Composite coating detection method and system

Abstract

This application relates to the field of coating inspection technology, and discloses a method and system for inspecting composite coatings. The method includes: emitting a terahertz signal to a moving electrode coating; converting the terahertz signal into a uniform stripe of light irradiating the electrode coating; receiving terahertz signals reflected from each layer of the electrode coating; acquiring real-time movement information of the electrode coating; synchronizing the reflected terahertz signals based on the movement information; and calculating the physical parameters of each layer based on the synchronized reflected terahertz signals. This method not only accurately analyzes the thickness of each individual coating layer, but also simultaneously derives key process properties such as porosity, areal density, and conductivity. Furthermore, the entire process is non-contact, involves no ionizing radiation, and requires no downtime for calibration.

Description

Technical Field

[0001] This application relates to the field of coating inspection technology, and in particular to a composite coating inspection method and system. Background Technology

[0002] Composite coating is a coating technology that has emerged in recent years. It refers to the simultaneous or sequential coating of two or more functional pastes (such as conductive layer / active layer / adhesive layer) onto an aluminum foil substrate to form a multilayer heterogeneous electrode structure. The core lies in the precise control of the independent thickness of each layer, the interface bonding state, and the pore distribution.

[0003] Currently, the two most commonly used thickness measurement methods in coating processes are beta-ray and X-ray. Beta-ray uses a high-speed electron beam to penetrate the coating, and the overall thickness is estimated by measuring the transmission intensity. X-ray uses high-energy photons to penetrate and be absorbed by the material, and the overall thickness is calculated by the amount of absorption. Both methods have the advantage of fast response and suitability for online overall thickness monitoring. However, they usually only provide the "overall thickness," making it difficult to decompose multi-layer coatings into the thickness of each layer and identify defects and porosity between layers. Furthermore, both are radioactive and harmful to human health, requiring additional protection and professional maintenance. Summary of the Invention

[0004] In view of this, embodiments of this application provide a composite coating detection method and system, which can effectively solve the technical problem that traditional thickness detection methods are unable to decompose multilayer coatings into the thickness of each layer and identify defects and pores between layers.

[0005] In a first aspect, embodiments of this application provide a composite coating detection system, the system comprising: A terahertz transmitter, positioned above the electrode coating, is used to transmit terahertz signals to the electrode coating in a moving state. A light spot forming component is disposed along the emission path of the terahertz transmitter and after the emission end of the terahertz transmitter, for converting the terahertz signal into a uniform strip light spot; A signal acquisition unit is disposed on the same side as the terahertz transmitter above the electrode coating, and is used to receive terahertz signals reflected by each layer of the electrode coating; A device encoder is used to acquire the movement information of the electrode coating in real time; A synchronization control unit is electrically connected to the terahertz transmitter, the signal acquisition unit, and the device encoder, respectively, and is used to synchronize the reflected terahertz signal according to the movement information; A real-time computing unit, which is communicatively connected to the signal acquisition unit, is used to calculate the physical parameters of each layer based on the reflected terahertz signal.

[0006] Secondly, embodiments of this application provide a method for detecting composite coatings, the method comprising: A terahertz signal is emitted toward the moving electrode coating; The terahertz signal is converted into a uniform stripe of light that illuminates the electrode coating; Receive terahertz signals reflected by each layer of the electrode coating; The movement information of the electrode coating is acquired in real time; The reflected terahertz signal is synchronized based on the movement information; The physical parameters of each layer are calculated based on the reflected terahertz signal after synchronization.

[0007] The embodiments of this application have the following beneficial effects: A uniform strip-shaped light spot is generated by a terahertz transmitter and a light spot forming component to achieve full-width continuous coverage; a common optical path reflection structure is constructed by setting the signal acquisition unit on the same side as the transmitter to ensure phase accuracy; hard triggering synchronization is achieved by the equipment encoder and synchronization control unit to eliminate sampling misalignment caused by belt jitter; finally, the real-time calculation unit can not only accurately analyze the thickness of each coating layer, but also simultaneously derive key process physical parameters such as porosity, areal density and conductivity, and the whole process is non-contact, non-ionizing radiation, and requires no downtime for calibration. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 This diagram illustrates the placement of the composite coating detection system according to an embodiment of this application. Figure 2 A schematic diagram illustrating the transmission and reception process of terahertz signals according to an embodiment of this application is shown; Figure 3 A schematic flowchart of the composite coating detection method according to an embodiment of this application is shown. Detailed Implementation

[0010] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0011] The components of the embodiments of this application described and illustrated in the accompanying drawings can be arranged and designed in a variety of different configurations. Therefore, the following detailed description of the embodiments of this application provided in the drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0012] In the following text, the terms "comprising," "having," and their cognates, which may be used in various embodiments of this application, are intended only to indicate a particular feature, number, step, operation, element, component, or combination thereof, and should not be construed as primarily excluding the presence of one or more other features, numbers, steps, operations, elements, components, or combinations thereof, or adding the possibility of one or more combinations thereof. Furthermore, the terms "first," "second," "third," etc., are used only for distinguishing descriptions and should not be construed as indicating or implying relative importance.

[0013] Unless otherwise specified, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which the various embodiments of this application pertain. Terms (such as those defined in a generally used dictionary) shall be interpreted as having the same meaning as in the context of the relevant technical field and shall not be interpreted as having an idealized or overly formal meaning, unless clearly defined in the various embodiments of this application.

[0014] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0015] The following specific embodiments illustrate the composite coating detection method and system.

