Ultrasonic Synchronous Measurement Method and System for Lubricating Oil Film Thickness and Coating Thickness

Abstract

An ultrasonic synchronous measurement method and system for lubricating oil film thickness and coating thickness, belonging to the technical field of lubrication state monitoring. The present invention aims at the problems that the existing methods for simultaneously monitoring the oil film thickness and coating wear are difficult to measure the oil film thickness in the blind area and are not applicable to thin coating structures. It includes collecting the reflection signal a of the solid I - coating - air interface and the reflection signal b of the solid I - coating - oil film - solid II interface; performing Fourier transform to obtain the frequency-domain complex spectrum; setting the search range of the coating thickness, and calculating the frequency-domain complex spectrum of the reference signal corresponding to each coating thickness search value based on the frequency-domain complex spectrum of the reflection signal a; calculating the transfer function of the four-layer structure by using the frequency-domain complex spectrum of the reflection signal b and the frequency-domain complex spectrum of the reference signal; then calculating the phase shift of the oil film layer corresponding to different coating thickness search values, obtaining the coating thickness measurement value through linear fitting, and further obtaining the oil film thickness measurement value. The present invention is used to realize the synchronous measurement of the oil film thickness and the coating thickness.

Description

Technical Field

[0001] The present invention relates to a method and system for ultrasonic synchronous measurement of lubricating oil film thickness and coating thickness, and belongs to the technical field of lubrication state monitoring. Background Art

[0002] In mechanical equipment, the lubricating oil film is an important factor to prevent the direct contact between the surfaces of the friction pairs of components. When the equipment is in the starting and stopping stage or operates under non-steady-state conditions, the oil film between the friction pairs is prone to rupture, and the coating on the surface of the friction pairs will gradually become thinner due to transient rubbing, which is extremely likely to cause lubrication failure of the equipment, thus leading to major safety hazards. Therefore, carrying out synchronous measurement of lubricating oil film thickness and solid coating thickness is of great significance for the condition monitoring and health management of mechanical equipment.

[0003] Electrical measurement method, optical measurement method and ultrasonic measurement method are three mainstream measurement means in the field of lubrication state monitoring. Among them, the electrical measurement method requires the sensor to be in direct contact with the lubricating oil film, which changes the original lubrication state of the friction pair; the optical measurement method needs to install a visualization window on the test piece, which destroys the mechanical structure of the friction pair. Therefore, the electrical measurement method and the optical measurement method are usually used for basic research in the laboratory environment and are difficult to be applied to metal friction pairs in industry. Due to its advantages such as non-invasive measurement and friendly test environment, the ultrasonic measurement method can overcome the limitations of the electrical measurement method and the optical measurement method, and is the most promising in-situ measurement means for lubrication state, and has been widely applied in mechanical components such as rolling bearings, sliding bearings, and internal combustion engines.

[0004] The application of the ultrasonic measurement method in the solid I - oil film - solid II three-layer structure has been relatively mature, but the application of this method in the solid I - coating - oil film - solid II four-layer structure still faces certain challenges. The ratio method and the echo separation method were first applied to the four-layer structure, but they can only measure the oil film thickness in a small range in the spring model area, and the test process is relatively complex. Subsequently, researchers proposed a method for simultaneously monitoring the oil film thickness and coating wear, calculating the oil film thickness using the amplitude of the reflection coefficient, and calculating the coating wear amount using the phase of the reflection coefficient. However, due to the insensitivity of the amplitude of the reflection coefficient in the blind area to the change of the oil film thickness, this method can only calculate the oil film thickness in the spring model area and the resonance model area, and it is difficult to obtain the oil film thickness in the blind area. In addition, this method requires the reflected echoes at both interfaces of the coating to be separated from each other in the time domain, so it is difficult to be applied to the four-layer structure with a thin coating.

