A method for evaluating the quality of resistance spot welding points of a motor vehicle body

Abstract

The application discloses a kind of automobile body resistance spot welding spot quality evaluation methods, it is related to welding quality evaluation technical field, including obtaining dynamic resistance time sequence signal in welding process;Obtain the time sequence signal of physical quantity synchronous with nugget growth process;By comparing the preset feature point of the dynamic resistance time sequence signal with the preset feature point of the physical quantity time sequence signal, the characteristic time sequence relationship between the two is determined;The change trend of the characteristic time sequence relationship in continuous multiple welding spots is analyzed, the quality is judged by comparing the time relationship of two signals by simultaneously detecting electric signal and physical signal in the application, and the change trend of this time relationship is tracked, so that evaluation result is more accurate and more stable.

Description

Technical Field

[0001] This invention belongs to the field of welding quality assessment technology, specifically a method for assessing the quality of resistance spot welds on automobile bodies. Background Technology

[0002] Resistance spot welding is a core joining process in automotive body assembly, and the quality of its welds directly affects the structural strength, safety, and durability of the vehicle body. Current weld quality assessment technologies are mainly divided into two categories: offline inspection and online monitoring. Offline inspection, such as ultrasonic testing and X-ray inspection, must be performed after welding, which suffers from low inspection efficiency, lack of real-time feedback, and potential damage to the vehicle body. Online monitoring technologies largely rely on a single dynamic resistance signal, judging quality by extracting features such as signal peaks and slopes. However, dynamic resistance signals are easily affected by factors such as electrode wear, workpiece surface cleanliness, and power grid fluctuations, and single-mode information cannot fully reflect the true state of weld nugget formation.

[0003] Some solutions attempt to introduce multi-physical quantity signals to assist in evaluation, but most remain at the level of parallel signal acquisition, failing to establish characteristic correlations between different signals related to weld nugget growth, and thus unable to capture the dynamic process of weld nugget formation through signal temporal coordination. Furthermore, existing technologies often focus on isolated analysis of individual weld points, neglecting the quality correlations between consecutive weld points, as well as the commonalities and individual differences in multi-station production lines. This makes it difficult to identify common upstream factors or individual station degradation issues, and lacks targeted correction and maintenance mechanisms, resulting in a high rate of quality misjudgment and failing to meet the efficient and precise quality control requirements of automotive production lines. Summary of the Invention

[0004] The purpose of this invention is to provide a method for evaluating the quality of resistance spot welding joints in automobile bodies, so as to solve the problems mentioned in the background art.

[0005] A method for evaluating the quality of resistance spot welds on automobile bodies, comprising:

[0006] Acquire dynamic resistance timing signals during the welding process;

[0007] Acquire the time-series signals of physical quantities synchronized with the nucleus growth process;

[0008] The characteristic timing relationship between the two is determined by comparing the preset characteristic points of the dynamic resistance timing signal with the preset characteristic points of the physical quantity timing signal.

[0009] Analyze the changing trend of the aforementioned temporal relationship across multiple consecutive solder joints;

[0010] Based on the characteristic temporal relationship itself and its changing trend, the quality grade of the weld and the stability state of the welding system are determined.

[0011] Specifically, existing solder joint quality assessment methods only detect a single electrical signal and use fixed judgment criteria. When the electrodes wear out, solder joints that were originally qualified are easily misjudged as unqualified. Therefore, by simultaneously detecting electrical and physical signals and judging the quality by comparing the time relationship between the two signals, and tracking the changing trend of this time relationship, the assessment results can be made more accurate and stable.

[0012] Furthermore, determining the temporal relationships of the features includes:

[0013] From the dynamic resistance timing signal, the critical point at which the rate of resistance decrease changes from increasing to decreasing is identified as the first time marker for the start of metal melting.

[0014] From the time-series signal of the physical quantity, the critical point at which the rate of increase of the signal value changes from increasing to decreasing is identified as the second time marker for the beginning of the expansion of the molten core volume.

[0015] Calculate the delay time between the second time identifier and the first time identifier, and use it as the quantization value of the characteristic timing relationship.

[0016] The existing technology does not clearly correlate the signal features (such as current magnitude and resistance value) extracted with the actual formation process of the weld nugget, making it difficult to truly reflect the welding quality. The above technical solution can accurately locate the two key moments of "metal begins to melt" and "weld nugget begins to expand". The time difference between the two can be used as the basis for judgment, directly reflecting the efficiency of heat conversion into the weld nugget. The physical meaning is clear and the sensitivity is high.

[0017] Further analysis of the changing trends includes the following steps:

[0018] In continuous production, the delay time of each weld point is recorded according to the welding sequence, forming a time sequence.

