MICRO-INTEGRATED DOUBLE AXIS PLANAR FLUXGATE

Size: px

Start display at page:

Download "MICRO-INTEGRATED DOUBLE AXIS PLANAR FLUXGATE"

Deborah Barnett
5 years ago
Views:

1 MICRO-INTEGRATED DOUBLE AXIS PLANAR FLUXGATE Andrea Baschirotto Dept. of Innovation Engineering, University of Lecce, Lecce Italy Enrico Dallago, Piero Malcovati, Marco Marchesi, Giuseppe Venchi Power Electronics Laboratory, Department of Electrical Engineering, University of Pavia, Pavia Italy Abstract In this paper a double axis micro-integrated Fluxgate magnetic sensor is presented. The device was designed using a PCB dual axis sensor previously realized as a starting point. In the PCB version the experimental results exhibit excellent agreement with the simulations performed using a tool based on the finite element method. Using the same design approach a version of the double axis Fluxgate structure to be realized in IC technology is here proposed. The accurate study of the magnetic field distribution allows a 75% area saving for the IC version with respect to a direct scaling of the PCB version. Imposing an external magnetic field of about 60 µt, the simulated differential output voltage of the micro-integrated Fluxgate sensor achieves a peak value of 1 mv with 5 ma sinusoidal excitation current peak at 100 khz. Keywords Planar Fluxgate; Amorphous Magnetic Core; Magnetic Sensors; INTRODUCTION In the realization of planar fluxgate magnetic sensors [1] the main technologies used are the Printed Circuit Board technology [2-4] and the integrated process [5]. The choice in favour of PCB technology derives primarily from its reduced cost and efficient fabrication, due above all to the very thin commercially available ferromagnetic material used for the core. On the other hand, the low power consumption and small dimensions are indeed the fundamental requirements for a magnetic field sensor to be embedded in portable devices. Therefore a micro-integrated version is mandatory for these applications. In this last case a study to foresee the characteristics of the micro-integrated sensor can be very useful. In this paper, starting with both the simulation results obtained by using the Flux3D [6] FEM tools and the experimental results for the PCB version of a planar fluxgate, an evaluation of a micro-integrated version of the same planar fluxgate will be presented. SENSOR STRUCTURE The schematic of the PCB structure realized for the double axis Fluxgate is shown in Fig. 1. It consists of an excitation coil and four sensing coils. The actual implementation is shown in the photo of Fig. 2. The planar excitation coil is characterized by 30 µm thickness, 30 turns and 400 µm pitch. The current in 1

For each component of the magnetic field the output voltage is obtained from two sensing coils placed in differential configuration, having 17 µm thickness, 21 turns and 400 µm pitch.

2 2 the excitation coil should be enough to saturate the ferromagnetic material glued over the PCB (see Fig. 2). This material is shaped as a cross in order to make the device sensitive to the two components of a magnetic field coplanar with the PCB. For each component of the magnetic field the output voltage is obtained from two sensing coils placed in differential configuration, having 17 µm thickness, 21 turns and 400 µm pitch. The excitation and sensing coils are realized on two different metal layers of the multilayer PCB structure, at a distance of 50 µm from each other. The total device size is 57.3 mm x 58.1 mm. The ferromagnetic material core is the amorphous alloy Vitrovac 6025 [7]. This material was chosen because of its extremely high relative permeability (µr = 105) and low losses. Its saturation magnetic induction is 0.55 T. SIMULATION MODEL The main issue in the setup of the model was to create a good mesh after taking into account the geometrical configuration of the ferromagnetic material, which is a cross sheet with dimensions of about µm x 3400 µm x 25 µm on the diagonal (Fig. 2). The critical dimension is the thickness of only 25 µm, which leads to a large number of elements in the mesh ( volume elements). The minimum peak value of the excitation current that guarantees the saturation of the ferromagnetic material was then evaluated. Using different magnetostatic analyses we verified that a peak current of about 600 ma is necessary to saturate the Vitrovac, as shown in Fig. 3. After this, the transient response of the device was investigated considering the geometrical model proposed in Fig. 1 with a 10 khz, 600 ma peak amplitude sinusoidal excitation current together with a field of 20 µt coplanar to the PCB and parallel to the ferromagnetic material. The results of this transient magnetic simulation, for one axis of sensitivity, are shown in Fig. 4, that reports also the comparison with the experimental result mm 3400 µm µm 58.1 mm VITROVAC 6025 Fig. 1 Schematic of the PCB double axis Fluxgate realized Fig. 2 Photograph of the double axis planar Fluxgate proposed

