force, and bending moment. Their reliability directly determines the lifespan of the wind turbine, so it is necessary to measure the backlash of wind turbine bearings.

[0003] Existing wind turbine bearing backlash measurement methods suffer from the following technical drawbacks: Traditional feeler gauge methods require machine shutdown and component disassembly, with each measurement taking 2-3 hours, severely impacting wind turbine power generation efficiency. Furthermore, the operation relies on manual experience, easily introducing subjective errors. Laser displacement sensor solutions are expensive (over 100,000 RMB per unit) and require specialized calibration environments, making them unsuitable for harsh outdoor conditions and resulting in high maintenance costs. Conventional mechanical measuring instruments have the following problems: limited measurement range (typically <5mm), failing to cover the wide backlash requirements of large bearings; insufficient accuracy (error >0.1mm), unable to meet the high-precision maintenance requirements of modern wind turbines; and inability to adapt to outdoor working environments ranging from -30℃ to 50℃, leading to performance degradation or failure at extreme temperatures. Data recording relies on manual transcription, which is prone to errors and cannot establish a historical database, affecting fault prediction and long-term maintenance planning. Therefore, improvements are needed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a wind turbine precision bearing backlash measurement system and method, which has the advantage of high measurement efficiency.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a wind turbine precision bearing backlash measurement system, comprising: a laser ranging module, a data processing module, and auxiliary components, wherein the laser ranging module enhances the laser triangulation method by employing orthogonal dual laser emitters (wavelength 650nm, Class...). (Level II safety), improving axial / radial displacement detection accuracy through the moiré fringe principle and reducing ambient light interference; integrating a CCD linear sensor with a resolution of 0.01mm and a sampling frequency of 100Hz, combined with adaptive exposure control to ensure stable capture of micron-level backlash changes under high-speed rotating bearings; also includes a dynamic alignment mechanism: the laser beam is precisely positioned by the clamp bracket, automatically calibrating the target surface for each measurement to avoid human error and improve repeatability; the data processing module uses an embedded ARM processor to run a backlash calculation algorithm: acquiring distance data from the dual laser units; performing phase correction based on the bearing rotation period; separating axial / radial backlash components through Fourier transform; also equipped with a 2.4-inch color touchscreen to display backlash values, waveforms, and over-limit alarms in real time, with data storage compatible with DDR3 modules; the auxiliary components include a magnetic foldable tripod, a laser target, a calibration block assembly, and a removable lithium battery.

[0006] As a preferred embodiment of the present invention, the phase correction of the embedded ARM processor is achieved through the following steps: Rotation cycle identification: The system detects the change in the angular velocity of the bearing rotation and uses the signals from the accelerometer or encoder to determine the current rotation cycle position, providing a time reference for subsequent corrections; Phase alignment: The laser measurement value at each sampling time is correlated with the corresponding rotation phase angle. An interpolation algorithm is used to convert the discrete phase data into a continuous phase function. The specific formula is as follows: In the formula, θ(t) is the phase angle at time t, θ0 is the initial phase, and ω(τ) is the angular velocity function; Phase compensation: Based on the aligned phase data, the original distance measurement value is compensated to eliminate the periodic error caused by rotation. The compensated distance value dcomp(t) can be expressed as: In the formula, d(t) is the original measurement value, and Δd(θ(t)) is the phase-related error correction term.

[0007] As a preferred embodiment of the present invention, the specific steps for separating the axial / radial clearance components by Fourier transform are as follows: Signal preprocessing: Filtering and denoising the phase-corrected distance data to ensure signal quality is suitable for spectrum analysis, including eliminating high-frequency noise and low-frequency drift; Fast Fourier Transform (FFT): Converts a time-domain signal into a frequency-domain representation, revealing different frequency components. The formula for Fast Fourier Transform (FFT) is: In the formula, x(n) is the discrete time domain signal, X(k) is the frequency domain component, and N is the number of sampling points; Component separation: Based on bearing vibration characteristics, axial clearance exhibits low-frequency components, while radial clearance contains higher-frequency components. Through frequency domain filtering and peak detection, these components are identified and separated. The separated axial clearance d... axial and radial clearance d radial Reconstructed through inverse transformation.

[0008] As a preferred embodiment of the present invention, the calculation formula of the tooth gap calculation algorithm is as follows: In the formula, Backlash represents the backlash value, which is a comprehensive reflection of axial and radial clearance; d axial Indicates axial clearance; d radial Indicates radial clearance.

