A system and method for measuring oil film thickness on strip surface based on infrared multi-point differential absorption

Abstract

The present invention discloses a system and method for measuring the thickness of oil film on the surface of a plate and strip based on multi-point differential absorption of infrared light, which is used to measure the thickness of the oil film to be measured coated on the plate and strip to be stamped, and includes an infrared light source, a measuring light path window, a focusing lens, a motor, a filter wheel, and a detector; infrared light sources are respectively provided on both sides above the plate and strip to be stamped, and light source windows are provided on the infrared light sources; the measuring light path directly above the plate and strip to be stamped is provided with a measuring light path window, a focusing lens, and a detector in sequence from bottom to top; a motor is provided on one side of the measuring light path, and the output shaft of the motor is connected to the center of the filter wheel to drive the filter wheel to rotate, and six circular reference signal windows are evenly distributed on the filter wheel, and the six circular reference signal windows are respectively sealed with oil samples to be measured with increasing thickness through two transparent windows. When the filter wheel rotates, the six circular reference signal windows can pass through the measuring light path between the focusing lens and the detector in sequence.

Description

Technical Field

[0001] The present invention relates to the field of non-contact high-precision measurement of film thickness based on the principle of infrared light absorption by materials, and in particular to a high-precision measurement system and method for surface oil film thickness during automobile body stamping processing. Background Art

[0002] Stamping is a widely used part processing method in modern industry, offering higher precision and production efficiency than traditional methods. Stamping dies often have complex shapes, and during the stamping process, a very thin film of lubricant must be applied to the surface of the stamped part. This has a significant impact on the dissipation of deformation force during the stamping process, the surface quality and internal quality of the metal workpiece, its service life, machining accuracy, and die wear.

[0003] The oil film thickness on the surface of stamped parts is a major factor affecting product quality and machining accuracy. An oil film that is too thin can cause surface scratches, cracking the metal during stretching, and compromise the product's corrosion resistance, severely impacting its appearance and quality. An oil film that is too thick can lead to localized wrinkling. Excessive oil accumulation during the stamping process can create bulging on the surface, forming oil pockets. Furthermore, it increases the vacuum effect during the stamping process, hindering air expulsion from the mold and exacerbating part sticking. For example, incomplete statistics show that incomplete oil film thickness causes over 1,300 sheet metal scratches daily, resulting from over 1000 defects. High-precision online measurement of oil film thickness on stamped parts is a pressing issue that needs to be addressed. By accurately measuring the oil film thickness on stamped parts, timely adjustments to production processes and procedures for substandard parts can significantly reduce costs and the likelihood of defective products being shipped.

[0004] In actual production, the oil film thickness applied to stamping parts is currently at the micron level, requiring very high measurement accuracy. Furthermore, the complex and interfering environment of the production workshop presents significant challenges. Rapid, online, and high-precision measurement of the ultra-thin oil film thickness on stamping parts in such a high-interference environment is a key issue and technical challenge that needs to be addressed.

[0005] Many methods can be used to measure oil film thickness, but their limitations hinder their application in the field. Contact measurement can damage the oil film, so non-contact measurement methods are the primary method used. Among these non-contact measurement methods, electrical methods based on resistance, capacitance, or eddy current principles are susceptible to noise from oil characteristics and electromagnetic interference. Furthermore, the measurement principles of electrical methods limit their application in long-distance measurements within industrial production lines.

[0006] In recent decades, ultrasonic nondestructive measurement has made significant progress and is widely used in oil film thickness measurement. However, measurement accuracy is significantly affected by the acoustic properties and thermal effects of the sensor. In industrial production lines, these methods are more susceptible to interference from ultrasonic waves generated by mechanical vibration. In contrast, optical methods are not limited by measurement distance and are not affected by electromagnetic interference, offering significant advantages for measuring oil film thickness on metal surfaces in industrial production. Currently, commonly used optical detection methods include optical interferometry, laser-induced fluorescence, and infrared absorption, all of which offer high measurement accuracy and resolution. However, the measurement principles and data processing procedures of optical interferometry and laser-induced fluorescence are relatively complex. Infrared absorption detection methods primarily include spectrophotometry and dual-wavelength detection. Dual-wavelength detection requires narrowband filters with different central wavelengths as the measurement and reference channels. Narrowband filters meeting these requirements in the mid-infrared band are difficult to obtain and are relatively costly. While spectrophotometry has a relatively simple optical path and data processing, the common method requires measuring dark noise and the light intensity of a blank sample as a reference, which complicates the measurement process and can easily introduce unnecessary errors. Summary of the Invention

