DESIGN AND CHARACTERIZATION OF A FAMILY OF FLUXGATE MAGNETIC SENSORS IN PCB TECHNOLOGY

Size: px

Start display at page:

Download "DESIGN AND CHARACTERIZATION OF A FAMILY OF FLUXGATE MAGNETIC SENSORS IN PCB TECHNOLOGY"

Lorin Barrett
5 years ago
Views:

1 DESIGN AND CHARACTERIZATION OF A FAMILY OF FLUXGATE MAGNETIC SENSORS IN PCB TECHNOLOGY Andrea Baschirotto Dept. of Innovation Engineering, University of Lecce, 731 Lecce Italy Enrico Dallago, Piero Malcovati, Marco Marchesi, Giuseppe Venchi Power Electronics Laboratory, Department of Electrical Engineering, University of Pavia, 271 Pavia Italy Fluxgates magnetic sensors are the most suitable magnetic sensors for applications requiring a resolution down to.1 nt and an absolute precision in the order of 1 nt. For consumer applications the trade-off between cost and performance can be optimized by using fluxgate sensors realized with printed circuit board (PCB) technology. In this work, a family of planar single and dual axis fluxgate sensors realized in PCB technology, featuring a sensitivity suitable for detecting the earth s magnetic field, is presented and experimentally characterized. The proposed sensors achieve a magnetic sensitivity of about.46 mv/µt and.35 mv/µt for a single and double axis structure respectively. Introduction In recent years fluxgate magnetic sensors [1-6] in planar configuration [7, 8] have undergone significant developments. A planar fluxgate structure comprehends a magnetic film (core), an excitation and two pick up coils, as shown in Fig.1. The excitation coil creates a magnetic field that periodically saturates the core in both directions. When no external magnetic field is applied, the two pick up coils placed in differential configuration show an output voltage that ideally is zero. In contrast, when an external field component is present and parallel to the core, the differential output voltage increases its value, resulting in an amplitude modulation. Magnetic Film Excitation coil Pick-up coils Fig. 1 Planar fluxgate A critical step in fluxgate sensor realization, is the choice of ferromagnetic materials and the generation of a layer of this material over the coils. A possible solution is to use amorphous material like VITROVAC 625X [9], which is commercially available with a thickness of 25 µm, that can be glued onto the 1

2 2 coils. In this work, a family of single and double axis PCB planar fluxgates realized using the above approach has been investigated. SINGLE AXIS SENSORS A family of single axis fluxgates, that include three different configurations, was realized. The basic structure, defined here as Structure 1-3, is shown in Fig.2a and consists of one planar excitation coil (3 turns, 3 µm thickness and 4 µm pitch) and two pick-up coils (3 turns, 17 µm thickness and 4 µm pitch) placed in a differential configuration. The ferromagnetic sheet core with a thickness of 25 µm corresponds to the above mentioned amorphous alloy known under the trade name of VITROVAC 625X, which was chosen primarily because of its extremely high relative permeability (µ r 1). The material, for which the magnetic induction at saturation is.55 T, was glued onto the PCB structure in a shape 17 µm long and 7 µm wide, as shown in Fig.2b. The other two structures, defined Structure 1-25 and Structure 1-2, were obtained by removing, from Structure 1-3, five and ten turns placed on the outside of the excitation and pick-up coils. The ferromagnetic material maintained the same geometrical dimensions for the three different structures. a) b) VITROVAC 625X Fig. 2 Single axis fluxgate magnetic sensor (structure 1-3) The sensors were analyzed with a sinusoidal excitation current having a 7mA peak at a frequency of 1kHz and setting an external magnetic field in the range of ±4 µt with a pair of Helmholtz coils. In order to measure the sensitivity of the sensor to the external magnetic field, the differential output voltage from the pick-up coils was read with a 3562A HP Dynamic Signal Analyzer. Under the aforesaid conditions, the sensor s output for Structures 1-3, 1-25 and 1-2, is the amplitude of the harmonic at 2kHz of the differential output voltage. This voltage is plotted in Fig.3 against the external magnetic field. The voltage output shows a linearity error of.83 % full scale in the range of ±6 µt with a maximum sensitivity of about.48 mv/µt for the Structure 1-3 turns. The response of the sensor, when the peak of the excitation current was varied between 5 ma and 8 ma, was analyzed and the results are shown in Fig.4. All fluxgate structures have a precise current excitation value that maximizes