[0009] As a preferred embodiment of the present invention, the distance calculation content of the laser triangulation method is as follows: The core formula of laser triangulation is based on the principle of similar triangles. For an oblique projection configuration (the angle α between the laser beam and the surface normal): In the formula, d is the measured distance, f is the focal length of the lens, Base is the distance between the laser and the CCD linear sensor, and x is the displacement of the light spot on the CCD linear sensor.

[0010] As a preferred embodiment of the present invention, the data processing module further incorporates an error correction model: In the formula, k and b are calibration coefficients, and ΔT and ΔI are the changes in temperature and ambient light intensity.

[0011] As a preferred embodiment of the present invention, the magnetic foldable tripod has a load-bearing capacity of ≥5kg and is equipped with a horizontal adjustment knob. The laser target is magnetically adsorbed, with a diameter of Φ50mm and a high-reflectivity film coated on the surface. The calibration block assembly includes standard gap plates of 0.05mm, 0.10mm, and 0.15mm.

[0012] To achieve the aforementioned other objective, the present invention provides the following technical solution: a method for measuring the backlash of precision wind turbine bearings, comprising the following specific steps: S1: Preparation before measurement S1.1: Instrument Calibration Place the 0.10mm calibration block in the measurement path; start the instrument self-test program and confirm that the displayed value is within the range of 0.098-0.102mm; S1.2: Bearing condition inspection Confirm that the bearing is in a free state (no axial preload); clean the measurement area to remove oil / rust (wipe with anhydrous ethanol); S2: Bearing spacing measurement S2.1: Reference Point Setting Outer ring measurement point: 1 / 3 width from the end face, marked as point A; Inner ring measurement point: 2mm from the chamfer, marked as point B; S2.2: Operating Procedures Power on and warm up for 5 minutes — Install the magnetic bracket — Aim the laser at point A — Press the HOLD key to lock the reading — Move the laser to point B — Record the displayed value ΔL; S2.3: Data Processing Spacing calculation formula: In the formula, L0 is the design spacing; α is the coefficient of linear expansion of the material (12.5 × 10⁻⁶). -6 / ℃); ΔT represents the temperature difference (the temperature difference between the measurement point and the ambient temperature); S3: Bearing clearance measurement S3.1: Dedicated Measurement Mode Switch to "GAP" mode, increase the sampling frequency to 100Hz; set the tolerance band: ±0.03mm (automatic trigger of audible and visual alarm). S3.2: Multi-point measurement method Measurement position: 12 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. Measurement position: 3 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. Measurement position: 6 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. S3.3: Dynamic Gap Verification Manually rotate the outer ring of the bearing (speed ≤ 5 rpm) and record the maximum / minimum clearance values: In the formula, F represents the actual load (kN); C0 represents the rated static load (kN).

[0013] As a preferred embodiment of the present invention, during the pre-measurement preparation described in S1, it is necessary to ensure that the wind speed is ≤8m / s, the relative humidity is ≤85%, and there are no strong electromagnetic interference sources in the measurement area (≥5m away from the frequency converter).

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: The advantages of this invention are as follows: improved measurement efficiency, reducing the single measurement time from 2 hours to 3 minutes, significantly reducing downtime and improving wind turbine availability; improved data reliability, automatically generating encrypted reports containing timestamps, GPS coordinates, and operator IDs, reducing human error and ensuring data traceability; reduced maintenance costs, extending the bearing replacement cycle from 5 years to 7 years, reducing the annual maintenance cost of a single wind turbine by 120,000 yuan, and avoiding sudden failures through preventative maintenance; and improved safety performance, avoiding the risk of manual entry into the gearbox in traditional methods, reducing the probability of operator injury, and meeting industry safety standards. Detailed Implementation

[0015] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] This invention provides a precision bearing backlash measurement system for wind power systems, comprising: a laser ranging module, a data processing module, and auxiliary components. The laser ranging module enhances the laser triangulation method by employing orthogonal dual laser emitters (wavelength 650nm, Class II safety level) and improving axial / radial displacement detection accuracy through the moiré fringe principle, while reducing ambient light interference. It integrates a CCD linear sensor with a resolution of 0.01mm and a sampling frequency of 100Hz, combined with adaptive exposure control to ensure stable capture of micron-level backlash changes under high-speed rotating bearings. It also includes a dynamic alignment mechanism: the laser beam is precisely positioned using a clamp bracket, automatically calibrating the target surface for each measurement to avoid human error and improve repeatability. The data processing module uses an embedded ARM processor to run a backlash calculation algorithm: acquiring distance data from the dual laser units; performing phase correction based on the bearing rotation cycle; and separating axial / radial backlash components through Fourier transform. It is also equipped with a 2.4-inch color touchscreen that displays backlash values, waveforms, and over-limit alarms in real time, with data storage compatible with DDR3 modules. The auxiliary components include a magnetic foldable tripod, a laser target, a calibration block assembly, and a removable lithium battery.

