Plastic pipe wall thickness detection method based on millimeter wave technology

Abstract

The invention discloses a plastic pipe wall thickness detection method based on a millimeter wave technology, and relates to the technical field of millimeter waves, and the method comprises the steps: S1, transmitting an original signal to a target plastic pipe, and calculating the overall dielectric constant of the target plastic pipe; s2, attenuating a slope value, and calculating a loss tangent value and an attenuation coefficient; s3, constructing a time domain matched filter to generate a corrected reflection signal and an original time delay set; and S4, calculating an optimal time delay value based on the outer wall peak and the optimal peak, and calculating a real thickness value by using the optimal time delay value and the dielectric constant. Compared with the prior art, by means of attenuation coefficient calculation and time domain matched filter compensation and introduction of reflection coefficient inversion dielectric constant and loss angle tangent value estimation, the detection capability of inner wall weak reflection signals is improved; the method has the advantages that the probability of inner wall recognition failure or misrecognition is remarkably reduced, and the thickness calculation precision is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of millimeter wave, in particular to a plastic pipe wall thickness detection method based on millimeter wave technology. BACKGROUND

[0002] In the manufacturing process of large plastic pipes, accurate online measurement of pipe wall thickness is crucial for quality control and cost optimization. Although millimeter wave technology has become an emerging measurement solution due to its non-contact, environmental interference resistance, and multi-parameter synchronous measurement capabilities, there are inherent technical defects in using millimeter wave technology to detect plastic pipes containing high dielectric loss additives, such as incorporating high dielectric loss materials such as carbon black, metal powder, glass fiber, or polar flame retardants into the material, which severely restricts measurement accuracy. When millimeter waves (usually set to 30-300 GHz) penetrate the pipe wall, the presence of polar molecules or conductive particles in high dielectric loss materials, such as carbon black content greater than 2%, will cause strong dielectric loss and eddy current absorption effects, resulting in exponential decay of electromagnetic wave energy, making the penetration depth of millimeter waves insufficient for high dielectric loss material plastic pipes, and the intensity of the inner wall reflected signal may be below the system detection threshold, resulting in the system only capturing the outer wall reflection, and the algorithm mistaking noise artifacts or the second reflection of the outer wall as "inner wall signals", outputting false wall thickness values. This error can typically range from 0.5mm to 1mm in the positive and negative ranges, far exceeding the 0.1mm tolerance size allowed by the ASTM D3035 industry standard. Therefore, using millimeter wave detection in existing conventional millimeter wave technology for high dielectric loss material plastic pipes may result in significant problems such as quality control or excessive design cost increases. SUMMARY

[0003] The present application provides a plastic pipe wall thickness detection method based on millimeter wave technology, which solves the problem of large error in the final detection result caused by insufficient millimeter wave penetration depth when detecting the pipe wall thickness of high dielectric loss material plastic pipes in the prior art.

[0004] The present application is implemented by the following technical solutions: A plastic pipe wall thickness detection method based on millimeter wave technology, the method comprising: Step S1: using a millimeter wave radar to emit an original signal to a target plastic pipe, collecting an initial reflection signal generated on the target plastic pipe, and calculating the overall dielectric constant of the target plastic pipe based on the reflection coefficient of the initial reflection signal; Step S2: obtaining an attenuation slope value after FFT transformation of the initial reflection signal, calculating the loss tangent value of the target plastic pipe using the attenuation slope value and the overall dielectric constant, and calculating the attenuation coefficient of the initial reflection signal based on the loss tangent value; Step S3: constructing a time domain matched filter using the attenuation coefficient, generating a corrected reflection signal by performing matched filtering on the initial reflection signal using the time domain matched filter, and generating an original time delay set representing the candidate reflection peaks based on the corrected reflection signal; Step S4: labeling the outer wall peak in the original time delay set, removing the artifact peak and screening the optimal peak from the candidate peak, calculating the optimal time delay value based on the outer wall peak and the optimal peak, and calculating the real thickness value using the optimal time delay value and the dielectric constant.

[0005] When the millimeter wave penetrates the pipe wall, the polar molecules or conductive particles in the high dielectric loss material will cause strong dielectric loss and eddy current absorption effect, resulting in exponential attenuation of electromagnetic wave energy, so that the penetration depth of the millimeter wave to the high dielectric loss material plastic pipe is insufficient, and the inner wall reflection signal strength may be lower than the system detection threshold, resulting in that the system can only capture the outer wall reflection, and thus the algorithm will misidentify the noise artifact or the outer wall secondary reflection as the "inner wall signal", and output a false wall thickness value. Therefore, in the existing conventional millimeter wave technology, when the millimeter wave is used to detect the high dielectric loss material plastic pipe, it may cause obvious problems such as quality out of control or excessive design cost increase. Based on this, the present application provides a plastic pipe wall thickness detection method based on millimeter wave technology, which solves the problem that in the prior art, when the pipe wall thickness of the high dielectric loss material plastic pipe is detected, the insufficient penetration depth of the millimeter wave easily leads to a large error in the final detection result.

