Single-frequency terahertz steady-state coherent phase high-precision thickness measurement system

Abstract

The present application belongs to the technical field of industrial nondestructive testing, and particularly relates to a single-frequency terahertz steady-state coherent phase high-precision thickness measuring system, which comprises a single-frequency continuous terahertz emission module, a coherent reflection receiving module, a phase demodulation module, a reference calibration module, a signal processing module, an industrial communication module and a low-expansion support and optical path fixing structure. The present application adopts a single-frequency continuous steady-state terahertz wave and a coherent phase thickness measuring method, which is different from the pulse time-domain flight time thickness measuring route, and the hardware structure is more simplified, without the need of complex wide-spectrum pulse emission, high-speed time-domain sampling and peak value positioning hardware, and the response speed is faster. The thickness is converted through the phase and analog potential signal, the complex real-time waveform operation is reduced, and the present application is suitable for narrow-tolerance detection. For the material with thickness fluctuation concentrated in a single interference period, the present application has high phase sensitivity and better industrial stability.

Description

Technical Field

[0001] This invention belongs to the field of industrial non-destructive testing technology, specifically relating to a single-frequency terahertz steady-state coherent phase high-precision thickness measurement system. Background Technology

[0002] In the production of lithium battery electrodes, functional films, precision coated materials, and composite sheets, material thickness directly affects product capacity, consistency, coating adhesion, insulation performance, and subsequent processing quality. Existing industrial production lines typically require continuous online thickness monitoring, with the results fed back in real time to coating, rolling, tension control, or quality sorting systems.

[0003] Current terahertz thickness measurement technologies mostly employ pulsed terahertz time-domain spectroscopy, which involves irradiating the material under test with a broadband terahertz pulse, collecting the reflected echoes from the upper and lower surfaces of the material, and calculating the material thickness using the time-of-flight difference of the echoes. While this technology offers advantages such as non-contact and non-destructive testing, it still has the following shortcomings for use in high-speed industrial production lines:

[0004] 1. The system hardware is complex, requiring a pulse terahertz source, high-speed sampling, time-domain waveform reconstruction, and peak identification unit, resulting in high equipment costs;

[0005] 2. The detection process relies on wide-spectrum pulses and time-of-flight differences, resulting in a long signal processing link and a large amount of real-time computation.

[0006] 3. The time-domain peak localization of minute thickness variations on high-speed production lines is easily affected by noise, material absorption, and multi-interface reflection;

[0007] 4. The conventional equipment structure mostly uses aluminum alloy, ordinary steel or a hybrid support structure. Changes in ambient temperature can cause slight drifts in the optical path spacing, transmission and reception angle or reference position.

[0008] 5. When the thickness tolerance of the material under test is very narrow and the thickness variation is mainly concentrated within a single interference period, continuing to use a complex pulse time-domain route will increase the burden on hardware and algorithms.

[0009] Therefore, it is necessary to propose a terahertz thickness measurement system that differs from the pulse time-domain flight time path, enabling it to form coherent interference through a single-frequency continuous steady-state terahertz wave, and to achieve high-precision online thickness detection by utilizing phase and simulated potential signal changes. Summary of the Invention

[0010] The purpose of this invention is to provide a high-precision single-frequency terahertz steady-state coherent phase thickness measurement system, which can solve the problems of existing pulse terahertz thickness measurement systems, such as complex structure, high cost, heavy real-time algorithm burden, and insufficient stability of industrial field calibration.

[0011] The specific technical solution adopted by this invention is as follows:

[0012] A single-frequency terahertz steady-state coherent phase high-precision thickness measurement system includes:

[0013] A single-frequency continuous terahertz transmitting module is used to output continuous steady-state terahertz electromagnetic waves with a fixed wavelength.

[0014] A coherent reflection receiving module is used to receive coherent interference signals formed by reflections from the upper and lower surfaces of the material under test, and to convert the coherent interference signals into electrical signals;

[0015] The phase demodulation module is used to analyze the phase shift or simulate the potential change of the coherent interference based on the electrical signal.

[0016] The reference calibration module is used to store the calibration correspondence between thickness, phase, and potential;

[0017] The signal processing module is used to call the calibration correspondence and calculate the thickness of the material to be measured based on the phase offset or the simulated potential change.

[0018] The industrial communication module is used to output the thickness detection results to the production line control system;

[0019] The low-expansion support and optical path fixing structure is used to fix the transmitting optical path, receiving optical path and detection reference position.

