Developing Gradiometer to Reduce the Low Frequency Noise of Magnetic Sensor with Amorphous Wire

Abstract

Pico-Tesla level high sensitivity magnetic sensor has been developed using FeCoSiB amorphous wire as the core of a coil, but the low frequency noise of the magnetic sensor is somewhat high, and some applications of magnetic sensors, such as magnetic microscope or deep defect detection, require that the sensors have low noise at low frequency. To reduce the noise, it is necessary to know the noise sources of the magnetic sensor. By measuring the noise spectrum of each part of the magnetic sensor with FeCoSiB amorphous wire was measured, it was found that the white noise of the magnetic sensor was mainly determined by the noise of the preamplifier and the low frequency noise was mainly determined by the low frequency noises of the DC bias current and AC bias current of the FeCoSiB magnetic sensor. To reduce the low frequency noise of the magnetic sensor caused by the bias current noise, an electrical gradiometer composed of two magnetic sensors sharing the same DC bias current and AC bias current was developed. The results proved that the gradiometer was not only effective to reduce the low frequency noise caused the bias current, but also effective to reduce the environmental noise caused by the 50 Hz line interference. Using the gradiometer, an eddy current testing (ECT) system were constructed, and the deep defect with the depth of about 8 mm in an aluminum plate was successfully detected by the ECT system.