[0005] In summary, there is an urgent need to establish a new measurement method that is applicable to thin coating structures and can completely cover the oil film change range to overcome the limitations of the existing methods in the four-layer structure, so as to provide important support for the lubrication state monitoring and failure prediction of mechanical equipment friction pairs. Summary of the Invention

[0006] Aiming at the problems that the existing methods for simultaneously monitoring the oil film thickness and coating wear are difficult to measure the oil film thickness in the blind area and are not applicable to thin coating structures, the present invention provides an ultrasonic synchronous measurement method and system for lubricating oil film thickness and coating thickness.

[0007] An ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness according to the present invention includes:

[0008] For a friction pair including solid I and solid II, the surface of solid I has a coating;

[0009] An ultrasonic signal is emitted from the side of solid I into the friction pair through an ultrasonic probe;

[0010] When there is no lubricating oil film between the coating and solid II, the reflection signal a of the solid I - coating - air interface is collected; then when there is a lubricating oil film between the coating and solid II, the reflection signal b of the solid I - coating - oil film - solid II interface is collected;

[0011] The reflection signal a and the reflection signal b are subjected to spectral analysis by using Fourier transform to obtain the frequency - domain complex spectrum of the reflection signal a and the frequency - domain complex spectrum of the reflection signal b;

[0012] The search range of the coating thickness is set from 0 to 1.1 times the initial coating thickness; the frequency - domain complex spectrum of the reference signal corresponding to each coating thickness search value within the search range is calculated based on the frequency - domain complex spectrum of the reflection signal a;

[0013] By using the frequency - domain complex spectrum of the reflection signal b and the frequency - domain complex spectrum of the reference signal, the transfer function of the ultrasonic signal in the solid I - coating - oil film - solid II structure is obtained for each coating thickness search value; according to the theoretical relationship between the oil film thickness, coating thickness and the transfer function, the phase shift of the oil film layer corresponding to each coating thickness search value is calculated;

[0014] Taking the coating thickness search value as a variable, the phase shift of the oil film layer within the - 6dB bandwidth is linearly fitted by using the least - squares method, and the coating thickness search value corresponding to the minimum fitting error is extracted as the coating thickness measurement value; then the oil film thickness measurement value is calculated according to the coating thickness measurement value.

[0015] According to the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness of the present invention, the ultrasonic probe generates an ultrasonic signal under the excitation of the excitation signal of the pulse transceiver.

[0016] According to the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness of the present invention, the calculation method of the frequency - domain complex spectrum of the reference signal is:

[0017]

[0018] Where Fref is the complex spectrum of the reference signal in the frequency domain, F a is the complex spectrum of the reflected signal a, z1 is the acoustic impedance of ultrasonic waves in solid I, z2 is the acoustic impedance of ultrasonic waves in the coating, z3 is the acoustic impedance of ultrasonic waves in air, f is the ultrasonic frequency, d2 is the search value of the coating thickness, and c2 is the sound velocity of ultrasonic waves in the coating.

[0019] According to the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness of the present invention, the transfer function is expressed as R:

[0020]

[0021] where F b is the complex spectrum of the reflected signal b in the frequency domain.

[0022] According to the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness of the present invention, the theoretical relationship between the oil film thickness, the coating thickness and the transfer function is:

[0023]

[0024] where R 12 is the reflection coefficient at the interface between solid I and the coating, R 23 is the reflection coefficient at the interface between the coating and the oil film, R 34 is the reflection coefficient at the interface between the oil film and solid II, d3 is the oil film thickness, and c3 is the sound velocity of ultrasonic waves in the lubricating oil film.

[0025] According to the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness of the present invention, the calculation method for the phase shift of the oil film layer is:

[0026]

[0027] where is the phase shift of the oil film layer;

[0028] Finally, the measured value of the oil film thickness is calculated based on the measured value of the coating thickness:

[0029]

[0030] The present invention also provides an ultrasonic synchronous measurement system for lubricating oil film thickness and coating thickness, which is used to implement the ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness, and includes a pulse transceiver, an ultrasonic probe, a data acquisition card and a computer;

[0031] where the pulse transceiver is used to generate an ultrasonic excitation signal and receive the reflected signal a and the reflected signal b;

[0032] The ultrasonic probe is used to achieve the electro - acoustic conversion between the excitation signal and the ultrasonic signal, as well as the acoustic - electric conversion of the reflected signal; to realize the transmission and reception of ultrasonic waves;

[0033] The data acquisition card is used to record the time - domain information of reflected signal a and reflected signal b;

[0034] The computer is used to process reflected signal a and reflected signal b, and calculate the measured value of the coating thickness and the measured value of the oil film thickness according to the theoretical relationship between the oil film thickness, the coating thickness and the transfer function.