[0020] In a preferred embodiment, the single-frequency continuous terahertz transmission module includes a continuous wave transmission unit and a wavelength selection unit. The continuous wave transmission unit is used to output a continuous sinusoidal steady-state terahertz wave and does not employ a broadband pulse transmission method.

[0021] The wavelength selection unit is used to set the output wavelength range from 100μm to 1000μm.

[0022] In a preferred embodiment, the coherent reflection receiving module includes:

[0023] The optical path arrangement unit is used to employ a coaxial or near-coaxial optical path structure so that the reflected signals from the upper and lower surfaces of the material under test form a steady-state coherent interference at the receiving end.

[0024] The receiving and processing unit includes a collimating lens, a focusing lens, a terahertz light transmission window, a dustproof sealing structure, and a low-noise preamplifier circuit, used for collimating, focusing, light transmission isolation, dustproof sealing of terahertz waves, and low-noise amplification of the received electrical signals.

[0025] The light intensity matching interface unit is used to reserve interfaces for light intensity matching, weak signal amplification, and dynamic baseline cancellation.

[0026] In a preferred embodiment, the phase demodulation module includes a low-noise amplifier circuit, a filter circuit, an analog-to-digital converter circuit, a phase comparison circuit, and a baseline correction circuit. It is used to perform low-noise amplification, filtering, analog-to-digital conversion, phase comparison, and baseline correction on the electrical signal, and incorporates the fixed phase constant formed by the interface reflection phase, the initial optical path length, and the circuit phase delay into the calibration correspondence in the reference calibration module.

[0027] In a preferred embodiment, the reference calibration module includes:

[0028] The calibration data storage unit is used to store thickness-phase-potential calibration curves, piecewise fitting functions, calibration lookup tables, or multidimensional calibration data containing temperature compensation parameters.

[0029] The calibration generation unit is used to complete the full-cycle calibration using a standard thickness template, and to establish the phase and potential values ​​corresponding to different thickness points.

[0030] In a preferred embodiment, the signal processing module includes:

[0031] The data acquisition and calibration unit is used to read real-time potential signals and phase offset signals, and to call the calibration curves corresponding to the product specifications.

[0032] The thickness calculation and compensation unit is used to calculate the thickness deviation based on the reference potential value or reference phase value, output the actual thickness value, thickness deviation value, alarm status and quality judgment result, and compensate for ambient temperature and signal baseline drift.

[0033] In a preferred embodiment, the signal processing module further includes:

[0034] The working range selection unit is used to perform thickness conversion within a single coherent interference period and select a working range in which the slope of the phase-potential curve is greater than a preset threshold based on the preset nominal product thickness.

[0035] In a preferred embodiment, the industrial communication module includes:

[0036] The protocol interface unit is used to output thickness data, thickness deviation, alarm status, or quality judgment results to the production line control system via Ethernet, Industrial Ethernet, serial port, CAN, EtherCAT, Profinet, or Modbus protocols.

[0037] In a preferred embodiment, the low-expansion support and optical path fixing structure includes:

[0038] The main support unit, made of Invar steel, is used to fix the transmitter, receiver, lens, and the detection position of the material under test.

[0039] The positioning unit is used to maintain the geometric relationship between the transmitting optical path, the receiving optical path, and the detection reference position.

[0040] The present invention also provides an electronic device, the electronic device comprising:

[0041] At least one processor;

[0042] and a memory communicatively connected to the at least one processor;

[0043] The memory stores a computer program that can be executed by the at least one processor, and the computer program is executed by the at least one processor to enable the at least one processor to execute the above-described single-frequency terahertz steady-state coherent phase high-precision thickness measurement system.

[0044] The technical effects achieved by this invention are as follows:

[0045] This invention employs a single-frequency continuous steady-state terahertz wave and coherent phase thickness measurement method, which differs from the pulse time-of-flight thickness measurement approach. It features a simplified hardware structure, eliminating the need for complex wide-spectrum pulse transmission, high-speed time-domain sampling, and peak positioning hardware, resulting in faster response. Thickness is calculated using phase and analog potential signals, reducing complex real-time waveform calculations. It is suitable for narrow tolerance detection and exhibits high phase sensitivity for materials with thickness fluctuations concentrated within a single interference period. It also offers better industrial stability. The low-expansion support structure reduces temperature-induced optical path and calibration reference drift, facilitating production line integration. It can be connected to existing production line control systems via an industrial communication module. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the system modules of the present invention;

[0047] Figure 2 This is a schematic diagram of the electronic device structure of the present invention. Detailed Implementation

[0048] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0049] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0050] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in a preferred embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that mutually excludes other embodiments.

