Eddy current testing instrument is a key equipment based on the principle of electromagnetic induction to realize non-destructive testing of conductive materials, which is widely used in metallurgy, aerospace, machinery manufacturing and other fields, and can complete various tasks such as defect detection, thickness measurement, and conductivity testing. Although instruments for different purposes have their own functional emphasis, their core working mechanisms are highly consistent with their basic composition.
Basic composition
The workflow of eddy current detection instrument revolves around "magnetic field excitation-eddy current generation-signal feedback-analysis and display", and the core is composed of five functional modules, and the performance of each part directly determines the detection accuracy and reliability.
(1) Incentive and testing units
The unit contains an oscillator and a sense coil (probe) that is the basis for generating a sense signal. Oscillators usually consist of an oscillation stage and a power amplification stage, and their core indicator is frequency stability - according to the "JJF (Beijing) 191-2025 Eddy Current Detector Calibration Specification", frequency stability needs to calculate the deviation through multiple measurements to ensure the accuracy of the detection frequency. Detection frequency directly affects detection depth: high frequency (>1MHz) is suitable for detecting small cracks on the surface, low frequency (<100kHz) can penetrate deep defects in materials, and modern instruments have covered the frequency range from 1Hz to 20MHz, which can meet different detection needs.
As a component that acts directly with the workpiece, the design of the detection coil must match the characteristics of the inspection object. Common types include single-coil, double-coil and multi-coil structures, with shapes such as circular, point-shaped, fan-shaped, etc., which can be customized according to the shape of workpieces such as flat plates, pipes, and complex surfaces. The number of turns, size, and material selection of the coil (e.g., high permeability core-enhanced magnetic field) directly affect inspection sensitivity, such as through-through probes for pipe inner wall inspection and flat probes for plate surface inspection.
(2) Signal detection and amplification unit
The signal detection circuit mostly uses a bridge circuit (such as the Wheatstone bridge), which can convert the impedance difference caused by the change of eddy current of the detection coil into a measurable voltage signal. The amplification gain can generally be adjusted in the range of 0 to 80dB, and the step accuracy can reach 0.1dB, which can not only ensure the signal strength, but also avoid excessive noise amplification.
(3) Signal processing unit
The unit is the core of suppressing interference and extracting effective information, and separating defect signals from interference signals through specific algorithms.
Phase-sensitive detection method - through the combination of phase-shifting circuit and detection circuit, the alternating signal is converted into a DC signal, and the phase difference is used to resolve the defect depth, especially suitable for distinguishing the signal change caused by defects and surface finish;
Frequency analysis method - using an adjustable bandpass filter (pass frequency bandwidth 1kHz to 1MHz), filtering out noise according to the frequency difference between defect signal and interference signal, and dynamically adjusting the filtering parameters according to the transmission speed of the workpiece in the automatic flaw Detector;
Amplitude analysis method - set a threshold by the rejection circuit to suppress the interference signal below the threshold, which is suitable for scenarios where the amplitude of the defect signal is obvious, but it is necessary to avoid missed detection caused by too high a threshold.
Modern instruments also incorporate digital signal processing technologies, such as wavelet transform to improve signal-to-noise ratio, and neural networks to achieve automatic defect identification to further improve signal resolution capabilities.
(4) Display and recording units
The display device needs to visualize the test results, and the common methods include impedance plan (XY mode), time-base scan map (YT mode) and multi-window combination display, which can display signal amplitude, phase and defect location information at the same time. The recording and storage functions have been intelligent, which can save inspection programs, signal waveforms and defect data, and some instruments support data export and printing to meet the needs of quality traceability.
Main types and applications
As the most commonly used eddy current detection equipment, eddy current flaw Detectors can be divided into manual and automatic according to the operation mode, which are suitable for different detection scenarios.
Manual eddy current flaw Detector
This type of instrument is scanned along the surface of the workpiece by the operator with a handheld probe, and most of them are designed with probe coils, which have the characteristics of strong portability and flexible operation. Due to the slow detection speed (usually < 0.5 m/s), there is no need for modulation frequency interference caused by fast motion, so there is no need for complex frequency filters, and the signal processing is mainly based on phase analysis or amplitude analysis.