[0016] Figure 1 A schematic diagram showing the placement of a composite coating detection system according to an embodiment of this application is illustrated. Exemplarily, the composite coating detection system includes: A terahertz transmitter, positioned above the electrode coating, is used to transmit terahertz signals to the moving electrode coating.

[0017] A light spot forming component is located along the emission path of the terahertz transmitter and after the emitting end of the terahertz transmitter, and is used to convert the terahertz signal into a uniform strip light spot.

[0018] The signal acquisition unit is located on the same side as the terahertz transmitter, above the electrode coating, and is used to receive terahertz signals reflected by each layer of the electrode coating.

[0019] The device encoder is used to acquire real-time information about the movement of the electrode coating.

[0020] The synchronization control unit is electrically connected to the terahertz transmitter, signal acquisition unit, and equipment encoder, respectively, and is used to synchronize the reflected terahertz signal according to the movement information.

[0021] The real-time computing unit, which communicates with the signal acquisition unit, is used to calculate the physical parameters of each layer based on the reflected terahertz signal.

[0022] Among them, the terahertz transmitter refers to a tunable multi-frequency continuous wave (CW) terahertz source, which is used to emit terahertz electromagnetic waves (i.e., terahertz signals) with controllable frequency (e.g., scanning in the range of 1-200THz) and strong coherence to the moving electrode coating; the output is shaped into a strip-shaped light spot after being shaped by the light spot forming component, realizing line-scan non-contact excitation.

[0023] Electrode coating refers to a composite functional layer coated on the surface of a metal current collector (such as aluminum foil or copper foil) in the manufacturing of lithium-ion batteries. It includes at least two slurry coatings with different compositions / ratios (such as a positive electrode active layer + conductive layer, or a negative electrode main material layer + binder layer), and is a wet / dry multilayer structure that is either not yet dried or has been dried but not rolled.

[0024] The beamforming module is an optical module consisting of an optical focusing lens (with a narrow rectangular focal spot design) and a beam shaper connected in series. It is set after the terahertz transmitter and is used to shape point-like or divergent terahertz waves into uniform, elongated strip-shaped beams that extend along the coating width, ensuring that a single scan covers the entire width of the film.

[0025] A signal acquisition unit is a receiving system built on a terahertz detector. It includes a signal detector (which directly responds to reflected terahertz waves and outputs weak electrical signals) and a signal converter (including low-noise amplification, A / D conversion and spectrum reconstruction modules). It is used to synchronously acquire the amplitude and phase information of the reflected signal pixel by pixel and output the frequency domain complex reflection coefficient spectrum or the time domain equivalent echo.

[0026] Each layer refers to an independent coating sublayer (e.g., coating 1, coating 2) with different physical / chemical properties, which are stacked sequentially from top to bottom along the thickness direction in the electrode coating. Each layer has a distinguishable dielectric constant, refractive index, thickness, and porosity.

[0027] The equipment encoder refers to a high-precision rotary encoder installed on the conveyor roller of the coating machine. It is used to collect the linear velocity, position and motion phase information of the electrode coating in real time, and output it to the synchronization control unit in the form of a level signal as a reference clock source for time-space coordinate mapping.

[0028] The synchronization control unit is a hardware triggering and logic coordination module that receives position / speed pulse signals from the device encoder, frequency scanning timing signals from the terahertz transmitter, and frame synchronization signals from the signal acquisition unit. Through a hard synchronization mechanism, it ensures that each frame of acquired data strictly corresponds to a unique and continuous spatial position segment on the electrode coating, eliminating sampling misalignment caused by belt jitter.

[0029] The real-time computing unit refers to an embedded or industrial control computing system equipped with a GPU acceleration platform. It communicates with the signal acquisition unit to perform data preprocessing (deconvolution, noise filtering, phase unrolling, inverse discrete Fourier transform reconstruction), physical model fitting (transfer matrix method inversion calculation), parameter solving (thickness, complex refractive index, surface density, conductivity, porosity) and defect identification algorithms, and outputs a full-area multi-dimensional physical parameter distribution map and anomaly warning signal updated in milliseconds.

[0030] Specifically, the composite coating inspection system consists of an adjustable multi-frequency terahertz emitter (terahertz emission system), an optical narrow rectangular focusing lens, beam shaper and baffle, a TeraFAST linear array camera (terahertz high-speed linear array focal plane imaging sensor), a data acquisition unit, and an encoder. The terahertz source used in this composite coating inspection system employs frequency scanning, with the linear array camera moving along the electrode coating to rapidly acquire the reflection intensity of each pixel at the used frequency; providing controllable frequency components allows for the acquisition of the spectrum (amplitude and phase) for each pixel. Under multi-frequency conditions, the spectral information of the waveform entering the coating can be obtained.

[0031] A tunable multi-frequency continuous wave (CW) terahertz source, also known as a terahertz transmitter, emits a coherent terahertz signal. Through a focusing lens and a beam shaper, the terahertz wave is formed into a uniform linear spot and irradiates the coating surface.

[0032] In one example, reference Figure 2 Before coating and winding, there is enough space above the coating layer to place the moving crossbar and the terahertz emitter. According to the electrode mold width, two terahertz emitters A and B are arranged on the crossbar at equal intervals according to the mold width length.

[0033] The following calculations show that the scanning range of this imaging system can cover the entire coating die width, achieving real-time full coverage of the coated film area. For example, if the fastest speed of the coating conveyor belt is 70 m / min (approximately 1.16 m / s), and the coating die width is 700 mm, the desired spatial resolution (sampling interval) of this system along the coating conveyor belt direction is... s=0.25mm=0.00025m.

[0034] Preferably, the two terahertz transmitters A and B share the coating mold width equally and are spliced ​​with a 5% overlap to improve the accuracy of the measurement.

