A rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux.

Abstract

This invention discloses a rapid tomographic method for detecting defects in ferromagnetic materials based on AC leakage magnetic flux, belonging to the field of nondestructive testing technology. The method includes the following steps: In an AC leakage magnetic flux detection system, a time-stamped band is set around the instant of excitation frequency switching; frequency jump intervals are determined based on the time-stamped bands; the start and end positions of abrupt changes in magnetic flux response are marked within the frequency jump intervals, forming a transient boundary sequence; a short-time frequency sweep is performed based on the transient boundary sequence; during the short-time frequency sweep, magnetic flux tail texture information and magnetic flux direction offset information are extracted to construct a continuous tail feature sequence. This invention achieves accurate separation of steady-state and transient responses by constructing a transient boundary sequence and frequency overlap identification, avoiding misjudgment of phantom defects; and through pre-excitation, delay compensation, and breathing-style frequency laminar flow rearrangement, the magnetic flux disturbance energy is gradually dissipated, improving the stability of leakage magnetic flux detection and the accuracy of defect identification.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, specifically to a rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux. Background Technology

[0002] Rapid tomography of ferromagnetic material defects based on AC leakage magnetic field refers to the use of AC excitation to create an alternating magnetic field within a ferromagnetic material. When this magnetic field encounters defects such as cracks, pores, or inclusions, it produces characteristic changes in the leakage magnetic field distribution. High-sensitivity sensors collect these AC leakage magnetic field signals, which exhibit different response characteristics with frequency variations. Combined with tomographic imaging algorithms, the spatial distribution of the leakage magnetic field is inverted and reconstructed, allowing for the rapid recovery of the shape, location, and size information of internal defects without damaging the material. The core idea is to leverage the shallow penetration, frequency tunability, and high sensitivity to minute disturbances of AC magnetic fields to extend traditional magnetic field detection from a single surface response to a rapid imaging process with three-dimensional tomographic features. This results in more precise defect identification, faster imaging speed, and greater adaptability to complex structures.

[0003] The existing technology has the following shortcomings:

[0004] In existing technologies, when AC magnetic flux leakage detection systems perform multi-frequency excitation scanning, dynamic switching between different excitation frequencies is typically required to extract the material's response characteristics to different magnetic field depths. However, at the instant of dynamic frequency jumps, the magnetic flux density inside the material cannot achieve an immediate steady-state transition, easily resulting in a brief overlap between the magnetic flux distributions corresponding to the two frequencies, thus forming a cross-frequency transient coupling signal. This signal is not a magnetic flux leakage response caused by a real defect, but rather a spurious magnetic field disturbance caused by the frequency jump process. Since existing tomographic algorithms are usually based on steady-state magnetic flux leakage distribution characteristics for inversion and reconstruction, they lack the ability to identify and suppress such transient couplings, easily misjudging this spurious signal as an internal void or structural separation region, leading to phantom defects in the detection results. Once a misjudgment occurs, the system will output a serious defect alarm, thereby triggering unnecessary shutdowns for maintenance, forcing large production equipment to stop operating without substantial damage, causing significant economic losses and affecting the continuity of equipment operation and maintenance plans.

[0005] The information disclosed in the background section is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux, so as to solve the problems in the background art mentioned above.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux, comprising the following steps:

[0008] In the AC leakage flux detection system, a time stamp band is set around the instant of excitation frequency switching. The frequency jump interval is determined based on the time stamp band. The start and end positions of the magnetic flux response change are marked within the frequency jump interval to form a transient boundary sequence.

[0009] Short-time frequency sweeping traction is performed based on transient boundary sequences. During the short-time frequency sweeping traction process, magnetic flux trailing texture information and magnetic flux direction offset information are extracted to construct a continuous trailing feature sequence.

[0010] A bidirectional sliding alignment is performed on the continuous trailing feature sequence. Based on the sliding alignment results, the frequency overlap core region caused by frequency jump is identified, and the continuous data sequence within the frequency overlap core region is extracted as the basis for subsequent analysis.

[0011] Injecting small-amplitude pre-excitation signals and delayed-compensation excitation signals into the frequency overlap core region expands the amplitude difference between the real defect response trajectory and the transient disturbance response trajectory, forming a shadow response sequence.

[0012] The shadow response sequence is mapped onto the sensor array, and the reverse excitation marker unit is set by combining the trailing feature and the amplitude difference feature. Based on the reverse excitation marker unit, an identification anchoring map is constructed to identify and mark potential false response intervals.

[0013] Based on the identified anchoring spectrum, a phased breathing-style frequency laminar flow rearrangement is performed. The frequency transition excitation intensity is reduced in the first stage of unloading and slow release, the transient energy peak is reduced in the middle stage of silent peak reduction, and the new frequency signal is smoothly introduced in the last stage of extended injection and slow conduction, so that the overlapping magnetic flux disturbance energy is gradually dissipated.

[0014] Preferably, the steps in the AC leakage flux detection system for setting a time stamp band around the instant of excitation frequency switching, determining the frequency jump interval based on the time stamp band, and forming a transient boundary sequence are as follows:

[0015] A time stamp band is set at the instantaneous change point of the excitation current waveform, with the starting point being the moment when the previous excitation frequency begins to decay and the ending point being the moment when the next excitation frequency establishes a stable amplitude, so that the time stamp band covers the entire process of magnetic field energy transition.

[0016] The magnetic flux response signal within the time range covered by the time stamp band is continuously acquired to form a time mapping line containing magnetic flux density, magnetic induction direction and response delay;

[0017] Identify the sustained rising point and stabilizing point of the rate of change of magnetic flux density along the time mapping line, and determine the starting and ending positions of the sudden change in magnetic flux response, respectively.

[0018] Using the start and end positions of the mutation as boundaries, magnetic flux change data of the mutation segment within the time-stamped band are extracted, and a continuous transient boundary sequence containing the pre-mutation preparation stage, the main mutation change segment, and the post-mutation stable stage is constructed in chronological order.

[0019] Preferably, the steps for performing short-time frequency sweeping traction based on transient boundary sequences and constructing continuous trailing feature sequences are as follows:

[0020] Using the transient boundary sequence as the time reference, a short-time sweep frequency traction interval is defined between the start and end times of the sudden change in magnetic flux response, and the short-time sweep frequency traction interval is divided into several continuous sweep frequency units;

[0021] Within each frequency sweeping unit, the excitation source frequency is changed sequentially while maintaining a constant sweeping duration, and data on the changes in magnetic flux density, magnetic induction direction, and magnetic flux distribution on and near the surface of the ferromagnetic material are continuously collected.

[0022] Based on the temporal distribution of the transient boundary sequence, the delayed decay process of the magnetic flux density peak is tracked, magnetic flux tail texture information is extracted, and the main extension direction and decay pattern of the magnetic flux are determined.

[0023] By combining magnetic flux direction offset information, the trailing texture information is correlated with time, frequency and magnetic flux density to form a continuous trailing feature sequence with temporal continuity and directional recognizability.

[0024] Preferably, during the short-time frequency sweep traction process, the excitation frequency changes continuously in a fixed step increment manner, the magnetic flux response data is collected at a constant sampling interval during each frequency stabilization stage, the magnetic flux tail texture information is extracted synchronously based on the magnetic flux density change rate and the direction offset angle, and the continuous tail feature sequence is spliced ​​in time with the magnetic flux extension direction as the main axis to form a complete magnetic flux delay response trajectory.