[0007] The purpose of the present invention is to overcome the shortcomings of the existing technology in the application of oil film thickness measurement on the surface of plates and strips in industrial sites, and to provide a plate and strip surface oil film thickness measurement system and method based on multi-point differential absorption of infrared light. The measurement system and method are based on the characteristics of the selective absorption of infrared light by the oil film to be measured and the Lambert-Beer law, and design a measurement optical path structure of dual-light source common optical path reflection absorption. The common absorption of infrared light by the oil film to be measured and the oil film of specified thickness is used as the measurement signal. The measurement signal is collected and differential operation is performed to solve the values of some unknown parameters. The values of the remaining unknown parameters are obtained by calibration, and the thickness value of the oil film to be measured is solved to obtain the thickness value of the oil film to be measured, thereby realizing online measurement of the thickness of ultra-thin oil film on the surface of the plate and strip to be stamped under complex industrial environments.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] A plate and strip surface oil film thickness measurement system based on infrared multi-point differential absorption is used to measure the thickness of the oil film to be measured on the plate and strip to be stamped. It includes an infrared light source, a measurement light path window, a focusing lens, a motor, a filter wheel, and a detector.

[0010] Infrared light sources are respectively provided on both sides above the plate strip to be punched, and light source windows are provided on the infrared light sources; a measuring light path directly above the plate strip to be punched is provided with a measuring light path window, a focusing lens and a detector from bottom to top; a motor is provided on one side of the measuring light path, and the motor output shaft is connected to the center of the filter wheel to drive the filter wheel to rotate, and six circular reference signal windows are evenly distributed on the filter wheel, and the six circular reference signal windows are respectively sealed with oil samples to be tested with increasing thickness through two transparent windows. When the filter wheel rotates, the six circular reference signal windows can pass through the measuring light path between the focusing lens and the detector in sequence.

[0011] Furthermore, the focusing lens includes a first focusing lens and a second focusing lens which are sequentially arranged from bottom to top on the measuring light path, and the second focusing lens is located on a conjugate image plane of the first focusing lens.

[0012] The present invention also provides a method for measuring the oil film thickness on the strip surface based on infrared multi-point differential absorption, comprising:

[0013] S1. Use a Fourier transform infrared spectrometer to measure the infrared absorption spectrum of the oil sample to be tested, determine the maximum absorption band range of the oil sample to be tested and the theoretical minimum oil thickness d required for the light within the maximum absorption band to be completely absorbed. min ;

[0014] S2. Based on the theoretical value of minimum oil film thickness d min , determine the thickness d1-d6 of the oil sample to be tested sealed in the six circular reference signal windows <d min , and the thickness of d1-d6 increases successively;

[0015] S3. The surface of the sheet to be stamped is blank, that is, the thickness of the oil film to be measured is zero, and the light emitted by the infrared light source is irradiated onto the sheet to be stamped through the light source window. The light reflected by the sheet to be stamped passes through the measurement light path window, focusing lens, and filter wheel on the measurement light path and enters the detector; the filter wheel is driven by a motor to rotate, and the detector receives 6 groups of reference light intensity signals of the oil samples to be tested of different thicknesses d1-d6;

[0016] S4. Based on the reference light intensity signal absorbed by the oil samples of different thicknesses, the constant parameters of the measurement system are obtained using a differential algorithm and a linear least squares fitting calibration method;

[0017] S5. Based on the calibrated constant parameters of the measurement system, the thickness of the oil film to be measured is measured online in real time, and the thickness value of the oil film to be measured is obtained by calculation.

[0018] Furthermore, in step S3, the Beer-Lambert law is used to calculate the absorption of infrared light by the oil film to be tested, and the formula is:

[0019]

[0020] Where, I ab is the light intensity transmitted after being absorbed by the oil film to be measured, I d is the intensity of light incident on the surface of the oil film to be tested after diffuse reflection, ε is the absorption coefficient of the oil sample to be tested in the infrared light band, C is the concentration of the oil sample to be tested, and n oil is the refractive index of the oil sample to be tested, d x is the thickness of the oil film to be measured, θ is the angle between the light source and the horizontal direction of the measurement system; for a certain measurement system and oil sample to be measured, εCn oil and θ are constants, and Q is also a constant.