[0035] In one embodiment, the signal acquisition unit includes a signal detector and a signal converter; the signal detector is used to receive terahertz signals reflected by each layer of the electrode coating and convert the reflected terahertz signals into time-domain analog signals at corresponding locations in each layer; the signal converter is electrically connected to the signal detector and is used to perform signal conversion on the time-domain analog signals to obtain the spectral information of the time-domain analog signals.

[0036] Among them, the signal detector refers to the photoelectric sensing unit used to receive terahertz waves reflected by each layer of the electrode coating and directly convert the reflected terahertz waves into time-domain analog electrical signals (such as voltage / current waveforms) at the corresponding spatial location points.

[0037] A signal converter is an electronic module that is electrically connected to a signal detector and is used to perform analog-to-digital conversion (ADC) and spectrum analysis on time-domain analog signals. Its output is discretized, computable spectrum data (such as amplitude-frequency / phase-frequency curves). Its functions include fast Fourier transform, window function weighting, noise suppression, and dynamic range compression.

[0038] The transmitting end refers to the physical output interface or equivalent phase center of the terahertz electromagnetic wave in the terahertz transmitter. It can be located at the coupling position between the radiation source (such as a photoconductive antenna, quantum cascade laser, or frequency doubler) and the optical path starting optical element (such as a collimating lens). It is the geometric and electromagnetic reference origin of the emitted beam.

[0039] In one embodiment, there are multiple signal detectors that are equally spaced along the width of the electrode coating, and there is a preset overlap between the ranges of the reflected terahertz signals received by adjacent signal detectors.

[0040] The width direction refers to the horizontal axis that is perpendicular to the electrode coating conveying direction and parallel to the edge of the coating area.

[0041] The preset ratio refers to the percentage (e.g., 20% to 50%) of the effective detection area overlap of two adjacent signal detectors in the width direction to the theoretical detection width of a single detector.

[0042] In one embodiment, the light spot forming assembly includes a light focusing mirror and a beam shaper arranged sequentially along the emission path of the terahertz transmitter.

[0043] refer to Figure 2A light focusing lens is a refractive optical element (such as a plano-convex lens, aspherical lens, or cylindrical focusing lens) or a reflective element (such as a parabolic mirror) with a specific curvature and refractive index distribution. It is used to collimate or converge the diverging beam output from the TeraFAST linear array camera (terahertz imaging system A and terahertz imaging system B), and to control the position and size of the beam waist spot. It is a primary optical device in the light spot forming assembly that realizes the spatial concentration of energy.

[0044] A beam shaper is an optical device used to modulate the cross-sectional intensity distribution of an incident terahertz beam from a Gaussian pattern to an approximately uniform rectangular distribution. Its main function is to improve the uniformity and edge steepness of the beam stripe. The beam stripe is then processed by a shutter and attenuator before being sent to the GPU for real-time processing.

[0045] Figure 3 A schematic flowchart of the composite coating detection method according to an embodiment of this application is shown. Exemplarily: Step S302: A terahertz signal is emitted to the electrode coating in the moving state.

[0046] Step S304: The terahertz signal is converted into a uniform stripe of light that irradiates the electrode coating.

[0047] Step S306: Receive the terahertz signals reflected by each layer of the electrode coating.

[0048] Step S308: Acquire the movement information of the electrode coating in real time.

[0049] Step S310: Synchronize the reflected terahertz signal based on the movement information.

[0050] Terahertz signals refer to electromagnetic wave signals with frequencies ranging from 0.1 terahertz to 200 terahertz. They are generated by tunable multi-frequency continuous wave terahertz transmitters and have the physical characteristics of being non-ionized, having strong penetration into polymer coatings, and being highly reflective into metal foils. They are used for non-destructive testing of the internal structure of composite coatings on electrodes.

[0051] A uniform strip-shaped light spot refers to a narrow rectangular irradiation area formed on the surface of the electrode coating after the combined action of a focusing lens and a beam shaper. The length direction is along the coating width direction, the width direction is perpendicular to the tape direction, and the energy distribution is uniform.

[0052] Motion information refers to the pulse sequence signal output in real time by the device encoder, which characterizes the instantaneous position, velocity and acceleration of the electrode coating in the belt transport direction. This pulse sequence signal triggers terahertz transmission and linear array acquisition at fixed time intervals (e.g., microseconds) to achieve a one-to-one correspondence between spatial sampling points and physical positions. It is the core reference for achieving "belt transport-scanning" synchronization.

[0053] Specifically, S1 data acquisition: The reflected terahertz signal is converted into a weak electrical signal by the detector, amplified with low noise, and then enters the high-speed acquisition system for A / D conversion. The linear array is read out pixel by pixel, and each sampling point generates the following raw data items, which are acquired in digital form: amplitude spectrum per frequency, phase spectrum per frequency, and time-domain waveform.

[0054] S2 data processing: It is recommended to use GPU for real-time preprocessing, including but not limited to: synchronization and clock alignment, deconvolution, noise filtering, phase unrolling, de-jumping, and temporal reconstruction.

[0055] S3 Data Analysis: Based on the processed information, the thickness, areal density, porosity, and conductivity of the composite coating can be calculated in real time using relevant formulas.

[0056] In one embodiment, the reflected terahertz signal is converted into a time-domain analog signal corresponding to the location point in each layer, and the time-domain analog signal is converted to obtain the spectrum information of the time-domain analog signal; the preset correction function of the layer where the spectrum information is located is obtained, and the ratio between the spectrum information and the preset correction function is used as the corrected spectrum information.

