High resolution magnetic imaging system for industrial quality control

This paper describes an effective non-destructive measurement system for the visualization of magnetic anomalies in magnetizable objects. For the realization of the system highly sensitive sensors based on the GMR effect (GMR = giant magneto resistance) are used. The developed sensing head can be mounted easily on a positioning unit and measure the magnetic flux density or its gradient using an ADC (analog digital converter) which measures the sensor output voltage and a microcontroller for signal processing. After data transfer via USB to a PC the measurement results can be visualized and further signal processing steps (e.g. error pattern recognition algorithms) can be applied. This probe is suitable especially for magnetic non-destructive testing (NDT) systems with high inspection speeds and can work with different types of GMR-sensors (magnetometer, gradiometer) exhibiting several sensitivities. For observing ferromagnetic objects either the object´s magnetization due to the earth´s magnetic field is large enough for a meaningful measurement result or the object has to be magnetized actively during the measuring process by a magnet. For adjusting the magnetic excitation easily electro magnets are preferred. Exemplary measurement results obtained by the sensing head implemented in an MFL (magnetic flux leakage) testing system are presented.