Version 1, June 17, 2000
submitted for Sensors and Actuators
Micro-fluxgate sensor with closed core
P. Ripka1, S. Kawahito2, S.O.Choi3, A. Tipek1, M. Ishida4
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Dept. of Measurement, Fac. of Electr. Eng. CTU, Technicka 2, 166 27 Praha 6, Czech
Republic, e-mail:
[email protected] , phone +4202 24353945, fax +4202 3119929 2
Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-
8011, Japan, e-mail:
[email protected] 3
Samsung Advanced Institute of Technology, P. O. Box 111, Suwon, Korea, 440-600
4
Dept.of Electr. and Electronic Eng. Toyohashi Univ. of Technology. Tempaku-cho,
Toyohashi, Japan
Abstract: Micro-fluxgate sensor with symmetrical core on both sides of the planar rectangular excitation and pick-up coils has substantially improved parameters compared to its single-core predecessor. Almost closed magnetic path for the excitation field allows much deeper saturation of the sensor core, which reduces the perming effect, hysteresis and noise. Perming of max. 5 µT for 6 mT field shock, 28 V/T sensitivity and 24 nT rms noise were achieved for 2 mm long sensor excited by 1 MHz, 180 mA rms sinewave current, without magnetic feedback.
Keywords: Magnetic sensors, fluxgate sensor, magnetometers, micro-fluxgate, flat coils
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Introduction
Fluxgate sensors serve for the measurement of the magnetic field up to 1 mT with a resolution of 0.1 nT to 10 nT. They have broad application range from precise geophysical instruments to rugged detection sensors for security and military applications. The latest competitors of fluxgate sensors are AMR and newly developed GMR magnetoresistors. The main advantage of precise fluxgate is its high temperature stability: 30 ppm/0C temperature coefficient of sensitivity and 0.1 nT/0C offset tempco are easily achievable, while the same parameters for any other solid-state vectorial magnetic sensors (including semiconductor and magnetoresistors) are at least 10-times worse. The main disadvantage of fluxgate is their high size and price. The effort to manufacture fluxgate sensors using microtechnology is very complex as their parameters dramatically degrade when reducing the core size. Micro-fluxgate sensor with double permalloy core on both sides of the planar rectangular excitation and pick-up coils was developed at the Department of Electrical and Electronic Engineering of the Toyohashi University of Technology. The new sensor design results in improved performance. Hysteresis, perming and crossfield effect errors are reduced if compared to traditional planar micro-fluxgate with single-sided open core [1]. The sensor described here is based on the concept of flat coils, so it substantially differs from the fluxgates having micro-machined solenoid coils and open core [2,3]. Another class of micro-fluxgates with flat coils and open core consisting of strips made of etched amorphous tape was described in [4] and [5]. Fluxgate sensors with closed core geometry and solenoid coils were made by PCB technology [6], which did not allowed real miniaturization of the sensor.
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Sensor construction The new sensor has one centrally located rectangular excitation coil and two antiserially connected pick-up coils. Each coil has 40 turns made of 3 µm thick aluminum layer. The coil pitch is 20µm (15 µm conductor width/ 2 µm space). The excitation and pick-up coil dc resistance is 150 and 250 Ω respectively. The electroplated permalloy core is 2*700 µm long and 1000 mm wide and both layers are 4 µm thick. The new magnetic circuit has only a small airgap so that the core material can be deeply saturated. The sensor structure is shown in Fig. 1. The centrally located excitation coil magnetises ferromagnetic core strips in opposite directions, so that the structure forms two closed magnetic paths. Thus one double-layered planar fluxgate microsensor is an equivalent of two standard fluxgates with ring core. The advantage of the present structure can be expressed in terms of the effective permeability: the demagnetisation with respect to the excitation field and the corresponding apparent (effective) permeability is high. This allows deep saturation of the sensor core with reasonable power consumption and sensor heating. On the other hand, the demagnetisation with respect to the measured field is different and it is mainly given by the strip geometry. This is substantial advantage for the open-loop mode, as the sensor linear range can be set to the required value and a compromise between the sensor linearity, sensitivity and stability can be reached for each particular application.
