GNGTS 2013 - Atti del 32° Convegno Nazionale

MEMS magnetic field sensors can be based on two different principles: Lorentz-force or Anisotropic Magneto Resistance (AMR) materials. In the former case, the sensor relies on the mechanical motion of the MEMS structure due to the Lorentz force acting on the current- carrying conductor in the magnetic field. The mechanical motion of the micro-structure is sensed either electronically or optically. The mechanical structure is often driven to its resonance in order to obtain the maximum output signal. In the latter case, AMR effects, which are commonly found in both magnetic and ferromagnetic materials, are enhanced using a specific geometry of the magnetic element, such that a net residual magnetization is induced by the balance among energetic terms (magnetostatic, magnetoelastic, magnetocrystalline). Anisometric thin films exhibit a strong effect and enable the realization of simple, cost-effective sensors where the electrical resistance is a function of the angle between magnetization (function of the external magnetic field) and current density. The device employed for the experiment is LSM303DHLC, manufactured by STMicroelectronics. It is a system-in-package featuring a triaxial digital linear acceleration sensor and a triaxial digital magnetic sensor based on AMR. The magnetic full-scale is selectable from 130,000 to 810,000 nT, with a maximum sensitivity of 90 nT/digit. The device also includes readout circuitry providing a digital output, easy to be employed in a microprocessor-based system. Its size is 3x5x1mm and its average power consumption is under 400 μW. The experiment. We made a model of a “restored arm” with a bioclastite cylindrical sample (height= 18 cm; diameter=10 cm) in which an iron pin (length= 9 cm; diameter= 5 mm) was inserted out of the sample axis and obliquely (Fig. 2a). A rotating platform surrounded by a graduated scale allowed us to rotate the sample in step of 10°. A sliding beam was used to move vertically the magnetometer so that, combining the two movements, the sample was scanned along horizontal circles 1 cm apart along its height (Fig. 2b) . At each measuring point we recorded 50 times at 50 Sample/s the three magnetic components (Vertical, Radial and Tangent) with the height and the angle. At each point mean, standard deviation and standard deviation of the mean over the 50 readings were calculated and the means were then taken as raw data. With this experimental setup, aimed to perform a preliminary test of the effectiveness of the sensor, the pin rotated within the sample and was always south of the sensor, therefore we did not get the reconstruction of the magnetic field around the sample as it would be in an acquisition, for example, around a statue arm. Results. With a software developed in Matlab© we plotted on cylindrical surfaces all of the components and the modulus. By subtracting the average value of each component, we estimated the anomaly caused by the iron pin. Among the many representations we made of the results the most meaningful with respect to the question “where and how long is the iron pin” seem to be the plots of the total field TF (Fig. 3a) and the horizontal vector H (Fig. 3b) anomalies onto a cylindrical surface mapping the lateral surface of the sample. The TF vector imaging seems to reasonably describe the magnetic field flux plus some noise. The top view of all the vectors is more suitable to guess the position of the pin with respect to the sample axis. Fig. 2 – The physical model of a limestone statue “restored arm” with the iron pin fitted inside (a). The system, with the vertical sliding arm and the rotating platform, made to realize the cylindrical scanning of the model (b). 171 GNGTS 2013 S essione 3.2