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.
Highlights
Room temperature pT-level high sensitivity magnetic sensors have been developed [1,2,3,4,5]
The high-sensitivity magnetic sensors using a coil with the (Fe0.06Co0.94)72.5Si2.5B15 (FeCoSiB) amorphous wire as the core were reported [17,18,19] and the white noise spectrum of about 6 pT/√Hz was achieved
To measure the noise contribution of the preamplifier, the preamplifier was connected to an inductor of 2 μH, which was similar with the inductance of the magnetic sensor with amorphous wire
Summary
Room temperature pT-level high sensitivity magnetic sensors have been developed [1,2,3,4,5]. For some applications of magnetic sensors, such as the magnetic microscope [13] or the deep defect detection using eddy current testing (ECT) method [14], magnetic sensor with low noise at low frequency is needed. Different from normal MI sensors, the amorphous wire of this kind of magnetic sensor had no any electrical connection with the cable. Dongfeng He: Developing Gradiometer to Reduce the Low Frequency Noise of Magnetic Sensor with Amorphous Wire. This kind of magnetic sensor includes high frequency circuit and low frequency circuit. An electrical gradiometer was developed to reduce the low frequency noise of the magnetic sensor. Different from normal electrical gradiometer [20,21,22] with independent driving circuits for two sensors, the two sensors of this gradiometer shared the DC bias current and the AC bias current
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