[0017] Application case of a 2MW double-fed induction generator unit at a wind farm: Measurement object: main shaft bearing (model FAG 24172-M-C3), as a key component, its condition directly affects the operational stability of the fan; Measurement results: The backlash value is 1.23 mm (standard value 0.8-1.5 mm), indicating that the bearing is within the normal wear range; Diagnostic recommendation: The backlash is within the normal range, but the vibration spectrum shows that the third harmonic component is too large. It is recommended to check the gearbox alignment during the next maintenance. This warning is based on data analysis to help users plan maintenance in advance. Subsequent verification: Three months later, the gearbox exhibited abnormal vibrations. The early warning system prevented a major failure, confirming the effectiveness of the invention in real-time monitoring and fault prediction. This case demonstrates how the invention can achieve efficient maintenance and support the sustainable development of the wind power industry.

[0018] The phase correction of the embedded ARM processor is achieved through the following steps: Rotation cycle identification: The system detects the change in the angular velocity of the bearing rotation and uses the signals from the accelerometer or encoder to determine the current rotation cycle position, providing a time reference for subsequent corrections; Phase alignment: The laser measurement value at each sampling time is correlated with the corresponding rotation phase angle. An interpolation algorithm is used to convert the discrete phase data into a continuous phase function. The specific formula is as follows: In the formula, θ(t) is the phase angle at time t, θ0 is the initial phase, and ω(τ) is the angular velocity function; Phase compensation: Based on the aligned phase data, the original distance measurement value is compensated to eliminate the periodic error caused by rotation. The compensated distance value dcomp(t) can be expressed as: In the formula, d(t) is the original measurement value, and Δd(θ(t)) is the phase-related error correction term.

[0019] Phase correction is a key technology to eliminate the influence of bearing rotation cycle on measurement. Because bearings vibrate periodically during rotation, the directly acquired distance data will contain errors caused by the rotation phase.

[0020] The specific steps for separating the axial / radial clearance components using Fourier transform are as follows: Signal preprocessing: Filtering and denoising the phase-corrected distance data to ensure signal quality is suitable for spectrum analysis, including eliminating high-frequency noise and low-frequency drift; Fast Fourier Transform (FFT): Converts a time-domain signal into a frequency-domain representation, revealing different frequency components. The formula for Fast Fourier Transform (FFT) is: In the formula, x(n) is the discrete time domain signal, X(k) is the frequency domain component, and N is the number of sampling points; Component separation: Based on bearing vibration characteristics, axial clearance exhibits low-frequency components, while radial clearance contains higher-frequency components. Through frequency domain filtering and peak detection, these components are identified and separated. The separated axial clearance d... axial and radial clearance d radial Reconstructed through inverse transformation.

[0021] Clearance separation uses Fourier transform to decompose axial / radial components, combined with Kalman filtering to suppress vibration noise, and outputs three parameters: displacement, velocity, and acceleration. Fourier transform is used to separate axial and radial clearance components from composite vibration signals.

[0022] The calculation formula for the tooth gap calculation algorithm is as follows: In the formula, Backlash represents the backlash value, which is a comprehensive reflection of axial and radial clearance; d axial Indicates axial clearance; d radial Indicates radial clearance.

[0023] This formula, based on geometric relationships, synthesizes orthogonal components into total tooth backlash, ensuring comprehensive measurement.

[0024] The distance calculation method using laser triangulation includes: The core formula of laser triangulation is based on the principle of similar triangles. For an oblique projection configuration (the angle α between the laser beam and the surface normal): In the formula, d is the measured distance, f is the focal length of the lens, Base is the distance between the laser and the CCD linear sensor, and x is the displacement of the light spot on the CCD linear sensor.

[0025] This formula allows the light spot displacement to be converted into distance with a resolution of 0.01 mm.

[0026] The data processing module also incorporates an error correction model: In the formula, k and b are calibration coefficients, and ΔT and ΔI are the changes in temperature and ambient light intensity.

[0027] By introducing an error correction model, accuracy can be improved, and by determining the coefficients through calibration experiments, micron-level accuracy can be ensured.

[0028] Among them, the magnetic foldable tripod has a load-bearing capacity of ≥5kg and is equipped with a horizontal adjustment knob. The laser target adopts magnetic adsorption, with a diameter of Φ50mm and a high-reflection film on the surface. The calibration block group includes standard gap plates of 0.05mm, 0.10mm, and 0.15mm.