[0025] Preferably, the steps for performing bidirectional sliding alignment on continuous trailing feature sequences and identifying frequency-overlapping core regions are as follows:

[0026] Based on the continuous tailing feature sequence as input, the sequence is divided into forward response sequence and backward response sequence according to the mutation time point as the central axis, and the time unit and data structure are standardized.

[0027] The backward response sequence is slid forward along the time axis with a fixed time step, and the magnetic flux density, magnetic induction direction and spatial distribution of the corresponding time points in the two sequences are compared one by one in each sliding step.

[0028] Among all the sliding window comparison results, the time period with consistent magnetic flux change, minimal directional shift and strongest energy superposition was selected as the core region of frequency overlap.

[0029] Magnetic flux density, magnetic induction direction, and spatial distribution data for all time points within the frequency overlap core region are extracted to form a continuous data sequence as the basis for subsequent analysis.

[0030] Preferably, the step of injecting a small-amplitude pre-excitation signal and a delay-compensated excitation signal into the frequency overlap core region to form a shadow response sequence is as follows:

[0031] A pre-excitation injection time interval is established in the frequency overlap core region. During the unstable magnetic flux response stage, a pre-excitation signal with an amplitude of 5% to 10% of the main excitation signal is applied to enable the magnetic domains inside the ferromagnetic material to respond in advance.

[0032] Inject a delay compensation excitation signal within the delay time window after the pre-excitation ends, so that the real defect response is extended in time, forming a dual-time excitation structure that echoes the pre-excitation.

[0033] During the pre-excitation and delayed compensation excitation injection process, magnetic flux density, magnetization, magnetic flux direction offset angle and response duration are continuously collected to form a time-continuous magnetic flux change curve.

[0034] Based on the response data of pre-excitation and delayed compensation excitation, a shadow response sequence containing an early enhancement segment, a stable response segment, and a delayed decay segment is constructed in chronological order to distinguish between the real defect response and the transient disturbance response.

[0035] Preferably, the steps for mapping the shadow response sequence to the sensor array and constructing the identification anchoring map are as follows:

[0036] Based on the shadow response sequence, the time-continuous magnetic flux response data are established according to the spatial arrangement of the sensor array to form a correspondence, so that the time information forms a continuous spatial distribution trajectory on the array plane;

[0037] Combining the characteristics of magnetic flux tailing and amplitude difference, the direction of magnetic flux extension and abnormal region are determined based on the response duration and magnetic flux amplitude distribution, and a reverse excitation marker unit is set in the overlapping region to generate phase reversal;

[0038] An identification anchoring map is constructed based on the amplitude, duration, and directional offset characteristics of the reverse response of the reverse excitation marker unit, and the spatiotemporal distribution of the false response region is marked in the map.

[0039] The coordinates of spurious response regions in the identified anchor map are projected back onto the original flux map, and the corresponding regions are spatially labeled and isolated to eliminate spurious responses and retain real defect response data.

[0040] Preferably, the weak-amplitude magnetic field signal applied by the reverse excitation marker unit gradually decays in space along the direction of magnetic flux tailing, and remains in phase opposite to the delay compensation stage of the shadow response sequence in time, so that the false response region forms a stable anti-phase response region, thereby forming a continuous and identifiable spatial reverse response trajectory in the identification anchoring spectrum, which is used to accurately distinguish between the real defect response and the transient disturbance response.

[0041] Preferably, the steps for performing a phased breathing-style frequency laminar flow rearrangement and gradually dissipating overlapping magnetic flux disturbance energy based on the identified anchoring pattern are as follows:

[0042] Based on the identified anchoring spectrum, the time interval and spatial distribution of overlapping magnetic flux disturbance energy are determined, and the time point of energy peak occurrence and the range of energy concentration area are clarified.

[0043] An unloading and slow-release operation is performed in the front part of the energy transition region. By gradually reducing the excitation current amplitude and setting the energy slow-release interval time, the magnetic flux energy diffuses in a gradient form.

[0044] In the middle of the energy transition region, a silent peak-shaving operation is performed to keep the excitation amplitude constant and reduce the peak magnetic flux energy through natural diffusion, so that the magnetic field distribution tends to be balanced.

[0045] In the latter part of the energy transition zone, a delayed injection and slow conduction operation is performed to control the excitation frequency to gradually increase to the target frequency and slowly restore the excitation amplitude, so that the magnetic flux energy is converted smoothly.

[0046] Throughout the laminar rearrangement process, the decay trend of magnetic flux disturbance energy is continuously monitored to ensure that the energy continues to decrease along the time axis and gradually diffuses in space until it completely dissipates.

[0047] The technical effects and advantages provided by the present invention in the above technical solution are as follows:

[0048] This invention establishes a time-stamped band and constructs a transient boundary sequence at the instant of excitation frequency switching, enabling precise time-segmented capture of the magnetic flux response during frequency jumps. This effectively separates the steady-state response from transient disturbance signals during multi-frequency excitation switching. Through short-time frequency sweeping and bidirectional sliding comparison, it accurately identifies the core region of frequency overlap, allowing the true defect response and spurious transient response to form a distinguishable temporal interval. This fundamentally avoids the misjudgment of "phantom defects" caused by frequency overlap, improving the stability and accuracy of defect tomography results.

[0049] This invention introduces pre-excitation and delayed compensation excitation in the frequency overlap core region, and combines this with a reverse excitation marker unit and a staged breathing-style frequency laminar flow rearrangement. This allows magnetic flux disturbance energy to gradually dissipate in time and be released uniformly in space, achieving a smooth energy transition during frequency switching. This process effectively suppresses the interference of transient coupling energy on the leakage magnetic signal, enabling the detection system to maintain a continuous and controllable magnetic flux response under multi-frequency scanning conditions, thereby improving the resolution and reliability of defect identification. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0051] Figure 1 This is a flowchart of the rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux, as described in this invention. Detailed Implementation

[0052] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that the description of this disclosure will be more complete and fully convey the concept of the exemplary embodiments to those skilled in the art.

[0053] This invention provides, for example Figure 1 The rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux, as shown, includes the following steps:

[0054] In the AC leakage flux detection system, a time stamp band is set around the instant of excitation frequency switching. The frequency jump interval is determined based on the time stamp band. The start and end positions of the magnetic flux response change are marked within the frequency jump interval to form a transient boundary sequence.

[0055] To accurately capture transient magnetic flux changes within ferromagnetic materials during multi-frequency excitation switching, a transient boundary sequence with clear start and end time characteristics is established. This provides a temporal reference and spatial constraint for subsequent magnetic flux tail feature extraction and frequency overlap identification. This step is achieved by setting a time-marked band around the instant of excitation frequency switching, determining the frequency jump interval based on the time-marked band, and marking the start and end positions of the abrupt change in magnetic flux response within the frequency jump interval. The specific implementation method is as follows:

[0056] During AC leakage flux detection, when a multi-frequency excitation coil prepares to transition from one frequency to another, a time-stamped band covering this instantaneous change in the excitation current waveform is established, using the moment of change as the core reference. The starting point of the time-stamped band is determined by recording the time when the current amplitude of the magnetic field excitation source begins to decrease, based on the moment the previous excitation frequency begins to decay. The ending point is determined by monitoring the time range required for the new frequency current signal to rise to a stable amplitude, based on the moment the new excitation frequency begins to establish. The total duration of the time-stamped band is determined based on the response speed of the excitation source, the coil inductance characteristics, the material permeability, and the hysteresis response delay, ensuring it covers both the excitation decay phase and the new frequency establishment phase, thus completely enveloping the energy transition region of the magnetic field during the switching process. During this phase, the magnetic flux density inside the ferromagnetic material gradually transitions from a steady-state distribution at the old frequency to a steady-state distribution at the new frequency, and the magnetic induction intensity, magnetization rate, and hysteresis loop gradually change. In this way, the time-stamped band not only defines the time range for frequency switching but also forms a time frame reflecting the transition characteristics of the material's internal magnetic response.

