Linear magnetic stray flux array based on GMR gradiometers

A drastic increase in storage density of magnetic media had a strong impact on the development of new sensor concepts for the measurement of weak magnetic fields. Two prerequisites were the miniaturization and the efficiency of the used effect. A number of magnetic field sensors based on various mechanisms have been developed and found their applications in information data-storage technology, mechanical engineering, and automative industry. The use of such sensors is quite attractive for special nondestructive testing (NDT) applications, e.g. eddy current testing with high penetration and online monitoring capabilities. Compared to most conventional sensors, the sensitivity of giant magneto-resistance (GMR) based sensors is higher; therefore, weaker magnetic fields can be detected. Based on a commercial GMR chip, we have designed an array with 16 gradiometer sensors arranged in a line with a pitch of 4mm. A preamplifier and a multiplexer are integrated into the sensor array. A PC data acquisition board digitizes the output signals from the array. The sensor array has been tested on surface and sub-surface defects. This paper presents an overview of the equipment design, including Sensor-On-Chip (SOC) technologies especially suitable for industrial application.