[0021] Furthermore, according to the Lambert-Beer law, the light intensity received by the detector through the six circular reference signal windows is expressed as

[0022]

[0023] Where, I mean is the light intensity received by the detector through n circular reference signal windows, d n The thickness of the oil sample to be measured is encapsulated inside the circular reference signal window;

[0024] The loss of light entering the measurement optical path before reaching the detector's photosensitive surface includes not only the normal absorption of the oil film, but also the absorption and scattering of the measurement system, optical devices, and air. The light intensity I absorbed by the oil sample to be measured that is not encapsulated in the circular reference signal window is oil The light intensity I after reflection and absorption of the measured oil film ab The relationship between

[0025] I oil =I ab β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4)

[0026] Where, β is the ratio coefficient of infrared light entering the measurement optical path after being reflected and absorbed by the oil film to be measured; ρ0 is the loss rate caused by the measurement system structure, including absorption, reflection, and scattering of measurement system components; ρ1 is the loss rate caused by the optical components themselves in the measurement system, including absorption and reflection of the optical path windows and focusing lenses; ρ2 is the loss rate caused by absorption and scattering of dust and oil on the surface of the optical device; ρ3 is the loss rate caused by the measurement system environment, including absorption and scattering of air and suspended particles in the air; ρ4 is the loss rate caused by absorption and reflection of the sheet to be punched.

[0027] The light intensity of the circular reference signal window received by the above detector is expressed as

[0028]

[0029] Where k = β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4), which represents the overall transmittance of the measurement system.

[0030] According to Fresnel's law of reflection, the reflection of light on the surface of different media is only related to the refractive index. Under non-ideal conditions, the light intensity received by the detector is expressed as

[0031]

[0032] Where Δd is the thickness of the absorbing oil equivalent to the reflection and refraction losses;

[0033] For all channels, there is the same reflection, which is equivalent to the measurement system error. The above formula is equivalent to

[0034]

[0035] For a fixed measurement system, k and I are obtained d are all constants, I k1 It is also a constant.

[0036] Furthermore, the specific steps of step S4 are as follows:

[0037] The surface of the sheet to be stamped is blank, and the thickness of the oil film to be measured is d x = 0, the light intensity I of the six circular reference signal windows received by the detector mea1 -I mea6 Expressed as:

[0038]

[0039] Where d1-d6 are the thicknesses of the oil samples sealed in the six circular reference signal windows, Q and I are k1 is the constant parameter of the measurement system; the value of the constant Q and I are obtained from the above formula k1 Irrelevant, avoid I during fitting k1 The value of affects Q, and Q is solved directly; combining the above equations, we get:

[0040]

[0041] Q i Indicates several calculation results, where m and n have Combination forms can be calculated The final Q value is obtained by taking the average value as shown below:

[0042]

[0043] On the basis of obtaining the Q value, As a variable combination, linear least squares fitting is used to obtain I k1 value.

[0044] Furthermore, the specific steps of step S5 are as follows:

[0045] During the measurement process, an oil film is applied on the surface of the sheet to be stamped. The constant parameters of the measurement system obtained by calibration are used. According to the value of the light intensity received by the detector through the circular reference signal window, the thickness of the six oil films to be measured d xi The calculation formula is as follows:

[0046]

[0047] In the measurement system, the detector converts the received infrared light into a voltage output. The final value is the voltage value, not the light intensity value. The light intensity is expressed as the optical power per unit area, in W / m 2 , the conversion relationship between voltage and light intensity is expressed as:

[0048] V=R·P o =R·I·S

[0049] Where R is the light response of the detector, I is the light intensity received by the detector, and S is the photosensitive area of the detector; then d xi Expressed as:

[0050]

[0051] Where V mean is the voltage value corresponding to the circular reference signal window light intensity signal output by the detector, V k1 is the constant parameter I k1 Corresponding voltage value; d n is the thickness of the oil sample to be tested sealed in the nth circular reference signal window; n = 1.2…6;

[0052] The six thickness values calculated by the above formula are averaged to obtain the final oil film thickness d x As shown below, it has higher accuracy and reliability than a single calculated value;

[0053]

[0054] Furthermore, the thickness of the oil samples to be tested sealed in the six circular reference signal windows ranges from 50 to 100 μm.