[0019] Calculate the average delay time of the latest consecutive weld points in the timing sequence, and use it as the reference value for the current operating condition;

[0020] Calculate the average delay time of the earliest consecutive solder joint in the timing sequence, and use it as the initial reference value;

[0021] The difference between the current operating condition reference value and the initial reference value is defined as the system cumulative offset;

[0022] When the cumulative offset of the system continuously deviates from zero and its variation exceeds a preset stability threshold, it is determined that the welding system has a deterministic offset trend.

[0023] This step compares the average delay time over a current period with the average delay time of the device in its initial state, and uses the cumulative offset to determine whether the device has undergone deterministic degradation. This method has a low false alarm rate and reliable judgment results.

[0024] Furthermore, the solder joint quality grade is determined based on the characteristic temporal relationship and its changing trend, including the following steps:

[0025] A dynamic acceptable range is set centered on the current operating condition benchmark value;

[0026] The real-time delay time of the current solder joint is compared with the dynamic acceptable range. If it exceeds the range, it is directly judged as unacceptable.

[0027] This invention focuses on the actual performance of the equipment under its current state, dynamically sets the acceptable range, and matches the judgment criteria with the actual capabilities of the equipment. This reduces misjudgments and extends the effective service life of the electrodes while ensuring quality.

[0028] Furthermore, for solder joints that do not exceed the dynamic acceptable range, determining their quality level also includes the following steps:

[0029] Calculate the instantaneous deviation between the solder joint delay time and the current operating condition reference value;

[0030] Obtain the average rate of change of the system's cumulative offset over a recent period;

[0031] When both the instantaneous deviation and the average rate of change are small, the weld is rated as a high stability level.

[0032] When either the instantaneous deviation or the average rate of change is large, the weld is classified as a level of concern.

[0033] When both the instantaneous deviation and the average rate of change are large, the weld is classified as a critical level.

[0034] Specifically, traditional methods can only determine whether a weld is "currently qualified" and cannot predict whether the weld is prone to early failure during the subsequent use of the vehicle. Therefore, this technical solution classifies currently qualified welds into three levels: high stability, concern, and critical, based on the degree of deviation from the normal value and the speed of equipment degradation. This allows for early prediction of which welds have potential risks and provides OEMs with more complete quality information.

[0035] Furthermore, the nature of the deterministic offset trend is determined based on the direction of the system's cumulative offset:

[0036] If the cumulative offset of the system continues to develop in the positive direction, it is determined that there is a gradual loss of welding thermal efficiency.

[0037] If the cumulative offset of the system continues to develop in the negative direction, it is determined that the basic conditions of the welding process have undergone a systematic change.

[0038] This invention directly determines the fault type based on whether the delay time increases or decreases. An increase indicates a decrease in thermal efficiency (electrode wear), while a decrease indicates a change in basic conditions (power supply or contact status), thus enabling rapid location of the root cause of the fault.

[0039] Furthermore, it also includes the following steps:

[0040] When the thermal efficiency is determined to be progressively reduced, the output maintenance command is directed to the inspection and handling of the welding electrode working surface.

[0041] When it is determined that the basic conditions have changed systematically, the output maintenance command is directed to check the welding power supply output characteristics or the workpiece-electrode contact interface.

[0042] Based on the specific fault type diagnosed, this invention directly outputs inspection instructions targeting specific components, such as checking the electrode end face, measuring power output, or inspecting workpiece contact. Maintenance personnel can then perform precise operations upon receiving the instructions.

[0043] Furthermore, the physical quantity time sequence signal is the electrode axial thermal expansion displacement signal; the step of identifying the second time marker specifically includes: performing smoothing filtering on the displacement signal, and then locating the peak point on its derivative curve, which is the second time marker.

[0044] This step first uses smoothing filtering to remove noise, then calculates the derivative to find the point of fastest change, thus stably and accurately locating the starting point of the molten core expansion.

[0045] Furthermore, the method also includes the step of:

[0046] The evaluation method is performed simultaneously on multiple parallel workstations on the car body welding production line.

[0047] Compare the consistency of the cumulative system offset obtained from each workstation in terms of both numerical value and development stage;

[0048] If most workstations exhibit consistency, it is determined that there are common influencing factors upstream of the production line.

[0049] If the performance of each workstation is discrete, it is determined that the individual characteristics of the workstation have deteriorated.

[0050] Furthermore, it also includes:

[0051] Based on the determination results of the common influencing factors or individual characteristic degradation, a systematic correction plan and an individualized maintenance queue are generated respectively, wherein the systematic correction plan is executed in priority over the individualized maintenance queue.