[0029] In addition, it adopts an aviation aluminum alloy frame (weighing only 2.5kg), with an overall weight of ≤2.5kg, which can be packed into a standard toolbox, thus meeting the needs of single-person carrying and operation in confined spaces, ensuring easy operation and reducing on-site installation time.

[0030] This invention also provides a method for measuring the backlash of precision wind turbine bearings, the specific steps of which are as follows: S1: Preparation before measurement S1.1: Instrument Calibration Place the 0.10mm calibration block in the measurement path; start the instrument self-test program and confirm that the displayed value is within the range of 0.098-0.102mm; S1.2: Bearing condition inspection Confirm that the bearing is in a free state (no axial preload); clean the measurement area to remove oil / rust (wipe with anhydrous ethanol); S2: Bearing spacing measurement S2.1: Reference Point Setting Outer ring measurement point: 1 / 3 width from the end face, marked as point A; Inner ring measurement point: 2mm from the chamfer, marked as point B; S2.2: Operating Procedures Power on and warm up for 5 minutes — Install the magnetic bracket — Aim the laser at point A — Press the HOLD key to lock the reading — Move the laser to point B — Record the displayed value ΔL; S2.3: Data Processing Spacing calculation formula: In the formula, L0 is the design spacing; α is the coefficient of linear expansion of the material (12.5 × 10⁻⁶). -6 / ℃); ΔT represents the temperature difference (the temperature difference between the measurement point and the ambient temperature); S3: Bearing clearance measurement S3.1: Dedicated Measurement Mode Switch to "GAP" mode, increase the sampling frequency to 100Hz; set the tolerance band: ±0.03mm (automatic trigger of audible and visual alarm). S3.2: Multi-point measurement method Measurement position: 12 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. Measurement position: 3 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. Measurement position: 6 o'clock position; 3 measurements; allowable deviation ≤ 0.02 mm. S3.3: Dynamic Gap Verification Manually rotate the outer ring of the bearing (speed ≤ 5 rpm) and record the maximum / minimum clearance values: In the formula, F represents the actual load (kN); C0 represents the rated static load (kN).

[0031] The laser rangefinder used must meet the following accuracy requirements: axial resolution ≤ 0.01 mm, repeatability ± 0.02 mm (compliant with GB / T 21389-2008 standard); protection rating: IP65 or higher, suitable for ambient temperatures from -20℃ to 50℃; ranging range: 0.05 m to 40 m, supporting single-point / continuous measurement modes; when recording data, the measurement report must include: ambient temperature and humidity, bearing model / number, schematic diagram of measurement points, and original data and calculation formulas.

[0032] In the preparation for measurement in S1, it is necessary to ensure that the wind speed is ≤8m / s, the relative humidity is ≤85%, and there are no strong electromagnetic interference sources in the measurement area (≥5m away from the frequency converter).

[0033] If the "E07" error code appears during measurement: check if the laser path is blocked; verify the reflectivity (standard target ≥90%); restart the instrument and recalibrate.

[0034] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A wind turbine precision bearing backlash measurement system, characterized by, It comprises: a laser distance measuring module, a data processing module and an auxiliary assembly, the laser distance measuring module enhances laser triangulation: adopts orthogonal double laser emitters (wavelength 650nm, Class II safety level), improves axial / radial displacement detection accuracy through Moire fringe principle, reduces environmental light interference; integrates CCD linear sensor, resolution 0.01mm, sampling frequency 100Hz, and combines with adaptive exposure control to ensure stable capture of micron-level backlash changes under high-speed rotating bearings; further comprises a dynamic alignment mechanism: the laser beam is precisely positioned through the hoop support, the target surface is automatically calibrated each time, manual errors are avoided, and the repeatability is improved; the data processing module adopts an embedded ARM processor to run a backlash calculation algorithm: collects distance data of the double laser units; phase correction is performed according to the bearing rotation period; axial / radial backlash components are separated through Fourier transform; it is also equipped with a 2.4-inch color touch screen, which displays backlash values, waveform diagrams and overrun alarms in real time, and data storage is compatible with DDR3 modules; the auxiliary assembly includes a magnetic foldable tripod, a laser target, a calibration block set and a detachable lithium battery.

2. The wind power precision bearing backlash measurement system of claim 1, wherein: The phase correction of the embedded ARM processor is realized through the following steps: ‌Rotation period identification‌: the system detects the angular velocity change of the bearing rotation, determines the current rotation period position using an acceleration sensor or encoder signal, and provides a time reference for subsequent correction; ‌Phase alignment‌: associate the laser measurement value at each sampling time with the corresponding rotation phase angle, convert the discrete phase data into a continuous phase function through an interpolation algorithm, and the specific formula is: In the formula, θ(t) is the phase angle at time t, θ0 is the initial phase, and ω(τ) is the angular velocity function; Phase compensation‌: based on the aligned phase data, compensate the original distance measurement value to eliminate the periodic error caused by rotation, and the compensated distance value dcomp(t) can be expressed as: In the formula, d(t) is the original measurement value, and Δd(θ(t)) is the phase-related error correction term.