[0057] Among them, the time-domain analog signal refers to the continuous voltage signal that changes with time after the signal detector (such as TeraFAST linear array) receives the reflected terahertz signal (wave), amplifies it with low noise and conditions it with the analog front end; the time-domain analog signal completely preserves the time delay, amplitude attenuation and phase jump characteristics formed by the multiple reflections of the terahertz wave at the interface of each coating, and each pixel corresponds to an independent time-domain waveform.

[0058] Spectral information refers to the complex numerical dataset obtained after performing a discrete Fourier transform on a time-domain analog signal. It contains the amplitude value (reflecting the reflection intensity) and phase value (reflecting the propagation delay) corresponding to each frequency point, forming a two-dimensional matrix with frequency as the horizontal axis and complex amplitude as the vertical axis. It is the direct data basis for inverting physical parameters such as thickness and refractive index.

[0059] The preset correction function refers to the comprehensive gain and phase shift of the terahertz emission source, optical path transmission, detector response, and electronic link at each frequency point. This preset correction function serves as a reference standard to eliminate systematic errors and ensure that the reflection coefficient calculated subsequently truly reflects the optical characteristics of the sample under test.

[0060] Specifically, after completing the terahertz signal transmission and reflection reception, the time-domain analog signals output by each pixel are input into the signal converter; the signal converter performs a fast Fourier transform operation to convert each time-domain waveform into its corresponding spectral information, obtaining a complex spectrum matrix including the amplitude spectrum and the phase spectrum.

[0061] At the same time, a pre-stored preset correction function is retrieved. This preset correction function is obtained by averaging multiple measurements on a standard high-reflectivity metal surface under the same hardware configuration and environmental conditions, and covers the amplitude response curve and phase offset curve of the system across the entire operating frequency band.

[0062] Subsequently, a frequency-point complex division operation is performed between the spectral information and the preset correction function: that is, the complex value of each frequency point in the spectral information is used as the dividend, and the complex value of the correction function at the corresponding frequency point is used as the divisor. The resulting quotient is the spectral information after system response correction. Understandably, this correction process effectively eliminates the nonlinear distortion and frequency-selective attenuation introduced by the instrument itself, significantly improving the accuracy and repeatability of subsequent physical parameter inversion.

[0063] Through the above embodiments, a preset correction function based on measured standard components is introduced, and the original spectrum information is subjected to complex normalization processing at each frequency point, thereby achieving accurate compensation for the inherent response characteristics of the terahertz detection system.

[0064] In one embodiment, calculating the physical parameters of each layer based on the reflected terahertz signal includes the following steps: Based on the flight time and the refractive index of each layer, determine the target thickness of each layer, or; The frequency intervals of the interference fringes formed at the corresponding locations are obtained based on the corrected spectral information, and the target thickness of each layer is determined based on the frequency intervals of the interference fringes and the refractive index of each layer.

[0065] The time of flight refers to the length of time it takes for a terahertz signal to travel from the transmitter to a specific interface of the electrode coating, be reflected back, and be received by the signal acquisition unit. This time is directly measured by the system's built-in high-precision timing module, reflecting the actual time it takes for the terahertz signal to travel back and forth within that layer.

[0066] The refractive index refers to the degree to which the various layers of the electrode coating affect the propagation speed of the terahertz signal. The larger the value, the slower the terahertz signal propagates in that material.

[0067] Interference fringes refer to the regular alternating patterns of strong and weak changes in the frequency distribution of the received signal after a terahertz signal is repeatedly reflected and transmitted between multiple layers of the electrode coating, resembling stripe-like undulations.

[0068] Frequency spacing refers to the frequency difference between two adjacent points of highest or lowest intensity in the frequency distribution diagram of the above interference fringes. The magnitude of this frequency difference is closely related to the thickness of the corresponding layer; the thinner the layer, the greater the difference.

[0069] The target thickness refers to the actual physical thickness of a certain layer in the electrode coating along the direction perpendicular to the coating surface at the time of detection.

[0070] Specifically, a terahertz signal is first emitted to the moving electrode coating, and the signal is shaped into a uniform stripe light spot covering a certain width by a light spot forming component, so as to ensure that information on the entire bandwidth can be obtained in one scan.

[0071] The signal acquisition unit receives terahertz signals reflected from each layer of the electrode coating. The synchronization control unit marks each reflected signal with a precise position based on the real-time movement information transmitted from the device encoder, achieving a one-to-one correspondence between the signal and the physical location.

[0072] After receiving the synchronized reflected signal, the real-time computing unit calculates the target thickness of each layer using two complementary methods: The first method is based on time-of-flight calculation. It identifies the arrival time sequence of reflected signals from different interfaces, calculates the time difference between the reflected signal and the emission time for each interface, and then, combined with the known refractive index of the layered material, estimates the actual thickness of the layer. Understandably, this method offers rapid response and is suitable for real-time monitoring in high-speed, continuous production scenarios.

[0073] The second method is based on frequency interval calculation. The reflected signal is converted into a frequency distribution map, and interference fringes belonging to a specific layer are extracted from the map, with their frequency intervals measured. Combined with the refractive index of the layer material, the thickness of the layer can be deduced. Understandably, this method offers higher accuracy and is particularly suitable for precise measurements of ultrathin functional layers that are sensitive to thickness variations.

[0074] Optionally, one of the methods can be automatically selected based on the current working conditions, or two methods can be run simultaneously and the results can be compared and fused to finally output a stable and reliable target thickness for each layer.

[0075] In one example, the target thickness of each layer is calculated using the following formula: Where n is the refractive index of each layer, c is the speed of light, and d is the target thickness of each layer. For frequency intervals.

