A Measuring Device and Method for the Thickness of Oil Film on the Surface of Stamped Parts Based on Correlation Filtering

Abstract

The present invention relates to a device and method for measuring the surface oil film thickness of a stamping part based on correlation filtering. The measuring device includes an infrared light source, a focusing lens group, a filter wheel disc, and a detector. A normal absorption window and a saturation absorption window are provided on the filter wheel disc. The rotation of the filter wheel disc can make the normal absorption window and the saturation absorption window rotate to the optical path that can be detected by the detector. Both the normal absorption window and the saturation absorption window are sealed by two transparent window pieces, and no filling is done between the two transparent window pieces that encapsulate the normal absorption window. A measured oil film sample with a certain thickness is encapsulated between the two transparent window pieces of the saturation absorption window.

Description

Technical Field

[0001] The present invention relates to the field of non-contact high-precision film thickness measurement based on optical principles, and more particularly to the field of high-precision measurement of the surface oil film thickness during the stamping process of automobile body parts. Background Art

[0002] Stamping is a commonly used part processing method in modern industry, with higher processing accuracy and production efficiency than traditional methods. Stamping dies usually have complex shapes, and it is necessary to coat a very thin lubricating oil film on the surface of the stamped parts during the stamping process, which has an important impact on the consumption of deformation force during stamping, the surface quality, internal quality, service life, processing accuracy of metal workpieces, and the wear of dies. At the same time, the oil film can increase the lubrication between the die and the surface of the stamped part, and can play roles such as reducing wear, decreasing surface roughness, reducing the friction coefficient, and improving fatigue life.

[0003] The oil film thickness on the surface of the stamped part is the main factor affecting product quality and processing accuracy. An overly thin oil film will cause surface scratches, cracking of the metal during the stretching process, and also affect the corrosion resistance of the product, seriously affecting the appearance and quality of the product. An overly thick oil film will cause local wrinkling and other situations. Excessive oil accumulation during the stamping process will cause bulging on the surface to form oil pockets. Secondly, it will increase the vacuum effect during the stamping process, affect the air discharge of the die, and lead to an aggravation of the phenomenon that the part holds the upper die. Taking the stamping forming of automobile body as an example, according to incomplete statistics, the number of sheet metal injuries caused by the oil film thickness not meeting the requirements reaches more than 1300 times per day. Online high-precision measurement of the surface oil film thickness of stamped parts is a pain point problem that needs to be solved urgently. On the premise of high-precision measurement of the surface oil film thickness of stamped parts, timely adjustment of the production process and flow for stamped parts that do not meet the requirements can greatly reduce the cost and the probability of defective products overflowing.

[0004] Currently, in actual production, the oil coating requirement on the surface of the stamped part is usually 3 - 5 g / m 2 , and the oil film thickness is at the μm level, with high requirements for measurement accuracy, and the production workshop environment is complex with large interference. How to achieve rapid online high-precision measurement of the ultra-thin oil film thickness on the surface of the stamped part in a strong interference environment is the key problem and technical difficulty that needs to be solved urgently at present.

[0005] Currently, the commonly used oil film thickness measurement methods include resistance method, capacitance method, eddy current method, optical detection method, ultrasonic detection method, quality control method, X-ray method, etc.

[0006] The principle of the resistance method is simple, but the resistance of the oil film is very large, making it difficult to calibrate and not easy to obtain the absolute value of the oil film thickness. The capacitance method is determined by the distance between two surfaces and is prone to interference in situations with a large distance. When the oil film is too thin, it is prone to breakdown. The eddy current method utilizes the eddy current effect of metal conductors and is easily interfered by surrounding metal objects on the production line. The X-ray method uses the penetrability of X-rays. Since most stamped parts are metal components and are not easily penetrated, X-rays also have strong radiation and low safety. The attenuation and reflection characteristics of ultrasonic waves change significantly when passing through a thin layer, resulting in complex signal processing and difficulty in accurately judging the true thickness of the oil film.

