- Email: [email protected]
Planar fluxgate layouts using a single layer of coils
Accepted Manuscript Title: Planar fluxgate layouts using a single layer of coils Author: Tobias Heimfarth Marcelo Mulato PII: DOI: Reference:
S0924-4247(14)00165-4 http://dx.doi.org/doi:10.1016/j.sna.2014.03.040 SNA 8741
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-6-2013 26-3-2014 31-3-2014
Please cite this article as: Tobias Heimfarth, Marcelo Mulato, Planar fluxgate layouts using a single layer of coils, Sensors & Actuators: A. Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.03.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tobias Heimfartha,∗, Marcelo Mulatoa,∗∗ a
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Planar fluxgate layouts using a single layer of coils
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Department of Physics, Ribeir˜ ao Preto School of Philosophy, Science and Literature, University of S˜ ao Paulo, 14040-901, 3900 Bandeirantes Av., Ribeir˜ ao Preto-SP, Brazil
Abstract
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This work describes the development of planar fluxgate magnetometers consisting of a single coils’ layer. The constructed sensors can achieve analogous noise levels to a similar double layer device, while reducing the manufacture complexity. The tread-off is an early limit in noise reduction by increasing the excitation amplitude, caused by the non-directly excited core part. But placing extra excitation coils parallel to the pick-up ones can avoid this minimum threshold. Fabrication and characterization are presented and discussed.
1. Introduction
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Keywords: Fluxgate, NiFe, Magnetic sensor, Planar layout, Electrodeposition, Noise analysis
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Magnetometers enable a wide range of applications such as navigation systems, magnetic-based medical diagnostics, data storage, magnetic ink reading and space exploration [1]. Among various magnetic sensors types, fluxgate is a general use vectorial magnetometer with good trade-off between resolution, power consumption, thermal offset and easy of use. They are able to measure from static magnetic fields up to low frequencies in the 10−3 to 10−11 T range [2]. In order to develop cheaper, smaller and more energy efficient fluxgates, miniaturization using thin films technology is one way to go. However, there ∗
Corresponding author. Corresponding author. Tel.: +55 1636024874. Email addresses: [email protected] (Tobias Heimfarth ), [email protected] (Marcelo Mulato ) ∗∗
Preprint submitted to Sensors and Actuators A
March 26, 2014
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is an important loss in performance, especially in resolution due to the increase of noise [2]. Also, the sensor layout has some new constrains brought by the thin film process: all parts have to be accommodated in parallel layers, that is, a planar layout. A fluxgate sensor uses at least two inductors (one for the core excitation and another for picking up the output signal) but usually three or more [3]. Two main approaches are the most common for manufacturing these elements in planar layouts: planar spiral coils [4] and 3D solenoids [5]. Although the planar spiral coils have worst coupling with the sensor’s core, they are simpler to make, using only a single metal layer. But commonly excitation elements and the pick-up ones are placed in at least two layers, for coupling reasons. That takes away the fewer layers advantage over the 3D solenoids, that intrinsically needs two. In this work some important parameters (sensitivity and noise) of several fluxgates planar layouts were studied. The main idea was to place all the planar coils in a single layer, reducing the building process complexity. To better understand the drawbacks and further consequences of this approach the results are here compared with an already know layout [6]. Additionally, the results for duplicated coil layers, all using the same manufacturing process, are also presented. The tested devices use PCB (printed circuit board) for the coils structures and electroplated NiFe alloy for the sensor’s cores. 2. Layouts Description
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The starting point layout was taken from [6] and labeled reference layout (RL, see figure 1). By simply moving apart the pick-up coils in order to make room for the excitation one in the same metal layer, layout 1 (L1) is introduced. This give raise to a single coil’s layer fluxgate, but with the disadvantage of keeping half of the core length not immersed in an intense excitation field. A second configuration called layout 2 (L2) was also developed. Two extra excitation coils were inserted in the middle of the pick-up ones. They should help the excitation of the outmost core areas. The tradeoff is that both types now have half the spire density when compared to the original ones. By staking two mirrored coil layers the perpendicular excitation field is attenuated and also some excitation amplitude limits due to overheating and equipment power can be overcome, thus this parameter can be studied in a wider rage. This double layer layouts are labeled, double layout 1 (L1d) and double layout 2 (L2d).