[0035] Advantages of the present invention: According to the theoretical relationship between the oil film thickness, the coating thickness and the ultrasonic transfer function, the present invention realizes the high - precision synchronous monitoring of the oil film thickness and the coating thickness of the friction pair. Compared with the existing methods, the present invention has the following beneficial effects: 1) It can completely cover the change range of the oil film thickness, and overcome the problem of blind - area measurement of the oil film thickness caused by the insensitive change of the reflection coefficient amplitude in the prior art; 2) It considers the coupling influence mechanism of the oil film layer and the coating on the ultrasonic propagation behavior in the four - layer structure, without the need for the reflected echoes from the two interfaces on both sides of the coating to be separated from each other, and is applicable to the four - layer structure with a relatively thin coating; 3) It effectively solves the problem of synchronous on - line measurement of the oil film thickness and the coating thickness faced by the existing ultrasonic measurement methods, and has important theoretical significance and practical value for the lubrication state monitoring and failure analysis of mechanical equipment under non - steady working conditions. Description of the Drawings

[0036] Figure 1 is the flow chart of the ultrasonic synchronous measurement method for the lubricating oil film thickness and the coating thickness of the present invention;

[0037] Figure 2 is the schematic diagram of the ultrasonic synchronous measurement system for the lubricating oil film thickness and the coating thickness of the present invention; In the figure, 1 is the ultrasonic probe, 2 is the pulse transceiver, 3 is the data acquisition card, 4 is the computer, 5 is solid II, 6 is solid I, 7 is the coating, and 8 is the lubricating oil film;

[0038] Figure 3 is the time - domain diagram of reflected signal a and reflected signal b when the real coating thickness is 500μm and the real oil film thickness is 50μm; In the figure, the reference signal a is reflected signal a, and the reference signal b is reflected signal b;

[0039] Figure 4 is the schematic diagram of the phase shift of the oil film layer corresponding to different search values of the coating thickness;

[0040] Figure 5 is the change diagram of the least - squares fitting error with the search value of the coating thickness;

[0041] Figure 6It is a schematic diagram of the measurement results of the coating thickness measurement value and the oil film thickness measurement value when the true coating thickness is 500μm;

[0042] Figure 7 It is a schematic diagram of the measurement results of the coating thickness measurement value and the oil film thickness measurement value when the true coating thickness is 400μm;

[0043] Figure 8 It is a schematic diagram of the measurement results of the coating thickness measurement value and the oil film thickness measurement value when the true coating thickness is 300μm;

[0044] Figure 9 It is a schematic diagram of the measurement results of the coating thickness measurement value and the oil film thickness measurement value when the true coating thickness is 200μm. Detailed implementation manners

[0045] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0046] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0047] Next, the present invention will be further described in conjunction with the accompanying drawings and specific embodiments, but it is not a limitation of the present invention.

[0048] Detailed implementation manner 1. In combination with Figure 1 As shown, this implementation manner provides an ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness, including:

[0049] For a friction pair including solid I and solid II, the surface of solid I has a coating;

[0050] An ultrasonic signal is emitted from the side of solid I into the friction pair through an ultrasonic probe;

[0051] When there is no lubricating oil film between the coating and solid II, the reflection signal a of the solid I - coating - air interface is collected; then when there is a lubricating oil film between the coating and solid II, the reflection signal b of the solid I - coating - oil film - solid II interface is collected;

[0052] The reflection signal a and the reflection signal b are subjected to spectral analysis by Fourier transform to obtain the frequency-domain complex spectrum of the reflection signal a and the frequency-domain complex spectrum of the reflection signal b;

[0053] Set the search range of the coating thickness to be from 0 to 1.1 times the initial coating thickness; calculate the frequency-domain complex spectrum of the reference signal corresponding to each search value of the coating thickness within the search range based on the frequency-domain complex spectrum of the reflection signal a.