[0076] The above embodiments solve the signal distortion problem caused by material movement during high-speed coating. By strictly synchronizing the equipment movement information with the terahertz signal acquisition process, it is ensured that each set of data accurately corresponds to the specific location of the electrode coating, avoiding measurement drift caused by speed fluctuations in traditional methods.

[0077] In one embodiment, the complex refractive index of each layer is calculated based on the reflected terahertz signal, including the following steps: An electromagnetic propagation model is obtained, which includes initial thickness and optical parameters for each layer. The initial optical parameters describe the phase delay and energy absorption of terahertz waves by the coating material of each layer. The complex reflection coefficient spectrum is obtained by simulating the transmission and multiple reflections of terahertz waves between layers under multi-frequency conditions. The complex reflection coefficient spectrum is compared with the corrected spectrum information at each frequency point to determine the complex error. Based on the complex error, the initial optical and initial thickness parameters of each layer are iteratively adjusted until the complex reflection coefficient spectrum converges to the preset convergence condition. Based on the converged optical parameters of each layer, the complex refractive index of each layer is determined. The real part of the complex refractive index characterizes the phase propagation characteristics of the coating material, and the imaginary part characterizes the energy dissipation capability of the coating material for terahertz waves.

[0078] Among them, the electromagnetic propagation model refers to a mathematical simulation model based on the physical laws of electromagnetic wave propagation in multilayer media, used to simulate the process of terahertz waves being reflected, transmitted and interfered sequentially in a composite coating structure composed of multiple different materials.

[0079] The initial thickness parameter refers to the estimated thickness of each coating layer set at the beginning of the modeling process, which serves as the starting variable for the inversion calculation.

[0080] The initial optical parameters refer to the initial complex refractive index values ​​set for each coating layer during the initial modeling stage, including the real part (reflecting the phase delay of terahertz wave propagation in the layer) and the imaginary part (reflecting the absorption and attenuation of terahertz wave energy by the layer).

[0081] Coating materials refer to the functional slurry system that is actually coated on the surface of the metal current collector (such as aluminum foil) in lithium-ion battery electrodes. They typically contain active substances, conductive agents, binders, and residual solvents.

[0082] The complex reflection coefficient spectrum refers to the theoretical reflection signal corresponding to a series of discrete frequency points calculated by the electromagnetic propagation model. The data at each frequency point contains both amplitude and phase information, and together they form a complex numerical sequence that can characterize the reflection characteristics of the multilayer interface of the coating.

[0083] Multi-frequency conditions refer to the operating state of a terahertz transmitter when it continuously scans multiple monochromatic waves of different frequencies, enabling the system to acquire a complete reflection response covering a certain frequency range at the same spatial location, thereby supporting thickness resolution, refractive index extraction, and defect identification.

[0084] Complex error refers to the difference obtained by performing complex subtraction at each frequency point between the complex reflection coefficient spectrum calculated by the model and the measured corrected reflection spectrum. This difference is itself a complex number, and its magnitude reflects the model fitting accuracy, while its direction reflects the combined characteristics of phase and amplitude deviation.

[0085] The preset convergence condition refers to the stopping criterion set during the iterative optimization process. When the average modulus of the complex error decreases below a certain threshold, or when the error change rate is less than a specified limit in several consecutive iterations, it is determined that the model parameters have stabilized and updates are stopped.

[0086] Optical parameters refer to the set of physical quantities that describe the essential characteristics of the interaction between the coating material and terahertz waves. They mainly include the real and imaginary parts of the complex refractive index. The real part determines the propagation speed and phase accumulation of terahertz waves in the material, while the imaginary part determines the energy loss of terahertz waves during propagation.

[0087] Specifically, firstly, an electromagnetic propagation model suitable for composite electrode structures is constructed. This electromagnetic propagation model considers the electrode under test as being composed of N horizontally stacked uniform media, which are arranged from top to bottom as the first functional layer, the second functional layer, and finally the bottom metal current collector. Each layer is assigned initial thickness parameters and initial optical parameters, and boundary conditions are set, such as the top layer being the air-coating interface and the bottom layer being the metal foil interface.

[0088] Secondly, based on this electromagnetic propagation model, within the actual operating frequency range of the terahertz system used, the theoretical reflection response formed by the superposition of multiple reflections and transmissions of the terahertz wave between each layer is calculated point by point, generating a set of complex reflection coefficient spectra covering the entire frequency band.

[0089] Next, the theoretical spectrum and the measured reflection spectrum after system correction are compared at the pixel level: for each frequency point, the difference between the two on the complex plane is calculated to obtain the complex error; using the least squares principle, with the sum of squares of the magnitude of the complex error as the objective function, the initial thickness parameters and initial optical parameters of each layer are adjusted synchronously through a nonlinear optimization algorithm.

[0090] After each parameter update, the complex reflection coefficient spectrum is recalculated and the error change is evaluated. When the average error magnitude is lower than 0.02 (optional, this value of 0.02 can be dynamically set according to the system signal-to-noise ratio), or the error decrease is less than 0.1% in five consecutive iterations (optional, this value of 0.1% can also be dynamically set according to the system signal-to-noise ratio), the preset convergence condition is met, and the iteration stops.

[0091] Finally, the converged optical parameters of each layer are directly read as the complex refractive index of the corresponding layer: the real part characterizes the dominant influence of the material of that layer on the phase propagation of terahertz waves, and the imaginary part characterizes its dominant influence on the energy dissipation of terahertz waves. Understandably, this result can be directly used for subsequent calculations of areal density, porosity, and conductivity without additional calibration, achieving end-to-end closed-loop analysis from the original signal to key physical property parameters.