[0006] Further, the calculation process of the overall dielectric constant is set to calculate the overall dielectric constant based on the reflection coefficient inversion of the outer wall, and the process includes: The received initial reflection signal is subjected to envelope detection and phase demodulation to obtain a fluctuating reflection coefficient corresponding to the position of the outer wall, the fluctuating reflection coefficient being generated based on the electromagnetic wave impedance discontinuity of the target plastic pipe outer wall; the target plastic pipe outer wall is set as a half-space boundary of the overall dielectric constant, and the overall dielectric constant is calculated based on the fluctuating reflection coefficient using the plane electromagnetic wave incidence process in the half-space boundary precondition.

[0007] Further, the process of calculating the overall dielectric constant using the fluctuating reflection coefficient includes: Let the overall dielectric constant be represented as E1, and the fluctuating reflection coefficient be represented as T; set an incident adjustment coefficient and represent it as alpha, set a projection adjustment factor and represent it as beta, and set a reflection compensation amount and represent it as kz, The equation for representing the fluctuating reflection coefficient using the overall dielectric constant is set as: , Then the calculation formula of the overall dielectric constant is represented as: .

[0008] Further, the projection adjustment factor β is used to adjust the multi-layer interface refraction superposition effect of the high dielectric loss layer in the target plastic pipe; the energy transmission weight parameter of the target plastic pipe material is denoted as ω, the incidence angle of the plane electromagnetic wave in the half-space boundary relative to the normal direction is denoted as θ; the local dielectric constant of the high dielectric loss layer in the target plastic pipe is denoted as E2, The variable calculation formula of the projection adjustment factor β is set as: , Wherein, the N represents the total number of equivalent division electromagnetic sub-sections of the target plastic pipe, the E2 n represents the local dielectric constant of the nth electromagnetic sub-section, the (sinθ) 2 represents the mapping influence component of the incidence angle on the local dielectric constant; the ω n represents the energy transmission weight parameter of the electromagnetic wave when passing through the nth layer electromagnetic sub-section energy.

[0009] Further, the attenuation coefficient calculation process of the initial reflection signal is set as: The loss angle is denoted as μ, and the tangent value of the loss angle is denoted as tanμ; the attenuation coefficient is denoted as σp, the frequency of the initial reflection signal is denoted as f0, and the speed of light is denoted as c, then the attenuation coefficient σp calculation formula is: .

[0010] Further, the tangent value calculation process of the loss angle is set as: extracting the imaginary part value and the real part value of the overall dielectric constant of the target plastic pipe, the ratio of the imaginary part value to the real part value is denoted as Q, the attenuation slope value is denoted as ka, and the attenuation adjustment coefficient is denoted as Then the calculation formula of the tangent value of the loss angle is: .

[0011] Further, the process of generating the corrected reflection signal includes: The initial reflection signal is subjected to a direct current offset processing, a echo response signal is set based on the processed initial reflection signal, a time conjugate signal of the echo response signal is constructed and used as a matching template signal of a time domain matching filter; the original signal and the echo response signal are subjected to convolution operation, a template threshold value is set for the signal parameters of the matching template signal, and the echo response signal whose signal parameters reach the template threshold value is extracted and labeled as the corrected reflection signal.

[0012] Further, the form of using the attenuation coefficient to construct the time domain matching filter is set as: the attenuation coefficient is used as the weighting factor of the time domain matching filter, and the point-by-point multiplication operation is performed with the matching template signal to generate the weight function of the time domain matching filter.

[0013] Furthermore, the process of extracting candidate peaks from the original time delay set is set as follows: search for local maxima of all candidate peaks in the envelope energy curve, set a candidate peak critical value to screen out the candidate reflection peaks that meet the requirements, and set a propagation time delay of a corresponding candidate reflection wave for each screened candidate peak.

[0014] Furthermore, a weighted score is used to screen out the optimal peak, wherein the weighting terms of the weighted score include peak intensity, time delay approximation, and the proportion of the main energy of the spectrum.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. By calculating the attenuation coefficient and using time-domain matched filter compensation, the detection capability of weak reflection signals on the inner wall is improved, enabling millimeter waves to effectively recover the signal characteristics of the inner wall even when the penetration depth is limited in high-loss materials such as high carbon black and metal powder, significantly reducing the probability of inner wall recognition failure or misidentification. 2. By introducing the reflection coefficient to estimate the dielectric constant and loss tangent, compared with the traditional coarse model of "fixed wave velocity and fixed dielectric constant", the thickness inversion parameters can be adjusted in real time for each pipe material, making the detection more in line with the intrinsic characteristics of the material and significantly improving the accuracy of thickness calculation. 3. A time-domain matched filter is constructed to improve the signal-to-noise ratio of the inner wall peaks, and an artifact removal algorithm is used to select the optimal peaks. This effectively suppresses the problem of false peak identification caused by secondary reflection from the outer wall or system noise, significantly reduces the error risk of pipe wall thickness measurement, and ensures the reliability of the wall thickness detection results. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings: Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0018] Example 1, as Figure 1 As shown, this embodiment is a method for detecting the wall thickness of plastic pipes based on millimeter-wave technology. The method includes: Step S1: Use millimeter-wave radar to transmit the original signal to the target plastic tube, collect the initial reflection signal generated on the target plastic tube, and calculate the overall dielectric constant of the target plastic tube based on the reflection coefficient of the initial reflection signal; Step S2: obtain the attenuation slope value after FFT transformation of the initial reflection signal, calculate the loss tangent value of the target plastic pipe using the attenuation slope value and the overall dielectric constant, and calculate the attenuation coefficient of the initial reflection signal based on the loss tangent value; Step S3: construct a time-domain matched filter using the attenuation coefficient, generate a corrected reflection signal after matched filtering of the initial reflection signal using the time-domain matched filter, and generate an original time delay set representing the candidate reflection peaks based on the corrected reflection signal; Step S4: label the outer wall peak in the original time delay set, remove the artifact peak and screen out the optimal peak from the candidate peak, calculate the optimal time delay value based on the outer wall peak and the optimal peak, and calculate the true thickness value using the optimal time delay value and the dielectric constant.