[0054] Using the frequency-domain complex spectrum of the reflection signal b and the frequency-domain complex spectrum of the reference signal, obtain the transfer function of the ultrasonic signal in the solid I - coating - oil film - solid II structure at each search value of the coating thickness; according to the theoretical relationship between the oil film thickness, the coating thickness, and the transfer function, calculate the phase shift of the oil film layer corresponding to each search value of the coating thickness.

[0055] Taking the search value of the coating thickness as a variable, perform a linear fitting on the phase shift of the oil film layer within the -6dB bandwidth using the least squares method, and extract the search value of the coating thickness corresponding to the minimum fitting error as the measured value of the coating thickness; then calculate the measured value of the oil film thickness based on the measured value of the coating thickness.

[0056] Furthermore, the ultrasonic probe generates an ultrasonic signal under the excitation of the excitation signal of the pulse transceiver.

[0057] In this embodiment, the calculation method of the frequency-domain complex spectrum of the reference signal is as follows:

[0058]

[0059] where F ref is the frequency-domain complex spectrum of the reference signal, F a is the frequency-domain complex spectrum of the reflection signal a, z1 is the acoustic impedance of the ultrasonic wave in the solid I, z2 is the acoustic impedance of the ultrasonic wave in the coating, z3 is the acoustic impedance of the ultrasonic wave in the air, f is the ultrasonic frequency, d2 is the search value of the coating thickness, and c2 is the sound velocity of the ultrasonic wave in the coating.

[0060] According to the frequency-domain complex spectrum of the reference signal corresponding to different search values of the coating thickness, calculate the transfer function R:

[0061]

[0062] where F b is the frequency-domain complex spectrum of the reflection signal b.

[0063] On this basis, calculate the phase shift of the oil film layer at different search values of the coating thickness, and perform a least squares fitting on it. Extract the search value of the coating thickness corresponding to the minimum fitting error as the measurement result of the coating thickness, and calculate the measurement result of the oil film thickness according to the theoretical relationship between the oil film thickness, the coating thickness, and the transfer function, so as to realize the synchronous monitoring of the oil film thickness and the coating thickness under the unsteady working conditions of the friction pair.

[0064] When ultrasonic waves propagate in the four-layer structure of solid I - coating - oil film - solid II, reflections and transmissions will occur at the interfaces of solid I - coating, coating - oil film, and oil film - solid II. According to the acoustic wave equation and acoustic boundary conditions, the transfer function reflecting the quantitative relationship between ultrasonic waves and the thickness of the oil film and coating thickness is expressed as:

[0065]

[0066] In the formula, R 12 is the reflection coefficient of ultrasonic waves at the solid I - coating interface, R 23 is the reflection coefficient of the coating - oil film interface, R 34 is the reflection coefficient of the oil film - solid II interface, d3 is the thickness of the oil film, and c3 is the sound velocity of ultrasonic waves in the lubricating oil film.

[0067] According to the transfer function, the phase shift of the oil film layer is extracted. The calculation method of the phase shift of the oil film layer is:

[0068]

[0069] In the formula is the phase shift of the oil film layer;

[0070] From the above relationship, it can be seen that there is a linear relationship between the phase shift of the oil film layer and the ultrasonic frequency. Therefore, the right - hand side of the equation is also a linear function with the ultrasonic frequency as the independent variable. When the search value of the coating thickness is closest to the true value, the linear fitting error of the phase shift of the oil film layer is the smallest. Based on this characteristic, the coating thickness can be measured. Subsequently, according to the measurement result of the coating thickness, the slope of the phase shift of the oil film layer is calculated, and further the measurement result of the oil film thickness is obtained:

[0071]