[0007] Optical measurement methods can take into account both the measurement distance and accuracy, are not easily affected by electromagnetic interference, and are more suitable for industrial on-site environments compared to the above methods. Summary of the Invention

[0008] The object of the present invention is to address the deficiencies of the prior art and propose a method for measuring the surface oil film thickness of stamped parts based on correlation filtering. This measurement method is based on the fact that the measured oil film has a maximum absorption peak in the infrared band and Lambert-Beer's law. A reflection-absorption optical path structure is designed to detect the light intensity signals of the measured oil film for infrared light under normal absorption and saturated absorption conditions. Through correlation filtering technology, it is possible to measure the thickness of the ultra-thin oil film on the surface of stamped parts in a complex industrial environment. At the same time, the measurement error caused by the loss of light intensity due to light reflection, refraction, etc. under saturated absorption conditions is corrected. The technical solution is as follows:

[0009] An apparatus for measuring the surface oil film thickness of stamped parts based on correlation filtering includes an infrared light source, a focusing lens group, a filter wheel disk, and a detector. A normal absorption window and a saturated absorption window are provided on the filter wheel disk, and the rotation of the filter wheel disk can make the normal absorption window and the saturated absorption window rotate to the optical path that the detector can detect; both the normal absorption window and the saturated absorption window are sealed by two transparent window pieces, and no filling is done between the two transparent window pieces encapsulating the normal absorption window; a certain thickness of the measured oil film sample is encapsulated between the two transparent window pieces of the saturated absorption window.

[0010] Further, a band-pass filter with a central wavelength and a full width at half maximum equivalent to the maximum absorption wavelength and the full width at half maximum measured for the measured oil film sample is detachably installed at the front end of the detector.

[0011] Further, the focusing lens group includes a first focusing lens and a second focusing lens, and the second focusing lens is located on the conjugate image plane of the first focusing lens.

[0012] Further, when detecting the thickness of the measured oil film, the distance between the two transparent window pieces encapsulating the normal absorption window is equal to the thickness of the oil film encapsulated in the saturated absorption window.

[0013] The present invention also provides a method for measuring the oil film thickness on the surface of a stamping part by using the above-mentioned measuring device, which is characterized by including the following steps:

[0014] Use a Fourier transform infrared spectrometer to measure the infrared absorption spectrum of the oil film sample to be measured, determine the maximum absorption band range of the sample, and determine the absorption coefficient k of the sample;

[0015] Select a continuous infrared light source whose emission wavelength can cover the maximum absorption band range of the oil film sample to be measured;

[0016] The infrared light source generates broadband infrared light, which is reflected and absorbed by the oil film to be measured, and then imaged after passing through the focusing lens group;

[0017] The filter wheel rotates, so that the normal absorption window and the saturation absorption window sequentially sweep across the beam focused by the lens group that can be detected by the detector;

[0018] Select a band-pass filter whose central wavelength and full width at half maximum are equivalent to the maximum absorption wavelength and full width at half maximum measured by the sample, and install it in front of the detector to ensure that the light intensity received by the detector is only the band that can be absorbed by the oil film sample to be measured. When the oil film thickness in the saturation absorption window is 0, record the signal amplitude output by the detector, denoted as V oil0 ;

[0019] Remove the band-pass filter installed in front of the detector. When the oil film thickness in the saturation absorption window is 0, record the signal amplitude output by the detector, denoted as V0;

[0020] Calculate the theoretical value d of the minimum thickness of the oil film required to be encapsulated in the saturation absorption window according to the Lambert-Beer law min ; Calibrate the absorption coefficient K of the saturation absorption window;

[0021] Determine the saturation absorption window for measuring the oil film thickness to be measured;

[0022] Rotate the filter wheel so that the detector alternately receives the light intensities passing through the normal absorption window and the saturation absorption window;