[0055] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0056] 1. High measurement accuracy and strong anti-interference ability: This invention is based on an optical measurement method that uses the infrared light absorption characteristics of the oil film to be measured in a specific band. In combination with the Lambert-Beer law, it uses a multi-point differential absorption algorithm to reduce the electromagnetic interference problem of the electrical measurement method and the mechanical vibration influence of the ultrasonic measurement method, significantly improving the measurement accuracy and environmental adaptability.

[0057] 2. A multi-point differential algorithm simplifies the calibration process: By encapsulating multiple reference oil film samples of known thickness in the filter wheel, the measurement system can simultaneously perform calibration and measurement, eliminating the need for additional calibration standards. The multi-point differential algorithm directly calculates some of the parameters to be solved, converting nonlinear fits into linear fits. This reduces the number of calibration parameters, simplifies the calibration process, and improves fit accuracy.

[0058] 3. Dual-light source common optical path design unifies system errors: The measurement system adopts a dual-light source common optical path reflection and absorption structure, integrating the multi-point measurement structure within the measurement system. This unifies all errors caused by measurement system errors, device and environmental interference into a common error. The measurement system parameters can be obtained through a single calibration, avoiding the need for separate measurement of the reference signal, thereby simplifying the measurement process and reducing the error caused by reference signal measurement.

[0059] 4. Signal modulation based on correlation filtering improves signal-to-noise ratio: The filter wheel modulates the optical signal to a fixed frequency, facilitating subsequent filtering and signal acquisition. Combined with correlation filtering, this effectively reduces the impact of industrial field noise on signal acquisition, significantly improving the measurement system's signal-to-noise ratio.

[0060] 5. Real-time online measurement, adaptable to various environments: The system uses key constant parameters obtained through calibration to achieve real-time online measurement of the oil film thickness to be measured. It can operate stably in industrial assembly line environments and meet the requirements of high-precision and fast measurement. The system is also suitable for other film thickness measurement scenarios.

[0061] 6. The measurement results are stable and highly accurate: The present invention calculates multiple thickness values through a differential algorithm and takes the average value as the final result, eliminating the random error in a single measurement. The verification process is simple and easy to implement, with high measurement accuracy, further improving the stability and reliability of the measurement results.

[0062] 7. Broad Application Prospects: The measurement method designed in this invention is not only suitable for high-precision measurement of oil film thickness on strip surfaces during the automotive body stamping process, but can also be expanded to non-contact measurement of the thickness of other industrial films, providing reliable technical support for diverse fields. It can meet the needs of measuring oil film thickness on any large stamping part and has great application prospects in measuring the thickness of other films besides oil films. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 FIG. 1 shows a schematic diagram of the optical path structure of the measurement system in this embodiment.

[0064] Figure 2 Shown is a schematic diagram of the detector output signal after modulation and processing.

[0065] Figure 3 Shown is a schematic flow chart of the oil film thickness measurement method of this embodiment.

[0066] Figure numerals: 101-infrared light source, 102-light source window, 103-plate to be punched, 104-oil film to be measured, 105-measuring light path window, 106-focusing lens, 107-focusing lens, 108-motor, 109-filter wheel, 110-circular reference signal window, 111-circular reference signal window, 112-circular reference signal window, 113-circular reference signal window, 114-circular reference signal window, 115-circular reference signal window, 116-detector. DETAILED DESCRIPTION

[0067] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0068] Example 1

[0069] See Figure 1 This embodiment provides a plate and strip surface oil film thickness measurement system based on infrared multi-point differential absorption, which is used to measure the thickness of a to-be-measured oil film 104 coated on a plate and strip 103 to be punched. The system includes an infrared light source 101, a measurement light path window 105, a focusing lens 106, a focusing lens 107, a motor 108, a filter wheel 109, and a detector 116.

