Method and device for measuring thickness of paint film based on terahertz time-domain spectroscopy

Abstract

The invention discloses a paint film thickness measurement method and device based on terahertz time-domain spectroscopy, and belongs to the technical field of terahertz thickness measurement. First, reference signals are collected, the reference signals within the left and right time ranges of a main peak are intercepted, and then upper and lower surface reflection spectrums are calculated; moving the main peak of the upper surface reflection signal forward relative to the main peak of the lower surface reflection signal to obtain lower surface reflection signals corresponding to the paint films with different thicknesses, adding the upper surface reflection spectrum signals and the lower surface reflection spectrum signals to obtain theoretical terahertz reflection spectrum signals corresponding to the paint films with different thicknesses, and collecting sample signals of a to-be-detected paint film sample; and calculating the correlation degree between the sampling point and the theoretical terahertz reflection spectrum signal, wherein the thickness corresponding to the theoretical terahertz reflection spectrum signal corresponding to the maximum correlation degree is the thickness of the sampling point of the unknown paint film sample. The thickness of a paint film below 80m, especially below 30m, can be detected, and the sensitivity of paint film thickness detection is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of terahertz spectroscopy technology, specifically relating to a method and apparatus for measuring the thickness of wet paint coatings based on terahertz time-domain spectroscopy. Background Technology

[0002] Spray painting is a crucial process in the production of automobiles, ships, and other industries. Paint film thickness is a key factor affecting coating quality. Inappropriate thickness can lead to defects such as runs, paint streaks, and sanding marks, and further affect the appearance of the paint film. Therefore, real-time thickness measurement of the paint film during spray painting is essential for controlling coating quality. Existing methods for measuring paint film thickness include electromagnetic / eddy current methods, ultrasonic methods, and terahertz wave methods. Among these, the terahertz time-domain spectroscopy-based thickness measurement method is a powerful non-contact, non-destructive testing method, particularly suitable for measuring the thickness of wet paint coatings (films). For example, patent CN105588516A discloses a paint film thickness measurement method based on terahertz pulse spectroscopy, which measures the time delay between the pulses reflected back from the upper and lower surfaces of the sample by the terahertz pulse. Based on the formula Calculate the thickness of the paint film. ,in The refractive index of the paint film material is used, but when the thickness of the paint film is less than 80 micrometers, the reflected pulses from the upper and lower surfaces will overlap into a single wave packet, making the time delay impossible to read directly with the naked eye. Thickness measurement is difficult. Therefore, this invention provides a method and apparatus capable of measuring the thickness of thinner paint films. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the existing technology and to design a method and device for measuring the thickness of wet paint coatings (films) based on terahertz time-domain spectroscopy, which enables the measurement of thinner paint film thickness and significantly improves the sensitivity of paint film thickness detection.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] A method for measuring paint film thickness based on terahertz time-domain spectroscopy includes steps S1 to S6.

[0006] S1. Acquire the terahertz time-domain spectral signal of the uncoated metal sample. As a reference signal.

[0007] S2, Extract the left and right sides of the main peak The reference signal within the time range is denoted as .

[0008] S3. Calculate the reflection coefficient of the upper surface according to the reflectivity formula 2 and transmittance formula 3 in the optical transmission model. and transmission coefficient .

[0009] Formula 2

[0010] Formula 3

[0011] in, The refractive index of air, denoted as the refractive index of the paint film.

[0012] Calculate the reflected signals from the upper and lower surfaces using formulas four and five respectively. and .

[0013] Formula 4

[0014] Formula 5

[0015] S4. Reflect the signal from the upper surface. The main peak reflects the signal from the lower surface. By shifting the main peak forward, the lower surface reflection signals corresponding to metal samples with different paint film thicknesses are obtained. Then, the upper and lower surface reflection spectral signals are added to obtain the theoretical terahertz reflection spectral signals corresponding to different paint film thicknesses. A paint film thickness model is constructed by mapping the paint film thickness to the corresponding theoretical terahertz reflection spectral signals. The theoretical terahertz reflection spectral signals... Corresponding paint film thickness Calculate according to Formula 6.

[0016] Formula Six

[0017] in, Represents the speed of light. express The time when the maximum value occurs express The time when the maximum value occurs This indicates the refractive index of the paint film.