[0052] Compared with the prior art, the beneficial effects of the present invention are:

[0053] This invention uses the delay time from the start of metal melting to the start of weld nugget expansion as a quality indicator. The length of this delay time is directly determined by the heat conduction efficiency and the interface contact state, and has a unique correlation with the weld strength. Experiments have shown that under interference such as electrode wear and coating changes, the traditional resistance curve has deviated from the standard shape, while the delay time still stably reflects the actual size of the weld nugget, possessing an anti-interference capability that traditional characteristic quantities cannot replace. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the method framework structure of the present invention. Detailed Implementation

[0055] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] Please see Figure 1 This application provides a method for evaluating the quality of resistance spot welds on automotive bodies, including:

[0057] Acquire dynamic resistance timing signals during the welding process;

[0058] Specifically, by installing resistance sensors on the electrode arms of the resistance spot welding equipment, dynamic resistance data of the entire welding process can be collected in real time.

[0059] The sensor sampling frequency is set to no less than 1kHz (the lowest frequency that can fully reflect the resistance change process), and the sampling duration covers the entire welding cycle from the start of power-on to the end of power-off, ensuring that the details of resistance change during the formation, growth and cooling stages of the weld nugget are captured.

[0060] It should be noted that the sensor measurement accuracy is no less than 0.1Ω (meeting the detection requirements for minute resistance changes). The data acquisition process employs the anti-interference circuit specified in GB / T26951-2011 standard to suppress power grid noise and electromagnetic coupling interference. The acquired dynamic resistance time-series signal undergoes preprocessing, including power frequency notch filtering, moving average denoising (window size set to 5 sampling points), and baseline correction, to remove noise and drift components from the signal, thereby improving the accuracy and reliability of subsequent feature extraction.

[0061] Acquire the time-series signals of physical quantities synchronized with the nucleus growth process;

[0062] It is understood that physical quantities that grow synchronously with the melt nucleus can directly or indirectly reflect the temperature, size and mechanical state of the melt nucleus. In this embodiment, the physical quantity timing signal is the electrode axial thermal expansion displacement signal. This signal is acquired by a high-precision displacement sensor installed in the axial direction of the electrode rod, with a measurement accuracy of not less than 0.01 mm and a sampling frequency that is consistent with the dynamic resistance signal (not less than 1 kHz).

[0063] To achieve synchronization with the nucleus growth process, the electrode axial thermal expansion displacement signal and the dynamic resistance signal are started at the same trigger time and the timestamps are aligned through the device synchronization interface. The time synchronization error does not exceed 10μs, so that the signals maintain a stable time correspondence.

[0064] In the preprocessing stage, the electrode axial thermal expansion displacement signal is processed by Gaussian filtering (standard deviation set to 0.5), which can retain the effective information related to the growth of the melt nucleus in the signal, laying the foundation for subsequent characteristic time series relationship analysis.

[0065] By comparing the preset characteristic points of the dynamic resistance timing signal with the preset characteristic points of the physical quantity timing signal, the characteristic timing relationship between the two is determined.

[0066] Specifically, the preset feature points are selected based on the physical mechanism of melt nucleus growth. The preset feature points of the dynamic resistance timing signal include the initial resistance value, the minimum resistance value, the peak resistance value, and the occurrence time of each feature point.

[0067] The preset feature points of the physical quantity time-series signal (electrode axial thermal expansion displacement signal) include the initial displacement value, peak displacement value, peak displacement derivative value, and the occurrence time of each feature point. Feature point extraction adopts a combination of threshold and slope change. Taking the minimum resistance value as an example, the slope judgment threshold is set to -0.5Ω / ms. When the slope is less than the threshold for three consecutive sampling points and the slope of subsequent sampling points reverses, the point is determined as the target feature point and the corresponding time is recorded.

[0068] Furthermore, in order to more accurately reflect the intrinsic physical process of molten nucleus formation and determine characteristic timing relationships, including: identifying the critical point from the dynamic resistance timing signal where the rate of resistance decrease changes from increasing to decreasing, as the first time marker for the start of metal melting;

[0069] Specifically, the rate of decrease in resistance is calculated using the sliding window method. Five consecutive sampling points are selected to form a calculation window, and the rate of change is determined by the slope difference between adjacent windows. The threshold for determining the rate of change is set to 0.1Ω / (ms²). When a sampling point satisfies the condition that the slope difference of the previous window is positive, the slope difference of the current window is negative, and the absolute value is greater than the threshold, that point is determined to be the critical point where the rate of decrease in resistance changes from increasing to decreasing. This critical point corresponds to the physical process in which the metal begins to melt, and its timestamp is the first time marker.

[0070] From the time-series signals of physical quantities, the critical point at which the rate of increase of the signal value changes to decrease is identified as the second time marker for the beginning of the expansion of the molten core volume.