Sensitivity The small-field sensitivity as a function of frequency is shown in Fig. 2. The sensor excitation current was 200 mA rms sinewave. The maximum sensor sensitivity for sinewave excitation is 28 V/T for 1 MHz frequency and 180 mA p-p current. Fig. 3 shows the sensor open-loop characteristics for high fields as a function of the excitation current amplitude. We can observe that the position of the output maximum shifts with increasing excitation towards
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the higher fields. This is fully in agreement with the theory of the closed-core fluxgate sensor [7], while this effect was not pronounced in case of micro-fluxgate sensors with single-sided core [8]. The sensitivity as a function of the excitation amplitude for 1 MHz sinewave is shown in Fig. 4. The optimum sensor working point does not correspond with the maximum sensitivity: larger excitation current amplitudes decrease the perming effect and increase zero stability. In our case the sensitivity maximum was reached for 160 mA p-p current, but 200 mA p-p was necessary to erase the perming effect.
Perming effect Perming is the change of the sensor zero (or even distortion of its characteristics) after the shock of large magnetic field. Although it is rarely mentioned in the datasheets, perming is a weak point of all magnetic sensors, which contain ferromagnetic material either in the form of the sensor core or in the form of the field concentrators. In the former case the core should be periodically re-magnetized in order to erase its magnetic history; it is usually no way how to prevent perming of the field concentrators. The linear part of the single-cored sensor characteristics is shown in Fig. 5. First curve (A-B-C-D-A) was measured on carefully demagnetized sensor. The corresponding hysteresis was 20 µT. Than the sensor was subjected to the in-axis field shock of - 6 mT: the output skipped to point E, which corresponds to the gross perming effect error of 50 µT. The following characteristics is (E-B-C-D-A). After another field shock of reverse polarity (+6 mT) the characteristics is similar to the virgin one. It can be shown that the sensor response depends in a complex way on its magnetic history. This effect is caused by low saturation of the core. As the excitation current cannot be further increased because of the temperature limitations and the coil thickness is limited by the metallization technology, the only way to increase the saturation level is to decrease the demagnetization of the core by decreasing of its
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width. Such change simultaneously decreases the linear range, which is unacceptable for industrial applications; however, 60 µT range may be still high enough for compass [5]. Fig. 6 shows the same characteristics measured for novel sensor with double-sided core (X-Y-X-Z-X). As the magnetic core is almost closed, the coupling between the coils and magnetic material is much stronger and the cores are saturated more deeply. As a result, hysteresis and perming were suppressed below 5 µT. By using the double-sided core, the sensor noise was 10-times reduced to 24 nT rms (20 mHz.. 10 Hz). The sensor performance may be further improved by using pulse excitation instead of sinewave.
Conclusions Using the symmetrical magnetic core on both side of the excitation coil improves the properties of the micro-fluxgate sensor. The magnetic circuit is almost closed with respect to the excitation field. Thus using the same excitation current, the core is magnetized much deeply. This results in reduced noise, hysteresis and perming error. 24 nT rms noise, and perming below 2.5 µT was achieved for 2 mm long sensor excited by 1 MHz sinewave current. The sensor is working without a magnetic feedback. The main obstacle is the selfheating of the 3 µm thick flat coils from 180 mA excitation current. The possible solution (besides increasing the metallic layer thickness) is to use pulse excitation waveform and more complex signal processsing.
Acknowledgment This work was partly supported by the Grant Agency of the Ministry of Education of the Czech Republic under No. ME 275.