[0018] S5. Calculate the thickness of a certain sampling point of an unknown paint film sample.

[0019] A reflective terahertz time-domain spectroscopy coating thickness measurement device was used to acquire the terahertz time-domain spectral signal of the coating film at a certain sampling point of the coating film sample to be tested. , As a sample signal, extract the left and right sides of the main peak. Sample signals in the picosecond range are denoted as ,Will The location of the main peak has been moved to At this point, the aligned terahertz reflection spectrum signal of the sample was obtained. According to Formula 7, calculate the theoretical terahertz reflection spectrum of each signal in step S4 for different paint film thicknesses. correlation between Maximum relevance The thickness corresponding to the theoretical terahertz reflection spectrum signal is the thickness D of this unknown paint film sample at that sampling point.

[0020]

[0021]

[0022] Formula 7

[0023] in, The terahertz reflection spectrum signal being compared at the present time. The Middle Terahertz reflection spectrum signal at time location, The terahertz reflection spectrum signal being compared at the present time. The average value, for The Middle Terahertz reflection spectrum signal at time location, for The average value of all terahertz reflection spectrum signals.

[0024] S6. Scan the entire paint film point by point, calculate the thickness of each sampling point using step S5, and then obtain the thickness of all sampling points of the unknown paint film sample and generate a thickness distribution map.

[0025] Step S2 specifically involves: First, calculating the position and corresponding time of the main peak of the reference signal according to Formula 1. Then, extracting the left and right sides of the main peak. The reference signal within the time range is denoted as , The duration is , The amount of data is , , This represents the time interval between two adjacent data points.

[0026] Formula 1

[0027] in, This indicates the time when the maximum value of the reference signal occurs.

[0028] Step S4 includes S401 to S403.

[0029] S401, Construct the upper surface reflection signal Time-shifted sequence.

[0030] The maximum time range in which the main peak can move forward is That is, the maximum amount of data to move forward is Based on this, a time-shifted sequence of the upper surface reflection signal is constructed. , .

[0031] S402, Construct the upper surface reflectance spectrum matrix.

[0032] Reflecting signals from the upper surface The main peak reflects the signal from the lower surface. The main peak shifted forward. Each time series in the data, specifically, will include the upper surface reflected signal. The main peak reflects the signal from the lower surface. The main peak shifted forward. The reflected signal from the translated upper surface is obtained. The final result The upper surface reflectance spectral signals constitute the upper surface reflectance spectral signal matrix. .

[0033] S403, Construct a paint film thickness model.

[0034] The upper surface reflectance spectral signal matrix Each upper surface reflection signal in the middle and Adding these together yields a series of theoretical terahertz reflection spectrum signals corresponding to paint films of different thicknesses, such as the upper surface reflection signal. and Adding them together yields the theoretical terahertz reflection spectrum signal for the corresponding thickness of the paint film. .

[0035] The paint film thickness measurement device based on terahertz time-domain spectroscopy includes a terahertz thickness measurement system, a terahertz thickness measurement probe, a nozzle, and a spraying device. Both the terahertz thickness measurement probe and the nozzle are directly facing the upper surface of the paint sample. The terahertz thickness measurement system is connected to the terahertz thickness measurement probe and acquires the terahertz signal of the paint film obtained by the terahertz thickness measurement probe in real time. The thickness of the paint film is calculated based on the paint film thickness measurement method of terahertz time-domain spectroscopy. When the spraying thickness is reached, a control signal is sent to the spraying device to control the nozzle to stop spraying.

[0036] Compared with the prior art, the present invention has the following advantages: (1) It can realize the real-time detection of paint film thickness and stop spraying in time when the coating thickness is reached to avoid overspraying; (2) It can realize the detection of paint film thickness below 80µm, especially below 30µm, and the detection accuracy is high, ensuring the uniformity of spraying; (3) Through thickness map drawing and online measurement, the edge position of the workpiece can be accurately detected, and the edge position can be accurately sprayed. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of a reflective terahertz time-domain spectroscopy coating thickness measurement device.

[0038] Figure 2 This is the reference signal for the terahertz reflection spectrum.

[0039] Figure 3 The terahertz reflection spectrum signals are from the upper and lower surfaces of the paint film.

[0040] Figure 4 The theoretical terahertz reflection spectrum signals are for paint films of different thicknesses.