[0057] After the time-marked band is formed, the magnetic flux response signal within the time range covered by the time-marked band is continuously acquired, and these signals are unfolded chronologically to form a complete time-map sequence. The specific process includes: acquiring the magnetic flux baseline signal in a stable state before the start of the time-marked band; continuously recording the entire process of magnetic flux density change over time within the time-marked band; and continuing to acquire the magnetic flux response at a stable new frequency after the time-marked band ends. Through this continuous acquisition, a complete magnetic flux change trajectory curve can be obtained, which starts from a stable pre-state, passes through a violently fluctuating transition section, and finally stabilizes at a new response state. Then, these time-series data are arranged chronologically to form a time-map line that reflects the entire process of frequency jumps. At this point, the magnetic flux value, magnetic induction direction, and response delay at each time point are clearly recorded, thus constructing a detailed distribution of the frequency jump interval in the time dimension. By observing the continuous change trend of magnetic flux density in the time-map line, the energy accumulation region, energy transition region, and energy release region can be clearly distinguished, laying a data foundation for identifying the start and end boundaries of magnetic flux abrupt changes.

[0058] After obtaining the complete frequency jump interval, the trend of the rate of change of magnetic flux density in the time map is analyzed segment by segment to identify the start and end positions of the magnetic flux response abrupt change. The specific steps are as follows: starting from the beginning of the marked band along the time axis, the rate of change of magnetic flux density is compared point by point; when the rate of change starts to increase continuously from a near-zero stable state and experiences a rapid rise within a short period, the first significant acceleration point is determined as the start position of the magnetic flux response abrupt change. Then, the process continues to trace backward along the time axis; when the rate of change of magnetic flux density gradually decreases and tends to stabilize, the point at which the rate falls back to a stable level is defined as the end position of the magnetic flux response abrupt change. In this way, the start and end times of the magnetic flux response abrupt change are clearly defined. During this process, the distribution of magnetic flux lines in the material changes from concentrated to diffused, the direction of magnetic induction shifts slightly, and hysteresis loss exhibits a brief increase. The formation of the abrupt change interval signifies that the material's magnetization state gradually breaks free from the magnetic flux constraint of the previous frequency and enters the response-dominated stage of the new frequency. The length of this interval reflects the material's response delay characteristics to frequency switching. The width of the abrupt change interval under different materials or different excitation intensities can be used as an important parameter to distinguish transient magnetic behavior.

[0059] After accurately determining the start and end positions of the magnetic flux response abrupt change, using these two time points as boundaries, all magnetic flux change data related to the abrupt change segment within the time-stamped band are extracted and rearranged in chronological order to form a continuous transient boundary sequence. The specific process includes: first, starting from the abrupt change point, all magnetic flux change information from the start to the end point is recorded point by point; second, characteristic parameters such as magnetic flux density, magnetic induction direction, magnetization rate, and phase shift are extracted within this time range and organized into a continuous time series in chronological order; third, data from several time points before and after the abrupt change interval are spliced ​​together to smoothly connect the transitional changes at both ends of the abrupt change boundary, thus ensuring the continuity and integrity of the transient boundary sequence; finally, using the center time of the abrupt change interval as a reference point, the magnetic flux response trends at the leading and trailing edges of the time-stamped band are incorporated into the sequence description, so that the entire transient boundary sequence includes not only the main change segment of the abrupt change interval but also the preparatory stage before the abrupt change and the stable stage after the abrupt change. Through this processing, the transient boundary sequence can truly reflect the entire process of magnetic flux from stability to disturbance and then back to stability during frequency switching, and its time structure, magnetic flux amplitude changes and directional shifts are all fully recorded.

[0060] Short-time frequency sweeping traction is performed based on transient boundary sequences. During the short-time frequency sweeping traction process, magnetic flux trailing texture information and magnetic flux direction offset information are extracted to construct a continuous trailing feature sequence.

[0061] To fully utilize the generated transient boundary sequence and comprehensively capture and structurally describe the delay characteristics of magnetic flux within ferromagnetic materials during excitation frequency jumps, a short-time frequency sweeping method based on the transient boundary sequence is proposed. This method guides the magnetic flux to gradually respond to different frequency excitations on the surface and near-surface regions of the ferromagnetic material by applying continuous and controllable frequency sweeps within the time range defined by the transient boundary sequence. This extracts magnetic flux tail texture information and magnetic flux direction offset information, and constructs a continuous tail feature sequence, providing a temporal basis and spatial reference for subsequent frequency overlap core region identification and phantom response interval calibration. The specific technical steps include the following:

[0062] Using the established transient boundary sequence as a time reference, the effective range of short-time frequency sweeping is determined. The transient boundary sequence clearly marks the start and end times of the abrupt change in magnetic flux response during the transition from the original frequency to the new frequency. Therefore, short-time frequency sweeping should be strictly limited to this time range. Specifically, between the start and end times of the abrupt change, the entire interval is divided into several consecutive time periods according to the temporal distribution of the rate of change of magnetic flux in the transient boundary sequence. Each time period corresponds to an independent frequency sweeping unit. Within each frequency sweeping unit, the frequency of the excitation source changes stepwise according to a preset step size, transitioning from the previous frequency to the next frequency in a fixed increment. To ensure that the magnetic flux inside the ferromagnetic material can fully respond to each frequency state, a constant frequency sweeping duration is maintained in each frequency sweeping unit, allowing the magnetic flux to reach a stable response at that frequency before entering the next frequency stage. By using this segment-by-segment frequency sweeping method, the magnetic flux inside the ferromagnetic material can exhibit a continuous changing trend throughout the entire transition range, thereby avoiding energy accumulation and local disturbance concentration caused by frequency abrupt changes. This allows the magnetic flux delay response characteristics to be smoothly unfolded in the time dimension, creating a continuous and traceable time-domain structure for subsequent tail identification.

[0063] During short-time frequency sweeping, the magnetic flux response of the ferromagnetic material's surface and near-surface layers is continuously acquired and its entire variation with time and frequency is recorded. Specifically, within each sweeping unit, the changes in magnetic induction intensity on the material surface are captured in real time through uniformly distributed magnetic flux acquisition channels, while simultaneously recording the instantaneous shift in magnetic flux direction and the local density changes of magnetic field lines. As the frequency gradually transitions from a lower to a higher value, the permeability inside the ferromagnetic material gradually decreases, the eddy current distribution changes accordingly, the penetration depth of the magnetic flux decreases, and the surface magnetic flux density exhibits a delayed decay over time. When the frequency falls back from a higher to a lower value, the penetration depth of the magnetic flux in the material gradually recovers, but due to hysteresis, the magnetic flux distribution does not immediately return to a stable state, but maintains a residual magnetization state for a short period, exhibiting a magnetic flux tail. This process forms a bidirectional trajectory of magnetic flux variation with time and frequency. After each sweeping unit, the acquired magnetic flux data is correlated with the corresponding time and frequency parameters, ensuring a clear correspondence between the magnetic flux response at each time point in both the time and frequency domains. This continuous acquisition and mapping method can form a record of magnetic flux changes in both time and frequency dimensions, providing a complete physical response trajectory for trailing texture extraction.