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References
[1] S. Kawahito, H. Satoh, M. Sutoh, and Y. Tadokoro: “High-resolution micro-fluxgate sensing elements using closely coupled coil structures," Sensors and Actuators A, Vol. 54, pp. 612-617, 1996 [2] T. M. Liakopoulos, C.H. Ahn, A micro-fluxgate magnetic sensor using micromachined planar solenoid coils, Sensors and Actuators A, Vol. 77, 1999, pp. 66-72. [3] Gottfried R., et al., "A miniaturized magnetic-field sensor system consisting of a planar fluxgate sensor and a CMOS readout circuity," Sensors and Actuators A, Vol. 54, 1996, pp. 443-447. [4] Kejik P., et al., "A new compact 2D planar fluxgate sensor with amorphous metal core," Sensors and Actuators A, Vol. 81, 2000, pp. 200-203 [5] L. Chiesi, P. Kejik, B. Janossy, R.S. Popovic: CMOS planar 2D Microfluxgate Sensor, Proc. Transducers 99 Conf., Sendai, p. 160, to appear in Sensors and Actuators [6] O. Dezuari, E. Belloy, S.E. Gilbert, M.A.M. Gijs: Printed circuit board integrated fluxgate sensor, Sensors and Actuators A, Vol. 81, 2000, pp. 200-203. [7] P. Ripka (ed.), Magnetic Sensors and Magnetometers, Artech, Boston - London, to be published in Dec. 2000. [8] P. Ripka, S.O.Choi, A. Tipek, S. Kawahito, M. Ishida: Symmetrical core improves microfluxgate sensors, Proc. Eurosensors 2000, Copenhagen
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BIOGRAPHY
Pavel Ripka Received an Ing. degree in 1984, a CSc (equivalent to PhD) in 1989 and Doc. degree in 1996 at the Czech Technical University, Prague, Czech Republic. During 1991/3 he was a visiting researcher at the Danish Technical University. He works at the Department of Measurement, Faculty of Electrical Engineering, Czech Technical University as a lecturer, teaching courses in Electrical Measurements and Instrumentation, Engineering Magnetism and Sensors. His main research interests are Magnetic Measurements and Magnetic sensors, especially Fluxgate. He is a member of Elektra society, Czech Metrological Society, Czech National IMEKO Committee and Eurosensors Steering Committee.
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Figure captions
Fig. 1: Structure of the fluxgate microsensor
Fig. 2: Frequency dependence of the double-core sensor sensitivity for sinewave 200 mA rms excitation
Fig. 3: Large field characteristics of the micro-fluxgate with symmetrical double-sided core
Fig. 4: Sensor sensitivity vs. excitation amplitude for 1 MHz sinewave excitation
Fig. 5: Hysteresis and perming of the single-core sensor in the 400 µT range
Fig. 6: Hysteresis and perming of the sensor with double-sided core
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Pickup coil
Ferromagnetic film (top)
Ex citation coil
3µM
4µm
15µM 5µM Ferromagnetic film (bottom) Silicon substrate
Ex citation coil
Pickup coil
Fig. 1
Sensitivity [V/T]
30
25
20
waveform - SIN Iexc=200mA
15
10
5
f [KHz] 0 200
400
600
800
1000
1200
Fig. 2
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1400
1600
1800
2000
2200
20
Vout [mV]
waveform - SIN fexc=1MHz
Iexc
15
10
5
0 -3000
-2000
-1000
0
1000
2000
-5
3000
B [µT]
Iexc=220mA Iexc=200mA Iexc=180mA -10
Iexc=160mA Iexc=140mA
-15
Iexc=120mA Iexc=100mA
-20
Fig. 3
sensitivity [V/T]
25
20
15
waveform - SIN fexc=1MHz 10
5
Iexc [mA] 0 90
100
110
120
130
140
150
160
Fig. 4
10
170
180
190
200
210
220
230
Uout [V]
12
waveform - SIN fexc=1MHz Iexc=180mA
D 10 8 6 4
C
2
A 0 -400
-300
-200
-100
0
100
E
200
300
400
-2
B [uT] 1- initial curve
-4
after -6mT shock I
-6
after +6mT shock -8
after -6mT shock II
-10
B -12
Uout [mV]
Fig. 5
waveform - SIN fexc=1MHz Iexc=200mAp-p
Z 8
3
X -500
-400
-300
-200
-100
0
100
200
300
-2
-7
B [µT]
1- initial curve after -6mT shock (first)
Y B -12
Fig. 6
11
400
500