[0071] The steps for identifying the second time marker specifically include: smoothing and filtering the electrode axial thermal expansion displacement signal (using a 5-point moving average smoothing filter), then calculating the first derivative curve of the displacement signal using numerical differentiation (with a differentiation step size of 2 sampling points), locating the peak point on the derivative curve (setting the peak value threshold to 0.005 mm / ms; the point that exceeds this threshold and where the derivative of the sampling points before and after it decreases is the peak point), and the timestamp corresponding to the peak point is the second time marker.

[0072] It is understandable that the peak value of the derivative of the electrode axial thermal expansion displacement signal directly corresponds to the maximum value of the melt core volume expansion rate, which is the core physical indicator that the melt core begins to expand significantly.

[0073] By employing steps such as smoothing filtering to eliminate signal noise, numerical differentiation to calculate the derivative curve, and threshold determination to locate the peak point, the second time marker can be accurately extracted, avoiding marker offset caused by noise interference and ensuring the accuracy of the feature time sequence relationship.

[0074] Calculate the delay time between the second time marker and the first time marker, and use it as the quantization value of the characteristic temporal relationship;

[0075] In one implementation of this step, the difference between the second time marker and the first time marker is used as the delay time. This delay time directly reflects the time interval from the start of metal melting to the start of molten core expansion. It can quantitatively characterize the continuity and stability of molten core growth and is the core quantitative indicator of characteristic time sequence relationship.

[0076] In addition to these, the temporal relationship of features also includes conventional time difference parameters, correlation parameters, and temporal sequence consistency indicators. The time difference parameter is used to reflect the temporal interval between different feature points, reflecting the sequential relationship of various physical changes during the nucleus growth process.

[0077] The correlation parameter is used to characterize the correlation strength between dynamic resistance characteristics and electrode axial thermal expansion displacement characteristics. The correlation threshold is set to 0.8. If the correlation is greater than this threshold, the correlation strength is considered to be good.

[0078] The temporal sequence consistency indicator is used to determine whether each feature point conforms to the temporal pattern corresponding to the normal growth of the melting nucleus. The threshold for the number of abnormal occurrences is set to 3 times. If the temporal disorder occurs 3 times in a row, it is judged as abnormal.

[0079] In one implementation of this step, a distributed computing architecture is used to complete the parameter calculation, ensuring the analysis efficiency of a single set of signals (the calculation time does not exceed 10ms). The coordinated change relationship between dynamic resistance and electrode axial thermal expansion displacement is fully characterized by multi-dimensional time-series parameters, providing a reliable basis for quality assessment.

[0080] The analysis examines the changing trends of characteristic temporal relationships across multiple consecutive solder joints. Specifically, the number of consecutive solder joints is set to 50 (determined based on the welding cycle time of the production line and the sensitivity of fault identification, ensuring that long-term changes are reflected without compromising real-time performance due to excessive data volume).

[0081] Trend analysis was conducted separately for time difference parameters, correlation parameters, time sequence consistency indicators, and the aforementioned delay time, using a combination of linear fitting and prediction models.

[0082] The analysis of the trend includes the following steps: During continuous production, the delay time of each weld point is recorded according to the welding sequence to form a time sequence; the average delay time of the latest continuous weld points (the latest 10 weld points) in the time sequence is calculated as the current working condition reference value; the average delay time of the earliest continuous weld points (the earliest 10 weld points) in the time sequence is calculated as the initial reference value.

[0083] The difference between the current operating condition baseline value and the initial baseline value is defined as the system cumulative offset;

[0084] The stability threshold is set to ±20% of the initial reference value. When the cumulative offset of the system continues to deviate from zero and the variation of 10 consecutive weld points exceeds the stability threshold, it is determined that the welding system has a deterministic offset trend.

[0085] The nature of the deterministic offset trend is determined based on the direction of the cumulative offset of the system: if the cumulative offset of the system continues to develop in the positive direction, it is determined that there is a gradual loss of welding thermal efficiency.

[0086] If the cumulative offset of the system continues to develop in the negative direction, it is determined that the basic conditions of the welding process have undergone a systematic change.

[0087] When the thermal efficiency is determined to be progressively reduced, the output maintenance command is directed to the inspection and handling of the welding electrode working surface.

[0088] When it is determined that the basic conditions have changed systematically, the output maintenance command is directed to check the welding power supply output characteristics or the workpiece-electrode contact interface.

[0089] Understandably, the current operating condition baseline value is used to characterize the latest working state of the welding system, while the initial baseline value is used to characterize the initial stable state of the equipment. The difference between the two, forming the system cumulative offset, can intuitively reflect gradual anomalies such as electrode wear, pressure decay, and changes in power supply characteristics. When the system cumulative offset continues to develop towards a positive value, it means that the interval between metal melting and weld nugget expansion is prolonged. The core reason is a decrease in welding thermal efficiency (such as electrode oxidation, reduced thermal conductivity due to wear on the working surface, and power supply decay). Therefore, maintenance instructions focus on cleaning, grinding, or replacing the welding electrode working surface.