[0070] An infrared light source 101 is provided on both sides above the plate strip 103 to be punched, and a light source window 102 is provided on each of the infrared light sources 101; a measuring light path directly above the plate strip 103 to be punched is provided with a measuring light path window 105, a focusing lens 106, a focusing lens 107 and a detector 116 from bottom to top, wherein the focusing lens 107 is located on the conjugate image plane of the focusing lens 106; a motor 108 is provided on one side of the measuring light path, and the output shaft of the motor 108 is connected to the center of the filter wheel 109 to drive the filter wheel 109 to rotate, and six circular reference signal windows 110-115 are evenly distributed on the filter wheel 109, and the six circular reference signal windows are respectively sealed with oil samples to be tested with increasing thickness through two transparent windows. When the filter wheel 109 rotates, the six circular reference signal windows can pass through the measuring light path between the focusing lens 107 and the detector 116 in sequence.

[0071] Example 2

[0072] In order to measure the oil film thickness on the strip surface, we first need to understand what parameters need to be obtained for the measurement results. The following formula and existing technical principles are used to derive the parameters required for the measurement process:

[0073] First, the infrared absorption spectrum of the oil sample to be tested is measured using a Fourier transform infrared spectrometer to determine the maximum absorption band range of the oil sample to be tested and the theoretical value d of the minimum oil thickness required for the light within the maximum absorption band to be completely absorbed. min .

[0074] Then select a continuous infrared light source whose emission wavelength can cover the maximum absorption band range of the sample.

[0075] The infrared light source 101 generates a broad spectrum of infrared light, which passes through the light source window 102 and reaches the oil film to be measured on the surface of the stamping part. The light intensity emitted by the infrared light source 101 is expressed as

[0076]

[0077] Where I0 is the total light intensity emitted by the infrared light source, I(λ) is the light intensity distribution function, λ1 is the lower limit of the wavelength of the continuous light source emission spectrum, and λ2 is the upper limit of the wavelength of the continuous light source emission spectrum.

[0078] The light reaches the oil film 104 to be measured coated on the surface of the plate strip 103 to be stamped, and enters the measurement light path after being reflected and absorbed by the oil film to be measured. When the light passes through the surface of the plate strip 103 to be stamped and the oil film to be measured, it follows the Lambertian diffuse reflection. The light intensity after diffuse reflection is I d Expressed as

[0079] I d =2δK d I0 max(0,cosθ) (2)

[0080] Where, I d is the light intensity after diffuse reflection, δ is the transmittance of the light source window 102, K d is the diffuse reflection coefficient, I0 is the total light intensity emitted by the infrared light source, that is, the incident light intensity, and θ is the angle between the incident light and the surface normal of the sheet to be stamped.

[0081] The infrared light reaching the oil film to be tested on the surface of the plate to be stamped includes two parts: diffuse reflection and absorption. The absorption of infrared light by the oil film to be tested 104 satisfies the Lambert-Beer law, which is expressed as

[0082] I ab =I d e -εCL (3)

[0083] Where, I abis the intensity of light transmitted after being absorbed by the oil film to be tested, ε is the absorption coefficient of the oil sample to be tested in the infrared light band, C is the concentration of the oil sample to be tested, and L is the optical path length.

[0084] like Figure 1 As shown, from the definition of optical path and the path of reflection and absorption, we can get

[0085]

[0086] Where n oil is the refractive index of the oil sample to be tested, d x is the thickness of the oil film to be measured, and θ is the angle between the incident light and the surface normal of the sheet to be stamped.

[0087] According to the above formula, the absorption of infrared light by the oil film to be measured satisfies the Lambert-Beer law and can be described as

[0088]

[0089] For a certain measurement system and oil film to be measured, εCn oil and θ are constants, then Q is a constant.

[0090] The light reflected and absorbed by the oil film to be measured 104 passes through the measuring optical path window 105 , is imaged by the focusing lens 106 , and is imaged again by the focusing lens 107 .

[0091] The measurement optical path is approximately an ideal optical system, and the imaging of focusing lens 106 and focusing lens 107 satisfies the Gaussian formula, which is expressed as

[0092]

[0093] Where l is the object distance, l' is the image distance, f is the object focal length, and f' is the image focal length.

[0094] The light beam enters the filter wheel 109 , and the motor 108 drives the filter wheel 109 , which is provided with n circular reference signal windows, to rotate, and sequentially scan the light beam focused by the lens group.

[0095] According to the Lambert-Beer law, the light intensity received by the detector 116 through the n circular reference signal windows is expressed as

[0096]

[0097] Where, I mean is the light intensity received by the detector through n circular reference signal windows, d n is the thickness of the oil sample to be measured encapsulated inside the circular reference signal window.