[0051] Please see Figure 1 As shown, this invention provides a high-precision single-frequency terahertz steady-state coherent phase thickness measurement system, comprising:

[0052] A single-frequency continuous-wave terahertz (CWT) transmitter module is used to output continuous steady-state terahertz electromagnetic waves with a fixed wavelength. The module includes a CWT transmission unit and a wavelength selection unit. The CWT transmission unit generates a continuous sinusoidal steady-state terahertz wave under DC bias drive, without employing a wideband pulse transmission method. The wavelength selection unit precisely sets the output wavelength within the range of 100 μm to 1000 μm by adjusting the oscillation cavity size, bias voltage, or an external filter. The actual wavelength is selected based on the thickness range, refractive index, absorption characteristics, and target resolution of the material under test. For example, for lithium batteries with a thickness of 50–200 μm... Electrodes are typically selected with a wavelength of 300μm to obtain a suitable penetration depth and reflectivity. For polymer films with a thickness of 200-500μm, a wavelength of 500μm is selected to reduce absorption loss, thereby ensuring a stable interference signal between different materials. After the system is powered on, the wavelength selection unit reads the current product specification configuration file and outputs the corresponding control signal to the continuous wave transmitting unit to make it work stably at the specified frequency. The terahertz wave output by the transmitting unit is converted into a parallel beam through a collimating lens after being coupled by a waveguide or space, and is perpendicularly irradiated to the material under test. The entire transmitting link is free of pulse modulation, ensuring that the output is a pure single-frequency continuous wave.

[0053] It should be noted that the specific basis for the above wavelength selection is as follows:

[0054] To ensure sufficient reflected signals and clear coherent interference on both the upper and lower surfaces of the material under test, the selected wavelength should satisfy the following conditions: the penetration depth of the terahertz wave in the material should be greater than the material thickness to ensure sufficient reflected signal strength on the lower surface, but not much greater than the thickness to avoid signal complexity caused by multiple reflections or internal scattering. Typically, a penetration depth of 2 to 5 times the material thickness is selected. The reflectivity of the upper and lower surfaces should not differ too much to ensure the modulation depth of the interference signal. The ratio of reflectivity of the upper and lower surfaces is usually required to be no more than 10:1. The selected wavelength should avoid the strong absorption peaks of the material under test in that frequency band to reduce energy attenuation. For a material with thickness d and refractive index n, the relationship between its absorption coefficient α and penetration depth δ is δ = 1 / α. To ensure the reflected signal on the lower surface, δ > d must be satisfied. Meanwhile, the reflectivities R1 and R2 of the upper and lower surfaces can be approximately calculated using the Fresnel formula. For example, for lithium electrode sheets with d = 50–200 μm, their typical refractive index n ≈ 2.5. At a wavelength of 300 μm, the penetration depth is about 600 μm, which meets the requirement of 2–5 times the thickness. The ratio of the upper and lower surface reflectivities is about 3:1, resulting in good signal modulation depth. If a wavelength of 500 μm is selected, the penetration depth will be too large (>1500 μm), which may introduce substrate reflection interference. Therefore, a wavelength of 300 μm is preferred. For another example, for polymer films with d = 200–500 μm, their typical refractive index n ≈ 1.5. At a wavelength of 500 μm, the penetration depth is about 800 μm, which meets the penetration requirement. Moreover, the absorption loss in this band is low, which is beneficial for signal transmission. If a wavelength of 300 μm is selected, the penetration depth will be less than 400 μm, and the signal on the lower surface will be too weak to form effective interference. Therefore, a wavelength of 500 μm is preferred.

[0055] A coherent reflection receiving module is used to receive coherent interference signals formed by reflections from the upper and lower surfaces of the material under test, and to convert the coherent interference signals into electrical signals. The coherent reflection receiving module includes:

[0056] The optical path arrangement unit is used to employ a coaxial or near-coaxial optical path structure to enable the reflected signals from the upper and lower surfaces of the material under test to form steady-state coherent interference at the receiving end. Specifically, it adopts a coaxial or near-coaxial optical path structure so that the transmitting optical path, the reflecting optical path, and the receiving optical path share a single lens group, ensuring that the reflected waves from the upper and lower surfaces have a stable geometric relationship and a fixed optical path difference at the receiving end. This structure can effectively suppress the influence of vibration and installation deviation on interference. At the same time, the beam splitter separates the transmitted wave from the reflected wave to avoid signal crosstalk.