[0064] After obtaining continuous time and frequency flux response data, the flux delay response occurring during short-time frequency sweeping is analyzed in depth based on the time distribution in the transient boundary sequence, extracting flux tail texture information. Specifically, each frequency sweeping unit is used as a time window, and the trajectory of the flux density peak value is tracked along the time direction. When the local flux peak value does not disappear synchronously with the frequency transition, but instead exhibits delayed decay in subsequent time periods, this delayed decay process constitutes flux tail. By analyzing the flux intensity change at each time point during the tail formation stage, it can be observed that the flux linear density gradually transitions from a concentrated state to a diffused state, the flux peak value exhibits a certain degree of hysteresis along the time axis, and the local magnetic field direction shows a slight rotation. Furthermore, by comparing the spatial distribution of flux lines in different directions within multiple time windows, the main extension direction and decay pattern of the flux tail can be identified. In the surface region of ferromagnetic materials, the flux tail typically appears as a strip-shaped region extending along the magnetization direction, while in the near-surface region, it appears as a fan-shaped distribution diffusing from the inside out. Through continuous observation and recording, complete information on magnetic flux tail texture can be obtained, including the rate of change of magnetic flux density, extension direction, attenuation length, time lag amplitude, and energy distribution trend. This texture information constitutes a spatial mapping of the material's magnetic field inertial response during frequency jumps, providing a physical basis for establishing a tail feature sequence in the next step.

[0065] After extracting the flux tail texture information, a continuous tail feature sequence is constructed by combining it with the flux direction offset information recorded during the frequency sweeping process. Flux direction offset refers to the angular deflection of the principal direction of the flux line due to changes in permeability and hysteresis delay during frequency changes. To accurately describe this direction change, the offset angle of the flux direction relative to the original excitation direction is measured in each time window, using the center line of the flux tail as the reference axis. These offset angles are correlated with time, frequency, and flux density to form a direction change sequence. Then, the tail texture information and the direction change sequence are sequentially mapped point by point in time and continuously spliced ​​together to form a complete tail feature curve. This curve starts at the abrupt change point and ends in the stable region after the abrupt change point. The first part is a transition region where the flux direction remains stable while the density gradually decreases; the middle part is an extension region where the flux direction shifts and the density decays with delay; and the last part is a convergence region where the flux direction gradually returns to the original magnetization direction. By sequentially connecting these time periods, a continuous trailing characteristic sequence with temporal continuity, spatial consistency, and directional identifiability is formed. This characteristic sequence can not only accurately reflect the dynamic process of magnetic flux delay response, but also clearly describe the trajectory of magnetic flux direction change during frequency jumps, providing a traceable basis for distinguishing between steady-state leakage magnetic response caused by real defects and transient disturbances caused by frequency transitions.

[0066] A bidirectional sliding alignment is performed on the continuous trailing feature sequence. Based on the sliding alignment results, the frequency overlap core region caused by frequency jump is identified, and the continuous data sequence within the frequency overlap core region is extracted as the basis for subsequent analysis.

[0067] To address the challenge of distinguishing between genuine defect responses and spurious tail signals during frequency transitions, it is necessary to further extract the region of highest concentration of magnetic flux interference based on the constructed continuous tail feature sequence. This step employs a bidirectional sliding comparison technique, performing forward and backward time-series sliding on the continuous tail feature sequence. Within each sliding step, a detailed comparison of magnetic flux response features is conducted to identify the temporally overlapping intervals of the magnetic flux responses at two different frequency stages during the frequency transition—the core frequency overlap region. Subsequently, a continuous data sequence within this core frequency overlap region is extracted as the basis for subsequent analysis, providing temporal and spatial references for the generation of shadow response sequences, the construction of inverse markers, and the rhythmic division of frequency laminar flow rearrangement. The entire technical process includes the following steps:

[0068] Using the constructed continuous tailing feature sequence as input, the initial structure for bidirectional sliding alignment is established. The continuous tailing feature sequence is a set of magnetic flux response data recorded in chronological order. Each data point includes magnetic flux density value, magnetic induction direction angle, spatial extension shape, and response time. For sliding alignment, this feature sequence is divided into two symmetrical sequences along the abrupt change time point as the central axis. The first sequence is the forward response sequence, covering the time period after the frequency jump when the magnetic flux begins to produce a tail; the second sequence is the backward response sequence, covering the time period after the frequency transition when the magnetic flux response gradually stabilizes. These two sequences are temporally adjacent and structurally similar, both containing the response behavior of magnetic flux decaying with time delay. The two sequences are named the main sequence and the comparison sequence, respectively, and are standardized according to a unified time unit and data structure format to ensure that each corresponding sliding position of the two sequences is comparable during subsequent sliding.

[0069] The main sequence and the comparison sequence are stepped across the time axis, with a complete response feature comparison performed at each step. The main sequence remains stationary as a reference, while the comparison sequence slides towards the main sequence at fixed time intervals. In each step, data at corresponding time points in the two sequences are compared, including whether the trends in magnetic flux density are consistent, whether the angles of the magnetic induction directions converge, and whether there are overlapping regions in the spatial extension of the magnetic field. The comparison process must cover the entire time interval of the trailing sequence, with each step corresponding to a comparison window. The size of the comparison window is set to cover a complete set of trailing delay response change cycles. Structural analysis is performed on the comparison results within each window, marking time periods with high overlap and recording the start and end times, spatial response range, directional angle difference magnitude, and magnetic flux density peak shift for each time period. These comparison results will serve as the direct basis for subsequent determination of the core region of frequency overlap.

[0070] Among all the comparison results of the sliding windows, the time period with the strongest overlap, the most consistent magnetic flux change, and the smallest directional offset was selected as the core region of frequency overlap. This region is where the responses of the two frequency excitation states overlap in time during the frequency jump process. Specifically, it is manifested as follows: before the magnetic flux response of the main sequence has completely decayed, the magnetic flux response of the comparison sequence has already begun to increase, forming a magnetic field region of superimposed energy; in terms of spatial distribution, the magnetic induction lines show a bidirectional intersecting direction, the magnetic field boundary is blurred, and the magnetic flux peaks show mutual influence; in terms of directional performance, the magnetic induction direction originally dominated by a single excitation has a transition zone, and the direction vector is in a switching state. This core region is usually located in the time overlap segment between the middle of the tail response curve and the initial stage of frequency establishment during the frequency jump process. In the time dimension, this core region does not belong to the stable response before the frequency change, nor to the new stable state after the frequency change, but is in the transition stage of the interaction of the energies of the two frequency excitations. The magnetic flux change generated in this stage often has a certain intensity, but it is not caused by the defect itself, but by the physical process of frequency transition. Therefore, accurately identifying this core region has practical engineering significance.

[0071] All time-point data within the frequency overlap core region are extracted to form a continuous data sequence, which serves as the input basis for subsequent processing. The extraction process must include all time points identified as part of the frequency overlap core region on the time axis. Each time point must fully retain its magnetic flux density value, magnetic induction direction, spatial distribution pattern, and corresponding frequency excitation state. Furthermore, to ensure the continuity and integrity of the data sequence, the time boundaries should be appropriately extended forward and backward during data extraction to supplement the magnetic flux transition information before and after, avoiding the omission of edge response features due to excessively narrow time windows. The extracted data sequence should maintain the same temporal granularity and spatial mapping accuracy as the original tail feature sequence to ensure data alignment and response consistency in subsequent analysis. Ultimately, this data sequence from the frequency overlap core region will serve as crucial data support for subsequent micro-amplitude pre-excitation injection, delayed compensation excitation superposition, inverse marker construction, and breathing-style frequency laminar flow rearrangement rhythm design, establishing a stable intermediate connection point for the entire defect identification and disturbance suppression process.