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3. Simulations, Fabrication and Experimental set-up configuration
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As can be seen in the finite element simulations (figure 2a), the magnetic field generated by a single planar coils is very inhomogeneous and almost vanish in the regions not directly upon the element. Similar results are found in [4]. In L2 (figure 2c) a significant improvement in the outer cores region is achieved by placing the extra coils. In both cases, the perpendicular component Hz has the same magnitude as the parallel one Hx , not expected for an ideal parallel fluxgate sensor. In the double version layouts the mirrored coils suppress Hz at the same time doubling Hx . Finite element calculations also showed that much higher Hexc to current ratio can be achieved scaling down the layouts. The cores of the sensors were made of electroplated NiFe alloys using the bath described in [7] with the replacement of the acid saccharin by the sodium saccharin: NiSO4 (0.7 mol/l), FeSO4 (0.03 mol/l), NiCl2 (0.002 mol/l), H3 BO3 (0.4 mol/l) and sodium saccharin (0.016 mol/l). The electroplating used a galvanostatic regime with 14 mA/cm2 for 40 minutes [8]. The electroplating process leads to a 7 µm tick, 3 mm wide and 13 mm long NiFe film on top of a 0.3 mm tick cooper sheet substrate. The coils have 18 spires with 220 µm width each and spaced by the same distance. The total coil width is 18 mm that leaves a space in the center where one of the contact pads stands. The cooper layer thickness is about 35 µm. The double coils used in L2 reduce the number of spires by half, but the other dimensions were not changed. The B(H) curves of the cores used are shown in figure 3. B(H) curves were also measured in the extremities of the core when excited by the actual planar coils, in order to investigate the magnetic state in these regions of interest. The sensors were characterized using the following experimental setup (figure 4).The system has an excitation current limit of 1 A. As the H/current ratio varies between layouts, the same occurs to the maximum applied excitation field. The excitation was done by a sine wave with a constant frequency of 100 kHz. 4. Sensitivity Results For each point in the sensitivity data (figure 5) five response curves were taken, containing 10 points in the field range from 0 to 50 µT . The sensitivity is the average of the linear fit slopes and the error bars correspond to the 3
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standard deviation. The maximum measured sensitivity, extracted from the curves, are (in V /T ): 65 at 1.1 kA/m for RL; 158 at 1.15 kA/m for L1; 115 at 0.8 kA/m for L2; 263 at 1.0 kA/m for L1d; 185 at 1.25 kA/m for L2d. RL presented the lowest sensitivity due to the half core length, with higher demagnetization (that can be seen in the lower Bsat in figure 3a). Furthermore the pick-up coils are slightly more distant than in the case of L1 and L2. The same occurs for the second layer of the doubled layouts, what is due to the core cooper substrate (300 µm thick). The coupling of these coils with the core is worse, both in pick-up and excitation ones. This later effect was compensated by decreasing the H/current ratio in L1d and L2d. The poorer couplings explain why doubling the pick-up coils does not double the sensitivity. Also, the sensitivity maximum occurs for higher Hexc in the double layer sensors, which can be explained by the attenuation of the Hz component. Let alone, sensitivity values are relevant in case of signal amplification limitations, but does not say much about the sensor resolution and other important parameters. Especially in the case of fluxgate sensors, these values can vary orders of magnitude. It depends on lots of factors like number of pick-up coil spires, core materials, pick-up coils/cores coupling, cores dimensions and excitation frequency. The devices were manufactured keeping these factors the same, as much as possible, so the values can be better directly compared. But the major role of these measurements are to normalize the noise values as seen in the next section. 5. Noise Results
For each Hexc , five 60 seconds long data stream were collected with a 2 milliseconds interval between points. The power spectral density were estimated by the Welch’s method using 10000 points per segment, followed by averaging the five resulting spectra. Figure 6 shows the rms noise present in the frequency range of 0.1 to 10 Hz. All layouts presented about the same noise level and behavior in the sub-saturation excitation region, rising an order of magnitude for excitations up to 0.5 kA/m. From 0.5 to 1 kA/m, the saturation region, the highest levels were found. Some strong low frequency fluctuations occur for higher excitations, particulary for RL (at 1 kA/m) and L2 (at 1.15 kA/m). It can be explained by hard to magnetize core volumes that do not consistently change its magnetization each excitation cycle. This kind of behavior was already described 4
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by [9], but for much higher H values. The instabilities seems to be related to single or few clusters because of their statistical properties: they are strongly related to a specific Hexc . Figure 7 shows a section of the time series measured in the L2 case, which is consistent with the few domain instability. One can expect to find this phenomena in the L2d at similar Hexc as the same cores were used in both sensors. In fact, at Hexc =1.2 kA/m, L2d presented a noise peak caused by the same two levels oscilation seen in its single counterpart. Likewise these patterns are related to the big noise increase in the RL sensor at about 1 kA/m. For excitation amplitudes >1 kA/m, while the noise for L1d stabilize at about 7 µVrms , L2d shows a slight decreasing trend. This finding is compatible with the expected noise level of the underexcited core part: once the fully excited part is saturated, a further increase in Hexc will not help in this outer region. This effect is mitigated in L2(d) because of the inserted excitation coils. As for the single layers layouts, due to the current limit of the experimental setup, it was not possible to scan this high excitation amplitude region. It can only be inferred that the behavior stands. For RL, the same experimental limitation persists, but in this case there are enough evidence in the literature to say that the noise decreases [1], despite the large fluctuations found this particular study. The sensitivity-normalized noise levels, relevant for the determination of the resolution of the devices, are shown in figure 8. They are the result of dividing the values of figure 5 by the ones in figure 6. A simple empirical 1/Hexc behavior describe fairly well the proposed layouts overall trends, but not RL. It has exponential-like decay but is much affected by the already decribed fluctuations. RL has the best case excitation profile, with no underexcited regions, therefore it is expected that the noise decays with a higher ratio. But its shorter core results in higher demagnetizing field, enough to keep its noise levels higher than the other layouts in the tested range. Note that doubling the core length results in a fourfold sensor area increase. Both L1 and L2 have the same overall behavior as their double counterparts in the measured region and do not differ significantly from each other also. Although L2 present some particular noise levels lower than L1 it also fluctuate to much higher ones. In the excitation amplitude region Hexc >1.2 kA/m (not achievable by RL, L1 and L2), L1d remains roughly constant with a sight increasing trend, deviating from the 1/Hexc reference. Its negative sensitivity’s slope (fig. 5b) in association with constant raw noise in the high excitation amplitudes region (fig. 6c) resulting in the slowly increasing normalized noise 5
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(fig. 8c). L2d, on the other hand, presented a constant saturated sensitivity for Hexc > 1 kA/m (fig. 5c), that allied with the decreasing raw noise level in this region (fig. 6e), leads to a decreasing normalized noise (fig. 8e) despite some small fluctuations. This seems to be a much better result. According to what was presented above, and considering the general trends, the bests measured noise characteristics were obtained for the L2 based sensors.