[0092] Through the above embodiments, an electromagnetic propagation model based on physical mechanisms and a multi-frequency complex reflection coefficient fitting method are introduced, achieving for the first time high-precision, pixel-by-pixel inversion of the complex refractive index of each layer in a composite-coated multilayer structure in an industrial-grade online inspection scenario. Compared to existing technologies that can only obtain the total thickness or a rough dielectric response, this application makes a breakthrough by elevating terahertz detection from macroscopic reflection intensity measurement to the analysis of electromagnetic constitutive parameters of microscopic materials.

[0093] In one embodiment, the areal density of each layer is calculated based on the reflected terahertz signal, including the following steps: Based on the preset material calibration relationship, the real part of the complex refractive index is converted into the volume density of each layer; the volume density of each layer is multiplied by the thickness to obtain the areal density of the targeted layer; the areal density of each layer is mapped according to the position of each point to generate a continuous areal density distribution map along the width direction of the electrode coating.

[0094] Among them, the preset material calibration relationship refers to the quantitative correspondence between the real part of the complex refractive index and the corresponding bulk density value in the material system, which is established by collecting a batch of standard electrode samples with known composition, known dry state and whose bulk density has been measured by an authoritative testing institution before the system is officially put into use, obtaining their real part data of complex refractive index under the same terahertz detection conditions, and then establishing the real part data of complex refractive index and corresponding bulk density value through statistical analysis.

[0095] Bulk density refers to the mass of solid components in a coating material per unit volume, that is, the actual density of dense material after excluding pore space. It reflects the degree of compaction of solid components such as active substances, conductive agents and binders in the coating. The higher the value, the denser the coating and the fewer pores.

[0096] Areal density refers to the mass of solid material coated per unit area, which is equal to the product of the volume density of the coating at that location and its physical thickness. It is a core control parameter in battery manufacturing that directly determines capacity consistency and rate performance.

[0097] Thickness refers to the vertical distance from the surface of the coating to the interface of the next layer (or the surface of the metal current collector).

[0098] A surface density distribution map is a two-dimensional image that arranges the surface density values ​​calculated from all locations obtained by continuous scanning along the width of the electrode coating in spatial order and visualizes them in grayscale, pseudo-color, or contour lines. It visually displays the uniformity, gradient changes, and location of abnormal areas of the entire coating in the horizontal direction.

[0099] Specifically, under the same terahertz detection environment, multi-frequency reflection scanning is performed on each sample to extract the average value of the real part of the complex refractive index and construct a calibration relationship table (preset material calibration relationship) that corresponds one-to-one with the real part of the complex refractive index and the bulk density.

[0100] For each pixel location, after completing the complex refractive index inversion, the real-time calculation unit immediately retrieves the calibration table corresponding to the coating type (e.g., the first layer of the cathode) of that pixel location, substitutes the currently inverted real part value of the complex refractive index into the table, and obtains the volume density value corresponding to that point by looking up the table or interpolation.

[0101] The volume density value is then multiplied by the inverted layer thickness value to obtain the areal density value of that layer at that point. Understandably, this process is executed pixel-by-pixel in parallel with GPU acceleration. Finally, the areal density values ​​of all pixels in the entire row are arranged in order of their physical coordinates in the coating width direction to generate a row of data; as the electrode continues to move, multiple rows of data are continuously collected at a set frame rate, and finally stitched together to form a two-dimensional areal density distribution map covering the entire coating width and detection period.

[0102] Through the above embodiments, compared with the limitations of traditional β-ray or X-ray, which can only provide single-point or line average surface density and cannot distinguish multilayer contributions, this application achieves for the first time a continuous full-width surface density mapping without contact, sampling, or downtime by embedding the calibration relationship between the real part of the complex refractive index and the bulk density into an online detection closed loop.

[0103] In one embodiment, the conductivity of each layer is calculated based on the reflected terahertz signal, including the following steps: Based on the spectral information and vacuum dielectric constant of each layer, the imaginary part of the complex refractive index is converted into an index of the response capability of the coating material of each layer to alternating electromagnetic fields. The conductivity of each layer is determined based on the response index of the coating material of each layer to alternating electromagnetic fields.

[0104] The vacuum dielectric constant is the inherent proportionality constant between the electric field and the displacement current when an electromagnetic wave propagates in a vacuum.

[0105] Alternating electromagnetic field refers to a coupling body of electric and magnetic fields in space where terahertz waves oscillate periodically at a specific frequency, with the oscillation frequency falling within the range of 0.1 to 10 terahertz; in this application, it specifically refers to a high-frequency electromagnetic excitation field generated by a terahertz transmitter for detecting the internal structure of an electrode coating.

[0106] The response capability index is a quantitative characterization of the overall electromagnetic response intensity exhibited by the internal free and bound charges of a coating material under the action of an alternating electromagnetic field. This response capability index is derived from the imaginary part of the complex refractive index after calibration with the vacuum dielectric constant, and its value directly reflects the strength of the material's ability to conduct current.

[0107] Coating materials refer to porous composite films formed on lithium-ion battery electrodes after coating and drying. They are mainly composed of active material particles, conductive carbon materials, polymer binders, and residual micropores.

[0108] Conductivity refers to the ability of a coating material per unit length and unit cross-sectional area to conduct current under DC or low-frequency limits, and is used to measure the integrity and connectivity of its internal electron transmission channels.

[0109] Specifically, the system software first presets the vacuum dielectric constant as a constant physical quantity and uses it as the standard conversion benchmark. Once the real-time calculation unit completes the complex refractive index inversion, it obtains the imaginary part value of the complex refractive index of each layer at each pixel location. This value itself reflects the material's absorption characteristics of electromagnetic wave energy in the terahertz band, but it does not yet have a clear electrophysiological meaning.