[0019] In the manufacturing process of high-performance plastic pipes, as higher mechanical, flame-retardant, anti-aging and electromagnetic shielding requirements are put forward for material performance, more and more plastic products begin to add functional fillers with strong dielectric response or conductive properties, such as carbon black, metal powder, glass fiber, ceramic particles or polar flame retardants. Such materials form plastic pipes, which are usually referred to as plastic pipes of high molecular dielectric loss materials. The main components of the plastic pipe of high molecular dielectric loss material are usually composed of matrix resin and high dielectric and conductive additives, and the high dielectric or conductive additives usually include carbon black, metal powder, glass fiber and polar flame retardants. High dielectric or conductive additives usually cause the overall dielectric constant of the plastic pipe to rise, and there is a significant dispersion effect, with a sharp change in dielectric constant and loss characteristics at different frequencies; when the additive concentration exceeds the limit threshold, the material exhibits conductor-like properties, producing strong reflection and absorption of high-frequency electromagnetic waves. Due to the characteristics of high dielectric loss, high composite structure inhomogeneity and other characteristics of the material itself, the traditional direct measurement method will encounter serious limitations in the penetration depth of electromagnetic waves and signal waveform distortion caused by strong reflection in actual application, making it difficult to accurately detect the inner wall of the pipe body and affecting time delay identification and thickness measurement.

[0020] In this embodiment, the transmitting unit of the millimeter-wave radar, such as the VCO and power amplifier, is activated to control the antenna array or lens structure, accurately projecting the millimeter-wave beam onto the target plastic pipe area. The original signal emitted by the millimeter-wave radar is an electromagnetic wave signal, a known waveform generated under system control, such as a frequency-modulated continuous wave or pulse wave, pointing towards the cross-section or wall of the target plastic pipe at a certain angle and power. When the emitted millimeter wave encounters the outer wall surface and the inner wall interface of the plastic pipe, it will produce partial reflection and partial transmission. The receiving module of the radar system collects these reflected echo signals, which are the initial reflection signals. The initial reflection signal consists of multiple time-delay components, representing reflections generated by different structural interfaces, and the signal parameters include different amplitude, frequency components, and phase characteristics, providing the original detection basis for subsequent signal processing and thickness inversion. The overall dielectric constant is an equivalent electromagnetic parameter used when modeling non-uniform dielectric materials. It represents the dielectric constant introduced when electromagnetic waves propagate in a non-uniform, composite material medium or object, and to simplify electromagnetic analysis, the medium or object is treated as a homogeneous material with average electromagnetic properties. The overall dielectric constant introduced in this case is the equivalent dielectric constant resulting from treating the target plastic tube as having uniform electromagnetic properties. In practical implementation, the overall dielectric constant is represented by a complex dielectric constant.

[0021] An FFT transform is performed on the initial reflected signal to convert it from a time-domain waveform to a frequency-domain spectrum. The result is a frequency-amplitude pair, representing the energy distribution of different frequency components in the signal. The attenuation slope value represents the rate at which the signal amplitude decreases as the analysis frequency increases, expressed as the slope of a linear trend of amplitude decreasing with frequency in a logarithmic coordinate system. The larger the slope, the more the high-frequency components are absorbed by the material, and the more severe the dielectric loss. A time-domain matched filter is constructed using attenuation coefficients. This involves extracting and utilizing the energy attenuation characteristics of the medium for millimeter waves, such as the high dielectric loss of plastic pipes leading to a rapid decrease in signal strength, to construct a time-domain matched filter using attenuation coefficients. Its function is to construct a filter that matches the signal attenuation trend, enabling it to enhance weak reflected signal components that still exist under the expected attenuation trend, while suppressing unexpected noise or artifact components. Matching the initial reflected signal with a time-domain matched filter maximizes the output value when the target signal matches the filter shape through correlation calculations, thereby maximizing the signal-to-noise ratio, suppressing unstructured noise, and improving signal clarity. The matching result is a corrected reflection signal with more concentrated signal energy. The corrected reflection signal represents the reflection waveform after removing interference and enhancing the signal, which is used for subsequent peak determination or feature extraction.