3. The wind power precision bearing backlash measurement system of claim 1, wherein: The specific steps for separating axial / radial backlash components through Fourier transform are as follows: Signal preprocessing‌: filter and denoise the phase-corrected distance data to ensure that the signal quality is suitable for spectral analysis, including eliminating high-frequency noise and low-frequency drift; Fast Fourier transform (FFT)‌: convert the time-domain signal to frequency-domain representation to reveal different frequency components, and the fast Fourier transform (FFT) formula is: In the formula, x(n) is the discrete time-domain signal, X(k) is the frequency-domain component, and N is the number of sampling points; Component separation: According to the bearing vibration characteristics, the axial clearance is represented as a low-frequency component, while the radial clearance contains higher frequency components. Through frequency domain filtering and peak detection, the components are identified and separated, and the separated axial clearance d axial and radial clearance d radial are reconstructed by inverse transformation.

4. The wind power precision bearing backlash measurement system of claim 1, wherein: The calculation formula of the backlash calculation algorithm is: In the formula, Backlash represents the backlash value, which is a comprehensive representation of the axial and radial play; d axial represents the axial play; d radial represents the radial play.

5. The wind power precision bearing backlash measurement system of claim 1, wherein: The distance calculation content of the laser triangulation method is: The core formula of the laser triangulation method is based on the principle of similar triangles, for oblique configuration (laser beam and surface normal angle α): In the formula, d is the measured distance, f is the lens focal length, Base is the distance between the laser and the CCD linear sensor, and x is the displacement of the light spot on the CCD linear sensor.

6. The wind power precision bearing backlash measurement system of claim 1, wherein: The data processing module also introduces an error correction model: In the formula, k and b are calibration coefficients, ΔT and ΔI are temperature and ambient light intensity changes.

7. The wind power precision bearing backlash measurement system of claim 1, wherein: The magnetic foldable tripod can bear ≥5 kg, with a horizontal adjustment knob, the laser target adopts a magnetic adsorption type, the diameter is Φ50 mm, and the surface is plated with a high reflection film. The calibration block set includes 0.05 mm, 0.10 mm and 0.15 mm standard gap sheets.

8. A method for measuring the backlash of a precision bearing for a wind turbine, characterized in that The specific steps are as follows: S1: preparation before measurement S1.1: instrument calibration Place the calibration block 0.10 mm in the measurement path; start the instrument self-checking program, and confirm that the display value is in the range of 0.098-0.102 mm; S1.2: bearing state inspection Confirm that the bearing is in a free state (without axial pre-tightening); clean the measurement area and remove oil stains / rust marks (wipe with anhydrous ethanol); S2: bearing spacing measurement S2.1: reference point setting Outer ring measurement point: 1 / 3 width away from the end face, marked as point A; inner ring measurement point: 2 mm away from the chamfer, marked as point B; S2.2: operation process Turn on and preheat for 5 minutes - install the magnetic support - laser aiming at point A - press the HOLD key to lock the reading - move the laser to point B - record the display value ΔL; S2.3: data processing Spacing calculation formula: In the formula, L0 is the design distance; a is the linear expansion coefficient of the material (12.5 x 10 -6 / ℃); and ΔT represents the temperature difference (difference between the measurement point and the ambient temperature). S3: bearing clearance measurement S3.1: special measurement mode Switch to "GAP" mode, and increase the sampling frequency to 100 Hz; set the tolerance band: ±0.03 mm (automatic trigger of sound light alarm); S3.2: multi-point measurement method Measurement position: 12 o'clock direction, measurement times 3, allowable deviation ≤0.02 mm; Measurement position: 3 o'clock direction, measurement times 3, allowable deviation ≤0.02 mm; Measurement position: 6 o'clock direction, measurement times 3, allowable deviation ≤0.02 mm; S3.3: dynamic clearance verification Manually rotate the bearing outer ring (rotation speed ≤5 rpm), and record the maximum / minimum clearance value: In the formula, F represents the actual load (kN); C0 represents the rated static load (kN).

9. The method of claim 8, wherein: In the preparation before measurement in S1, it is necessary to ensure that the wind speed is ≤8 m / s, the relative humidity is ≤85%, and there is no strong electromagnetic interference source (distance from the frequency converter ≥5 m) in the measurement area.

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