[0023] The detector converts the optical signal into an electrical signal, which is output as a modulation signal after conditioning, amplification, and acquisition, that is, the detector output voltage;

[0024] Determine the model for calculating the oil film thickness to be measured, and the expression is as follows:

[0025]

[0026] d oil is the oil film thickness to be measured, V mea is the voltage value corresponding to the normal absorption window output by the detector, V refis the voltage value corresponding to the saturation absorption window output by the detector, ρ4 is the transmittance of the saturation absorption window, and β is the unknown quantity to be calibrated;

[0027] Use n stampings with different specified thicknesses of the oil film to be measured sprayed on their surfaces as inputs to calibrate the measurement system. Record the voltages output by the measurement system respectively, and obtain the functional relationship between the oil film thickness and the voltage value output by the detector of the measurement system through fitting.

[0028] Furthermore, calibrate the absorption coefficient K of the saturation absorption window. Specifically, fabricate a group of saturation absorption windows, encapsulate oil films with different specified thicknesses between the two transparent window sheets for each saturation absorption window, and record the signal amplitudes output by the detector, denoted as V oilt ; among them, the thickness of each encapsulated oil film should be less than the theoretical value d min ; Perform least-squares fitting based on the data of the oil films to be measured with different thicknesses and the signal amplitudes output by the detector to obtain the absorption coefficient K.

[0029] Furthermore, the method for determining the saturation absorption window for measuring the thickness of the oil film to be measured is as follows:

[0030] Encapsulate an oil film inside the saturation absorption window with a thickness of the theoretically calculated value d min , and record the signal amplitude V output by the detector out ; If V out ≤V0 - V oil0 , it indicates that the thickness of the encapsulated oil film meets the usage requirements; if V out >V0 - V oil0 , then increase the thickness of the oil film, and the increased thickness d a is

[0031]

[0032] Adjust the thickness of the oil film encapsulated inside the saturation absorption window to d min +d a , record the signal amplitude output by the detector. If it does not meet the requirements, increase the thickness of the oil film encapsulated inside the saturation absorption window by d a , repeat the test until V out ≤V0 - V oil0 ; Obtain the saturation absorption window for measuring the thickness of the oil film to be measured.

[0033] Furthermore, adjust the distance between the two transparent window sheets for encapsulating the normal absorption window to ensure that it is equal to the thickness of the oil film encapsulated in the saturation absorption window for measuring the thickness of the oil film to be measured.

[0034] Further, when measuring the thickness of the oil film to be measured, after collecting the output voltage value of the detector, the measured value of the thickness of the oil film to be measured is obtained according to the fitted functional relationship.

[0035] The method for measuring the thickness of the oil film on the surface of a stamping part based on correlation filtering proposed by the present invention has the following beneficial effects:

[0036] (1) The method for measuring the thickness of the oil film on the surface of a stamping part based on correlation filtering adopted by the present invention is an optical measurement method based on the absorption characteristics of the oil film to be measured for infrared light in a specific band. Compared with the electrical measurement method, it has high measurement accuracy and strong anti-interference ability.

[0037] (2) The optical path structure designed by the present invention realizes complementary light sources and maximally ensures the light intensity reaching the oil film to be measured. At the same time, the measurement optical path and the reference optical path are integrated inside the system, making the system error and the errors caused by devices and environmental interference become common-mode errors. In addition, the separate measurement of the reference signal is avoided, simplifying the measurement process. At the same time, the reference signal can be measured in real time, reducing the error generated by the measurement of the reference signal and improving the measurement accuracy and environmental adaptability of the system.

[0038] (3) The modulation optical path based on correlation filtering designed by the present invention modulates the output effective signal into a fixed frequency through the filter wheel disk, which is convenient for subsequent filtering and signal acquisition, and greatly improves the signal-to-noise ratio of the system.