[0022] Based on the corrected reflection signal, a set of original time delays containing candidate reflection peaks is generated, a peak detection is performed on the corrected reflection signal, and a time delay value corresponding to each possible reflection point is extracted to form a set of time delay candidates. The purpose is to construct a time delay list containing all possible real reflection points, providing a basis for subsequent steps such as artifact removal, optimal peak selection, wall thickness calculation, etc. In millimeter wave thickness detection, "peak" generally refers to the apparent energy reflection point of millimeter wave in time domain reflection signal or frequency domain response signal, which corresponds to the energy mutation of millimeter wave propagation in the material and reflection at different medium interfaces (such as air-material, material-material or material-air), and these peaks contain the boundary information of the internal structure of the material. The outer wall peak represents the reflection signal peak formed when the millimeter wave or other radio frequency signal reaches the outer wall air interface after passing through the pipe wall and is reflected back, which is used as the reference signal peak in this embodiment. The artifact peak is a false peak caused by multiple reflections, scattering or system noise, which may interfere with the judgment of the real reflection position. The optimal peak refers to the reflection peak that is most likely to correspond to the inner wall reflection, which is the key peak value for thickness calculation. The peak is selected from the candidate set through multi-parameter evaluation. According to the propagation time delay difference Δt between the optimal peak (inner wall) and the outer wall peak, the propagation time of the signal in the pipe wall material for one round trip can be obtained. For example, if the inner wall peak time delay is t1 and the outer wall peak time delay is t2, the optimal propagation time delay Δt = t2-t1. The real thickness value is calculated using the optimal time delay value and the dielectric constant, i.e. using the electromagnetic wave propagation formula and the motion-based pipe wall thickness calculation formula. As a specific application, the process can be listed as follows: the electromagnetic wave propagation formula based on dielectric constant is , where v represents the electromagnetic wave speed, c represents the speed of light, and E represents the current dielectric constant; the pipe wall thickness calculation formula combined with the electromagnetic wave propagation formula is , where Δt is the propagation time delay difference between the optimal peak and the outer wall peak, and the final calculation result d is the real pipe wall thickness of the target plastic pipe.

[0023] In embodiment 2, the loss tangent value calculation process is set as follows: the ratio of the imaginary part value to the real part value of the overall dielectric constant of the target plastic pipe is denoted as Q, the attenuation slope value is denoted as ka, and the attenuation adjustment coefficient is denoted as , then the calculation formula of the loss tangent value is: . The attenuation adjustment coefficient represents the degree of influence of the attenuation slope value on the overall loss angle correction value, which is an artificially adjustable adjustment weight parameter; it can be related to material thickness, wavelength, process conditions, etc., and is suitable for obtaining through experimental fitting or adaptive learning. In the formula, The item represents a correction gain item for the traditional loss tangent tan mu, and considers the actual frequency domain attenuation trend of the signal in the material, so that the loss tangent can better reflect the attenuation behavior of the millimeter wave signal in the real propagation environment.

[0024] In this embodiment, as a feasible implementation process, the calculation process of the attenuation coefficient is set as follows: assuming that the attenuation slope value is represented as ka, the frequency domain amplitude of the initial reflected signal is represented as A, and the frequency domain frequency of the initial reflected signal is represented as fa, the calculation formula of the attenuation slope value can be represented as: The calculation formula represents the rate of the logarithmic amplitude of the frequency domain signal changing with the frequency, that is, the change amount of the logarithmic amplitude when the frequency changes by 1 Hz, and represents the downward trend in the frequency domain. The loss tangent represents the dielectric loss degree of the material, and reflects the ability of the material to convert electromagnetic wave energy into heat energy.

[0025] Further, as a feasible implementation, the calculation process of the overall dielectric constant is set to be based on the reflection coefficient of the outer wall to calculate the overall dielectric constant by inversion, and the process includes: The received initial reflected signal is subjected to envelope detection and phase demodulation to obtain a fluctuating reflection coefficient corresponding to the outer wall position, which is generated based on the electromagnetic wave impedance discontinuity of the outer wall of the target plastic pipe; the outer wall of the target plastic pipe is set as a half-space boundary of the overall dielectric constant, and the overall dielectric constant is calculated based on the fluctuating reflection coefficient using the plane electromagnetic wave incidence process in the half-space boundary precondition.