[0072] Injecting small-amplitude pre-excitation signals and delayed-compensation excitation signals into the frequency overlap core region expands the amplitude difference between the real defect response trajectory and the transient disturbance response trajectory, forming a shadow response sequence.

[0073] To more clearly distinguish between the steady-state leakage magnetic response caused by actual defects within ferromagnetic materials and the non-steady-state magnetic field disturbance response generated during frequency transitions, a pre-excitation signal with small amplitude and time-displacement characteristics, along with a delayed compensation excitation signal, is injected into the identified frequency overlap core region. This subjects the ferromagnetic material to dual temporal excitation within the same frequency transition time period. In this way, the actual defect response and the transient disturbance response exhibit amplitude differences in time response curves, energy distribution, and magnetic flux direction changes. These differences are recorded as a time series, constructing a shadow response sequence. The specific technical steps are as follows:

[0074] A pre-excitation injection time interval is established within the identified frequency overlap core region. The frequency overlap core region is determined by a bidirectional sliding comparison process, and its time range falls within the transitional phase where the energy of the previous excitation frequency has not fully decayed and the magnetic field of the next frequency begins to establish. This region typically represents the central period of the entire frequency switching process, where the magnetic flux distribution is most complex. Within this time interval, taking the start time of the core region as the injection starting point, a small pre-excitation signal is applied while the magnetic flux response is still unstable. The frequency of the pre-excitation signal is consistent with the main excitation signal, but its amplitude is controlled between 5% and 10% of the main excitation signal to avoid affecting the overall magnetic flux structure. The waveform of the excitation signal remains a continuous sinusoidal change to ensure a smooth temporal transition with the main excitation signal. By injecting the pre-excitation signal within the time window before the magnetic flux mutation has fully unfolded, the magnetic domains within the ferromagnetic material can respond earlier, thereby creating a temporal separation between the actual defect response and the transient disturbance response. The real defect response, originating from changes in the material's internal permeability, exhibits a relatively stable flux build-up rate, while the transient disturbance response, directly driven by abrupt frequency changes, shows a higher flux change rate. The pre-excitation-induced early response effect allows for a distinguishable gap between the two types of responses at their time starting points.

[0075] After the pre-excitation injection is completed, a delayed compensation excitation signal is injected in the latter half of the frequency overlap core region. The frequency of the delayed compensation excitation signal is also consistent with the main excitation signal, but its phase lags behind the main excitation signal by a certain angle, controlled between one-twentieth and one-tenth of the phase of the main excitation signal. The injection time is selected within the delay time window after the pre-excitation signal ends, and the duration covers the end of the frequency overlap core region to the early stage of steady-state recovery. The amplitude of the delayed compensation excitation signal is the same as that of the pre-excitation signal, remaining within a low proportion of the main excitation amplitude to prevent the introduction of new magnetic flux saturation or additional interference. The main function of this signal is to introduce a slight disturbance again during the stage when the magnetic flux response tends to stabilize, thereby giving the real defect response an additional extension in time, while the transient disturbance response, due to its energy decaying to a low level, cannot produce a sustained response to the delayed excitation. In this way, the real defect response is represented by a gradually lengthening magnetic flux change curve on the time axis, while the transient disturbance response only shows a peak in the initial short period and then decays rapidly. The pre-excitation and the delayed compensation excitation form a corresponding relationship in time, together constituting the time span extension of the magnetic flux response in the core region of frequency overlap, making the two types of responses clearly distinguishable in terms of time extension.

[0076] During the injection of the pre-excitation signal and the delayed compensation excitation signal, the magnetic flux response of the ferromagnetic material surface and near-surface layer is continuously acquired and recorded. The acquisition range covers the entire time interval of the frequency overlap core region, and the sampling interval is kept within a fixed time step to ensure the temporal continuity of the response curve. The acquired data includes changes in magnetic flux density, magnetization, magnetic flux direction offset angle, and response duration. Specifically, during the pre-excitation phase, the magnetic flux response exhibits a slight early enhancement, with the magnetic flux density rising rapidly and then slightly declining after reaching its peak, at which point the magnetic induction direction shows a slight offset. During the delayed compensation excitation phase, the magnetic flux response exhibits a delayed decay characteristic, with the magnetic flux density rising a second time after the main response decays and gradually stabilizing. By recording the entire response process at each time point, a time-continuous magnetic flux change curve containing the early response segment, the main response segment, and the delayed response segment can be formed. This curve shows different characteristics in the real defect region and the transient disturbance region: the magnetic flux response in the real defect region has a smooth rise and fall pattern, accompanied by a stable direction return process; the magnetic flux response in the transient disturbance region exhibits rapid fluctuations and sharp decay characteristics. By continuously collecting and recording data, this difference can be preserved quantitatively, providing a precise data source for the subsequent construction of shadow responses.

[0077] After obtaining a complete time-continuous magnetic flux response record, a shadow response sequence was constructed based on the effects of pre-excitation and delayed-compensation excitation. The construction process was time-based, organizing the magnetic flux response data chronologically. First, the early response data from the pre-excitation stage was placed at the beginning of the sequence, reflecting the difference between the actual defect response and the transient disturbance response in the initial stage of flux establishment. Then, the response data from the main excitation stage followed, reflecting the stable range response of the magnetic flux. Next, the delayed response data from the delayed-compensation excitation stage was placed at the end of the sequence, forming the delayed-attenuation segment. In this way, the shadow response sequence consists of three continuous parts, corresponding to the early enhancement segment, the stable response segment, and the delayed-attenuation segment. The recorded information at each time point includes magnetic flux density, magnetic flux direction, response duration, and magnetic flux energy decay rate. The time span of the shadow response sequence covers the entire frequency overlap core region, forming a continuous magnetic flux response chain on the time axis. Observation of the sequence reveals that the true defect response forms two stable peaks, corresponding to the response enhancement caused by premature excitation and the response extension caused by delayed excitation, respectively; while the transient disturbance response forms a single spike only at the frequency switching moment and disappears rapidly. Thus, the true defect and the spurious disturbance can be clearly separated in the shadow response sequence. After the shadow response sequence is constructed, its temporal structure and amplitude characteristics can be directly used for subsequent reverse labeling and breathing-style frequency laminar flow rearrangement steps, providing a complete, continuous, and traceable time-domain basis for flux interference suppression during the frequency switching phase.

[0078] The shadow response sequence is mapped onto the sensor array, and the reverse excitation marker unit is set by combining the trailing feature and the amplitude difference feature. Based on the reverse excitation marker unit, an identification anchoring map is constructed to identify and mark potential false response intervals.

