High Isotropic Magnetic Sensor Based on Ring Terfenol-D / PZT Composites

Abstract

The magnetoelectric sensor has the characteristics of high sensitivity, wide response frequency range, low power consumption and easy preparation. It has broad application prospects in the detection of weak magnetic field such as geomagnetic field, biological magnetic field etc. In recent years, the related research [1–4] shows that the output of the magnetoelectric composites vary with the angle of the measured magnetic field, in which the ratio of the maximum value to the minimum value of the output signal is denoted as K. When K is large, the magnetoelectric composites have high anisotropy. For example, Dong et al. [1] and our group [2] prepared magnetoelectric sensors with high anisotropy $( 100 \le \mathrm {K}\le 1000)$, and found that K values are related to the shape anisotropy of piezoelectric materials and magnetostrictive materials respectively. It means that the magnetoelectric sensor is very sensitive to the magnetic field in a specific direction and is suitable for vector magnetic field measurement. In contrast, when K is small, the magnetoelectric composites are highly isotropic, which means that their outputs are less affected by the direction of the magnetic field, and are suitable for scalar magnetic field measurements. However, highly isotropic magnetoelectric sensors were seldom reported. For this reason, this paper studies a Terfenol-D / PZT highly isotropic magnetoelectric sensor based on ringshaped composite structure. As shown in Figure 1 (a), we first prepared magnetoelectric composites using Terfenol-D and PZT-5H, both of which were annular in shape. The resonant frequency of the material is shown in Figure 1 (b). The sensor was placed in a solenoid with AC current, held by a foam, and fixed on a platform. In the case of the electromagnet to provide DC bias magnetic field, the spectrum analyzer measured the output signal of the magnetoelectric sensor. The frequency of the AC signal was changed continuously to record the output of the magnetoelectric composites, and the resonance frequency of the composites can be analyzed through the curve to be about 32 kHz. Fig. 1 (c) shows the relationship between the output of composites and the DC bias magnetic field, when the frequency of AC resonant magnetic field is 32 kHz, as well as the DC bias magnetic field Hdc is from 200 to 3360Oe. The result shows that the composites output increased first, then kept stable after. Next step, we measured the linearity and isotropy of the sample in a shield tube. Figure 2 (a) introduces the testing instruments, and the micro-sensor device was placed in the magnetic shield tube. Fig. 2 (b) show the linearity of the sensor. When the DC bias magnetic field is is about 280Oe by bias magnet, the output of the magnetoelectric sensor at the AC frequency of 32 kHz has a linear relationship with the amplitude of the AC signal, as well as the sensitivity is 24 mV / Oe, indicating that it has a good linear relationship. Finally, we analyzed this model with finite method, the finite element results show that the magnetoelectric isotropy could be influenced mainly by the shape of the magnetostrictive and piezoelectric material, and the position of the bias magnetic field. Then we measured the isotropic characteristics of the ring-shaped sensor. The result is shown in Figure 2 (c). We placed the sensor device in the shield tube and rotated the sensor 360° in steps of 30° to measure the change of the output signal. The isotropy ratio of the magnetoelectric sensor (K) was fitted with finite method, and the result shows that the result approximately is 13: 1. As can be seen from the figure, the test values were in agreement with the theoretical values with finite method. This is because both the Terfenol-D and PZT-5H materials are isotropic in shape, which greatly increases the isotropic effect of the magnetoelectric sensor.