[0057] The receiving and processing unit includes a collimating lens, a focusing lens, a terahertz transmission window, a dustproof sealing structure, and a low-noise preamplifier circuit. The collimating lens converts the reflected terahertz waves back into parallel light, while the focusing lens converges the parallel light onto the sensitive surface of the terahertz detector. The terahertz transmission window is made of high-resistivity silicon or polyethylene material, ensuring high transmittance while isolating external environmental interference. The dustproof sealing structure prevents dust and moisture from entering the optical path. The low-noise preamplifier circuit amplifies the weak electrical signal output by the detector with low noise. The process involves collimating, focusing, isolating, and sealing the terahertz wave, as well as amplifying the received electrical signal with low noise. In practice, the collimating lens, focusing lens, and terahertz detector are encapsulated in a metal cavity with a high-resistivity silicon transparent window. The cavity is filled with dry nitrogen to eliminate moisture absorption. The low-noise preamplifier circuit is placed close to the detector output to transmit the signal via the shortest path, reducing parasitic capacitance and noise introduction. The amplified signal is transmitted to the subsequent processing circuit through differential transmission to enhance the common-mode rejection ratio and further reduce noise interference.

[0058] The light intensity matching interface unit is used to reserve interfaces for light intensity matching, weak signal amplification and dynamic baseline cancellation. When the reflectivity difference between the upper and lower surfaces is large (such as metal substrate and coating), the gain can be adjusted or a compensation signal can be introduced through this interface to maximize the modulation depth of the interference signal, thereby improving the signal-to-noise ratio. This achieves amplitude matching of the reflected signals from the upper and lower surfaces and avoids the interference signal being submerged due to excessive reflectivity difference.

[0059] The specific operation procedure of the light intensity matching interface is as follows:

[0060] After the system is powered on, the light intensity matching interface unit first reads the preset product specification parameters, including the expected reflectivity R1 and R2 of the upper and lower surfaces. Before the test begins or after the product is changed, a standard sample (with known thickness and reflectivity of the upper and lower surfaces consistent with the product to be tested) is placed. The coherent reflection receiving module outputs an interference signal. After pre-amplification, the ADC of the light intensity matching interface unit acquires the waveform of the signal at a high sampling rate (e.g., 1MHz). Then, the acquired waveform is analyzed to calculate its maximum value Vmax and minimum value Vmin, thereby obtaining the modulation depth M of the interference signal (M=(Vmax-Vmin) / Vmax+Vmin). If M is lower than the minimum allowable threshold (e.g., 0.5), it is considered that the modulation depth is insufficient and light intensity matching is required.

[0061] If the absolute value of Vmax-Vmin is small, it indicates that the overall signal is weak. In this case, the gain of the low-noise preamplifier circuit in the receiving processing unit should be increased first.

[0062] If the signal amplitude is sufficient but the modulation depth is low, it indicates a severe mismatch in the intensity of the reflected signals from the upper and lower surfaces. In this case, the variable attenuator or amplifier in the optical intensity matching interface needs to be adjusted according to the ratio of R1 and R2 to attenuate the stronger reflection branch or amplify the weaker reflection branch. The M value should be re-evaluated after each adjustment until M exceeds the minimum allowable threshold and remains stable. If the signal baseline drifts significantly after adjusting the gain, exceeding the ADC input range, the optical intensity matching interface unit outputs a DC compensation voltage opposite to the direction of baseline drift to bring the signal center back to the optimal input range of the ADC. After completing the above adjustments, the final gain value, attenuation value, compensation voltage, and other parameters will be written into the non-volatile memory as the standard configuration of this product specification. They can be directly called up during subsequent testing without repeated adjustments.

[0063] Specifically, the parallel terahertz wave output by the single-frequency continuous terahertz emission module passes through the light-transmitting window and reaches the material under test. A first reflection wave is generated on the upper surface of the material, and some energy enters the interior of the material, generating a second reflection wave on the lower surface. The two reflected waves return along their original paths and converge at the detector via a lens group. The electrical signal output by the detector is the coherent interference signal carrying thickness information. This signal is amplified by a low-noise preamplifier circuit, and then the gain is adjusted and the baseline is compensated by the intensity matching interface unit. This achieves amplitude matching of the reflection signals from the upper and lower surfaces, ensuring that the interference signal has sufficient modulation depth. The coherent reflection receiving module receives the coherent interference signal formed by the reflections from the upper and lower surfaces of the material under test. The two reflected waves are superimposed at the receiving end, and their combined intensity can be approximately expressed as:

[0064] ;

[0065] in , These represent the reflected signal intensities from the upper and lower surfaces, respectively. The phase difference between the two waves is represented by the interference intensity, which varies periodically with the thickness. This interference is converted into an electrical signal by the detector and then sent to the phase demodulation module.