[0039] (4) The correlation filtering technology proposed by the present invention eliminates the errors caused by system, devices and environmental interference in principle, and also avoids the influence of environmental changes on the measurement results, improving the stability and measurement accuracy of the measurement system, and being more suitable for industrial sites with complex environments.

[0040] (5) The verification process of the measurement method proposed by the present invention is simple, easy to implement, and has high measurement accuracy, which can meet the needs of measuring the thickness of the oil film on the surface of any stamping part with a large area, and also has good application prospects in the measurement of the thickness of other thin films except the oil film. Description of the Drawings

[0041] Figure 1 Shown is a schematic diagram of the optical path structure.

[0042] Figure 2 Shown is a schematic diagram of the output signal of the detector after modulation and processing.

[0043] Figure 3 Shown is a flow chart of the method for measuring the thickness of the oil film described in the present invention.

[0044] Description of reference numerals in the figure: Infrared light source 101, light source window piece 102, oil film to be measured 103, measurement optical path window piece 104, focusing lens (L1) 105, focusing lens (L2) 106, motor 107, normal absorption window 108, saturation absorption window 109, filter wheel disc 110, detector 111. Detailed implementation mode

[0045] Based on the Lambert-Beer law, the present invention proposes a method for measuring the thickness of the oil film on the surface of a stamping part based on correlation filtering, which belongs to the category of optical measurement. The optical path structure is as Figure 1 shown, mainly including: infrared light source 101, light source window piece 102, oil film to be measured 103, measurement optical path window piece 104, focusing lens (L1) 105, focusing lens (L2) 106, motor 107, normal absorption (measurement signal) window 108, saturation absorption (reference signal) window 109, filter wheel disc 110, detector 111.

[0046] First, use a Fourier transform infrared spectrometer to measure the infrared absorption spectrum of the sample, determine the maximum absorption band range of the sample, and the absorption coefficient k of the sample.

[0047] Select a continuous infrared light source whose emission wavelength can cover the maximum absorption band range of the sample.

[0048] The infrared light source 101 generates broadband infrared light, which reaches the oil film to be measured on the surface of the stamping part after passing through the light source window piece 102. The light intensity emitted by the light source is expressed as

[0049]

[0050] In the formula, I0 is the total light intensity emitted by the light source, is the light intensity corresponding to the maximum absorption band of the sample to be measured, is the light intensity corresponding to the non-absorbing band of the sample to be measured.

[0051] The light reaches the surface of the oil film to be measured 103, and after being reflected and absorbed by the oil film to be measured, it enters the measurement optical path. When the light passes through the surface of the stamping part and the oil film to be measured, it follows Lambert diffuse reflection, and the reflected light intensity is expressed as

[0052] I d =K d I0max(0,cosθ)

[0053] In the formula, I d is the reflected light intensity, K d is the diffuse reflection coefficient, I0 is the incident light intensity, and θ is the angle between the incident light and the surface normal.

[0054] The light reflected and absorbed by the measured oil film 103 passes through the optical window sheet 104 of the measurement optical path part. After being imaged by the focusing lens (L1) 105, it is imaged again by the focusing lens (L2) 106, where the focusing lens (L2) 106 is located on the conjugate image plane of the focusing lens (L1) 105.

[0055] The measurement optical path part is approximately an ideal optical system, and the imaging of the focusing lenses L1 and L2 satisfies the Gaussian formula, expressed as

[0056]

[0057] The light beam enters the modulation optical path part, and the motor 107 drives the filter wheel disk provided with two circular light-transmitting windows to rotate, successively sweeping across the light beam focused by the lens group. Among them, the inside of the light-transmitting window is sealed by 2 transparent window sheets that allow the light beam to pass through normally. As Figure 1 shown, there is no filling between the transparent window sheets of the normal absorption window 108, and the detector receives only the light intensity absorbed by the measured oil film on the surface of the stamping part; a sample of the measured oil film with a certain thickness is encapsulated between the transparent window sheets of the saturation absorption window 109 to ensure that the light in the corresponding maximum absorption band of the measured oil film in the infrared light is completely absorbed, and the light received by the detector only contains the bands not absorbed by the measured oil film.