[0026] The received initial reflection signal is subjected to envelope detection and phase demodulation. In the millimeter wave thickness detection, the system transmits electromagnetic wave signals and receives signals reflected from the interfaces inside the measured medium. The received "initial reflection signal" is a high-frequency composite signal containing information such as amplitude, phase and carrier frequency. The envelope detection refers to extracting the envelope of the signal to obtain the amplitude profile representing the trend of the reflection energy strength, which is used to determine the position and strength of the reflection peak. The phase demodulation refers to extracting the phase information of the original signal, which is used to analyze the phase change of the reflected wave during propagation, and helps to more accurately determine the position and material electrical properties of the reflection surface. The wave reflection coefficient reflects the degree of reflection of electromagnetic waves from the air to the outer wall of the plastic pipe due to the change in dielectric properties. The wave reflection coefficient is based on the electromagnetic wave impedance discontinuity of the outer wall of the target plastic pipe. If the electromagnetic wave encounters a position where the medium impedance suddenly changes during propagation, it will be reflected. The outer wall interface of the target plastic pipe is such an impedance mutation point. When the electromagnetic wave enters the plastic from the air, part of the electromagnetic wave is reflected, and the strength and phase constitute the wave reflection coefficient. The wave reflection coefficient refers to the reflection coefficient of the reflected electromagnetic wave strength and phase varying with time, frequency or spatial position due to the existence of the outer wall interface during electromagnetic wave propagation. The wave reflection coefficient of the outer wall can be calculated by the ratio of the reflection signal to the incident signal of the electromagnetic wave on the outer wall.

[0027] More specifically, the outer wall of the target plastic pipe is set as a half-space boundary of the overall dielectric constant, which means that the outer wall of the plastic pipe is regarded as a medium mutation surface, and the inner wall of the pipe is equivalent to a "half-space" with a uniform overall dielectric constant. In the implementation process, the layered structure or non-uniformity of the plastic pipe is not considered, and it is only regarded as a uniform dielectric material filled overall region. Based on the plane electromagnetic wave incidence process in the half-space boundary precondition, it is assumed that the electromagnetic wave is incident vertically or approximately vertically to the outer wall surface in the form of a plane wave, and the boundary conditions in the electromagnetic wave reflection theory, such as the continuity condition of the electromagnetic field on both sides of the boundary.

[0028] Further, as a feasible implementation, the process of calculating the overall dielectric constant using the wave reflection coefficient includes: Let the overall dielectric constant be represented as E1, and the wave reflection coefficient be represented as T. Set the incident adjustment coefficient as α and the projection adjustment factor as β. Let the reflection compensation amount be represented as kz, The equation for representing the wave reflection coefficient using the overall dielectric constant is set as: , The calculation formula of the overall dielectric constant is represented as: .

[0029] The wave reflection coefficient T is a reflection intensity coefficient obtained by envelope phase processing of the actually measured reflection signal, and reflects the reflection intensity caused by the impedance discontinuity of the outer wall interface under the action of electromagnetic waves, which can be demodulated from the received signal in actual detection. The overall dielectric constant E1 represents the effective equivalent dielectric constant of the entire target region to be detected, and is used to represent the electrical response capability of the target plastic pipe to electromagnetic waves, which is usually a complex number, and in specific implementation, it can also be approximated as a real number. The incident adjustment coefficient a represents the system adjustment factor, which is related to the incident parameters of the system calibration. The projection adjustment factor β represents the electromagnetic projection impedance faced by the incident wave penetrating the boundary, which is related to the wave impedance of the background medium. The reflection compensation amount kz represents the average compensation correction factor of the target plastic pipe affected by the electromagnetic wave reflection effect, which is used to compensate for the loss caused by the transmission line, antenna, coupling structure, etc., which can be set based on the amplitude and phase information of the initial reflection signal using empirical rules in specific application. The part represents the incident medium factor, which can accurately describe the propagation characteristics of the wave in the plastic pipe by adjusting a and E1. part constitutes the impedance discontinuity reflection calculation formula, which reflects the mismatch between the incident medium factor and the reference impedance β. In the formula, the part represents the negative change adjustment of the reflected wave, that is, when the electromagnetic wave interacts with the material interface, the intensity of the reflected wave is affected by the propagation characteristics of the incident wave and the interface effect; in the formula, the part represents the adjustment of the potential transmitted wave, and the intensity and propagation characteristics of the transmitted wave are also affected by the interface effect after the electromagnetic wave passes through the material surface. If the intensity of the reflected wave is greater than that of the transmitted wave, the value of the entire molecular expression will tend to be a higher reflection coefficient; otherwise, if the intensity of the transmitted wave is greater, the reflection coefficient will be lower. The calculation formula of the overall dielectric constant is obtained by inverting the equation representing the wave reflection coefficient based on the overall dielectric constant.

[0030] Further, as a feasible implementation manner, the projection adjustment factor β is used to adjust the multi-layer interface refraction superposition effect of the high dielectric loss layer in the target plastic pipe; the energy transmission weight parameter of the target plastic pipe material is denoted as ω, the incident angle of the plane electromagnetic wave in the half-space boundary relative to the normal direction is denoted as θ; the local dielectric constant of the high dielectric loss layer in the target plastic pipe is denoted as E2, then the variable calculation formula of the projection adjustment factor β is set as: , wherein, the N represents the total number of equivalent electromagnetic sub-sections of the target plastic pipe, the E2 n represents the local dielectric constant of the nth electromagnetic sub-section, and the (sin θ) 2represents the mapping influence component of the incident angle on the local permittivity; ω n represents the energy transmission weight parameter of the electromagnetic wave when passing through the nth layer electromagnetic sub-section energy, which represents the contribution degree of the layer to the total projection adjustment factor.