[0090] When the value turns negative, it indicates that the connection between melting and expansion is abnormally accelerated. The essence is that the basic welding conditions have changed (such as abnormal stability of the output voltage / current of the welding power supply, sudden change in contact resistance caused by oil and rust on the workpiece surface, abnormal electrode pressure, etc.). Therefore, the maintenance instruction is directed to the calibration of the output characteristics of the welding power supply or the cleaning and assembly inspection of the workpiece-electrode contact interface. This targeted maintenance instruction can shorten the troubleshooting time and improve the efficiency of production maintenance.

[0091] The method also includes the following steps: simultaneously performing the evaluation method on multiple parallel stations (no less than 3 parallel stations) on the body welding production line, and collecting the cumulative system offset of each station in real time;

[0092] Set the consistency judgment threshold to ±10% of the initial baseline value, and compare the consistency of the cumulative system offset obtained by each station in terms of numerical value (difference not exceeding the consistency judgment threshold) and development stage (both in positive / negative offset, offset duration difference not exceeding 5 solder joints);

[0093] If more than 60% of the workstations (the majority of workstations) show consistency with the above values ​​and the development stage, it is determined that there are common influencing factors in the upstream of the production line (such as fluctuations in the unified power supply system, abnormal material / cleanliness of batch workpieces, unstable pressure of the shared air source of the production line, etc.).

[0094] If the numerical difference of the cumulative offset of each workstation system exceeds the consistency judgment threshold, and the development stage is discrete (partly positive, partly negative, or the difference in the duration of offset exceeds 10 solder joints), it is judged as a degradation of the individual characteristics of the workstation (such as wear of a single workstation electrode, independent power supply failure, positioning deviation of workstation-specific tooling, etc.).

[0095] Based on the determination results of common influencing factors or individual characteristic degradation, a systematic correction plan and an individualized maintenance queue are generated respectively, with the systematic correction plan taking precedence over the individualized maintenance queue.

[0096] The systematic correction plan includes common problem types, scope of impact, correction parameter adjustment schemes (such as adjusting the welding voltage compensation value by ±3% for power supply system fluctuations, and adding surface pretreatment processes for abnormal cleanliness of batch workpieces), execution process and time limit (starting within 2 hours after judgment and completing within 4 hours).

[0097] Individualized maintenance queues are sorted by the severity of workstation anomalies (workstations with severe threshold exceedances are given priority, while those with minor threshold exceedances are postponed), and the maintenance items (such as electrode replacement, power calibration, tooling adjustment), responsible persons, and completion deadlines for each workstation are clearly defined (workstations with severe anomalies should respond within 1 hour, and workstations with minor anomalies should be completed within 8 hours).

[0098] This multi-station synchronous evaluation, consistency comparison, and correction and maintenance mechanism can overcome the limitations of single-station evaluation: first, it identifies and locates unified problems upstream of the production line through common factors, and then solves them centrally with a systematic correction plan to avoid repeated maintenance.

[0099] Then, individualized maintenance queues are used to address anomalies at individual workstations, ensuring the rational allocation of resources. The design of prioritizing the execution of systematic correction plans can quickly eliminate the impact on most workstations, reduce batch quality risks, and then address individual issues, further improving the maintenance efficiency and control effectiveness of multi-workstation production lines, adapting to the needs of batch production scenarios.

[0100] For the time difference parameter and the delay time, the slope of the parameter change in continuous solder joints is obtained by linear fitting. The slope threshold is set to ±5% of the initial baseline value per solder joint. If the slope exceeds this threshold, it is determined that there is a significant trend of change.

[0101] Meanwhile, the LSTM prediction model is used to predict the parameters of the next 5 solder joints. The deviation threshold is set to ±10% of the initial baseline value. The degree of change is judged by the deviation between the predicted value and the historical average.

[0102] For correlation parameters, the standard deviation of parameters in consecutive solder joints is calculated, and a standard deviation threshold of 0.1 is set. If the standard deviation exceeds this threshold, the stability of the feature correlation strength is considered to have decreased. For temporal sequence consistency indicators, the number of times an anomaly indicator appears is counted, and a threshold of 5 times is set. If anomalies occur 5 times consecutively, the temporal sequence pattern is considered to be unstable.

[0103] It should be noted that the trend analysis process is equipped with a data caching mechanism. When communication is abnormal, the relevant data of the last 30 minutes is cached. After communication is restored, the analysis process is automatically completed to avoid data loss and evaluation errors. Through the trend analysis of continuous weld points, potential anomalies in the welding system can be detected in advance.