3 3 Bmax= 0.55 T Simulation Experimental Fig. 3 Simulated magnetic induction Fig. 4 Comparison between the simulation and experimental result Fig. 5 Transfer characteristic of the PCB Fluxgate obtained with a spectrum analyzer Fig. 6 Angle response for the two measurement axes (X and Y). EXPERIMENTAL RESULTS Imposing an external magnetic field with a pair of Helmholtz coils, the differential output voltage of the PCB structure has been evaluated. The comparison between the simulation and experimental results, shown in Fig. 4, gives an error of about 10 % on the amplitude of the output voltage. This error is extremely low in consideration of the simulation conditions adopted to reduce the computation time. To characterize the PCB structure, the external magnetic field has been varied in the range of ±100 µt. The differential output voltage from the pick-up coils was analyzed with a 3562A Hewlett Packard Dynamic Signal Analyzer to extract the value of the 2 nd order harmonic (at double the frequency of the excitation signal). This voltage is plotted in Fig. 5 against the external magnetic field. The sensor shows a linearity error of about 2.6% full scale in the range of ±60 µt with a sensitivity of 0.37 mv/µt. To evaluate the possibility of using the sensor as a 2D-vector magnetic field detector a suitable read-out circuit has been realized. Different analyses have been realized as reported in [4]. The voltage output of the sensor measured while rotating it in a horizontal plane is shown in Fig. 6. The angle error was smaller than 6 and included signal non-linearity, hysteresis and noise.

4 INTEGRATED MICRO STRUCTURE Using the experience with the PCB prototype an IC version of the double axis Fluxgate sensor for low power applications requiring small dimension has been analyzed with

4 4 INTEGRATED MICRO STRUCTURE Using the experience with the PCB prototype an IC version of the double axis Fluxgate sensor for low power applications requiring small dimension has been analyzed with the FEM simulator. The design features have been assumed in agreement with the IC realization of the coils in a 0.5 µm CMOS technology with a post processing for the deposition of the ferromagnetic material. The model created in Flux3D is shown in Fig. 7: it consists of an excitation coil and four sensing coils, of which only two are visualized in figure. The planar excitation coil is characterized by 5.5 µm thickness, 95 turns and 9 µm pitch (5 µm width and 4 µm spacing). The total area for the excitation coil is (1760 x 1760) µm 2. In order to optimize the device area occupation a number of magnetostatic analyses were performed varying the area of the sensing coil. The simulations showed that the component of magnetic induction associated with the ferromagnetic material perpendicular to the plane of the sensing coil is concentrated in a small area under each of the outer end of the core branches. Therefore the sensing coils were centered in these areas and their size was reduced to the minimum possible. This gives a 75% area saving with respect to the direct scaling of the PCB prototype, without any performance loss. The geometrical parameters of the sensing coils based on the used technology are: 1 µm thickness, 1.3 µm width and 1.6 µm spacing. The total area for the sensing coils, consisting of 75 turns, is 450 x 450 µm 2. The Vitrovac 6025 has been introduced in the simulation in cross shape with the dimensions of 2513 µm x 113 µm on the diagonal (Fig. 7). The minimum thickness for the ferromagnetic material that can be saturated with a peak current of about 5 ma (in order to limit the power consumption) was investigated. Using different magnetostatic analysis we established that the complete saturation condition is obtained with a thickness of 1 µm µm 113 µm Fig. 7 Simulation model of the integrated version of the PCB structure. Fig. 8 Differential output voltage simulated with Flux3D A transient time analysis was performed considering the geometrical model proposed in Fig. 7 with a 100 khz, 5 ma peak amplitude sinusoidal excitation

5 current together with a 60 µt magnetic field coplanar to the micro-integrated structure and parallel to the ferromagnetic material. The results of this transient magnetic simulation, for one axis of sensitivity, are shown in Fig. 8. The output voltage obtained has an amplitude of about 1 mv, sufficiently large to be processed by the readout circuit. CONCLUSIONS A PCB double axis Fluxgate magnetic sensor has been realized and analyzed with a software tool based on the finite element method. The simulations showed a good agreement with the experimental results, therefore the same software has been used to evaluate the performance of a micro-integrated version of the double axis Fluxgate, which is presently under fabrication. The simulation results predict that the micro-integrated sensor will have enough sensitivity for sensing the Earth s magnetic field, with a power consumption two orders of magnitude lower than the PCB version. 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 (2001). [2] A. Baschirotto, E. Dallago, P. Malcovati, M. Marchesi, G. Venchi Fluxgate magnetic sensor in PCB technology, Proceeding of Instrumentation and Measurement Technology Conference (IMTC-2004), Como 2004, Vol.2 pp [3] P. Kejik, L. Chiesi, B. Janossy, R. D. Popovic, A new compact 2D planar fluxgate sensor with amorphous metal core Sensors and Actuators 81 (2000) [4] A. Baschirotto, E. Dallago, P. Malcovati, M. Marchesi, G. Venchi, Precise vector-2d magnetic field sensor system for electronic compass, Proceeding of IEEE Sensors 2004, Wien October 2004, pp [5] S.O. Choi, S. Kawahito, Y. Matsumoto, M. Ishida, Y. Tadokoro, An integrated micro fluxgate magnetic sensor Sensors and Actuators A 55 (1996) pp [6] CEDRAT: Flux3D User s guide, Cedrat Meylan France (2002) [7] Vacuumschmelze Hanau Germany, Amorphous metals VITROVAC 5

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

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,