[0058] Select a band-pass filter with a center wavelength and a full width at half maximum that are exactly the same as the maximum absorption wavelength and the full width at half maximum of the sample measurement, and install it in front of the detector 111 to ensure that the light intensity received by the detector is only the band that can be absorbed by the measured oil film sample. When the oil film thickness encapsulated in the saturation absorption window 109 is 0, record the signal amplitude output by the detector, denoted as V oil0 .

[0059] Remove the filter installed in front of the detector 111. When the oil film thickness encapsulated in the saturation absorption window 109 is 0, record the signal amplitude output by the detector, denoted as V0.

[0060] The absorption of the infrared light by the oil film encapsulated inside the saturation absorption window 109 follows the Lambert-Beer law, expressed as

[0061]

[0062] In the formula, I oil0 represents the light intensity signal received by the detector when the oil film thickness encapsulated in the saturation absorption window 109 is 0, and I oilt represents the light intensity signal (transmission absorption) received by the detector when the oil film thickness encapsulated in the saturation absorption window 109 is not 0. G represents the photoelectric conversion coefficient of the detector, k represents the light absorption coefficient of the measured oil film, c represents the concentration of the measured oil film sample as a fixed value, l represents the optical path difference, and d represents the oil film thickness.

[0063] Calculate the theoretical value d of the minimum thickness of the encapsulating oil film required for the saturation absorption window 109 according to the above formula min 。

[0064] Calibrate the absorption coefficient K of the saturation absorption window 109. Specifically, encapsulate 5 groups of oil films with specified thicknesses between the transparent window sheets of the saturation absorption window 109. The thickness increases in increments of 0.5 mm, and record the signal amplitudes output by the detector, denoted as V oilt 。Among them, the thickness of the encapsulated oil film should be less than the theoretical value d min 。

[0065] Perform a least-squares fit on the data of the 5 groups of specified-thickness oil films and the detector output signals to obtain the absorption coefficient K

[0066] Encapsulate an oil film inside the saturation absorption window 109 with a thickness of the theoretically calculated value d min ,and record the signal amplitude V output by the detector out 。If V out ≤V0 - V oil0 ,it indicates that the thickness of the encapsulated oil film meets the usage requirements;

[0067] If V out >V0 - V oil0 ,it is necessary to increase the thickness of the oil film. The increased thickness d a is

[0068]

[0069] Adjust the thickness of the oil film encapsulated inside the saturation absorption window 109 to d min +d a ,and record the signal amplitude output by the detector until V out ≤V0 - V oil0 。

[0070] Adjust the distance between the transparent window sheets inside the normal absorption window 108 to ensure that it is equal to the thickness of the oil film encapsulated inside the saturation absorption window 109

[0071] The detector 111 alternately receives the light intensities passing through the two windows 108 and 109, which are respectively expressed as

[0072]

[0073] In the formula, is the light intensity received by the detector after I0 is absorbed by the measured oil film, and I ref is the light intensity received by the detector after saturation absorption, which does not include the light in the maximum absorption band of the oil film, that is

[0074] Detector 111 converts the optical signal into an electrical signal. After conditioning, amplification, and acquisition, a modulated signal is output. The output modulated signal contains two parts of effective signals, and both parts of the effective signals contain common-mode errors such as system errors, errors caused by ambient light interference, light intensity losses due to device fouling and reflection, etc. Among them, the frequency of the modulated signal is determined by the motor speed.

[0075] By using the correlation filtering technique for the acquired modulated signal, the value of the measured oil film thickness can be calculated as follows.