[0104] Based on the characteristic temporal relationship itself and its changing trend, determine the quality grade of the weld and the stability state of the welding system;

[0105] Specifically, weld quality is categorized into three levels: excellent, qualified, and unqualified. Welding system stability is categorized into three states: stable, warning, and fault. The determination is based on a pre-established hierarchical rule base. The rule base was trained using over 1000 weld sample sets under different working conditions, enabling it to adapt to welding scenarios with different vehicle models and sheet thicknesses.

[0106] Determining the weld quality grade based on the characteristic temporal relationship and its changing trend includes the following steps: setting a dynamic qualified range centered on the current working condition benchmark value;

[0107] When the system is stable, the interval width is set to ±15% of the current operating condition baseline value; when the system is under warning, the interval width is narrowed to ±8% of the current operating condition baseline value.

[0108] The real-time delay time of the current solder joint is compared with the dynamic acceptable range. If it exceeds the range, it is directly judged as unacceptable.

[0109] It should be noted that the dynamic qualified range is set based on the current operating condition benchmark value and the system stability state and is dynamically adjusted. The design consideration is that when there is a gradual deviation in the welding system, a fixed qualified range is prone to misjudgment, while a dynamic qualified range can adapt to the current working state of the system, which can avoid excessive stringency affecting production efficiency, and can accurately identify abnormal weld points that exceed the current operating condition capabilities.

[0110] For solder joints that do not exceed the dynamic acceptable range, determining their quality level also includes the following steps: calculating the instantaneous deviation between the solder joint delay time and the current operating condition reference value; obtaining the average rate of change of the system cumulative offset over a recent period (the most recent 10 solder joints);

[0111] Set a smaller threshold and a larger threshold for the instantaneous deviation. The smaller threshold is 30% of the width of the dynamic acceptable range, and the larger threshold is 70% of the width of the dynamic acceptable range.

[0112] The rate threshold for the average rate of change is set to ±3% of the initial reference value per weld point (determined based on the long-term drift tolerance allowed by the welding system).

[0113] When the instantaneous deviation is less than the smaller threshold and the average rate of change is less than the rate threshold, the weld is rated as a high stability level.

[0114] When the instantaneous deviation is greater than the larger threshold, or the average rate of change is greater than the rate threshold, the weld is classified as a level of concern.

[0115] When the instantaneous deviation is greater than a larger threshold and the average rate of change is greater than a rate threshold, the weld is rated as critical.

[0116] It is understandable that instantaneous deviation reflects the degree of instantaneous fluctuation of a single weld point, while the average rate of change reflects the long-term drift trend of the welding system. Combining the two for classification can simultaneously take into account both the quality of individual points and the system status, thus achieving more refined quality hierarchical management.

[0117] A high stability level indicates that the solder joint quality and system status are both in an ideal state. A concern level indicates that there are minor abnormalities that require continuous observation. A critical level indicates that the system is approaching the quality boundary and requires early intervention.

[0118] First, a preliminary quality assessment is made based on the characteristic temporal relationship. If the real-time delay time exceeds the dynamic acceptable range, it is directly judged as unacceptable.

[0119] For solder joints that do not exceed the range, they are further divided into high stability level, attention level and critical level according to instantaneous deviation and average rate of change, and comprehensively mapped to the final quality level of excellent, qualified and unqualified (high stability level corresponds to excellent, attention level corresponds to qualified, critical level corresponds to unqualified edge, and secondary verification is required).

[0120] In another implementation method, the preliminary judgment results are corrected by combining the trend of change, the consistency judgment results of multiple workstations and the correction and maintenance plan, and the system stability is determined.

[0121] Set the minor threshold to ±10% of the initial baseline value and the severe threshold to ±25% of the initial baseline value. When the trend shows no obvious abnormalities and the performance of multiple workstations is consistent, the system is considered stable, and the initial quality level is retained.

[0122] If common upstream influencing factors are identified, a systematic correction plan is generated simultaneously and implemented with priority (such as uniformly calibrating the power supply system and checking the quality of batch workpieces). After the implementation is completed, the status of each workstation is reassessed.

[0123] When some parameters show a slight abnormal trend, the slope of the delay time change, or the cumulative offset of the system exceeds the slight threshold, the system status is in warning mode, and the grade of qualified solder joints is downgraded.

[0124] If it has been determined that the welding thermal efficiency is gradually reduced, the welding power parameters can be adjusted first (adjustment range is ±5%) and an electrode working surface inspection command can be output.

[0125] If the basic conditions are systematically changed, then the electrode pressure (allowable deviation ±0.2MPa) and workpiece assembly status are checked, and power supply characteristics or contact interface inspection instructions are output.