[0072] Specific Embodiment 2, in combination with Figure 2 As shown, this embodiment provides an ultrasonic synchronous measurement system for the lubricating oil film thickness and coating thickness, which is used to implement the ultrasonic synchronous measurement method for the lubricating oil film thickness and coating thickness described in Specific Embodiment 1, including a pulse transceiver, an ultrasonic probe, a data acquisition card, and a computer;

[0073] Among them, the pulse transceiver is used to generate an ultrasonic excitation signal and receive the reflected signal a and the reflected signal b;

[0074] The ultrasonic probe is used to realize the electro - acoustic conversion between the excitation signal and the ultrasonic signal, and the acoustic - electro conversion of the reflected signal; to realize the emission and reception of ultrasonic waves;

[0075] The data acquisition card is used to record the time - domain information of the reflected signal a and the reflected signal b;

[0076] The computer is used to process the reflected signal a and the reflected signal b, and calculate the measured value of the coating thickness and the measured value of the oil film thickness according to the theoretical relationship between the oil film thickness, the coating thickness and the transfer function.

[0077] Example:

[0078] Establish a Figure 2 synchronous measurement system for the oil film thickness and the coating thickness of the friction pair as shown in the figure to verify the effectiveness of the method of the present invention. An excitation pulse is sent to the ultrasonic probe through a pulse transceiver. After the ultrasonic probe performs electro-acoustic conversion, ultrasonic waves are emitted to the friction pair. The reflected ultrasonic waves are received by the same ultrasonic probe and subjected to acoustic-electric conversion. The converted electrical signal is processed by the pulse transceiver, recorded by a data acquisition card, and transmitted to a computer for data processing and output.

[0079] First, a solid I specimen with a coating thickness of 500 μm is used for the test. The reflected signal of the ultrasonic wave in the solid I - coating - air structure is recorded as the reflected signal a. Then, lubricating oil is added between the coating and the solid II to form a lubricating oil film, and the thickness of the oil film is precisely set by adjusting the distance between the coating and the solid II. The reflected signals of the ultrasonic wave in the solid I - coating - oil film - solid II structure at different oil film thicknesses are recorded as the reflected signal b.

[0080] Figure 3 is the time domain diagram of the reflected signal a and the reflected signal b when the true coating thickness is 500 μm and the true oil film thickness is 50 μm. According to the initial value of the coating thickness, the search range of the coating thickness is set to 0 μm to 500 μm. The Fourier transform is used to perform spectral analysis and processing on the signal to obtain the phase shift of the oil film layer at different coating thickness search values, as shown in Figure 4 the figure. Subsequently, the least squares fitting is performed on the phase shift of the oil film layer corresponding to different coating thickness search values to obtain the variation law of the fitting error with the coating thickness search value, as shown in Figure 5 the figure. It can be seen that when the coating thickness search value is closest to the actual value, the fitting error of the phase shift of the oil film layer is the smallest. The search value when the fitting error is the smallest is extracted as the measurement result of the coating thickness, and the corresponding measurement result of the oil film thickness is obtained according to the theoretical relationship.

[0081] After completing the above tests, solid I specimens with different coating thicknesses are used for the tests respectively to simulate the change of the coating thickness caused by the rubbing of the friction pair. The same steps are used to test, record and process the signals to obtain the synchronous measurement results of the oil film thickness and the coating thickness at different true coating thicknesses. Figures 6 to 9The measurement results are respectively given when the true coating thicknesses are 500μm, 400μm, 300μm, and 200μm. It can be seen that when both the coating thickness and the oil film thickness change, the present invention can still achieve high-precision synchronous monitoring of the coating thickness and the oil film thickness, effectively overcoming the limitations of the existing ultrasonic measurement method in the four-layer structure of thin coatings, meeting the on-line monitoring requirements of the oil film thickness and the coating thickness of the friction pair, and fully reflecting the advantages of the present invention.

