Comparative study of the sensing performance of orthogonal fluxgate sensors with different amorphous sensing elements

Sensors and Actuators A 136 (2007) 90–94

Comparative study of the sensing performance of orthogonal fluxgate sensors with different amorphous sensing elements Z.J. Zhao a , X.P. Li a,b , J. Fan a , H.L. Seet a , X.B. Qian a , P. Ripka c,∗ a

Department of Mechanical Engineering, National University of Singapore, Singapore b Division of Bioengineering, National University of Singapore, Singapore c Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic Received 16 May 2006; received in revised form 14 September 2006; accepted 30 October 2006 Available online 28 November 2006

Abstract A comparative study of the sensing performance of orthogonal fluxgate sensors using different amorphous wires as sensing elements is presented. The sensing elements used are Co-based amorphous wire with and without glass-coating. The results show that the sensor with amorphous wire sensing element made by cold draw is more sensitive when the excitation ac passing through the wire is of low frequency, and the sensor with glass-coated wire is more sensitive when the excitation ac is of high frequency. The results are explained by considering the magnetic properties of the wires, such as the magnetic softness of the wires, indicated by the hysteresis loops, as well as by the magneto-impedance (MI) effect ratios that tell the ac magnetic properties of the wires under the influence of “skin effect”. The MI spectrum results show that the MI ratio of the cold-draw amorphous wire is larger at low frequency and smaller at high frequency, compared to that of the glass-coated wire, which reveals that the circumferential permeability plays a key role in the sensor sensitivity and the skin effect is related to the frequency characteristics. © 2006 Published by Elsevier B.V. Keywords: Magnetic sensors; Giant magneto-impedance effect; High sensitivity

1. Introduction Magnetic sensors have been of great assistance to mankind in a variety of applications. The sensors can be used to detect the strength or direction of magnetic fields and convert the field to a corresponding electrical signal. Currently, for weak magnetic field sensing, different kinds of magnetic sensors [1–4], such as SQUID, Hall Effect sensor, fluxgate sensor, magneto-inductive sensor, magneto-impedance sensor, magneto-resistance sensor, search coil, and newly developed spin-valve sensor, have been developed. Among these sensors, SQUID has the highest sensitivity, which can be used for measurement of bio-magnetic field, ranging from 10−10 to 10−15 T. Fluxgate sensors and magnetoimpedance sensors are the most appropriate and suitable if the required sensing resolution is 10−9 to 10−10 T. Orthogonal fluxgate sensors [5–9] have the advantage of small size, and yet high sensitivity depending on the sensing element. The sensor comprises of a ferromagnetic sensing ele∗

Corresponding author. Tel.: +420 2 2435 3945; fax: +420 2 3333 9929. E-mail address: [email protected] (P. Ripka).

0924-4247/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.sna.2006.10.054

ment driven by a high frequency ac current, a signal pickup coil, and an output LC circuit formed by the signal pickup coil and a capacitor. Its theoretical noise level could be at 10−13 T at room temperature [10]. The advantages of such a highly sensitive sensor are the micro scale sensing element and ambient operation temperature, unlike SQUID which is huge and requires a complicated cooling system for low temperature operation. Some of the optimal working parameters in relation to the sensor sensitivity of orthogonal fluxgate sensor have been investigated in previous papers [9]. In this study, the sensitivity of orthogonal fluxgate sensor in relation to two different kinds of sensing elements are tested and compared: cold-drawn amorphous wire (CDAW) of 30 ␮m in diameter [11] and glass-coated amorphous wire (GCAW) of 20 ␮m in diameter [12]. 2. Experimental details In the experiments, two samples, CDAW and GCAW in equal length of 15 mm were used. The metallic diameters of CDAW and GCAW were 30 and 20 ␮m, respectively. CDAW with composition of Co68.2 Fe4.3 Si12.5 B15 was produced by

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Fig. 1. Schematic of an orthogonal fluxgate sensor.