[0066] The phase demodulation module is used to analyze the phase shift or analog potential change of coherent interference based on the electrical signal. The module includes a low-noise amplifier circuit, a filter circuit, an analog-to-digital converter circuit, a phase comparator circuit, and a baseline correction circuit. Specifically, the low-noise amplifier circuit amplifies the pre-amplified signal a second time; the filter circuit removes out-of-band noise and power frequency interference; the analog-to-digital converter circuit converts the analog signal into a digital signal; the phase comparator circuit first mixes the digital signal after analog-to-digital conversion with a reference signal to obtain in-phase and quadrature components; by calculating the arctangent of the two components, the principal value of the phase difference is obtained; then, combined with a phase unwrapping algorithm, phase jumps are eliminated to recover the continuous phase shift; and the baseline correction circuit is used to eliminate phase shifts caused by temperature drift or other factors. To address the DC offset caused by component aging, and ensure the accuracy and stability of phase demodulation, the phase demodulation module can be further extended to multi-frequency or frequency-sweeping modes for multi-layer materials or coating structures. By utilizing the phase response differences at different frequencies, the reflected signals from each layer interface are decoupled, thereby analyzing the thickness of each layer layer by layer. This enables the measurement of the thickness distribution of multi-layer structures. The fixed phase constant formed by the interface reflection phase, initial optical path length, and circuit phase delay is incorporated into the calibration correspondence in the reference calibration module. Specifically, after the electrical signal enters the phase demodulation module, it undergoes amplification, filtering, and digitization processing. Then, the phase comparison circuit calculates the phase value of the current interference signal. The difference between this phase value and the calibration reference phase is the phase offset. Simultaneously, an analog potential value V (representing the interference light intensity) can be directly output for subsequent rapid conversion to the thickness value. The conversion relationship can be expressed as:

[0067] V=V b +K·cos(Δφ+φe);

[0068] Where V is the sampling potential, V b The circuit baseline is K, the photoelectric conversion and amplification coefficient is K, and the circuit phase delay constant is φe. Through low-noise amplification, filtering and phase comparison, the phase offset Δφ can be extracted or the analog potential value V can be directly output.

[0069] The reference calibration module is used to store the calibration correspondence between thickness, phase, and potential. The reference calibration module includes:

[0070] The calibration data storage unit uses non-volatile memory to store thickness-phase-potential calibration curves, piecewise fitting functions, calibration lookup tables, or multi-dimensional calibration data containing temperature compensation parameters.

[0071] The calibration generation unit completes full-cycle calibration using standard thickness templates before the equipment leaves the factory or when changing product specifications. It establishes the phase and potential values ​​corresponding to different thickness points. The specific process is as follows: a series of standard templates with known thicknesses are placed sequentially at the test position, and the phase and potential values ​​corresponding to each thickness point are recorded. A continuous correspondence is established through interpolation or curve fitting and stored in the storage unit. That is, when using the equipment for the first time or switching products, the calibration generation unit guides the operator to place the standard templates, automatically collects data and generates calibration curves. During online testing, the signal processing module directly calls the calibration data of the current specification from the storage unit without the need to calculate the interference equation in real time.

[0072] It needs to be clarified that the full-cycle calibration process is as follows:

[0073] First, prepare a set of standard samples with precisely known thicknesses, covering the entire calibration range of the product under test and spanning at least one complete interference cycle. There should be no fewer than 20 samples, evenly distributed along the thickness direction. The sample material should be the same as the product under test or have consistent optical properties (refractive index, absorption coefficient). Then, place the standard samples at the testing position in ascending order of thickness. For each sample, record the ambient temperature, the phase value output by the phase demodulation module, and the actual thickness of the sample. Also, plot a scatter plot of phase versus thickness. Since the phase changes periodically within [0, 2π), phase unwrapping is required to recover the continuous phase change curve. Simultaneously, plot the potential value versus thickness curve, which exhibits a cosine waveform within the interference cycle. Then, perform linear fitting of the unwrapped phase-thickness data using the least squares method to obtain the slope k = 4πn / λ0, thereby allowing for the inverse verification of the material's refractive index n. Finally, fit the potential-thickness data using a sine function model.

[0074] ;

[0075] in, For amplitude, For frequency, For the first phase, Using these parameters as a baseline, a continuous potential-thickness calibration curve can be obtained. The model parameters obtained from the fitting, the segmentation points and polynomial coefficients of the piecewise fitting function, or the calibration lookup table generated by interpolation, along with the temperature and compensation coefficients during calibration, are packaged together into a calibration file and stored in the non-volatile memory of the reference calibration module.