[0082] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. A method for ultrasonic synchronous measurement of lubricating oil film thickness and coating thickness, characterized in that: include: For the friction pair including solid I and solid II, the surface of solid I has a coating; The ultrasonic probe transmits an ultrasonic signal from the solid I side to the inside of the friction pair; When there is no lubricating oil film between the coating and the solid II, the reflection signal a of the interface of solid I-coating-air is collected; when there is a lubricating oil film between the coating and the solid II, the reflection signal b of the interface of solid I-coating-oil film-solid II is collected; Performing spectrum analysis on the reflected signal a and the reflected signal b by Fourier transform to obtain the frequency domain complex spectrum of the reflected signal a and the frequency domain complex spectrum of the reflected signal b; The coating thickness search range is set to 0 to 1.1 times of the initial coating thickness; the reference signal frequency domain complex spectrum corresponding to each coating thickness search value within the search range is calculated based on the frequency domain complex spectrum of the reflected signal a; The frequency domain complex spectrum of the reflected signal b and the frequency domain complex spectrum of the reference signal are used to obtain the transfer function of the ultrasonic signal in the structure of solid I-coating-oil film-solid II at each coating thickness search value. According to the theoretical relationship between oil film thickness, coating thickness and transfer function, the phase shift of the oil film layer corresponding to each coating thickness search value is calculated; Taking the coating thickness search value as a variable, the least square method is used to perform linear fitting on the phase offset of the oil film layer within the -6dB bandwidth, and the coating thickness search value corresponding to the minimum fitting error is extracted as the coating thickness measurement value; The oil film thickness measurement value is then calculated based on the coating thickness measurement value.

2. The ultrasonic synchronous measurement method for lubricating oil film thickness and coating thickness according to claim 1 is characterized in that: The ultrasonic probe generates an ultrasonic signal under the excitation of a pulse transceiver excitation signal.

3. The ultrasonic synchronous measurement method of lubricating oil film thickness and coating thickness according to claim 1 is characterized in that: The calculation method of the reference signal frequency domain complex spectrum is: Where F ref is the frequency domain complex spectrum of the reference signal, F a is the frequency domain complex spectrum of the reflected signal a, z1 is the acoustic impedance of the ultrasound in the solid Ⅰ, z2 is the acoustic impedance of the ultrasound in the coating, z3 is the acoustic impedance of the ultrasound in the air, f is the ultrasonic frequency, d2 is the coating thickness search value, and c2 is the sound velocity of the ultrasound in the coating.

4. The ultrasonic synchronous measurement method of lubricating oil film thickness and coating thickness according to claim 3 is characterized in that: The transfer function is denoted as R: Where F b is the frequency domain complex spectrum of the reflected signal b.

5. The ultrasonic synchronous measurement method of lubricating oil film thickness and coating thickness according to claim 4 is characterized in that: The theoretical relationship between oil film thickness, coating thickness and transfer function is: Where R 12 is the reflection coefficient of ultrasound at the solid I-coating interface, R 23 is the reflection coefficient of the coating-oil film interface, R 34 is the reflection coefficient of the oil film-solid II interface, d3 is the oil film thickness, and c3 is the sound velocity of the ultrasonic wave in the lubricating oil film.

6. The ultrasonic synchronous measurement method of lubricating oil film thickness and coating thickness according to claim 5 is characterized in that: The calculation method of the phase shift of the oil film layer is: In the formula is the phase shift of the oil film layer; Finally, the oil film thickness measurement value is calculated based on the coating thickness measurement value:

7. A lubricating oil film thickness and coating thickness ultrasonic synchronous measurement system, used to implement the lubricating oil film thickness and coating thickness ultrasonic synchronous measurement method according to any one of claims 1 to 6, characterized in that: Includes pulse transceiver, ultrasonic probe, data acquisition card and computer; The pulse transceiver is used to generate an ultrasonic excitation signal and receive a reflected signal a and a reflected signal b; The ultrasonic probe is used to realize the electric-to-acoustic conversion between the excitation signal and the ultrasonic signal, and the acoustic-to-electric conversion of the reflected signal; to realize the emission and reception of ultrasonic waves; The data acquisition card is used to record the time domain information of the reflection signal a and the reflection signal b; The computer is used to process the reflection signal a and the reflection signal b, and calculate the coating thickness measurement value and the oil film thickness measurement value according to the theoretical relationship between the oil film thickness, the coating thickness and the transfer function.

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