3 their sensitivity [1]. This value is related to the magnetic field that saturates the material and to the topology of the sensor. In the range of ±1 µt, the maximum sensitivity for Structures 1-3 and 1-25 is obtained with 7 ma (Fig.s 4a and 4b), while for Structure 1-2 it is with 8 ma (Fig.4c) ma 8 ma 6 ma turns 7 ma ma turns 7 ma 1 2 turns 8 ma Fig. 3 Voltage output comparison between the three single axis structures Fig. 4a) Structure ma 9 ma 8 ma 7 ma 7 ma 4 6 ma 6 ma 5 ma 5 ma Fig.4b) Structure 1-25 Fig. 4c) Structure 1-2 DOUBLE AXIS SENSORS A family of double axis planar fluxgates was studied and realized in three different configurations. The basic structure, denoted in the following as Structure 2-3, is shown in Fig.5a and consists of one excitation coil (3 turns, 3 µm thickness and 4 µm pitch) and four pick-up coils (21 turns, 17 µm thickness and 4 µm pitch). The VITROVAC 625X ferromagnetic core (25 µm thickness, 377 µm length and 34 µm width) was glued onto the PCB structure, as shown is Fig.5b. The other two structures, called Structure 2-25 and Structure 2-2, were obtained by removing from Structure 2-3, five and ten turns placed on the outside of the excitation and pick-up coils. The ferromagnetic material was adjusted in length, step by step for the new diagonal dimension of the excitation coil. The voltage output for the double axis fluxgates, with 7 ma of current excitation at 1 khz, was measured with an

4 external magnetic field in the range of ±15 µt and is shown in Fig.6. The voltage increments show a linearity error of 1.45 % full scale in the range of ±6 µt with a maximum sensitivity of about.

For Structure 2-3 the maximum sensitivity corresponds to an excitation current of 6 ma peak (Fig.7a). For Structure 2-25 the maximum sensitivity is achieved with 8 ma peak (Fig.

4 4 external magnetic field in the range of ±15 µt and is shown in Fig.6. The voltage increments show a linearity error of 1.45 % full scale in the range of ±6 µt with a maximum sensitivity of about.35 mv/µt for Structure 2-3. The response of the sensor, when the peak of the current excitation was varied between 5 and 9 ma, was analyzed and the results are shown in Fig.7. For Structure 2-3 the maximum sensitivity corresponds to an excitation current of 6 ma peak (Fig.7a). For Structure 2-25 the maximum sensitivity is achieved with 8 ma peak (Fig.7b), while for Structure 2-2 the maximum sensitivity is obtained with a 9 ma peak (Fig.7c). a) b) Fig. 5 Double axis Fluxgate magnetic sensors (structure 2-3) turns 6 ma ma 7 ma 6 ma 5 ma turns 8 ma turns 9 ma Fig. 6 Voltage output comparison between the three different double axis fluxgates Fig. 7 a) Structure ma 15 9 ma ma 7 ma 6 ma 5 ma ma 7 ma 6 ma Fig. 7b) Structure 2-25 Fig. 7c) Structure 2-2

5 5 CONCLUSIONS A family of single and double axis fluxgates has been presented and characterized. All prototypes in PCB technology showed good sensitivity and linearity in the range of ±6 µt, which is suitable for compass applications. For a given geometry, a reduced size preserved linearity but decreased sensitivity. To complete the identification of two components of a magnetic field in a plane using a smaller power consumption, the best solution is to adopt a structure with two sensitivity axes and 3 turns of excitation coil. ACKNOWLEDGMENTS The authors would like to thank STMicroelectronics, Cornaredo Italy, for technological support, Cedrat, Grenoble France, for allowing our use of their FEM simulator and Vacuumschmelze, Hanau Germany, for Vitrovac samples. References [1] Pavel Ripka, Magnetic Sensors and magnetometers, Artech House Boston, London (21). [2] James E. Lenz, A Review of Magnetic Sensors, Proceeding of the IEEE vol.78 no. 6 (199) pp [3] R.S. Popovic, J.A. Flanagan, P.A. Besse, The future of magnetic sensors, Sensors and Actuators A 56 (1996) pp [4] P. Ripka, New directions in fluxgate sensors, Journal of Magnetism and Magnetic Materials pp (2). [5] F. Kaluza, A. Grüger, H. Grüger, New and future applications of fluxgate sensors, Sensors and Actuators A 16 (23) pp [6] P. Ripka, Advances in fluxgate sensors, Sensors and Actuators A 16 (23) pp [7] L. Chiesi, P. Kejik, R.S. Popovic, CMOS planar 2D micro-fluxgate sensor, Sensors and Actuators 82 (2) [8] S.O. Choi, S. Kawahito, Y. Matsumoto, M. Ispida, Y. Tadokoro An integrated micro fluxgate magnetic sensor, Sensors and Actuators A 55 (1996) [9] Vacuumshmelze, Hanau Germany, Amorphous metals VITROVAC. [1] M. Schneider, CMOS Magnetotransistor and Fluxgate Vector Sensors, Physical Electronics Laboratory, ETH Zurich, CH, (1999).

MICRO-INTEGRATED DOUBLE AXIS PLANAR FLUXGATE

MICRO-INTEGRATED DOUBLE AXIS PLANAR FLUXGATE Andrea Baschirotto Dept. of Innovation Engineering, University of Lecce, 73100 Lecce Italy Enrico Dallago, Piero Malcovati, Marco Marchesi, Giuseppe Venchi