[0031] represents the energy transmission weight parameter of the electromagnetic wave when passing through the nth layer electromagnetic sub-section energy, which represents the contribution degree of the layer to the total projection adjustment factor. n represents the energy transmission weight parameter of the electromagnetic wave when passing through the nth layer electromagnetic sub-section energy, which represents the contribution degree of the layer to the total projection adjustment factor. E2 n represents the electromagnetic properties of the nth layer, for high dielectric loss layers, such as layers containing carbon black, metal powder, the value of E2 will be higher, so it will significantly affect the propagation of electromagnetic waves, resulting in more obvious interface effects between different layers. (sinθ) 2 represents the mapping influence component of the incident angle on the local permittivity, and the value change represents the different degrees of change of the local permittivity; when the incident angle is close to the normal, the influence of the permittivity is small; when the incident angle is large, the influence of the permittivity will increase significantly; (sinθ) 2 As a mapping factor, the introduction of this angle effect into the calculation ensures the compensation for the change of the dielectric constant under different incident angles. The total number of equivalent electromagnetic sub-sections of the target plastic pipe represents that the thickness or layer structure of the plastic pipe wall is divided into several equivalent thin layers or sub-sections in the thickness direction, and each sub-section is regarded as an electromagnetic sub-section with uniform electromagnetic properties. The purpose is to discretely decompose the continuously distributed permittivity and loss characteristics into uniform approximations on multiple sections to facilitate numerical calculation and weighted summation. By adjusting the size of N and the division method of each sub-section, a balance between precision and calculation complexity can be achieved, and the multi-layer interface effect of high dielectric loss body can be better simulated.

[0032] The multi-layer refraction superposition effect means that the propagation characteristics of refraction and reflection of electromagnetic waves passing through sub-sections with different permittivities are continuously modulated. The variable calculation formula of the projection adjustment factor β represents the effective refraction characteristics of the nth layer sub-section to the electromagnetic wave. The The part represents the effective projection component of the local permittivity in the normal direction, which represents the effective propagation contribution of the electromagnetic wave in the normal direction within the nth sub-section. When θ is close to 90°, the value tends to 0, indicating a decrease in penetration ability. The entire formula constructs a comprehensive adjustment factor by weighting and accumulating the dielectric directional projection of each sub-section, which combines local electromagnetic properties, spatial incident geometry, and energy transmission, and can describe the effective projection and refraction ability of the entire structure under electromagnetic wave incidence.

[0033] In this embodiment, the attenuation coefficient calculation process of the initial reflection signal is set as: Let the loss angle be denoted as μ, and the tangent value of the loss angle be denoted as tan μ; let the attenuation coefficient be denoted as σ p, the frequency of the initial reflected signal be denoted as f 0, and the speed of light be denoted as c, then the attenuation coefficient σ p calculation formula is: .

[0034] The loss angle μ represents the electromagnetic loss characteristic parameter of the target plastic pipe, and is usually directly related to the dielectric constant and loss factor of the material; the greater the loss angle, the stronger the absorption of the material to the electromagnetic wave, that is, the electromagnetic wave will produce more energy loss when it is transmitted in the material, which is an important parameter for describing the energy conversion and attenuation of the electromagnetic wave in the material. The tangent value of the loss angle tan μ represents the relationship between the dielectric loss and the wave propagation loss of the material; the loss angle μ of the material describes the absorption and energy loss of the electromagnetic wave when it propagates in the material, and tan μ is the quantitative value of this absorption effect. The attenuation coefficient σ p represents the amplitude attenuation degree of the reflected signal caused by the dielectric loss in the process of electromagnetic wave propagation, which is used to evaluate and correct the loss effect of signal energy when passing through the target plastic pipe, and then improve the accuracy of matched filtering or echo identification. The attenuation coefficient σ p is proportional to the loss capability of the material (i.e. the tangent value of the loss angle tan μ), and is also proportional to the propagation speed of the signal in the material. The calculation formula converts the time oscillation rate of the electromagnetic wave in the medium into the wave front change rate in space. Then multiply the tangent value of the loss angle tan μ, that is, the dielectric loss capability of the material is applied to the propagation constant, and the attenuation rate per meter of the wave in the material is obtained. By combining the tangent value of the loss angle tan μ representing the intrinsic loss of the material and the angular frequency and propagation constant representing the wave propagation characteristics, the attenuation rate of the millimeter wave when propagating in the high dielectric loss plastic pipe is obtained. The part represents the degree of electric field energy density enhancement or propagation speed slowing down compared with free space when the electromagnetic wave propagates in the medium, and represents the excess attenuation contribution of the electromagnetic wave in the medium relative to the vacuum in terms of dielectric energy storage. By setting the square root, a correction value reflecting the change of the dielectric constant of the material can be obtained, so that a more accurate attenuation coefficient can be calculated.