[0079] To ensure a complete spatial mapping of the obtained shadow response sequence and to identify and label spurious flux responses caused by frequency jumps within the surface and near-surface regions of ferromagnetic materials, a temporal and spatial integrated mapping relationship is established on the sensor array. Combining flux tailing and amplitude difference characteristics, reverse excitation marker units are set up to form reverse control identifiers in the spatial response distribution. Based on these markers, an identification anchoring spectrum is further constructed, clearly marking potential spurious response intervals within the spectrum. This allows transient disturbances generated during frequency jumps to be clearly defined and isolated in both temporal and spatial dimensions. The specific implementation process is as follows:

[0080] Based on the shadow response sequence, time-continuous magnetic flux response data is mapped onto the spatial coordinate range of the sensor array. The shadow response sequence records the entire process of magnetic flux response within the frequency overlap core region, including the early response stage generated by the pre-excitation, the steady-state response stage generated by the main excitation, and the delayed response stage generated by the delay compensation excitation. The data at each time point includes the magnetic flux density value, magnetic induction direction, energy decay rate, and response duration. To convert the temporal information into a spatial distribution, a one-to-one mapping relationship needs to be established according to the arrangement of the sensor array. The mapping process uses the center sensor of the array as the spatial reference point, projecting the earlier magnetic flux response data (corresponding to the pre-excitation stage) to the sensor position in front of the array center, the later response data (corresponding to the delay compensation stage) to the sensor position behind the array center, and the steady-state response data in the middle period to the sensor position in the middle of the array. In this way, the response characteristics in the time series are completely converted into a spatial distribution, so that the magnetic flux change forms a directional spatial trajectory on the array plane. To ensure the continuity of the mapping, the response data at each time point corresponds to a specific sensor location in space, and the spacing between different sensors is kept proportional to the time interval, so that the entire shadow response sequence presents a continuous transition structure in space. After mapping, the temporal evolution of the shadow response forms a magnetic flux extension path from front to back in the array plane, providing a clear spatial basis for subsequent feature analysis and labeling unit setting.

[0081] Based on the completed mapping, reverse excitation marker units are set within the spatial range of the sensor array, combining flux tailing characteristics and amplitude difference characteristics. Flux tailing characteristics reflect the time delay and energy diffusion characteristics of the flux response, while amplitude difference characteristics reflect the difference in flux energy levels between the true defect response and the transient disturbance response. To establish the spatial correlation between the two, the duration of the flux response of each sensor in the array is first statistically analyzed, and the direction and range of flux extension are determined by comparing the response times of different sensors. When the response time of a certain region is significantly longer than that of other regions, it indicates that there is a persistent tail of residual flux after a frequency jump in that region. Next, the flux amplitude of each sensor is compared with the overall average flux amplitude of the array. When the flux amplitude at a certain location suddenly increases or fluctuates rapidly within a short time interval, a transient disturbance response may exist at that location. The tailing characteristic region and the amplitude anomaly region are spatially superimposed to find the overlapping area, which is the region most likely to form a false flux response. In these regions, reverse excitation marker units are arranged, applying a weak-amplitude magnetic field excitation opposite in phase to the main excitation, causing a phase reversal in the local magnetic flux response. In this way, two types of contrasting responses are formed in the spatial distribution: the real defect response maintains its original phase direction, while the spurious perturbation response exhibits an inverse phase characteristic, thus creating a identifiable contrast between the two in the spatial signal distribution. This reverse excitation process not only produces an energy suppression effect but also establishes an identification benchmark in the spatial response, making spurious response regions clearly distinguishable in the magnetic flux image.

[0082] After the reverse excitation marking unit is established, an identification anchoring map is constructed based on the magnetic flux reverse response characteristics at each location. The identification anchoring map uses the spatial coordinates of the sensor array as the horizontal axis and time as the vertical axis, recording the response behavior of each sensor under reverse excitation. Specifically, the following steps are taken: First, the amplitude of the magnetic flux reverse response of each sensor in the array during reverse excitation is measured, and the amplitude value is numerically marked at the corresponding coordinate position. Then, the duration of the reverse response is recorded and arranged in a time series. Next, the offset angle of the magnetic induction direction during the reverse response is measured and converted into spatial direction change information. Finally, the amplitude, time, and direction parameters are visualized and arranged in a two-dimensional coordinate system to form a spatiotemporally corresponding identification map. In the identification anchoring map, false response areas are characterized by large reverse amplitude, short duration, and significant directional offset, while true defect response areas are characterized by small reverse amplitude, long duration, and gradual directional change. By analyzing the spatial distribution of these characteristic areas, the specific location and time span of the false response in the array can be obtained, thus forming an accurate false response marking area in the map. The construction of this identification anchoring spectrum makes the non-steady-state response behavior during frequency jumps traceable in both time and space, providing an operational basis for subsequent spurious response suppression and defect image optimization.

[0083] After establishing the identification anchor map, the marked spurious response regions are projected back onto the original flux response distribution map, and these regions are spatially labeled and isolated. Specifically, the coordinates of the inverse response regions marked in the identification anchor map are mapped one-to-one with the spatial coordinates of the original flux mapping map, and spurious response regions are distinguished using individual identification markers. In the spatial distribution map, spurious response regions typically exhibit discontinuous distribution and abrupt changes in response amplitude. For these regions, the flux response data within their time range are extracted separately as a spurious response data set and removed from the defect tomography reconstruction dataset, preventing these regions from participating in subsequent flux inversion and imaging. Simultaneously, the regions marked in the identification anchor map can also guide energy distribution adjustments in subsequent frequency laminar flow control processes, applying appropriate magnetic field mitigation operations in these regions to reduce residual transient disturbance energy. In this way, spurious responses are temporally constrained and spatially isolated, retaining only the effective flux information of the true defect response regions in the detection results, thereby obtaining stable, accurate, and verifiable defect spatial distribution results.

[0084] Based on the identified anchoring spectrum, a phased breathing-style frequency laminar flow rearrangement is performed. The frequency transition excitation intensity is reduced in the first stage of unloading and slow release, the transient energy peak is reduced in the middle stage of silent peak reduction, and the new frequency signal is smoothly introduced in the last stage of extended injection and slow conduction, so that the overlapping magnetic flux disturbance energy is gradually dissipated.

[0085] To eliminate the energy superposition effect during frequency transitions and ensure that overlapping magnetic flux disturbance energy is released in an orderly manner over time and dissipates gradually in space, thereby avoiding spurious response signals interfering with defect identification results, the timing and energy flow of frequency transition excitation are precisely rearranged based on the spurious response intervals and energy concentration areas identified in the anchoring spectrum. This step divides the entire frequency switching process into three continuous energy control segments by performing a phased, breathing-style frequency laminar flow rearrangement: a pre-unloading and slow-release stage, a mid-stage silent peak-shaving stage, and a post-delayed injection and slow-conduction stage. In each stage, the excitation amplitude, frequency change rate, and magnetic flux response rhythm are adjusted sequentially to allow the magnetic flux energy to be released and rebalanced gradually. The specific implementation process is as follows:

[0086] The temporal interval and spatial distribution of overlapping magnetic flux disturbance energy are determined based on the identification and anchoring spectrum. The identification and anchoring spectrum records the time period, duration, energy peak value, and spatial coordinates of the spurious response during frequency jumps within the sensor array. To perform breathing-type laminar flow rearrangement, it is necessary to clearly define the location of the energy peak in the temporal dimension and the range of the energy concentration region in the spatial dimension. Specifically, the time point with the largest energy peak in the identification and anchoring spectrum is selected as the frequency jump center point, and a complete excitation cycle before and after it is defined as the energy transition zone. The energy transition zone is further divided into three continuous stages: a rapid energy accumulation phase, a sustained energy peak phase, and a gradual energy decay phase. Simultaneously, the spatial coordinates in the identification and anchoring spectrum are converted into the relative positional distribution between the excitation coil and the sensor array to determine which region of the material surface the magnetic flux energy is mainly concentrated in. Through this bidirectional correspondence between time and space, an energy regulation benchmark is provided for subsequent frequency laminar flow rearrangement, enabling the excitation control process to implement differentiated adjustments for specific energy regions.