[0041] Figure 5 This is the correlation sequence between the theoretical terahertz reflection spectrum of paint films of different thicknesses and the terahertz reflection spectrum of the target paint film sample.

[0042] Figure 6 This is a comparison chart of the original terahertz reflection spectrum signal and the theoretical terahertz reflection spectrum signal when the paint film thickness is 28µm.

[0043] Figure 7 This is a comparison chart of the original terahertz reflection spectrum signal and the theoretical terahertz reflection spectrum signal when the paint film thickness is 45µm.

[0044] Figure 8 The thickness distribution diagrams and original terahertz reflection spectrum signals are shown for coating film thicknesses of 22µm, 28µm, 45µm, or 64µm. Detailed Implementation

[0045] The present invention will be further described below through specific embodiments.

[0046] Example 1

[0047] like Figure 1As shown, the reflective terahertz time-domain spectroscopy paint film thickness measurement device used in this embodiment includes a terahertz thickness measurement system, a terahertz thickness measurement probe, a nozzle, and a spraying device. Both the terahertz thickness measurement probe and the nozzle are directly facing the upper surface of the painted sample. The terahertz thickness measurement system is connected to the terahertz thickness measurement probe and acquires the terahertz signal of the paint film in real time. Based on the following terahertz time-domain spectroscopy method for measuring paint film thickness, the thickness of the paint film is calculated. When the spraying thickness is reached, a control signal is sent to the spraying device to stop the nozzle from spraying. The terahertz scanning probe is fixed on a two-dimensional translation frame, and the sample is placed on a sample stage below the terahertz scanning probe. Terahertz spectra are acquired while the metal sample is being sprayed with paint.

[0048] The coating thickness measurement method based on terahertz time-domain spectroscopy involved in this embodiment includes steps S1 to S6.

[0049] S1. Acquire the terahertz time-domain spectral signal of the uncoated metal sample. As a reference signal, such as Figure 2 As shown.

[0050] S2, Extract the left and right sides of the main peak The reference signal within the time range is denoted as .

[0051] Specifically, the position and corresponding time of the main peak of the reference signal are calculated according to Formula 1.

[0052] Formula 1

[0053] in, This indicates the time when the maximum value of the reference signal occurs.

[0054] Then cut off the left and right sides of the main peak The reference signal within the time range is denoted as , The duration is , The amount of data is , , This represents the time interval between two adjacent data points.

[0055] S3. Calculate the reflection spectrum of the upper and lower surfaces.

[0056] The reflection coefficient of the upper surface is calculated according to formulas two for reflectivity and three for transmittance in the optical transmission model. and transmission coefficient .

[0057] Formula 2

[0058] Formula 3

[0059] in, The refractive index of air, denoted as the refractive index of the paint film.

[0060] Calculate the reflected signals from the upper and lower surfaces using formulas four and five respectively. and ,like Figure 3 As shown.

[0061] Formula 4

[0062] Formula 5

[0063] S4. When using reflective terahertz time-domain spectroscopy to detect a point on a metal coating sample, the terahertz wave first reaches the upper surface of the coating, and a portion of it is reflected, generating a reflection signal from the upper surface. Another portion penetrates the paint film and reaches the lower surface of the paint film. Due to the characteristics of the metal substrate, it is completely reflected, generating a reflection signal from the lower surface. upper surface reflected signal and the signal reflected from the lower surface The two reflectance signals overlap and together constitute the terahertz reflection spectrum signal. Theoretically, for thin-film materials such as paint films (typically 10µm-300µm thick), with the sample stage position remaining constant (i.e., the lower surface of the sample remaining constant), the upper surface reflection signal can be detected earlier as the paint film thickness increases. The waveform is shown as Main peak relative The main peak shifts forward, that is and The longer the time interval between the main peaks, the better. For thin-layer materials, and The waveform remains essentially unchanged. Therefore, this invention uses the upper surface reflected signal... The main peak reflects the signal from the lower surface. By shifting the main peak forward, the lower surface reflection signal corresponding to the metal sample with different coating thickness is obtained. Then, the upper and lower surface reflection spectrum signals are added together to obtain the theoretical terahertz reflection spectrum signal corresponding to the coating film with different thickness.

[0064] The specific steps are as follows: S401, Construct the upper surface reflection signal Time-shifted sequence.