[0126] If the performance of multiple workstations is discrete, an individualized maintenance queue is generated and executed according to priority. When multiple parameters show significant abnormal trends, the slope of the delay time change, or the cumulative system offset exceeds the critical threshold, or a deterministic offset trend is determined, the system status is faulty, all solder joint levels are downgraded accordingly, and a shutdown inspection command is triggered.

[0127] Prioritize the implementation of systematic correction plans (if common factors exist), then handle remaining anomalies according to individualized maintenance queues, and quickly locate fault points by combining the nature of the offset trend with maintenance instructions.

[0128] It should be understood that the judgment process includes a secondary verification step to review cases where the real-time delay exceeds the dynamic acceptable range, is rated as critical, or the system status is faulty (the signal is collected three times for verification) to avoid misjudgment caused by a single abnormal signal.

[0129] Simultaneously, an adaptive adjustment mechanism for welding parameters and a maintenance execution tracking mechanism are established. When the system is in an early warning state, the corresponding welding parameters are automatically adjusted based on the nature of the deterministic offset trend and the consistency results of multiple workstations.

[0130] After maintenance is completed, continuous monitoring of no fewer than 10 solder joints at each workstation is conducted to confirm whether the abnormalities have been eliminated, thus forming a complete closed loop from assessment to judgment to maintenance and verification. In this embodiment, the above method can achieve integrated multi-workstation synchronous assessment, single-point solder joint quality grading, system status monitoring, fault root cause (common / individual) location, targeted correction maintenance, and execution tracking. While ensuring the reliability of individual solder joints, it improves the management efficiency, maintenance convenience, and quality stability of multi-workstation production lines, and reduces the probability of batch quality problems.

[0131] This invention compares the dynamic resistance signal and the electrode thermal expansion displacement signal during the welding process using a time sequence analysis of characteristic points. The time difference between these two key physical events is used as the criterion for judging weld quality. Based on the systematic variation of this time difference in consecutive welds, it simultaneously completes weld quality grading, equipment fault diagnosis, and production line maintenance decisions. Specifically, it manifests as follows:

[0132] First, the inherent physical timing of weld nugget formation replaces signal amplitude as a quality criterion. Traditional methods rely on the peak value or area of ​​current and resistance, which have no direct causal relationship with whether the weld nugget grows normally. This invention uses the delay time from the start of metal melting to the start of weld nugget expansion as a quality indicator. The length of this delay time is directly determined by heat conduction efficiency and interface contact state, and has a unique correlation with weld strength. This ensures that even under interference such as electrode wear and coating changes, when the traditional resistance curve deviates from the standard shape, the delay time still stably reflects the actual size of the weld nugget, possessing an anti-interference capability that traditional characteristic quantities cannot replace.

[0133] Second, this invention uses cumulative offset instead of a fixed threshold to achieve early identification of equipment degradation. Traditional methods independently judge each solder joint, and equipment performance decline is only manifested as a slow decrease in the pass rate, failing to quantify the degradation process. This invention compares the average delay time of the current consecutive solder joints with the average delay time of the equipment in its initial stage, defining a system cumulative offset. This offset exhibits a monotonically unidirectional drift with electrode wear and power supply aging, and its slope is directly related to the degradation rate, providing early warning of the equipment entering the degradation phase while all solder joints are still qualified. This identification method requires no statistical model or training samples, and can complete the status assessment using only the equipment's own historical data.

[0134] Third, the physical root cause of the fault is uniquely mapped by the direction of offset. This invention discovered and verified the following correspondence: when the delay time continuously increases, the fault source uniquely points to the increased heat dissipation caused by the increased diameter of the electrode working surface; when the delay time continuously decreases, the fault source uniquely points to abnormal welding circuit impedance or changes in workpiece clamping stiffness. This mapping relationship is derived from the welding heat balance equation and has physical determinism. Based on this, maintenance personnel can directly replace the electrode or check the power supply and fixture without sequential troubleshooting, reducing fault location time from hours to minutes.

[0135] Fourth, it achieves zero-cost differentiation between common and individual faults by leveraging the consistency of trends across multiple workstations. Traditional production lines, when experiencing quality fluctuations at multiple workstations simultaneously, cannot quickly determine whether the fault lies in a single workstation or an upstream common system anomaly. This invention compares the onset time and rate of change of the cumulative offset at each workstation. If all workstations offset synchronously, the problem is identified as a common system issue such as cooling water or the power grid; if the offset occurs only at an individual workstation, the problem is identified as a problem with the electrode or fixture at that workstation. This method requires no additional sensors and can complete production line-level fault tracing using only existing quality data.

[0136] Fifth, the invention eliminates false rejection by replacing fixed standards with dynamic acceptable boundaries. This invention sets an acceptable range centered on the current operating condition benchmark value. The range width is determined by process specifications, and the center position automatically shifts with the equipment status. When the electrode enters the later stages of wear, the acceptable range shifts to the right, ensuring that welds still possessing connection strength are no longer rejected due to deviation from the new electrode standard. This method extends the single-use lifespan of the electrode without compromising structural reliability.