[0076] Among them, in the saturated absorption state, the light intensity received by the detector that does not contain the maximum absorption band is expressed as

[0077]

[0078] In the normal absorption state, the light intensity received by the detector that does not contain the maximum absorption band is expressed as

[0079]

[0080] In the normal absorption state, the light intensity of the maximum absorption band received by the detector is expressed as

[0081]

[0082] Among them, ρ0, is the transmittance of the system, including the absorption, reflection, etc. of the scattered infrared light, lenses, optical windows and other system components.

[0083] ρ1, is the transmittance of the dust, oil, etc. on the surface of the optical elements in the system.

[0084] ρ2, is the transmittance of air and suspended particles in the air.

[0085] ρ3, is the transmittance of the surface of the stamping part to be measured.

[0086] ρ4 is the transmittance of the saturated absorption window 109.

[0087] is the transmittance of the oil film to be measured.

[0088] Dividing the signals of normal absorption and saturated absorption collected, the common-mode error is eliminated, which is expressed as

[0089]

[0090] After simplification, it is obtained that

[0091]

[0092] Among them,

[0093]

[0094] Since the complete absorption and the normal absorption are in the same working environment and within the same system, the light intensity losses caused by factors such as the system, optical devices, stray light interference, reflection of stamping parts, and light intensity that does not enter the measurement optical path are equal, that is

[0095]

[0096] Combined with the above formula, the transmittance of the oil film to be measured can be expressed as

[0097]

[0098] The selective absorption of the infrared light by the oil film to be measured satisfies the Lambert-Beer law, which is expressed as

[0099] I = I0·e -kcl

[0100] In the formula, I0 is the incident light intensity and I is the reflected light intensity.

[0101] The transmittance of the infrared light after being absorbed by the oil film to be measured is expressed as

[0102]

[0103] When light passes through a medium, there are three possibilities: transmission, absorption, and reflection, and T + A + R = 1. Among them, A is the absorption rate, T is the transmittance, and R is the reflectance.

[0104] Combining the above formula, in the measurement method proposed in the present invention, the Lambert-Beer law can be specifically expressed as

[0105]

[0106] In the formula, k is the absorption coefficient of the oil film to be measured for infrared light, c is the concentration of the oil film sample to be measured, and l is the optical path of the infrared light in the oil film to be measured.

[0107] Take as a constant, which will not change with the change of the emission light power. Therefore, the absorption rate can be accurately measured even if the emission light power is not absolutely stable.

[0108] The transmittance ρ4 of the saturation absorption window 109 can be calibrated. Under the condition of no oil film to be measured, a pure oil block is placed in the optical path to ensure that the light in the maximum absorption band is completely absorbed. The light intensity received by the detector is the transmitted light intensity, denoted as I oil . The light intensity of the optical path corresponding to the detector without the pure oil block placed is the incident light intensity, denoted as I noil,but

[0109]

[0110] Measured oil film thickness d oil is a function of the absorptivity and is expressed as

[0111] d oil =f(I ref ,I mea ).

[0112] Light intensity is a physical quantity that describes the radiation energy or flux of light passing through a unit area in unit time. The commonly used unit is watt / square meter (W / m 2 ), then the light intensity is expressed as the optical power per unit area. In the measurement system, the detector receives infrared light and then converts it into voltage output. The conversion relationship is expressed as

[0113] V=R·P o =R.I.S

[0114] Where R is the light responsivity of the detector, I is the light intensity received by the detector, and S is the photosensitivity area of ​​the detector.

[0115] Combining the detector photoelectric conversion relationship and the Lambert-Beer law specified above, the relationship between the system output voltage and the optical path length of infrared light in the measured oil film can be obtained, which is expressed as

[0116]

[0117] Where V mea V is the voltage value corresponding to the normal absorption window of the detector output collected by the system. ref The voltage value corresponding to the saturation absorption window of the detector output collected by the system.