[0087] During the initial period of the energy transition region, a slow-release operation is performed to reduce the excitation intensity of the frequency transition and release the initial energy. Specifically, just before the frequency switch begins, the excitation frequency is kept constant, and the amplitude of the excitation current is gradually decreased linearly, causing the magnetic field driving force to gradually transition from a high-energy state to a medium-to-low-energy state. During this process, the magnetic flux density decreases slowly from a high value, and the magnetic domain structure inside the ferromagnetic material gradually changes from a forced alignment state to a partially relaxed state. To avoid the instantaneous collapse of the magnetic flux lines due to a sudden drop in energy, a very short energy release interval is set after each amplitude decrease, allowing the magnetic flux to redistribute within the decrease interval, forming a continuous energy release channel. As the slow-release process continues, the magnetic flux energy diffuses towards the material surface in the form of a gradient, with the energy gradient gradually decreasing from the center outwards. The rate of decrease in magnetic induction intensity is synchronized with the rate of decrease in excitation amplitude. Through this process, the energy peak at the beginning of the frequency transition is dispersed in advance, allowing the magnetic field transition process to unfold smoothly and creating stable energy input conditions for the mid-stage silent peak clipping phase.

[0088] A silent peak-shaving operation is performed in the middle of the energy transition region to reduce the energy peak of overlapping magnetic flux disturbances and achieve magnetic flux energy balance. After the unloading and release phase, the excitation signal amplitude is kept at a low level and maintained constant for a certain period of time to stabilize the magnetic field energy. During this silent phase, the magnetic flux density inside the ferromagnetic material no longer changes rapidly, but achieves a localized balanced energy distribution through natural diffusion. The magnetic flux lines on the material surface change from a densely packed state to a relatively uniform distribution, the magnetic induction direction gradually returns to the main magnetization direction, and the local peak of magnetic flux intensity is weakened. In the latter half of the silent phase, the frequency of the excitation signal is increased slightly to be slightly higher than the original frequency value. At this time, the new excitation frequency generates a slight magnetic flux response oscillation, allowing the magnetic field distribution to re-establish a stable directional flow without triggering new energy superposition. Through this silent peak-shaving operation, the energy peak in the middle of the frequency switching is reduced, the magnetic flux energy achieves balanced diffusion in space, and the overlapping energy is effectively weakened. After this step, the magnetic flux distribution inside the ferromagnetic material enters a stable transition state, providing a balanced energy environment for the introduction of subsequent new frequency signals.

[0089] In the latter part of the energy transition region, a delayed induction and conduction operation is performed to smoothly introduce the new frequency signal and eliminate residual energy. This stage begins at the end of the silent peak clipping stage, gradually increasing the excitation frequency to the target frequency while slowly restoring the excitation amplitude. During this process, the frequency of the excitation signal increases linearly, maintaining a brief period of stability after each increase, allowing the magnetic flux within the ferromagnetic material to rearrange at the new frequency and form a continuous magnetic field structure. As the new frequency signal is gradually introduced, the residual magnetic energy at the original frequency is absorbed and transformed into a stable component of the new frequency magnetic field, the direction of the flux lines gradually unifies, and the fluctuation of magnetic flux density decreases. To avoid energy rebound or overlap, the introduction rate of the new frequency signal is controlled within a safe range of the magnetic flux relaxation time constant, ensuring a continuous and smooth magnetization process. Through this delayed induction and conduction process, the energy in the later stages of the frequency transition is redistributed, the magnetic flux disturbance energy decays naturally along the time axis, and ultimately the magnetic field structure completely transitions to a steady-state distribution at the new frequency.

[0090] After the entire phased breathing-style frequency laminar flow rearrangement process is completed, the decay process of magnetic flux disturbance energy is continuously monitored to ensure that the energy is released unidirectionally in time and gradually dissipates in space. Specifically, in the unloading and slow release phase, the magnetic flux energy diffuses from the high-energy region to the periphery, and the peak amplitude of the magnetic flux density continuously decreases; in the silent peak clipping phase, the magnetic flux energy weakens through balanced diffusion, and the energy peak is dispersed to a wider spatial range; in the extended conduction phase, the residual magnetic energy is reabsorbed by the new frequency excitation, and the magnetic flux energy tends to stabilize in time. Through this process, the overlapping magnetic flux disturbance energy shows a continuous downward trend on the time axis and forms a diffusion structure that gradually dilutes from the center outward on the spatial axis. The energy density in the false response region decreases significantly, and the false magnetic flux signal is completely eliminated. Finally, the magnetic field inside the ferromagnetic material recovers to a stable directional flow, the change in magnetic induction intensity is continuous and smooth, the magnetic flux lines are uniformly distributed, and the magnetic flux response under the new frequency signal is stable and controllable. This phased, breathing-like frequency laminar flow rearrangement process ensures the energy release during frequency switching has temporal continuity and spatial balance, thereby achieving complete dissipation of overlapping magnetic flux disturbance energy and providing a stable magnetic field foundation and reliable signal environment for rapid tomographic detection of defects in ferromagnetic materials.

[0091] This invention establishes a time-stamped band and constructs a transient boundary sequence at the instant of excitation frequency switching, enabling precise time-segmented capture of the magnetic flux response during frequency jumps. This effectively separates the steady-state response from transient disturbance signals during multi-frequency excitation switching. Through short-time frequency sweeping and bidirectional sliding comparison, it accurately identifies the core region of frequency overlap, allowing the true defect response and spurious transient response to form a distinguishable temporal interval. This fundamentally avoids the misjudgment of "phantom defects" caused by frequency overlap, improving the stability and accuracy of defect tomography results.

[0092] This invention introduces pre-excitation and delayed compensation excitation in the frequency overlap core region, and combines this with a reverse excitation marker unit and a staged breathing-style frequency laminar flow rearrangement. This allows magnetic flux disturbance energy to gradually dissipate in time and be released uniformly in space, achieving a smooth energy transition during frequency switching. This process effectively suppresses the interference of transient coupling energy on the leakage magnetic signal, enabling the detection system to maintain a continuous and controllable magnetic flux response under multi-frequency scanning conditions, thereby improving the resolution and reliability of defect identification.

[0093] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux, characterized in that, Includes the following steps: In the AC leakage flux detection system, a time stamp band is set around the instant of excitation frequency switching. The frequency jump interval is determined based on the time stamp band. The start and end positions of the magnetic flux response change are marked within the frequency jump interval to form a transient boundary sequence. Short-time frequency sweeping traction is performed based on transient boundary sequences. During the short-time frequency sweeping traction process, magnetic flux trailing texture information and magnetic flux direction offset information are extracted to construct a continuous trailing feature sequence. A bidirectional sliding alignment is performed on the continuous trailing feature sequence. Based on the sliding alignment results, the frequency overlap core region caused by frequency jump is identified, and the continuous data sequence within the frequency overlap core region is extracted as the basis for subsequent analysis. Injecting small-amplitude pre-excitation signals and delayed-compensation excitation signals into the frequency overlap core region expands the amplitude difference between the real defect response trajectory and the transient disturbance response trajectory, forming a shadow response sequence. The shadow response sequence is mapped onto the sensor array, and the reverse excitation marker unit is set by combining the trailing feature and the amplitude difference feature. The identification anchoring map is constructed based on the reverse excitation marker unit. Based on the identified anchoring spectrum, a phased breathing-style frequency laminar flow rearrangement is performed. The frequency transition excitation intensity is reduced in the first stage of unloading and slow release, the transient energy peak is reduced in the middle stage of silent peak reduction, and the new frequency signal is smoothly introduced in the last stage of extended injection and slow conduction, so that the overlapping magnetic flux disturbance energy is gradually dissipated.