[0035] Further, as a feasible implementation manner, the process of generating the corrected reflected signal comprises: The initial reflected signal is subjected to a direct current offset processing, a back echo response signal is set based on the processed initial reflected signal, a time conjugate signal of the back echo response signal is constructed and used as a matching template signal of a time domain matched filter; the original signal is subjected to a convolution operation with the back echo response signal, a template threshold value is set for the signal parameters of the matching template signal, and the back echo response signal whose signal parameters reach the template threshold value is extracted as the corrected reflected signal.

[0036] The DC offset processing is used to eliminate the constant offset in the signal, prevent the DC component from affecting the subsequent filtering and matching, and improve the dynamic range and matching accuracy of the signal. The echo response signal is used to treat the processed signal as the response of the system to the input, and to prepare for constructing the matching template. The time conjugate signal is constructed as the matching template, which is used to convolve with the original signal to effectively enhance the correlation between the signal and the template, and highlight the target signal component. In actual implementation, the time conjugate signal can be regarded as an ideal reference waveform. The convolution process is used to realize the matching filtering, extract the signal part most similar to the template by maximizing the correlation, and suppress noise and non-target components. Through template threshold screening, the signal components with high matching degree are retained, and the low matching degree noise artifacts are removed, realizing signal enhancement and purification. The template threshold can be evaluated and set by empirical rules for the signal waveform.

[0037] Further, as a feasible implementation, the form of constructing the time domain matching filter using the attenuation coefficient is set as follows: the attenuation coefficient is used as the weighting factor of the time domain matching filter, and a point-by-point multiplication operation is performed with the matching template signal to generate the weight function of the time domain matching filter.

[0038] The attenuation coefficient σp reflects the degree of energy attenuation of millimeter waves in the material during propagation, and using it as the weight can compensate for the filter response. Point-by-point multiplication with the matching template signal means multiplying the signal value of each time point of the matching template signal by the corresponding attenuation weight to form the weight function of the time domain matching filter. This enables the filter to give different degrees of attention to different time segments of the signal. The weight function combines the material attenuation characteristics and the signal template shape to constitute the matching characteristics of the filter, which is used for matching filtering with the received signal.

[0039] Further, as a feasible implementation, the process of extracting candidate peaks from the original time delay set is set as follows: search for all local maximum points of candidate peaks in the envelope energy curve, set a candidate peak threshold to screen out the candidate reflection peaks that meet the threshold, and set a propagation time delay for each screened candidate peak corresponding to a candidate reflection wave; use a weighted score to screen out the optimal peak, and the weighted items of the weighted score include peak intensity, time delay approximation degree and spectral main energy proportion value.

[0040] The envelope energy curve searches for local maximum points, which are used to traverse the pre-processed signal envelope curve to find all local peaks, i.e. points on both sides of which the signal value is lower, and these local maximum points represent potential reflection signal peaks. The candidate peak threshold screening process sets a threshold to exclude noise peaks with low amplitudes, and only the maximum points with peak amplitudes exceeding the threshold are included as candidate reflection peaks for subsequent processing. For each candidate peak, the corresponding electromagnetic wave propagation time delay is determined according to its position on the time axis, which reflects the time from transmission to reflection to reception, and is a key parameter for calculating the wall thickness. The peak strength represents the amplitude of the reflection peak, and the greater the peak value represents the stronger the reflection signal and the higher the reliability. The time delay approximation degree represents the closeness of the candidate peak time delay to the expected or prior time delay, which measures the reasonableness of the time delay. The spectral main energy proportion value represents the proportion of the main energy in the frequency spectrum of the peak, i.e. the proportion of the highest energy, which reflects whether the frequency domain characteristics of the peak conform to the spectral characteristics of the target reflection signal. According to the above indexes, a comprehensive score is calculated for each candidate peak, and the indexes can be given different weights; the peak with the highest score is selected as the optimal peak, i.e. the most likely to represent the target inner wall reflection signal. Through local maximum value and threshold screening, noise false peaks and non-target reflection peaks can be effectively eliminated; the weighted score multi-dimensionally comprehensively considers the time domain and frequency domain characteristics of the peak, which improves the accuracy and robustness of the inner wall reflection peak identification, provides a more reliable signal basis for subsequent thickness inversion and dielectric parameter calculation, and improves the peak identification capability and measurement accuracy in complex environments in the millimeter wave plastic pipe wall thickness detection.