[0098] The loss of light entering the measurement optical path before reaching the detector's photosensitive surface includes not only the normal absorption of the oil sample to be measured encapsulated inside the reference signal window, but also the absorption and scattering of the measurement system, optical devices, and air. The light intensity I oil The light intensity I after reflection and absorption of the measured oil film ab The relationship between

[0099] I oil =I ab β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4) (8)

[0100] Where β is the proportional coefficient of infrared light entering the measurement optical path after being reflected and absorbed by the oil film to be measured; ρ0 is the loss rate caused by the system structure, including absorption, reflection, and scattering of system components; ρ1 is the loss rate caused by the optical components themselves in the system, mainly due to the absorption and reflection of optical windows and lenses; ρ2 is the loss rate caused by the absorption and scattering of dust and oil on the surface of optical devices; ρ3 is the loss rate caused by the system environment, mainly including the absorption and scattering of air and suspended particles in the air; ρ4 is the loss rate caused by absorption and reflection of the sheet to be punched.

[0101] like Figure 1 As shown, the measurement system is a common optical path structure. The light intensity loss of all circular reference signal windows due to the system, optical devices and environment is equal. The light intensity of the circular reference signal window received by the detector is expressed as

[0102]

[0103] Here, k = β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4), which represents the overall transmittance of the measurement system.

[0104] The above formula only considers the light intensity received by the detector under the condition of absorption by the oil film encapsulating the circular reference signal window. In fact, when the light beam passes through an oil film of a certain thickness, in addition to absorption, there are also certain losses such as reflection and refraction. According to the Fresnel reflection law, the reflection of light on the surface of different media is only related to the refractive index. Therefore, the light intensity received by the detector under non-ideal conditions is expressed as

[0105]

[0106] Where Δd is the thickness of the absorbing oil equivalent to the loss due to reflection, refraction, etc.

[0107] As shown in the above formula, the same reflection exists for all channels, which can be equivalent to the system error. Therefore, the above formula can be equivalent to

[0108]

[0109] Where, for a fixed system k and I ab are all constants, so I k1 is a constant.

[0110] At this point, as shown in the above formula, it is known that if the unknown constant parameter I of the measurement system can be obtained k1 and Q values, the oil film thickness d to be measured can be calculated using the signal output by the detector x value.

[0111] like Figure 1 As shown, six light-transmitting circular reference signal windows are set on the filter wheel. The thicknesses of the oil sample to be tested encapsulated between each pair of transparent windows in the circular reference signal window 110 to the circular reference signal window 115 are d1, d2, d3, d4, d5, and d6, respectively. The detector receives the light intensity absorbed by the oil film to be tested and the encapsulated oil sample to be tested of known thickness.

[0112] Since the oil film thickness range commonly used in automobile body stamping is 5-20μm, based on this, the thickness range of d1-d6 is taken as 50-100μm (100μm <d min ), and the thickness of d1-d6 increases successively.

[0113] See Figure 3 , when the surface of the sheet to be stamped is blank, that is, the thickness of the oil film to be measured is d x Parameter calibration is performed when =0, and the light intensity of the reference signal window received by the detector is expressed as.

[0114]

[0115] From the above formula, we can get the value of constant Q and I k1 Irrelevant, avoid I during fitting k1 The value of affects Q, so we can solve Q directly. By combining the above equations, we can get:

[0116]

[0117] As shown above, Q i Indicates multiple calculation results, where m and n have Combination forms can be calculated The final Q value can be obtained by taking the average value as shown below

[0118]

[0119] On the basis of obtaining the Q value, As a variable combination, I can be obtained by linear least squares fitting. k1value.

[0120] During the measurement process, an oil film is applied to the surface of the stamping plate, and the Q and I are obtained using the above calibration. k1 According to the light intensity of the circular reference signal window received by the detector, the six oil film thicknesses d can be calculated as shown in formula (11): xi As shown below

[0121]

[0122] In the measurement system, the detector converts the received infrared light into a voltage output. The final value is the voltage value rather than the light intensity value. The light intensity is expressed as the light power per unit area, and the commonly used unit is (W / m 2 ), the conversion relationship between voltage and light intensity is expressed as

[0123] V=R·P o =R·I·S (16)

[0124] Where R is the light responsivity of the detector, I is the light intensity received by the detector, and S is the photosensitive area of the detector. Then, in practical engineering applications, formula (15) is expressed as

[0125]

[0126] Where V mean is the voltage value corresponding to the light intensity signal of the measurement window output by the detector, V k1 is the parameter I k1 The corresponding voltage value.