[0076] The signal processing module is used to call the calibration correspondence and calculate the thickness of the material under test based on the phase offset or the change in simulated potential. The signal processing module includes:

[0077] The data acquisition and calibration unit reads the real-time potential signal and phase offset signal at a fixed sampling rate, and retrieves the calibration curve corresponding to the product specification from the reference calibration module according to the current product specification.

[0078] The thickness calculation and compensation unit is used to calculate the thickness deviation based on the reference potential value or reference phase value, output the actual thickness value, thickness deviation value, alarm status and quality judgment result, and compensate for ambient temperature and signal baseline drift. For example, when the signal strength is detected to be too low or the fluctuation is too large, the data point is automatically marked as invalid and a warning is issued.

[0079] The compensation methods for temperature and baseline drift are as follows:

[0080] Temperature compensation:

[0081] During factory calibration, the equipment was placed in a temperature-controlled chamber, and the thickness-phase-potential calibration curve in the reference calibration module was recorded every 5°C. Data analysis revealed that the main causes were thermal expansion and contraction of the low-expansion support structure (although Invar steel was used, slight expansion still occurred) and temperature drift of electronic components, manifested as an overall shift in the calibration curve and slight changes in its slope. Therefore, a two-dimensional compensation model was established.

[0082] ;

[0083] In the formula, The thickness value after compensation. The initial thickness value is obtained from the calibration curve. The current ambient temperature (read in real time by a temperature sensor installed on the low-expansion support structure). The reference temperature for calibration. and The temperature compensation coefficient, obtained through fitting experimental data, corresponds to the translation and slope change of the calibration curve, respectively.

[0084] During each thickness calculation, the signal processing module reads the current temperature. Substitute the values ​​into the above formula to obtain the thickness value from the table or by calculation. Make corrections.

[0085] Baseline drift compensation:

[0086] Baseline drift mainly originates from fluctuations in light source power, changes in detector dark current, and zero-point drift in the amplifier circuit. It manifests as a slow overall shift in the potential signal. At the beginning of each measurement cycle (e.g., every 10ms), a brief zeroing phase is inserted. During this phase, the terahertz emission light path is briefly blocked by an optical switch or mechanical shutter, causing the detector to receive a zero-intensity signal. The voltage value recorded at this time is the current circuit baseline. By comparing this value with the initial baseline during calibration, the baseline drift can be obtained. In the subsequent normal measurement phase, the baseline drift is subtracted from all acquired potential signals to obtain the corrected potential value. Then, the thickness is looked up and calculated using the corrected potential value. To prevent jitter caused by single-zeroing measurement errors, a moving average filter is applied to the baseline drift to eliminate glitches caused by random noise and transient interference, resulting in a smooth baseline drift.

[0087] The relationship between coherent phase and thickness is as follows:

[0088] Under approximately normal incidence conditions, assuming the center wavelength of the terahertz wave in air is λ0, the equivalent refractive index of the material under test at this frequency is n, and the material thickness is d, there is a round-trip optical path difference between the first reflected wave formed by reflection from the upper surface of the material and the second reflected wave formed by reflection from the lower surface. Ignoring the fixed reflection phase term or incorporating it into the calibration constant, the phase difference between the two reflected waves can be approximately expressed as:

[0089] Δφ = 4πnd / λ0 + φc;

[0090] Wherein, φc is a fixed phase constant formed by the interface reflection phase, installation angle, initial optical path length, and circuit phase delay. Since φc can be absorbed by the calibration curve after the equipment structure is stable and calibration is completed, the relationship between the phase change Δφ and the thickness change Δd is the focus during online detection.

[0091] Δd≈λ0Δφ / (4πn);

[0092] This relationship shows that within a single interference period, a small thickness change corresponds to an analyzable phase change. For narrow tolerance products such as lithium electrode sheets, thin films, and coatings, thickness fluctuations are usually less than one equivalent interference period, thus avoiding the problem of phase multi-values ​​across periods.

[0093] Based on the above, the final output actual thickness value = reference thickness value + thickness deviation value, where the reference thickness value is obtained by looking up a table under the corresponding reference phase or reference potential of the calibration curve, and the thickness deviation value is obtained by converting the currently detected phase offset or potential change through the calibration curve, that is, actual thickness value = reference thickness value + (current phase offset - reference phase offset) × (λ0 / (4πn)) where λ0 is the center wavelength of the terahertz wave in air, and n is the equivalent refractive index of the material to be measured. This linear approximation holds when the thickness fluctuation is less than λ0 / (2n), which can guarantee the detection accuracy;

[0094] The working range selection unit is used to perform thickness conversion within a single coherent interference cycle. Based on the preset nominal product thickness, it selects a working range where the slope of the phase-potential curve is greater than a preset threshold. When the current working point is detected to be in a region with a smaller curve slope, it automatically switches to an adjacent interference cycle or adjusts the transmit power and receive gain to ensure that the working point is always in a high-sensitivity range, thereby improving the resolution and stability of thickness measurement. At the same time, it records the phase jump value before and after the switch, which is used to correct the phase shift introduced by the cycle switching in subsequent data processing, ensuring the continuity and consistency of the thickness value.