the in-rotating-water-quenching techniques and then colddrawn (UNITIKA). GCAW with the same composition of Co68.2 Fe4.3 Si12.5 B15 was obtained by the Taylor–Ulitovsky method. In the orthogonal sensor, the sensing element was placed inside a pickup coil winding with 70 ␮m copper wire and connected to a function generator (Agilent 3240A). An adjustable capacitor is connected in parallel to the pickup coil. A schematic diagram of the sensor is shown in Fig. 1, where the sensing element is driven by an ac current of optimum frequency and amplitude selected according to each sensing element for the maximum sensitivity. The external magnetic field to be measured, Hext , was generated by a Helmholtz coil. The induced sensing output Vout in variation with the external magnetic field was measured using an oscilloscope. The sensor output will be of second harmonic signal if the magnitude of the driving current is large enough to magnetize the sensing element to saturation. It will vary sensitively with respect to external magnetic field when the resonance frequency of the LC circuit is adjusted to be at the second harmonic frequency region. In this mode the output is sinewave and therefore it can be measured by digital oscilloscope instead of lock-in amplifier which must be used for untuned sensors. The hysteresis loops were measured by conventional induction method using digital integration. The magneto-impedance measurements for the sensing elements were carried out using a precision impedance analyzer (HP4294A). The RMS value of the ac driving current was kept constant at 10 mA, and its frequency was ranged from 100 kHz to 50 MHz. The relative change of impedance ratio was defined as Z Z(Hext ) − Z(Hmax ) = × 100% Z Z(Hmax )

(1)

where Z(Hext ) and Z(Hmax ) are the impedance values of the sample under an external magnetic field Hex and under the maximum external magnetic field Hmax , respectively. The external field was provided by a Helmholtz coil. All the above described measurements were conducted inside a shielding pipe, which consists of seven layers of FINEMET sheets, separated by insulators.

Fig. 2. Comparative study of sensor output for three different sensing elements: cold-drawn amorphous wire (CDAW) and glass-coated amorphous wire (GCAW). The inset shows the sensitivity of the sensors tested at low and high frequencies within the external field of 0.5 Oe.

3. Results and discussion Fig. 2 shows the sensor output curves for sensors with different sensing elements driven by low and high frequency ac. The low frequency results were obtained using a 1000-turn pick-up coil, and the high frequency results were obtained using a 50-turn pick-up coil. In both cases the coil was connected to a low value capacitor in parallel. It can be seen that for each of the sensing elements, the sensing output displays a centrosymmetric curve about the origin, with a peak at certain external field. The sensor output came from the timevariable inductance of the micro coil. The variation of the magnetic flux of the coil with time induced a voltage between the ends of the coil. According to Faraday inductive law, the output is dφ d(NAμ(t)Hext (t)) =− dt dt   dHext (t) dμ(t) = −NA Hext +μ dt dt

Vout = −

(2)

where φ and A are the magnetic flux in the coil and cross section area of the coil, respectively, μ(t) and Hext (t) the effective ac longitudinal permeability and measured external magnetic field, respectively, and N is the number of turns of the pickup coil. The sensing element was subjected to two magnetic fields. One was the ac circumferential field induced by the driving current and another was the external magnetic field. Since the field to be measured was almost steady, the second term in the right parenthesis in Eq. (2) can be ignored. In this case, the dynamic permeability is a tensor. Even without external magnetic field, the longitudinal permeability also varies with time, owing to the magnetization by the ac circumferential field. For zero instant value of the excitation field the longitudinal permeability is maximum. For both positive and negative maximum of the excitation current the large part of the core volume is saturated in circumferential direction, so that the longitudinal permeability reaches

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its minimum twice in one period. That is why the output signal is at the second harmonics. In our case both wires have circumferential anisotropy. The sensitivity to the longitudinal external field is high due to the softness and the low demagnetization factor in this direction. When the sensing element was subjected to the external field, the circumferential permeability increased with increasing the field until reaching the circumferential anisotropy field of the element. After that, the circumferential permeability decreases with further increases of the external field. For higher values of the measured field this dependence has some effects on the waveform of the excitation field. Compared to circumferential permeability, the situation of longitudinal permeability was just opposite. For small values of the measured field the permeability time dependence is not influenced, so that the induced voltage is almost linearly proportional to the measured field. For external field approaching anisotropy field the volume of the core which is saturated by the excitation field starts to decrease. This leads to increase of the minimum axial permeability and thus decrease of the sensitivity. As the sensitivity depends on the time derivative of the axial permeability, the mentioned dependence is not straightforward. The higher driving frequency gives higher sensing output and sensitivity. This tendency can be explained by Eqs. (2) and (3), dμ ∝ fdr dt