[0127] The six thickness values calculated from the above formula are averaged to obtain the final value of the oil film thickness to be measured, which has higher accuracy and reliability than a single calculated value.

[0128]

[0129] In summary, the infrared light multi-point differential absorption method proposed in this embodiment can well realize the measurement of the oil film thickness on the surface of the plate strip to be stamped. The optical path structure of the dual-light source common optical path reflection and absorption makes the system errors of all reference signal windows become unified, and the required parameter values can be obtained through a single calibration. In addition, the multi-point measurement structure is integrated into the measurement system to realize the integration of calibration and measurement, avoid the additional establishment of a standard source for calibration, and simplify the calibration process. The method of the present invention can realize multiple measurements of the measured value at one time, and greatly improves the stability and accuracy of the measurement results through averaging.

[0130] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solutions of the present invention. The above specific embodiments are merely illustrative and not restrictive. Without departing from the scope of the present invention and the scope of protection of the claims, those skilled in the art may make various specific modifications based on the teachings of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A method for measuring the oil film thickness on the surface of a plate and strip based on multi-point differential absorption of infrared light, comprising a system for measuring the oil film thickness on the surface of a plate and strip, the measuring system comprising an infrared light source, a measuring light path window, a focusing lens, a motor, a filter wheel, and a detector; infrared light sources are respectively provided on both sides above the plate and strip to be punched, and light source windows are provided on the infrared light sources; the measuring light path directly above the plate and strip to be punched is provided with a measuring light path window, a focusing lens, and a detector in sequence from bottom to top; a motor is provided on one side of the measuring light path, and the output shaft of the motor is connected to the center of the filter wheel to drive the filter wheel to rotate, and six circular reference signal windows are evenly distributed on the filter wheel, and the six circular reference signal windows are respectively sealed with oil samples to be tested with increasing thicknesses through two transparent windows, and when the filter wheel rotates, the six circular reference signal windows can pass through the measuring light path between the focusing lens and the detector in sequence, characterized in that include: S1. Use a Fourier transform infrared spectrometer to measure the infrared absorption spectrum of the oil sample to be tested, determine the maximum absorption band range of the oil sample to be tested and the theoretical minimum oil thickness d required for the light within the maximum absorption band to be completely absorbed. min ; S2. Based on the theoretical value of minimum oil film thickness d min , determine the thickness d1-d6 of the oil sample to be tested sealed in the six circular reference signal windows <d min , and the thickness of d1-d6 increases successively; S3. The surface of the sheet to be stamped is blank, that is, the thickness of the oil film to be measured is zero, and the light emitted by the infrared light source is irradiated onto the sheet to be stamped through the light source window. The light reflected by the sheet to be stamped passes through the measurement light path window, focusing lens, and filter wheel on the measurement light path and enters the detector; the filter wheel is driven by a motor to rotate, and the detector receives 6 groups of reference light intensity signals of the oil samples to be tested of different thicknesses d1-d6; S4. Based on the reference light intensity signal absorbed by the oil samples of different thicknesses, the constant parameters of the measurement system are obtained using a differential algorithm and a linear least squares fitting calibration method; S5. Based on the calibrated constant parameters of the measurement system, the thickness of the oil film to be measured is measured online in real time, and the thickness value of the oil film to be measured is obtained by calculation.

2. The method for measuring oil film thickness on a plate strip surface based on infrared multi-point differential absorption according to claim 1, characterized in that: In step S3, the Lambert-Beer law is used to calculate the absorption of infrared light by the oil film to be tested, and the formula is: Where, I ab is the light intensity after being absorbed by the oil film to be measured, I d is the intensity of light incident on the surface of the oil film to be tested after diffuse reflection, ε is the absorption coefficient of the oil sample to be tested in the infrared light band, C is the concentration of the oil sample to be tested, and n oil is the refractive index of the oil sample to be tested, d x is the thickness of the oil film to be measured, θ is the angle between the light source and the horizontal direction of the measurement system; for a certain measurement system and oil sample to be measured, εCn oil and θ are constants, and Q is also a constant.