[0118] Taking the logarithm of both sides, we get

[0119]

[0120] The above formula can be expressed as

[0121]

[0122] In the formula, is the proportionality factor

[0123] The measurement method proposed in the present invention is based on the reflection and absorption of infrared light by the measured oil film, and the optical path is related to the actual oil film thickness d oil The relationship is expressed as

[0124]

[0125] Wherein, θ2 is the refraction angle of the light on one side of the oil film to be measured, which can be obtained by calculation according to the refraction law, and n oil is the refractive index of the oil film to be measured.

[0126] The expression for the thickness of the oil film to be measured can be obtained as follows

[0127]

[0128] Wherein, d oil is the thickness of the oil film to be measured, θ2 is the refraction angle of the light on one side of the oil film to be measured, which can be obtained by calculation according to the refraction law, and n oil is the refractive index of the oil film to be measured, V mea is the voltage value corresponding to the normal absorption window 108 of the detector output collected by the system, V ref is the voltage value corresponding to the saturated absorption window 109 of the detector output collected by the system, ρ4 is the transmittance of the saturated absorption window 109, and β is an unknown quantity to be calibrated.

[0129] Using n stamped parts with different specified thicknesses of oil film sprayed on the surface as inputs, calibrate the measurement system, record the measurement results output by the system respectively, and obtain the accurate functional relationship between the oil film thickness and the output voltage value of the system through fitting.

[0130] After the system calibration is completed, measure the oil film with unknown thickness on the surface of the stamped part, process and display the measurement data in the upper computer, and obtain the final measurement result of the oil film thickness on the surface of the stamped part.

[0131] From the derivation process and the final result of the oil film thickness measurement expression, it can be seen that the systematic error, the errors caused by devices and the environment are filtered out and will not affect the final measurement result. The measurement process does not require calibration of the system dark noise and the incident light intensity, and the measurement errors caused by factors such as temperature change and the fouling of optical devices over time will not affect the final oil film thickness measurement result. It shows that the method for measuring the oil film thickness on the surface of a stamped part based on correlation filtering proposed by the present invention can avoid the influence of a complex working environment on the measurement result and can well realize the measurement of the ultra-thin oil film thickness on the surface of a stamped part in a complex industrial environment.

Claims

1. A method for measuring the oil film thickness on the surface of a stamping part, characterized in that The measurement device adopted includes an infrared light source, a focusing lens group, a filter wheel disc and a detector. A normal absorption window and a saturation absorption window are arranged on the filter wheel disc. The rotation of the filter wheel disc can make the normal absorption window and the saturation absorption window rotate to the optical path that can be detected by the detector; both the normal absorption window and the saturation absorption window are sealed by two transparent window pieces, and no filling is done between the two transparent window pieces that encapsulate the normal absorption window; a measured oil film sample with a certain thickness is encapsulated between the two transparent window pieces of the saturation absorption window. The method for measuring the surface oil film thickness of the stamping part includes the following steps: Use a Fourier transform infrared spectrometer to measure the infrared absorption spectrum of the measured oil film sample, determine the maximum absorption band range of the sample, and determine the absorption coefficient k of the sample; Select a continuous infrared light source whose emission wavelength can cover the maximum absorption band range of the measured oil film sample; The infrared light source generates broadband infrared light. After being reflected and absorbed by the measured oil film, it forms an image after passing through the focusing lens group; The filter wheel disc rotates, so that the normal absorption window and the saturation absorption window sequentially sweep across the beam focused by the lens group that can be detected by the detector; Select a band-pass filter with a central wavelength and full width at half maximum (FWHM) equivalent to the maximum absorption wavelength and FWHM of the sample measurement, and install it at the front end of the detector to ensure that the detector receives only the light intensity in the band that can be absorbed by the oil film sample to be measured. When the oil film thickness is 0 in the saturated absorption window, record the signal amplitude output by the detector, denoted as V oil0 ; Remove the band-pass filter installed at the front end of the detector. When the oil film thickness encapsulated in the saturation absorption window is 0, record the signal amplitude output by the detector, denoted as V0; Calculating the theoretical value d of the minimum thickness of the encapsulating oil film required for the saturation absorption window according to the Lambert-Beer law min Calibrating the absorption coefficient K of the saturation absorption window; Determine the saturation absorption window used for measuring the thickness of the measured oil film; Rotate the filter wheel disc so that the detector alternately receives the light intensities passing through the normal absorption window and the saturation absorption window; The detector converts the optical signal into an electrical signal, which is output as a modulation signal after conditioning, amplification and acquisition, that is, the output voltage of the detector; Determine the model for calculating the thickness of the measured oil film, and the expression is as follows: d oil is the oil film thickness to be measured, V mea is the voltage value corresponding to the normal absorption window output by the detector, V ref is the voltage value corresponding to the saturation absorption window output by the detector, ρ4 is the transmittance of the saturation absorption window, and β is the unknown quantity to be calibrated; Use n stamping parts with different specified thicknesses of the measured oil film sprayed on the surface as inputs to calibrate the measurement system, record the voltages output by the measurement system respectively, and obtain the functional relationship between the oil film thickness and the output voltage value of the detector of the measurement system through fitting.