(3)

where the differential result of permeability with respect to time is proportional to the driving frequency and so the output Vout is directly proportional to the driving frequency fdr . It should be noted that at higher frequencies this tendency is influenced by other factors such as frequency dependence of permeability and quality factor of the resonant circuit at the output. This is shown in Fig. 2. For higher frequency the number of turns was proportionally lower, so ideally the voltage sensitivity would remain the same. The observed changes in sensitivity (increase for GCAW and decrease for CDAW) are caused by the mentioned second-order effects. As can be seen from Fig. 2, both the CDAW and GCAW showed very sharp output signal increases with increasing external magnetic field in the weak field range. At low frequency, the sensor with CDAW sensing element seems to have the best sensitivity. At high frequency, however, the sensor with GCAW sensing element has the higher sensitivity. In order to explain different characteristics of the two sensor elements, we measured their basic magnetic properties. Fig. 3 shows the longitudinal hysteresis loops of the sensing elements tested at 300 Hz using a conventional induction method. It can be seen that GCAW has a coercivity of 0.10 Oe, smaller than that of CDAW, 0.32 Oe. As can be seen from Fig. 3, both of the two wires were not of pure circumferential anisotropy structure. The GCAW has an inner core with radial anisotropy and an outer shell with circumferential anisotropy [12]. So the coercivity of the GCAW is very small. The anisotropy of CDAW wire may have a larger deviation angle from circumferential direction to longitudinal direction, due to an inner core with longitudinal magnetization [11], which is one reason for the larger coerciv-

Fig. 3. Hysteresis loops of the cold-drawn amorphous wire (CDAW) and glasscoated amorphous wire (GCAW).

ity. The different magnetic anisotropy of the inner core of the GCAW and CDAW may also affect the frequency characteristic of the circumferential permeability μϕ , which has the similar trend with sensitivity of the orthogonal fluxgate as discussed above. We used magnetoimpedance effect (MI) to examine this frequency characteristic, since the sensitivity of MI sensors is √ proportional to the μφ [11]. Fig. 4(a) and (b) shows the MI ratios in variation with an external magnetic field for GCAW and CDAW, respectively. Both of the two kinds of sensing elements showed double-peak MI curves at high frequency. The MI ratio spectrums of GCAW and CDAW are shown in Fig. 5. It can be seen that in the lower frequency range, the MI ratio of GCAW was smaller than that of CDAW, but at higher frequency range the MI ratios for GCAW were higher than those of CDAW. This is consistent with the frequency dependence of sensing output for sensors using GCAW and CDAW, as shown in Fig. 2. It can be explained by the nature of MI, which is based on the circumferential permeability of the sensing element in variation with the external magnetic field. Driven by an ac current, the impedance of the sensing element depends on the skin-effect depth,  2 δ= (4) σμφ ω where σ is the conductivity and ω is the angular frequency of the ac current [13]. The MI ratio is related to the dependence of circumferential permeability on the external magnetic field, while the orthogonal fluxgate sensor sensitivity is dependent on the differential of the longitudinal permeability of the sensing elements with time. It can be clearly seen from Figs. 2 and 4 that the MI ratio and the sensor sensitivity have similar trends in variation with the external field. The external magnetic field affects the magnetization distribution and consequently affects the magnetic permeability of the wire. As a result, the impedance becomes a function of the magnetic field. Since GCAW and CDAW have the same composition, the difference in the MI ratio

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spectrum must be due to the differences in their diameter and anisotropy. Theoretically, the MI reaches the maximum while the skin depth is equivalent to the geometric size, therefore D2 ∝

1 μφ ω

(5)

where D is the diameter of the wire. Hence generally, if the magnetic properties of the wires were identical, for smaller diameter wire the optimum excitation frequency is higher. For CDAW the diameter was 30 ␮m and MI optimum frequency was 2.5 MHz, while for GCAW these values were 20 ␮m and 14 MHz, respectively. The ratio of the optimum frequencies would be (30/20)2 = 2.25, while the measured ratio was 14/2.5 = 5.6. The discrepancy between the measure ratio and the one calculated from the diameter ratio could be attributed to the difference between the circumferential permeabilities of CDAW and GCAW, as can be observed from the MI curves shown in Fig. 4. 4. Conclusion

Fig. 4. MI ratio in variation with an external magnetic field for: (a) glass-coated amorphous wire and (b) cold-drawn amorphous wire.