3. The method for measuring oil film thickness on a plate strip surface based on infrared multi-point differential absorption according to claim 2, characterized in that: According to the Lambert-Beer law, the light intensity received by the detector through the six circular reference signal windows is expressed as Where, I mean is the light intensity received by the detector through n circular reference signal windows, d n The thickness of the oil sample to be measured is encapsulated inside the circular reference signal window; The loss of light entering the measurement optical path before reaching the detector's photosensitive surface includes not only the normal absorption of the oil film, but also the absorption and scattering of the measurement system, optical devices, and air. The light intensity I absorbed by the oil sample to be measured that is not encapsulated in the circular reference signal window is oil The light intensity I after reflection and absorption of the measured oil film ab The relationship between I oil =I ab β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4) Wherein, β is the ratio coefficient of infrared light entering the measurement optical path after being reflected and absorbed by the oil film to be measured; ρ0 is the loss rate caused by the measurement system structure, including absorption, reflection, and scattering of the measurement system components; ρ1 is the loss rate caused by the optical components themselves in the measurement system, including absorption and reflection of the optical path windows and focusing lenses; ρ2 is the loss rate caused by absorption and scattering of dust and oil on the surface of the optical device; ρ3 is the loss rate caused by the measurement system environment, including absorption and scattering of air and suspended particles in the air; ρ4 is the loss rate caused by absorption and reflection of the sheet to be punched. The light intensity of the circular reference signal window received by the detector is expressed as Where k = β(1-ρ0)(1-ρ1)(1-ρ2)(1-ρ3)(1-ρ4), which represents the overall transmittance of the measurement system. According to Fresnel's law of reflection, the reflection of light on the surface of different media is only related to the refractive index. Under non-ideal conditions, the light intensity received by the detector is expressed as Where Δd is the thickness of the absorbing oil equivalent to the reflection and refraction losses; For all channels, there is the same reflection, which is equivalent to the measurement system error. The above formula is equivalent to For a fixed measurement system, k and I are obtained d are all constants, I k1 It is also a constant.

4. The method for measuring oil film thickness on a plate strip surface based on infrared multi-point differential absorption according to claim 1, characterized in that: The specific steps of step S4 are as follows: The surface of the sheet to be stamped is blank, and the thickness of the oil film to be measured is d x = 0, the light intensity I of the six circular reference signal windows received by the detector mea1 -I mea6 Expressed as: Where d1-d6 are the thicknesses of the oil samples sealed in the six circular reference signal windows, Q and I are k1 is the constant parameter of the measurement system; From the above formula, we can get the value of constant Q and I k1 Irrelevant, avoid I during fitting k1 The value of affects Q, and Q is solved directly; combining the above equations, we get: Q i Indicates several calculation results, where m and n have Combination forms can be calculated The final Q value is obtained by taking the average value as shown below: On the basis of obtaining the Q value, As a variable combination, I was obtained by linear least squares fitting. k1 value.

5. The method for measuring oil film thickness on a plate strip surface based on infrared multi-point differential absorption according to claim 1, characterized in that: The specific steps of step S5 are as follows: During the measurement process, an oil film is applied on the surface of the sheet to be stamped. The constant parameters of the measurement system obtained by calibration are used. According to the value of the light intensity received by the detector through the circular reference signal window, the thickness of the six oil films to be measured d xi The calculation formula is as follows: In the measurement system, the detector converts the received infrared light into a voltage output. The final value is the voltage value, not the light intensity value. The light intensity is expressed as the optical power per unit area, in W / m 2 , the conversion relationship between voltage and light intensity is expressed as: V=R·P o =R·I·S Where R is the light response of the detector, I is the light intensity received by the detector, and S is the photosensitive area of the detector; then d xi Expressed as: Where V mean is the voltage value corresponding to the circular reference signal window light intensity signal output by the detector, V k1 is the constant parameter I k1 Corresponding voltage value; d n is the thickness of the oil sample to be tested sealed in the nth circular reference signal window; n = 1.2…6; The six thickness values calculated by the above formula are averaged to obtain the final oil film thickness d x As shown below, it has higher accuracy and reliability than a single calculated value; 6. The method for measuring oil film thickness on a plate strip surface based on infrared multi-point differential absorption according to claim 1, characterized in that: The thickness of the oil samples to be tested sealed in the six circular reference signal windows ranges from 50 to 100 μm.

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