[0095] The specific operation logic for selecting the work area is as follows:

[0096] Based on the thickness-potential calibration curve in the reference calibration module, the slope dV / dd at the current operating point is calculated in real time. When the absolute value of the slope is greater than the lower limit of the allowable slope, the current operating point is determined to be in the high-sensitivity range, and the current operating state is maintained. If the absolute value of the slope is less than or equal to the lower limit of the allowable slope, the current operating point is determined to be in the low-sensitivity range, and an operating range switching operation is triggered. To avoid false switching caused by occasional noise, the switching operation will only be performed after at least three consecutive low-sensitivity states are detected. The preferred strategy is to switch the interference period. That is, if the current period is the Nth interference period, the switch to the N+1 or N-1th period will be attempted. The switching method is that the signal processing module demodulates the phase. The module sends a command to change the reference phase standard of the phase comparison circuit (increasing or decreasing the phase value of 2π corresponding to one cycle). After the switch, the new operating point slope is immediately detected, and the switch is only confirmed as successful when it is confirmed to be in the high sensitivity range. The phase jump value of 2π generated by this cycle switch is recorded for the continuity correction of subsequent thickness calculations. The alternative strategy is to adjust the transmit or receive parameters. If the thickness is close to the cycle boundary and cannot be improved by cycle switching (i.e., the adjacent cycle is also in the low slope region), parameter adjustment is initiated. This involves the signal processing module fine-tuning the output power of the single-frequency continuous terahertz transmit module or the receive gain of the coherent reflection receive module through the optical intensity matching interface unit to observe the slope change. The goal is to find a parameter combination that makes the slope greater than the allowable lower limit of the slope. After the adjustment is completed, the default transmit power and receive gain parameters under this product specification are updated. If neither strategy can increase the slope above the threshold, a low sensitivity alarm will be output, and the current operating parameters will be maintained, prompting the operator to check the optical path or recalibrate.

[0097] The industrial communication module outputs thickness measurement results to the production line control system. It includes a protocol interface unit that supports various industrial communication protocols, including Ethernet, Industrial Ethernet, serial port, CAN, EtherCAT, Profinet, or Modbus. It is responsible for outputting thickness data, thickness deviation, alarm status, or quality judgment results to the production line control system. For example, the Ethernet interface unit supports the TCP / IP protocol and can be configured with a static IP or DHCP to achieve real-time data interaction with the host computer or MES system. It supports the transmission of thickness measurement values, statistical information, and equipment status through standardized data formats (such as JSON, XML, or custom binary protocols). It also features data caching and reconnection mechanisms to ensure that critical measurement data is not lost during network fluctuations or communication interruptions, and automatically retransmits cached data after connection restoration, ensuring the integrity and traceability of production line data. The communication cycle can be set from 1ms to 10ms to meet the real-time requirements of high-speed production lines.

[0098] A low-expansion support and optical path fixing structure is used to fix the transmitting optical path, receiving optical path, and detection reference position, including:

[0099] The main support unit is made of Invar steel and is integrally machined. It is used to fix the transmitter, receiver, lens and test material detection positions to ensure that the relative positions of each optical element remain stable when the temperature changes.

[0100] The positioning unit is used to maintain the geometric relationship between the transmitting optical path, the receiving optical path, and the detection reference position. This ensures that the influence of changes in the optical path length on the thickness measurement results can be controlled under long-term operation or environmental temperature fluctuations, thereby guaranteeing the stability and repeatability of the measurement reference.

[0101] Please see Figure 2 An electronic device, comprising:

[0102] At least one processor;

[0103] and memory that is communicatively connected to at least one processor;

[0104] The memory stores a computer program that can be executed by at least one processor. The computer program is executed by at least one processor so that at least one processor can execute the above-mentioned single-frequency terahertz steady-state coherent phase high-precision thickness measurement system.

[0105] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, apparatus, article, or method that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, apparatus, article, or method. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, apparatus, article, or method that includes that element.