[0065] Based on Formula 4, we can understand... and The time length is the same. The maximum time range in which the main peak can move forward is That is, the maximum amount of data to move forward is Based on this, a time-shifted sequence of the upper surface reflection signal is constructed. .

[0066] S402, Construct the upper surface reflectance spectrum matrix.

[0067] Reflecting signals from the upper surface The main peak reflects the signal from the lower surface. The main peak shifted forward. Each time series in the data, specifically, will include the upper surface reflected signal. The main peak reflects the signal from the lower surface. The main peak shifted forward. The reflected signal from the translated upper surface is obtained. The final result The upper surface reflectance spectral signals constitute the upper surface reflectance spectral signal matrix. , .

[0068] S403, Construct a paint film thickness model.

[0069] The upper surface reflectance spectral signal matrix Each upper surface reflection signal in the middle and Adding these together yields a series of theoretical terahertz reflection spectrum signals corresponding to paint films of different thicknesses, such as the upper surface reflection signal. and Adding them together yields the theoretical terahertz reflection spectrum signal for the corresponding thickness of the paint film. ,like Figure 4 As shown, a paint film thickness model is constructed by mapping the paint film thickness to the corresponding theoretical terahertz reflection spectrum signal, where, Corresponding paint film thickness Calculate according to Formula 6.

[0070] Formula Six

[0071] in, Represents the speed of light. express The time when the maximum value occurs express The time when the maximum value occurs This indicates the refractive index of the paint film.

[0072] It should be noted that after the move Data at the back is missing, and If perfect alignment is not possible, the missing data shifted out during the forward movement can be added to the missing positions at the back, or the missing positions at the back can be filled with 0s to obtain the final result. .therefore, ,and as well as Both length and data volume are similar to same.

[0073] S5. Calculate the thickness of a certain sampling point of an unknown paint film sample.

[0074] use Figure 1 The reflective terahertz time-domain spectroscopy coating thickness measurement device shown acquires the terahertz time-domain spectral signal of the coating film at a certain sampling point of the coating film sample to be tested. , As the sample signal, it is obtained based on Formula 1. The location and corresponding time of the main peak, and the left and right sides of the main peak. Sample signals in the picosecond range are denoted as ,move Move its main peak position to place (i.e.) Main peak and (Main peak alignment) to obtain the aligned sample terahertz reflection spectrum signal According to Formula 7, calculate the theoretical terahertz reflection spectrum of each signal in step S4 for different paint film thicknesses. correlation between The correlation sequence is obtained, such as Figure 5 As shown.

[0075]

[0076]

[0077] Formula 7

[0078] in, The terahertz reflection spectrum signal being compared at the present time. The Middle Terahertz reflection spectrum signal at time location, The terahertz reflection spectrum signal being compared at the present time. The average value, for The Middle Terahertz reflection spectrum signal at time location, for The average value of all terahertz reflection spectrum signals.

[0079] It should be noted that after the move Data missing at the end, and the matrix middle If perfect alignment is not possible, the missing data shifted out during the forward movement can be added to the missing positions at the back, or the missing positions at the back can be filled with 0s to obtain the final result. .therefore, Length and data volume same.

[0080] Maximum Relevance The thickness corresponding to the theoretical terahertz reflection spectrum signal is the thickness D of this unknown paint film sample at that sampling point.

[0081] Figure 6 and Figure 7 The original and constructed signals for 28µm and 45µm thick paint films, respectively, show a very high degree of overlap, proving that the present invention can achieve detection of even thinner thicknesses.

[0082] S6. Scan the entire paint film point by point, calculate the thickness of each sampling point using step S5, and then obtain the thickness of all sampling points of the unknown paint film sample, and generate a thickness distribution map, such as... Figure 8 As shown.