Typical examples such as Mattel MET series portable flaw Detectors, equipped with 6-8 inch touch displays, support 10Hz-10MHz frequency adjustment, compatible with internal penetration, external penetration, flat surface and other probes, can not only detect metal surface cracks, folds and other defects, but also expand the coating thickness measurement and conductivity measurement functions, suitable for in-situ inspection of aviation parts, pipeline in-service inspection and other scenarios. Its digital filtering technology can effectively suppress environmental noise, and the alarm threshold can be flexibly set to ensure detection accuracy.
Automatic eddy current flaw Detector
The core advantage of the automatic flaw Detector is that it has high detection efficiency and good repeatability of results, and is suitable for online inspection of mass-produced workpieces such as pipe bars and wires. In addition to the basic components, it is equipped with special functional modules:
Transmission device - driven by servo motor to ensure that the workpiece is centered through the coil and the speed is uniform (fluctuation ≤±1%), avoiding the signal distortion caused by the difference in circumferential sensitivity and uneven speed caused by eccentricity;
Magnetic saturation device - composed of a coil with an iron yoke and a DC power supply, which saturates and magnetizes the ferromagnetic workpiece, eliminates the interference caused by uneven permeability, and improves the accuracy of defect detection;
Sorting and marking device - according to the test results, the workpiece is divided into three categories: qualified, to be inspected, and unqualified, and the defect position is marked by the pneumatic marking pen, and the sorting speed can reach dozens of pipes per minute.
Comparison of the main technical performance of eddy current testing instruments at home and abroad
The technical performance of eddy current testing instruments at home and abroad has its own focus, and the core differences are focused on accuracy, stability and cost performance. Foreign representatives such as Germany's Fischer SMP10 and the United States' GE AutoSigma3000, with frequencies covering 60kHz-480kHz, measurement accuracy of ±1% of the full scale, lift-off compensation (14mm probe compensation of 0.5mm) and extreme environment adaptability (-30°C~85°C) are stronger, suitable for high-end precision testing; Domestic products such as Xiamen Tianyan Sigma2008 and Mattel MET series, the mid-to-high-end models have approached the international accuracy (±0.5%), support multi-unit switching and multi-frequency detection (10Hz-10MHz), and can also expand the coating thickness measurement and other functions, although the entry-level products (such as Xiamen Foster FD101) are slightly weaker in continuous stability, but they are cost-effective, adaptable to Chinese operation, and more in line with the needs of small and medium-sized enterprises.
Use and maintenance specifications
Proper use and maintenance of the instrument is the key to ensuring the accuracy of the test
Preparation before use
1. Environmental inspection - the installation site should be kept away from strong magnetic fields (≤240A/m), the temperature should be controlled at 15-35°C, and the relative humidity should be 30%-90%, and good grounding should be ensured to avoid interference;
2. Workpiece treatment - remove oil, rust and dust on the surface of the workpiece, and make special backs for workpieces with too small edge size to eliminate the impact of "edge effect";
3. Instrument calibration - After 30 minutes of start-up warm-up, calibrate the zero point and sensitivity with a standard test block to ensure that the probe and the test block are well coupled, and the calibration point should cover the common defect depth (such as 0.5mm, 1.0mm, 1.5mm).
Operational precautions
1. The probe should move at a constant speed and maintain a consistent pressure (about 0.5N) during manual testing to avoid wear of the probe caused by excessive pressure.
2. When automatically detecting, the transmission speed needs to be debugged first to ensure that it matches the filtering frequency, and the general speed does not exceed 2m/s.
3. Monitor the alarm signal in real time during the detection process, and repeatedly verify the abnormal signal to avoid misjudgment.
Daily maintenance points
1. The instrument should be kept clean and dry, covered with a dust cover after use, and the probe should be stored separately in an anti-static box to avoid falling;
2. Regular power-on maintenance (at least once a week), long-term idleness needs to be turned on for more than 4 hours per month;
3. According to the requirements of the calibration specification, conduct a comprehensive calibration at least once a year, focusing on checking the frequency stability, gain error and noise level.
4. When a fault is found (such as no signal output or abnormal alarm), refer to the manual for troubleshooting, and it is forbidden to remove the cover in the on-on state.
summary
With the advantages of non-contact, no couplant, and fast detection speed, eddy current testing instruments have become the core equipment of modern industrial quality control. From basic manual flaw Detectors to intelligent automatic detection systems, its technological development has always revolved around "improving sensitivity, suppressing interference, and optimizing the operating experience". With the integration of digital signal processing and artificial intelligence technology, future instruments will realize automatic classification and in-depth quantification of defects, further expanding the application boundaries in high-end manufacturing fields such as aerospace and new energy.