[0106] The above description is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained in this invention are implemented according to conventional methods in the art unless otherwise specified or limited.

Claims

1. A single-frequency terahertz steady-state coherent phase high-precision thickness measurement system, characterized in that: include: A single-frequency continuous terahertz transmitting module is used to output continuous steady-state terahertz electromagnetic waves with a fixed wavelength. A coherent reflection receiving module is used to receive coherent interference signals formed by reflections from the upper and lower surfaces of the material under test, and to convert the coherent interference signals into electrical signals; The phase demodulation module is used to analyze the phase shift or simulate the potential change of the coherent interference based on the electrical signal. The reference calibration module is used to store the calibration correspondence between thickness, phase, and potential; The signal processing module is used to call the calibration correspondence and calculate the thickness of the material to be measured based on the phase offset or the simulated potential change. The industrial communication module is used to output the thickness detection results to the production line control system; Low-expansion support and optical path fixing structure are used to fix the transmitting optical path, receiving optical path and detection reference position; The single-frequency continuous terahertz transmission module includes a continuous wave transmission unit and a wavelength selection unit. The continuous wave transmission unit is used to output a continuous sinusoidal steady-state terahertz wave and does not use a wideband pulse transmission method. The wavelength selection unit is used to set the output wavelength range to 100μm to 1000μm; The coherent reflection receiving module includes: The optical path arrangement unit is used to employ a coaxial or near-coaxial optical path structure so that the reflected signals from the upper and lower surfaces of the material under test form a steady-state coherent interference at the receiving end. The receiving and processing unit includes a collimating lens, a focusing lens, a terahertz light transmission window, a dustproof sealing structure, and a low-noise preamplifier circuit, used for collimating, focusing, light transmission isolation, dustproof sealing of terahertz waves, and low-noise amplification of the received electrical signals. The light intensity matching interface unit is used to reserve interfaces for light intensity matching, weak signal amplification, and dynamic baseline cancellation. The benchmark calibration module includes: The calibration data storage unit is used to store thickness-phase-potential calibration curves, piecewise fitting functions, calibration lookup tables, or multidimensional calibration data containing temperature compensation parameters. The calibration generation unit is used to complete the full-cycle calibration using a standard thickness template, and to establish the phase and potential values ​​corresponding to different thickness points. The signal processing module includes: The data acquisition and calibration unit is used to read real-time potential signals and phase offset signals, and to call the calibration curves corresponding to the product specifications. The thickness calculation and compensation unit is used to calculate the thickness deviation based on the reference potential value or reference phase value, output the actual thickness value, thickness deviation value, alarm status and quality judgment result, and compensate for ambient temperature and signal baseline drift.

2. The single-frequency terahertz steady-state coherent phase high-precision thickness measurement system according to claim 1, characterized in that: The phase demodulation module includes a low-noise amplifier circuit, a filter circuit, an analog-to-digital converter circuit, a phase comparison circuit, and a baseline correction circuit. It is used to perform low-noise amplification, filtering, analog-to-digital conversion, phase comparison, and baseline correction on electrical signals, and incorporates the fixed phase constant formed by the interface reflection phase, the initial optical path length, and the circuit phase delay into the calibration correspondence in the reference calibration module.

3. The single-frequency terahertz steady-state coherent phase high-precision thickness measurement system according to claim 1, characterized in that: The signal processing module further includes: The working range selection unit is used to perform thickness conversion within a single coherent interference period and select a working range in which the slope of the phase-potential curve is greater than a preset threshold based on the preset nominal product thickness.

4. The single-frequency terahertz steady-state coherent phase high-precision thickness measurement system according to claim 1, characterized in that: The industrial communication module includes: The protocol interface unit is used to output thickness data, thickness deviation, alarm status, or quality judgment results to the production line control system via Ethernet, Industrial Ethernet, serial port, CAN, EtherCAT, Profinet, or Modbus protocols.

5. The single-frequency terahertz steady-state coherent phase high-precision thickness measurement system according to claim 1, characterized in that: The low-expansion support and optical path fixing structure includes: The main support unit, made of Invar steel, is used to fix the transmitter, receiver, lens, and the detection position of the material under test. The positioning unit is used to maintain the geometric relationship between the transmitting optical path, the receiving optical path, and the detection reference position.

6. An electronic device, characterized in that: The electronic device includes: At least one processor; and a memory communicatively connected to the at least one processor; The memory stores a computer program that can be executed by the at least one processor, and the computer program is executed by the at least one processor to enable the at least one processor to execute the single-frequency terahertz steady-state coherent phase high-precision thickness measurement system according to any one of claims 1 to 5.

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