For orthogonal fluxgate sensor, the sensitivity performance of sensing elements of two kinds of materials, cold-drawn amorphous wire and glass-coated amorphous wire, has been tested. It has been found that the circumferential permeability, and the skin effect of the sensing element are the key factors that dominate the sensor sensitivity. For better sensitivity, the cold-drawn amorphous wire sensing element should be driven by a lower frequency ac compared to that for the glass-coated amorphous wire. Such a behavior was caused both by different diameters and different magnetic properties of the wires. Therefore, a high frequency has to be used for the excitation current when a small diameter core is used and vice versa. Acknowledgements The authors wish to thank the Defence Science & Technology Agency of Singapore for the finance support of this project. Also thanks are given to Prof. H. Chiriac of National Institute of Research and Development for Technical Physics, Romania, for the glass-coated amorphous wire samples. References

Fig. 5. Maximum MI spectrum of the cold-drawn amorphous wire (CDAW) and glass-coated amorphous wire (GCAW).

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Biographies Z.J. Zhao was born in Hebei, China, in 1970. He received his Ph.D. degree in Condensed Matter Physics from Lanzhou University in 1999. He worked in National University of Singapore as a research fellow from May 2002 to August 2004. Currently, he is professor in Department of Physics, East China Normal University. His research interests include M¨ossbauer spectroscopy and its applications, nanomagnetism, magnetic sensors. X.P. Li received his Ph.D. in 1991 from University of New South Wales, Australia. He joined the National University of Singapore in 1992, where he is currently an associate professor in the Department of Mechaial Engineering, Division of Bioengineering, and Graduate Programme of Bioengineering. His research current interests include neurosensors and nanomaching. J. Fan received his B. Eng. Degree from University of Science and Technology of China, in 2003 and currently is a Ph.D. student in the Department of Mechanical Engineering in National University of Singapore. His research interests include the micromagnetism, magnetic sensors and magnetometers, and magnetic sources detection system. H.L. Seet received his B. Eng. degree in Mechanical Engineering from National University of Singapore (NUS), Singapore in 2002. He is currently pursu-

ing his PhD and is a research fellow in Neurosensors Laboratory, Department of Mechanical Engineering, NUS. His current research interests include magnetic sensor materials, nanomaterials fabrication processes and characterization methods. X.B. Qian received B.Sc. degree from Beijing Institute of Technology, PR China, in 1991, M.Sc. degree from Institute of Physics, Chinese Academy of Sciences, in 1996, and Ph.D. degree from National University of Singapore in 2005, with research direction on on-chip readout circuits for microbolometer focal plane array. From 1996 to 1999, she was a research engineer in Institute of Acoustics, Chinese Academy of Sciences, worked on sonar signal receiving and processing systems. Now she is employed by Department of Mechanical Engineering and Division of Bioengineering, National University of Singapore as a research fellow. Her research interest is low-noise low-power integrated circuits design and biomedical sensor electronics, including electroencephalography IC, magnetcardiography IC, low-noise amplifier, filter and data converters, etc. Pavel Ripka received an Ing degree in 1984, a CSc (equivalent to PhD) in 1989 and Prof. degree in 2001 at the Czech Technical University, Prague, Czech Republic. He works at the Department of Measurement, Faculty of Electrical Engineering, Czech Technical University as a full professor, teaching courses in electrical measurements and instrumentation, engineering magnetism and sensors. He also worked as visiting scientist at Danish Technical University (1990-93), National University of Ireland (2001) and in the Institute for the Protection and the Security of the Citizen, European Commission Joint Research Centre in Italy (2005/6). His main research interests are magnetic measurements and magnetic sensors, especially fluxgate. He is an author of >50 SCI journal papers and 5 patents. He is a member of IEEE, Elektra society, Czech Metrological Society, Czech National IMEKO Committee and Eurosensors Steering Committee. He served as an associate editor of the IEEE Sensors Journal. He was a General Chairman of Eurosensors 2002 conference.