1998 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4,JULY 2001 Pulse Excitation of Micro-Fluxgate Sensors Pavel Ripka, San On Choi, Alois Tipek, Shoji Kawahito, and Makoto Ishida Abstract—Miniature fluxgate sensors with symmetrical closed core elements on both sides of the planar coils were manufac- tured using standard microtechnology. The new sensors have shown substantial improvement over the standard single-sided microfluxgate sensors: for the same field range the sensor noise was reduced 10-times to 20 nT rms (20 mHz 10 Hz) and the perming suppressed below 5 T, for field shocks of 6 mT. The maximum sensitivity for sinewave excitation was 32 V/ T for 1 MHz frequency and 200 mA - excitation current amplitude. Pulse shape of the excitation current allows use of high current peaks to suppress perming, while the rms value is low. Using a 20% duty factor squarewave excitation with 180 mA - amplitude, the sensitivity was twice that of the sinewave excitation, while the chip temperature dropped from 80 C to 40 C. Index Terms—Fluxgate, magnetic field sensors, magnetometers, microfluxgate. I. INTRODUCTION T RADITIONAL fluxgate sensors are popular for measuring the magnetic field in the range of 1 nT to 1 mT. They can reach better than a 0.1 nT resolution and high precision such as 10 ppm linearity error and 30 ppm/ C temperature coefficient of sensitivity. These devices need to be manually adjusted and individually calibrated which causes manufacturability issues, and leads to expensive devices [1], [2]. Many applications require cheap sensors or sensor arrays with a 10 nT to 1 nT resolution. These include magnetic ink reading, detection of ferromagnetic objects such as weapons and vehicles, reading of magnetic labels, magnetic 3-dimensional position tracking for virtual reality systems and robots [3]. Some of these applications can be successfully addressed by ferromagnetic magnetoresistors: commercially available AMR with flipping and newly developed linear GMR with a AC bias [4]. But there is still a strong demand for development of cheap and small vectorial magnetic field sensors, which could offer better accuracy than magnetoresistors. Microelectronic technology has already been used to lower the production cost and further decrease the size of the flux- gate sensors. The first approach is to replace the excitation and Manuscript received October 13, 2000. This work was supported in part by the Czech Ministery of Education under Grant no. ME 275. P. Ripka is with the Czech Technical University of Measurement, Fac. of Electr. Eng. CTU, Czech Republic (e-mail: ripka@feld.cvut.cz). S. O. Choi is with Samsung Advanced Institute of Technology, Suwon, Korea, 440-600. A. Tipek is with the Czech Technical University of Measurement, Fac. of Electr. Eng. CTU, Czech Republic (e-mail: xtipeka@feld.cvut.cz). S. Kawahito is with Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-8011, Japan (e-mail: kawahito@idl.rie.shizuoka.ac.jp). M. Ishida is with Dept. Electrical and Electronic Eng. Toyohashi Univ. of Technol., Tempaku-cho, Toyohashi, Japan (e-mail: ishida@eee.tut.ac.jp). Publisher Item Identifier S 0018-9464(01)06198-2. Fig. 1. Double-sided microfluxgate sensor: (a) excitation coil and the flux path (pick-up coils are not shown for the clarity), (b) function of the pick-up coils, (c) cross-sectional view of the complete sensor. sensing wire coils by solenoids made by pcb-technology [5], micromachining [6], or standard planar process [7], [8]. This geometry is ideal for the sensor function; problem is the man- ufacturing complexity and a limited number of turns of such solenoids. Another approach, also used in the present paper, is to use flat coils made by the planar process [9], [10]. II. SENSOR DESIGN The new type of miniature fluxgate sensors with a symmet- rical closed core and planar coils was manufactured using stan- dard microtechnology [11]. The sensor geometry is shown in Fig. 1. The structure consists of two metallic layers made of 3 m aluminum, which are sandwiched between two ferromag- netic layers. The metallic layers form a flat excitation coil and two antiserially connected sensing coils. Four 0.7 mm long, 0.4 m thick core strips made of electroplated permalloy are positioned symmetrically on the both sides of the coils, so that they form two closed magnetic paths for the excitation field. Fig. 1(a) shows the centrally positioned excitation coil with two symmetrical excitation flux paths. The sensing coils are shown separaTely for clarity in Fig. 1(b). The cross-section of the com- plete sensor is shown in Fig. 1(c). The sensor is functionally similar to two double-core fluxgate sensors. The gain of this new design is that the sensor core has a high apparent perme- ability with respect to the excitation field so that the core may be deeply saturated to erase perming and reduce hysteresis. The apparent permeability with respect to the measured field is much lower, which results in higher sensor stability in the open-loop mode: temperature changes of the core material permeability 0018–9464/01$10.00 © 2001 IEEE