[0110] Next, the imaginary part of the complex refractive index is multiplied by the vacuum dielectric constant, and combined with the actual operating frequency range of the terahertz signal, it is converted into an index of the coating material's response capability to alternating electromagnetic fields through a physical mapping relationship.

[0111] Then, the calibration curve corresponding to the coating type (such as the negative conductive layer) at that location is retrieved. The current responsiveness index is substituted into the curve, and the equivalent conductivity value of that layer at that point is obtained by looking up a table or interpolation. This process is performed pixel-by-pixel under the GPU parallel architecture, ensuring that the conductivity calculation of the entire 256 pixels in a row is completed within a single frame.

[0112] Finally, the conductivity values ​​of all pixels are arranged according to their actual spatial coordinates in the coating width direction to generate a row of conductivity data; as the electrode moves continuously, multiple rows of data are continuously collected and stitched together to form a two-dimensional conductivity distribution map covering the entire width and all time periods.

[0113] In one embodiment, interlayer defects in the electrode coating are determined based on time-domain information corresponding to the reflected terahertz signal; stripe background defects in the electrode coating are determined based on frequency-domain information corresponding to the reflected terahertz signal; and at least one of the target thickness, complex refractive index, areal density, and conductivity is compared with an associated preset parameter threshold to determine whether parameter defects exist in the electrode coating.

[0114] Among them, interlayer defects refer to unexpected interface anomalies between adjacent functional layers in a composite coating structure, including but not limited to: complete absence of a layer (layer missing), foreign matter or air bubbles mixed between two layers (inclusion), insufficient interlayer adhesion leading to local peeling (delamination), or interlayer penetration and blurring caused by mismatch in the rheological properties of the slurry (interface diffusion). Such defects are manifested in time-domain reflectometry signals as identifiable temporal characteristics such as reduced echo number, abnormal echo time interval, sudden drop in echo amplitude, or phase jump.

[0115] Striped background defects refer to the non-uniformity of the microstructure inside the coating in the frequency domain response, including local abrupt changes in porosity, agglomeration of conductive components, uneven distribution of binder, or microcracks induced by drying stress. These defects do not change the macroscopic layered structure, but they will disturb the scattering path and interference conditions of terahertz waves inside the material, which is manifested in the spectrum as interference fringe baseline shift, decreased fringe contrast, abnormal increase in background scattering power, or phase distortion in a specific frequency range.

[0116] The preset parameter thresholds refer to the allowable fluctuation range of each physical parameter set based on the statistical analysis of historical good product data, including the upper limit, lower limit and spatial gradient limit.

[0117] Parameter defects refer to situations where any physical parameter (including target thickness, complex refractive index, areal density, and conductivity) calculated by the terahertz detection system exceeds its corresponding preset parameter threshold.

[0118] Specifically, identifying interlayer defects based on time-domain information: After completing the temporal echo reconstruction, the real-time computing unit performs structural analysis on the temporal signal at each pixel location. First, it identifies whether there are three or more clearly identifiable main echo peaks (corresponding to the coating surface, interlayer interface, and metal current collector, respectively). If only two echo peaks appear in a region and the middle peak is missing, it is determined to be a missing layer. If the time difference between two adjacent echo peaks significantly deviates from the theoretical flight time corresponding to the inverted thickness at that location, it is determined to be an abnormal layer thickness or interface shift. If the amplitude attenuation of an echo peak exceeds 70% and is accompanied by a phase jump, it is determined that there are inclusions or delamination at that interface.

[0119] Second, identify striped background defects based on frequency domain information: A two-dimensional analysis is performed on the corrected spectral information: First, the overall contrast of the interference fringes (i.e., the ratio of the main peak amplitude to the average background amplitude) is calculated. When the contrast is lower than the statistical lower limit for good products (e.g., 1.8), it indicates a decrease in the internal density of the coating or disordered porosity distribution. Second, the spectral background scattering power (i.e., the low-frequency energy integral after removing the main interference peak) is extracted. When this power value is higher than the mean of good products by two standard deviations, it indicates the presence of scattering enhancement sources such as conductive agent agglomeration or microcracks. The two types of indicators are weighted and fused to generate a background stability index. Regions below the threshold are marked as fringe background defects.

[0120] Third, parameter defects are identified based on multi-parameter threshold comparison: The target thickness, real part of complex refractive index, areal density, conductivity, and other parameters calculated for each pixel location are compared item by item with the preset parameter thresholds associated with its coating type and the current process stage. After spatial filtering and connectivity assessment, all pixels that trigger the threshold are used to generate defect type labels.

[0121] Through the above embodiments, by constructing a three-in-one defect identification framework of "time domain structure analysis, frequency domain background modeling, and parameter threshold linkage", a full-dimensional quality perception capability from apparent geometric anomalies to internal microstructure deterioration, and from single-point parameter deviations to multi-dimensional coupling failures has been realized for the first time in online inspection of lithium battery composite coating.

[0122] In the several embodiments provided in this application, it should be understood that the disclosed systems and methods can also be implemented in other ways. The system embodiments described above are merely illustrative. For example, the flowcharts and block diagrams in the accompanying drawings show the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of this application. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of code, which contains one or more executable instructions for implementing a specified logical function. It should also be noted that, in alternative implementations, the functions marked in the blocks may occur in a different order than those marked in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagram and / or flowchart, and combinations of blocks in the block diagram and / or flowchart, can be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0123] In addition, the functional modules or units in the various embodiments of this application can be integrated together to form an independent part, or each module can exist independently, or two or more modules can be integrated to form an independent part.