Claims

1. A method for measuring paint film thickness based on terahertz time-domain spectroscopy, characterized in that, Includes the following steps: S1. Acquire the terahertz time-domain spectral signal of the uncoated metal sample. As a reference signal; S2, Extract the left and right sides of the main peak The reference signal within the time range is denoted as ; S3. Calculate the reflection coefficient of the upper surface according to the reflectivity formula 2 and transmittance formula 3 in the optical transmission model. and transmission coefficient ; Formula 2 Formula 3 in, The refractive index of air, The refractive index of the paint film; Calculate the reflected signals from the upper and lower surfaces using formulas four and five respectively. and ; Formula 4 Formula 5 S4, reflect the signal from the upper surface The main peak reflects the signal from the lower surface. By shifting the main peak forward, the lower surface reflection signals corresponding to metal samples with different paint film thicknesses are obtained. Then, the upper and lower surface reflection spectral signals are added to obtain the theoretical terahertz reflection spectral signals corresponding to different paint film thicknesses. A paint film thickness model is constructed by mapping the paint film thickness to the corresponding theoretical terahertz reflection spectral signals. The theoretical terahertz reflection spectral signals... Corresponding paint film thickness Calculate according to Formula 6; Formula Six in, Represents the speed of light. express The time when the maximum value occurs express The time when the maximum value occurs Indicates the refractive index of the paint film; S5. Calculate the thickness of a sampling point on an unknown paint film sample; A reflective terahertz time-domain spectroscopy coating thickness measurement device was used to acquire the terahertz time-domain spectral signal of the coating film at a certain sampling point of the coating film sample to be tested. , As a sample signal, extract the left and right sides of the main peak. Sample signals in the picosecond range are denoted as ,Will The location of the main peak has been moved to At this point, the aligned terahertz reflection spectrum signal of the sample was obtained. According to Formula 7, calculate the theoretical terahertz reflection spectrum of each signal in step S4 for different paint film thicknesses. correlation between Maximum relevance The thickness corresponding to the theoretical terahertz reflection spectrum signal is the thickness D of this unknown paint film sample at that sampling point; Formula 7 in, The terahertz reflection spectrum signal being compared at the present time. The Middle Terahertz reflection spectrum signal at time location, The terahertz reflection spectrum signal being compared at the present time. The average value, for The Middle Terahertz reflection spectrum signal at time location, for The average value of all terahertz reflection spectral signals; S6. Scan the entire paint film point by point, calculate the thickness of each sampling point using step S5, and then obtain the thickness of all sampling points of the unknown paint film sample and generate a thickness distribution map.

2. The method for measuring paint film thickness based on terahertz time-domain spectroscopy according to claim 1, characterized in that, Step S2 specifically involves: first, calculating the position and corresponding time of the main peak of the reference signal according to Formula 1, and then extracting the left and right sides of the main peak. The reference signal within the time range is denoted as , The duration is , The amount of data is , , The time interval between two adjacent data points; Formula 1 in, This indicates the time when the maximum value of the reference signal occurs.

3. The method for measuring paint film thickness based on terahertz time-domain spectroscopy according to claim 1, characterized in that, Step S4 is as follows: S401, Construct the upper surface reflection signal Time-shifted sequence The maximum time range in which the main peak can move forward is That is, the maximum amount of data to move forward is Based on this, a time-shifted sequence of the upper surface reflection signal is constructed. , ; S402, Construct the upper surface reflectance spectral matrix Reflecting signals from the upper surface The main peak reflects the signal from the lower surface. The main peak shifted forward. Each time series in the data, specifically, will include the upper surface reflected signal. The main peak reflects the signal from the lower surface. The main peak shifted forward. The reflected signal from the translated upper surface is obtained. The final result The upper surface reflectance spectral signals constitute the upper surface reflectance spectral signal matrix. ; S403, Constructing a paint film thickness model The upper surface reflectance spectral signal matrix Each upper surface reflection signal in the middle and Adding these together yields a series of theoretical terahertz reflection spectrum signals corresponding to paint films of different thicknesses, such as the upper surface reflection signal. and Adding them together yields the theoretical terahertz reflection spectrum signal for the corresponding thickness of the paint film. . A coating film thickness measurement device based on terahertz time-domain spectroscopy, characterized in that, The system includes a terahertz thickness measurement system, a terahertz thickness measurement probe, a nozzle, and a spraying device. Both the terahertz thickness measurement probe and the nozzle are directly facing the upper surface of the painted sample. The terahertz thickness measurement system is connected to the terahertz thickness measurement probe and acquires the terahertz signal of the paint film obtained by the terahertz thickness measurement probe in real time. The thickness of the paint film is calculated based on the paint film thickness measurement method of terahertz time-domain spectroscopy as described in any one of claims 1-3. When the spraying thickness is reached, a control signal is sent to the spraying device to control the nozzle to stop spraying.

Images