[0137] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for evaluating the quality of resistance spot welds on automobile bodies, characterized in that, include: Acquire dynamic resistance timing signals during the welding process; Acquire the time-series signals of physical quantities synchronized with the nucleus growth process; The characteristic timing relationship between the two is determined by comparing the preset characteristic points of the dynamic resistance timing signal with the preset characteristic points of the physical quantity timing signal. Determining the temporal relationships of features includes: From the dynamic resistance timing signal, the critical point at which the rate of resistance decrease changes from increasing to decreasing is identified as the first time marker for the start of metal melting. From the time-series signal of the physical quantity, the critical point at which the rate of increase of the signal value changes from increasing to decreasing is identified as the second time marker for the beginning of the expansion of the molten core volume. Calculate the delay time between the second time identifier and the first time identifier, and use it as the quantized value of the characteristic temporal relationship; Analyze the changing trend of the aforementioned temporal relationship across multiple consecutive solder joints; Based on the characteristic temporal relationship itself and its changing trend, the quality grade of the weld and the stability state of the welding system are determined.

2. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 1, characterized in that, Analyzing trends involves the following steps: In continuous production, the delay time of each weld point is recorded according to the welding sequence, forming a time sequence. Calculate the average delay time of the latest consecutive weld points in the timing sequence, and use it as the reference value for the current operating condition; Calculate the average delay time of the earliest consecutive solder joint in the timing sequence, and use it as the initial reference value; The difference between the current operating condition reference value and the initial reference value is defined as the system cumulative offset; When the cumulative offset of the system continuously deviates from zero and its variation exceeds a preset stability threshold, it is determined that the welding system has a deterministic offset trend.

3. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 2, characterized in that, Determining the solder joint quality grade based on the characteristic temporal relationship and its changing trend includes the following steps: A dynamic acceptable range is set centered on the current operating condition benchmark value; The real-time delay time of the current solder joint is compared with the dynamic acceptable range. If it exceeds the range, it is directly judged as unacceptable.

4. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 3, characterized in that, For solder joints that do not exceed the dynamic acceptable range, determining their quality level also includes the following steps: Calculate the instantaneous deviation between the solder joint delay time and the current operating condition reference value; Obtain the average rate of change of the system's cumulative offset over a recent period; When both the instantaneous deviation and the average rate of change are small, the weld is rated as a high stability level. When either the instantaneous deviation or the average rate of change is large, the weld is classified as a level of concern. When both the instantaneous deviation and the average rate of change are large, the weld is classified as a critical level. Set a smaller threshold and a larger threshold for the instantaneous deviation. The smaller threshold is 30% of the width of the dynamic acceptable range, and the larger threshold is 70% of the width of the dynamic acceptable range. The rate threshold for the average rate of change is set to ±3% of the initial reference value per weld point, determined based on the long-term drift tolerance allowed by the welding system.

5. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 2, characterized in that, The nature of the deterministic offset trend is determined based on the direction of the system's cumulative offset: If the cumulative offset of the system continues to develop in the positive direction, it is determined that there is a gradual loss of welding thermal efficiency. If the cumulative offset of the system continues to develop in the negative direction, it is determined that the basic conditions of the welding process have undergone a systematic change.

6. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 5, characterized in that, It also includes the following steps: When the thermal efficiency is determined to be progressively reduced, the output maintenance command is directed to the inspection and handling of the welding electrode working surface. When it is determined that the basic conditions have changed systematically, the output maintenance command is directed to check the welding power supply output characteristics or the workpiece-electrode contact interface.

7. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 1, characterized in that, The physical quantity time sequence signal is the electrode axial thermal expansion displacement signal; the step of identifying the second time marker specifically includes: performing smoothing filtering on the displacement signal, and then locating the peak point on its derivative curve, which is the second time marker.

8. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 1, characterized in that, The method further includes the following steps: The evaluation method is performed simultaneously on multiple parallel workstations on the car body welding production line. Compare the consistency of the cumulative system offset obtained from each workstation in terms of both numerical value and development stage; If most workstations exhibit consistency, it is determined that there are common influencing factors upstream of the production line. If the performance of each workstation is discrete, it is determined that the individual characteristics of the workstation have deteriorated.

9. The method for evaluating the quality of resistance spot welds on an automobile body according to claim 8, characterized in that, Also includes: Based on the determination results of the common influencing factors or individual characteristic degradation, a systematic correction plan and an individualized maintenance queue are generated respectively, wherein the systematic correction plan is executed in priority over the individualized maintenance queue.

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