[0041] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for detecting the thickness of a plastic pipe wall based on millimeter wave technology, characterized by, The method comprises: Step S1: using a millimeter wave radar to emit an original signal to a target plastic pipe, collecting an initial reflection signal generated on the target plastic pipe, and calculating an overall dielectric constant of the target plastic pipe based on a reflection coefficient of the initial reflection signal; Step S2: obtaining an attenuation slope value after performing FFT transformation on the initial reflection signal, calculating a loss tangent value of the target plastic pipe using the attenuation slope value and the overall dielectric constant, and calculating an attenuation coefficient of the initial reflection signal based on the loss tangent value; Step S3: constructing a time-domain matched filter using the attenuation coefficient, generating a modified reflection signal after performing matched filtering on the initial reflection signal using the time-domain matched filter, and generating an original time delay set representing candidate reflection peaks based on the modified reflection signal; Step S4: labeling an outer wall peak in the original time delay set, removing an artifact peak, and screening an optimal peak from the candidate peaks, calculating an optimal time delay value based on the outer wall peak and the optimal peak, and calculating a true thickness value using the optimal time delay value and the dielectric constant.

2. The method for detecting the thickness of a plastic pipe wall based on millimeter wave technology according to claim 1, characterized in that, The calculation process of the overall dielectric constant is set to calculate the overall dielectric constant based on the reflection coefficient inversion of the outer wall, and the process comprises: Performing envelope detection and phase demodulation on the received initial reflection signal to obtain a fluctuating reflection coefficient corresponding to the position of the outer wall, the fluctuating reflection coefficient being generated based on the electromagnetic wave impedance discontinuity of the outer wall of the target plastic pipe; setting the outer wall of the target plastic pipe as a half-space boundary of the overall dielectric constant, and calculating the overall dielectric constant using the fluctuating reflection coefficient based on the plane electromagnetic wave incidence process in the half-space boundary precondition.

3. The method according to claim 2, wherein, The process of calculating the overall dielectric constant using the fluctuating reflection coefficient comprises: Let the overall dielectric constant be represented as E1, and the fluctuating reflection coefficient be represented as T; set an incident adjustment coefficient and represent it as α, set a projection adjustment factor and represent it as β, and set a reflection compensation amount and represent it as kz, The equation representing the wave reflection coefficient using the bulk dielectric constant is set as: , The calculation formula of the overall dielectric constant is represented as: .

4. The method for detecting the thickness of a plastic pipe wall based on millimeter wave technology according to claim 3, characterized in that, The projection adjustment factor β is used to adjust the multi-layer interface refraction superposition effect of the high dielectric loss layer in the target plastic pipe; Let the energy transmission weight parameter of the material of the target plastic pipe be represented as ω, and let the incidence angle of the plane electromagnetic wave in the half-space boundary relative to the normal direction be represented as θ; let the local dielectric constant of the high dielectric loss layer in the target plastic pipe be represented as E2, The variable calculation of the projection adjustment factor β is set as: , wherein, the N represents the total number of equivalent electromagnetic sub-sections of the target plastic pipe, the E2 n represents the local dielectric constant of the nth electromagnetic sub-section, the (sin θ) 2 represents the mapping influence component of the incident angle on the local dielectric constant; the ω n represents the energy transmission weight parameter of the electromagnetic wave when passing through the energy of the nth layer electromagnetic sub-section.

5. The method for detecting the thickness of a plastic pipe wall based on millimeter wave technology according to claim 3, characterized in that, The attenuation coefficient calculation process of the initial reflection signal is set to: Let a loss angle be denoted as μ, and a loss angle tangent value be denoted as tan μ; let an attenuation coefficient be denoted as σp, a frequency of an initial reflected signal be denoted as f0, and a light speed be denoted as c, then an attenuation coefficient σp calculation formula is expressed as: .

6. The method for detecting the thickness of a plastic pipe wall based on millimeter wave technology according to claim 5, characterized in that, The loss tangent value calculation process is set to extract the imaginary part value and the real part value of the overall dielectric constant of the target plastic pipe, express the ratio of the imaginary part value and the real part value as Q, set the attenuation slope value as ka, and set the attenuation adjustment coefficient as k , and the calculation formula of the loss tangent value is expressed as: .

7. The method for detecting the thickness of a plastic pipe wall based on millimeter wave technology according to claim 1, characterized in that, The process of generating the modified reflection signal comprises: Performing DC offset processing on the initial reflection signal, setting a return response signal based on the processed initial reflection signal, constructing a time conjugate signal of the return response signal as a matching template signal of the time-domain matched filter; performing convolution operation on the original signal and the return response signal, setting a template threshold for the signal parameters of the matching template signal, and extracting the return response signal whose signal parameter reaches the template threshold as the modified reflection signal. 8.The method of claim 7, wherein, The form of constructing the time-domain matched filter using the attenuation coefficient is set to use the attenuation coefficient as the weighting factor of the time-domain matched filter, perform point-by-point multiplication operation on the attenuation coefficient and the matching template signal, and generate the weight function of the time-domain matched filter. 9.The method of claim 7, wherein, The process of extracting candidate peaks from the original delay set is set to search for all candidate peak local maximum points in the envelope energy curve, set a candidate peak threshold to screen out the corresponding candidate reflection peaks, and set a propagation delay for each screened candidate peak. 10.The method of claim 1, wherein, The optimal peak is screened out using a weighted score, and the weighted items of the weighted score include peak intensity, delay approximation degree, and spectral main energy proportion value.

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