[0124] If a function is implemented as a software module and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a smartphone, personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of the various embodiments of this application.

[0125] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A composite coating detection system, characterized in that, The system includes: A terahertz transmitter, positioned above the electrode coating, is used to transmit terahertz signals to the electrode coating in a moving state. A light spot forming component is disposed along the emission path of the terahertz transmitter and after the emission end of the terahertz transmitter, for converting the terahertz signal into a uniform strip light spot; A signal acquisition unit is disposed on the same side as the terahertz transmitter above the electrode coating, and is used to receive terahertz signals reflected by each layer of the electrode coating; A device encoder is used to acquire the movement information of the electrode coating in real time; A synchronization control unit is electrically connected to the terahertz transmitter, the signal acquisition unit, and the device encoder, respectively, and is used to synchronize the reflected terahertz signal according to the movement information; A real-time computing unit, which is communicatively connected to the signal acquisition unit, is used to calculate the physical parameters of each layer based on the reflected terahertz signal.

2. The system according to claim 1, characterized in that, The signal acquisition device includes a signal detector and a signal converter; The signal detector is used to receive terahertz signals reflected by each layer of the electrode coating, and to convert the reflected terahertz signals into time-domain analog signals of corresponding positions in each layer. The signal converter, electrically connected to the signal detector, is used to perform signal conversion on the time-domain analog signal to obtain the spectral information of the time-domain analog signal.

3. The system according to claim 2, characterized in that, The signal detectors are multiple and equally spaced along the width of the electrode coating, and there is a preset overlap between the ranges of the reflected terahertz signals received by adjacent signal detectors.

4. The system according to claim 1, characterized in that, The light spot forming assembly includes a light focusing lens and a beam shaper arranged sequentially along the emission path of the terahertz transmitter.

5. A composite coating detection method, applied to the system according to any one of claims 1 to 4, characterized in that, The method includes: A terahertz signal is emitted toward the moving electrode coating; The terahertz signal is converted into a uniform stripe of light that illuminates the electrode coating; Receive terahertz signals reflected by each layer of the electrode coating; The movement information of the electrode coating is acquired in real time; The reflected terahertz signal is synchronized based on the movement information; The physical parameters of each layer are calculated based on the reflected terahertz signal after synchronization.

6. The method according to claim 5, characterized in that, The method further includes: The reflected terahertz signal is converted into a time-domain analog signal at the corresponding position point in each layer, and the time-domain analog signal is converted to obtain the spectral information of the time-domain analog signal. Obtain the preset correction function of the layer where the corresponding location point of the spectrum information is located, and use the ratio between the spectrum information and the preset correction function as the corrected spectrum information.

7. The method according to claim 6, characterized in that, The physical parameters include the target thickness; The calculation of the physical parameters of each layer based on the reflected terahertz signal includes: Based on the flight time and the refractive index of each layer, determine the target thickness of each layer, or; The frequency intervals at which interference fringes form at corresponding locations are obtained based on the corrected spectral information, and the target thickness of each layer is determined based on the frequency intervals of the interference fringes and the refractive index of each layer.

8. The method according to claim 7, characterized in that, The physical parameters also include complex refractive index; The calculation of the complex refractive index of each layer based on the reflected terahertz signal includes: An electromagnetic propagation model is obtained, in which initial thickness parameters and initial optical parameters are set for each layer. The initial optical parameters are used to describe the phase delay and energy absorption of terahertz waves by the coating materials of each layer. The complex reflection coefficient spectrum is obtained by simulating the superposition of transmission and multiple reflections of terahertz waves between the layers under multi-frequency conditions. The complex reflection coefficient spectrum is compared with the corrected spectrum information at frequency points to determine the complex error. Based on the complex error, the initial optical parameters and initial thickness parameters of each layer are iteratively adjusted until the complex reflection coefficient spectrum converges to the preset convergence condition. Based on the converged optical parameters of each layer, the complex refractive index of each layer is determined. The real part of the complex refractive index characterizes the phase propagation characteristics of the coating material, and the imaginary part characterizes the energy dissipation capability of the coating material for terahertz waves.

9. The method according to claim 8, characterized in that, The physical parameters also include areal density; The calculation of the surface density of each layer based on the reflected terahertz signal includes: According to the preset material calibration relationship, the real part of the complex refractive index is converted into the bulk density of each layer; Multiply the volume density of each layer by its thickness to obtain the areal density of the targeted layer. The areal density corresponding to each layer is mapped according to the position of each location point to generate a continuous areal density distribution map along the width direction of the electrode coating.

10. The method according to claim 9, characterized in that, The physical parameters also include conductivity; The step of calculating the conductivity of each layer based on the reflected terahertz signal includes: converting the imaginary part of the complex refractive index into an index of the coating material's response to alternating electromagnetic fields for each layer based on the spectral information and vacuum dielectric constant corresponding to each layer. The conductivity of each layer is determined based on the response index of the coating material of each layer to alternating electromagnetic fields.

11. The method according to any one of claims 5-10, characterized in that, The physical parameters include one or more of the following: target thickness, complex refractive index, areal density, and conductivity. The method also includes any one or more of the following: Based on the time-domain information corresponding to the reflected terahertz signal, the interlayer defects of the electrode coating are determined; Based on the frequency domain information corresponding to the reflected terahertz signal, the stripe background defects of the electrode coating are determined. The electrode coating is compared with at least one of the target thickness, the complex refractive index, the areal density, and the conductivity, and an associated preset parameter threshold to determine whether there are parameter defects.

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