Advance in Magnetoresistance Magnetometer Performances Applied in Eddy Current Sensor Arrays

The sensitivity of the magnetic sensor is important for many electromagnetic Non Destructive Evaluation (NDE) applications. Nevertheless, for successful defect detection, properties such as high linearity, large dynamic range and good spatial resolution are required. One solution comes in good using improved solidstate magnetic sensors based on Giant MagnetoResistance (GMR). In this way, we have implemented and improved elementary GMR magnetometer in order to develop NDE Eddy Current System (ECS) sensor arrays [1]. Elementary sensor operates in unshielded environment with very good performances, like high bandwidth (f > 100 kHz), very high slew-rate (> 30 T/s), high intrinsic dynamic (> 140 dB/√Hz at 100 Hz) and low total harmonic distortion (< 0.03 %). We compare their performances to some magnetic field sensors implemented in NDE System, focusing on the state of the art of high sensitivity magnetometer experimental investigation. The improved GMR magnetometer replaces here the detection coil used in conventional ECS, which is surrounding by an excitation setup using a classical coil. Besides the determination of its experimental performances in alternating current techniques or remanent field measurement, we present the implementation of improved GMR magnetometer dedicated to the measurement of deep lying cracks in conducting material like aluminum and ferromagnetic plates. Introduction: Classically, ElectroMagnetic NonDestructive Evaluation is based on probing local anomalies in the magnetic of the object under test or electromagnetic stray field of the object under test. This probing allows one to detect cracks, corrosion, inclusions or other materials defects, since the material properties in the void differ from those of unflawed material base materials. We can distinguish three measurement categories according to the type of excitation: remanent field measurement, direct current (DC) (magnetic flux leakage) or alternating current (AC) (eddy current testing). All of these NDE techniques need an effective, inexpensive way of detecting deeply buried or small cracks in metallic parts and structures. More, the detection of deep lying cracks, in conducting samples using eddy current testing, can be performed with magnetic sensors having high field sensitivity at low excitation frequencies, high field dynamic, high linearity, high spatial resolution, high reproducibility, low intrinsic magnetic noise, ruggedness, low interchannel matching and large integration potentials. In order to appreciate the needed performances, it necessary to put into perspective the relative amplitudes of NDE signals, the required spatial resolution and environmental magnetic noise. Indeed, it will be shown that the requirements for noise cancellation dictate these main specifications [2] both shielded and unshielded environments. The figure 1 gives a view of the problematic of detection.