2. The rapid tomographic method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 1, characterized in that, The steps for setting a time stamp band around the instant of excitation frequency switching in an AC leakage magnetic flux detection system, determining the frequency jump interval based on the time stamp band, and forming a transient boundary sequence are as follows: A time stamp band is set at the instantaneous change point of the excitation current waveform, with the starting point being the moment when the previous excitation frequency begins to decay and the ending point being the moment when the next excitation frequency establishes a stable amplitude, so that the time stamp band covers the entire process of magnetic field energy transition. The magnetic flux response signal within the time range covered by the time marker band is continuously acquired to form a time mapping line; Identify the sustained rising point and stabilizing point of the rate of change of magnetic flux density along the time mapping line, and determine the starting and ending positions of the sudden change in magnetic flux response, respectively. Using the start and end points of the mutation as boundaries, magnetic flux change data of the mutation segment within the time-stamped band are extracted, and a continuous transient boundary sequence is constructed in chronological order.

3. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 2, characterized in that, The steps for performing short-time frequency sweeping traction based on transient boundary sequences and constructing continuous trailing feature sequences are as follows: Using the transient boundary sequence as the time reference, a short-time sweep frequency traction interval is defined between the start and end times of the sudden change in magnetic flux response, and the short-time sweep frequency traction interval is divided into several continuous sweep frequency units; Within each frequency sweeping unit, the excitation source frequency is changed sequentially while maintaining a constant sweeping duration, and data on the changes in magnetic flux density, magnetic induction direction, and magnetic flux distribution on and near the surface of the ferromagnetic material are continuously collected. Based on the temporal distribution of the transient boundary sequence, the delayed decay process of the magnetic flux density peak is tracked, magnetic flux tail texture information is extracted, and the main extension direction and decay pattern of the magnetic flux are determined. By combining magnetic flux direction offset information, the trailing texture information is correlated with time, frequency and magnetic flux density to form a continuous trailing feature sequence.

4. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 3, characterized in that, During the short-time frequency sweeping traction process, the excitation frequency changes continuously in a fixed step increment manner. The magnetic flux response data is collected at a constant sampling interval during each frequency stabilization phase. The magnetic flux tail texture information is extracted synchronously based on the magnetic flux density change rate and the direction offset angle. The continuous tail feature sequence is spliced ​​in time with the magnetic flux extension direction as the main axis to form a complete magnetic flux delay response trajectory.

5. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 3, characterized in that, The steps for performing bidirectional sliding alignment and identifying frequency-overlapping core regions on continuous trailing feature sequences are as follows: Based on the continuous tailing feature sequence as input, the sequence is divided into forward response sequence and backward response sequence according to the mutation time point as the central axis, and the time unit and data structure are standardized. The backward response sequence is slid forward along the time axis with a fixed time step, and the magnetic flux density, magnetic induction direction and spatial distribution of the corresponding time points in the two sequences are compared one by one in each sliding step. Among all the sliding window comparison results, the time period with consistent magnetic flux change, minimal directional shift and strongest energy superposition was selected as the core region of frequency overlap. Magnetic flux density, magnetic induction direction, and spatial distribution data for all time points within the frequency overlap core region are extracted to form a continuous data sequence as the basis for subsequent analysis.

6. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 5, characterized in that, The steps for injecting a small-amplitude pre-excitation signal and a delay-compensated excitation signal into the frequency overlap core region to form a shadow response sequence are as follows: A pre-excitation injection time interval is established in the frequency overlap core region. During the unstable magnetic flux response stage, a pre-excitation signal with an amplitude of 5% to 10% of the main excitation signal is applied to enable the magnetic domains inside the ferromagnetic material to respond in advance. Inject a delay compensation excitation signal within the delay time window after the pre-excitation ends, so that the real defect response is extended in time, forming a dual-time excitation structure that echoes the pre-excitation. During the pre-excitation and delayed compensation excitation injection process, magnetic flux density, magnetization, magnetic flux direction offset angle and response duration are continuously collected to form a time-continuous magnetic flux change curve. Based on the response data of pre-excitation and delayed compensation excitation, a shadow response sequence containing an early enhancement segment, a stable response segment, and a delayed decay segment is constructed in chronological order to distinguish between the real defect response and the transient disturbance response.

7. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 6, characterized in that, The steps for mapping the shadow response sequence to the sensor array and constructing the identification anchoring map are as follows: Based on the shadow response sequence, the time-continuous magnetic flux response data are established according to the spatial arrangement of the sensor array to form a correspondence, so that the time information forms a continuous spatial distribution trajectory on the array plane; Combining the characteristics of magnetic flux tailing and amplitude difference, the direction of magnetic flux extension and abnormal region are determined based on the response duration and magnetic flux amplitude distribution, and a reverse excitation marker unit is set in the overlapping region to generate phase reversal; An identification anchoring map is constructed based on the amplitude, duration, and directional offset characteristics of the reverse response of the reverse excitation marker unit, and the spatiotemporal distribution of the false response region is marked in the map. The coordinates of spurious response regions in the identified anchor map are projected back onto the original flux map, and the corresponding regions are spatially labeled and isolated to eliminate spurious responses and retain real defect response data.

8. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 7, characterized in that, The weak-amplitude magnetic field signal applied by the reverse excitation marker unit gradually decays in space along the direction of magnetic flux tailing, and remains in phase opposite to the delay compensation stage of the shadow response sequence in time, so that the false response region forms a stable anti-phase response region, thereby forming a continuous and identifiable spatial reverse response trajectory in the identification anchoring spectrum, which is used to accurately distinguish between the real defect response and the transient disturbance response.

9. The rapid chromatography method for defects in ferromagnetic materials based on AC leakage magnetic flux according to claim 7, characterized in that, The steps for performing a phased breathing-style frequency laminar flow rearrangement and gradually dissipating overlapping magnetic flux disturbance energy based on the identified anchoring pattern are as follows: Based on the identified anchoring spectrum, the time interval and spatial distribution of overlapping magnetic flux disturbance energy are determined, and the time point of energy peak occurrence and the range of energy concentration area are clarified. An unloading and slow-release operation is performed in the front part of the energy transition region. By gradually reducing the excitation current amplitude and setting the energy slow-release interval time, the magnetic flux energy diffuses in a gradient form. In the middle of the energy transition region, a silent peak-shaving operation is performed to keep the excitation amplitude constant and reduce the peak magnetic flux energy through natural diffusion, so that the magnetic field distribution tends to be balanced. In the latter part of the energy transition zone, a delayed injection and slow conduction operation is performed to control the excitation frequency to gradually increase to the target frequency and slowly restore the excitation amplitude, so that the magnetic flux energy is converted smoothly. Throughout the laminar rearrangement process, the decay trend of magnetic flux disturbance energy is continuously monitored to ensure that the energy continues to decrease along the time axis and gradually diffuses in space until it completely dissipates.

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