2. The method for measuring the surface oil film thickness of a stamped part according to claim 1, characterized in that, The absorption coefficient K of the saturation absorption window is calibrated. Specifically, a group of saturation absorption windows are fabricated. An oil film with a different specified thickness is encapsulated between two transparent window pieces for each saturated absorption window. The signal amplitudes output by the detector are recorded respectively and denoted as V oilt ; among them, the thickness of each encapsulated oil film should be less than the theoretical value d min ; the absorption coefficient K is obtained by performing least-squares fitting based on the data of the measured oil films with different thicknesses and the signal amplitudes output by the detector.

3. The method for measuring the oil film thickness on the surface of a stamping part according to claim 1, wherein, The method for determining the saturation absorption window used for measuring the thickness of the measured oil film is as follows: Encapsulate an oil film inside the saturation absorption window, with a thickness of the theoretically calculated value d min , and record the amplitude V of the detector output signal out ; If V out ≤V0 - V oil0 , it indicates that the thickness of the encapsulated oil film meets the usage requirements; If V out >V0 - V oil0 , then increase the thickness of the oil film, and increase the thickness d a is Adjust the thickness of the internal encapsulated oil film in the saturation absorption window to d min +d a , record the amplitude of the detector output signal. If it does not meet the requirement, increase the thickness of the internal encapsulated oil film in the saturation absorption window by d a , repeat the test until V out ≤V0 - V oil0 ; Obtain a saturation absorption window for measuring the thickness of the oil film to be measured.

4. The method for measuring the oil film thickness on the surface of a stamped part according to claim 3, wherein Adjust the distance between the two transparent window pieces used to encapsulate the normal absorption window to ensure that it is equal to the thickness of the oil film encapsulated in the saturation absorption window used for measuring the thickness of the measured oil film.

5. The method for measuring the surface oil film thickness of the stamping part according to claim 1, wherein When measuring the thickness of the measured oil film, after collecting the output voltage value of the detector, obtain the measured value of the thickness of the measured oil film according to the fitted functional relationship.

6. The method for measuring the surface oil film thickness of the stamping part according to claim 1, wherein A band-pass filter with a central wavelength and a half-height width equivalent to the maximum absorption wavelength and half-height width measured of the measured oil film sample is detachably installed at the front end of the detector.

7. The method for measuring the surface oil film thickness of a stamping part according to claim 1, characterized in that, The focusing lens group includes a first focusing lens and a second focusing lens, and the second focusing lens is located on the conjugate image plane of the first focusing lens.

8. The method for measuring the oil film thickness on the surface of the stamping part according to claim 1, wherein When detecting the thickness of the measured oil film, the distance between the two transparent window pieces used to encapsulate the normal absorption window is equal to the thickness of the oil film encapsulated in the saturation absorption window.

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