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6. Conclusions
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The planar fluxgates with a single layer of coils presented similar to lower noise levels compared to a double layer reference with the same coils geometrical parameters in the low excitation amplitude region studied. Although the levels decreases with a higher ratio in the reference layout it was more affected by low frequency fluctuations caused by hard to magnetize core volumes. The simpler single layer layout proposed reaches a minimum noise level that can not be reduced just by increasing the excitation amplitude. But by inserting auxiliary excitation coils parallel to the pick-up ones this problem can be mitigated. These coils helps in the saturation of the outer parts of the magnetic core reducing the noise levels. 7. Acknowledgements
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This work was supported by FAPESP, CAPES and CNPq brazilian agencies. References
[1] P. Ripka, Magnetic Sensors and Magnetometers, Artech House Publishers, 2001. [2] P. Ripka, Advances in fluxgate sensors, Sensors and Actuators A: Physical 106 (1-3) (2003) 8–14. doi:10.1016/S0924-4247(03)00094-3. [3] P. Ripka, Review of fluxgate sensors, Sensors and Actuators A: Physical 33 (3) (1992) 129–141. doi:10.1016/0924-4247(92)80159-Z. [4] I. Vincueria, M. Tudanca, C. Aroca, E. Lopez, M. Sanchez, P. Sanchez, Flux-gate sensor based on planar technology, IEEE Transactions on Magnetics 30 (6) (1994) 5042 –5045. doi:10.1109/20.334293. 6
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[5] S. Kawahito, H. Satoh, M. Sutoh, Y. Todokoro, High-resolution microfluxgate sensing elements using closely coupled coil structures, Sensors and Actuators A: Physical 54 (1–3) (1996) 612–617. doi:10.1016/S09244247(97)80024-6.
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[6] S. Choi, S. Kawahito, Y. Matsumoto, M. Ishida, Y. Tadokoro, An integrated micro fluxgate magnetic sensor, Sensors and Actuators A: Physical 55 (2–3) (1996) 121–126. doi:10.1016/S0924-4247(97)80066-0.
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[7] J.-M. Quemper, S. Nicolas, J. Gilles, J. Grandchamp, A. Bosseboeuf, T. Bourouina, E. Dufour-Gergam, Permalloy electroplating through photoresist molds, Sensors and Actuators A: Physical 74 (1-3) (1999) 1–4. doi:10.1016/S0924-4247(98)00323-9.
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[8] T. C. dos Santos, M. Mulato, Analysis of electrodeposited NiFe thin films for the development of planar fluxgate magnetic sensors, MRS Online Proceedings Library 998. doi:10.1557/PROC-998-J05-21.
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[9] D. Scouten, Sensor noise in low-level flux-gate magnetometers, IEEE Transactions on Magnetics 8 (2) (1972) 223 – 231. doi:10.1109/TMAG.1972.1067284.
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Figure 1: Layouts schemes, exploded view. In black the excitation coils, in light gray the pick-up coils and in dark gray the cores of the sensors. (a) Reference layout (RL), taken from literature [6] with minimum change. The pick-up coils stands in the lower metal layer and the excitation one in the higher with the core between them. (b) Moving apart the pick-up coils from RL in order to place the excitation one in the middle we have L1, a single coil layer layout. Its double version L1d is achieved by staking and identical coil’s layer on top of it. (c) To help the L1 excitation coil to excite the not immediately above part of the core, two extra excitation coils were insert in the middle of the pick-up ones leading to L2 and L2d. Not shown here but present are the insulation layers between the coils and the cores.
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Figure 2: H field components generated by the planar coils (b) computed at a line (c. line) 150 µm above the coils plane aligned with the middle. This is the place where the sensors’ core was placed. A current of 1 A was used as a reference as the H field is linear with respect to the current. The single excitation coil of L1 (a) leads to near half core length overlap, a problem mitigated by placing a half spire density coil inside the pick-up ones as done in L2 (b). The region from where the reference Hexc field to applied current ratio was taken, based on Hx , are indicated by the arrows.
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Figure 3: B(H) curves taken from the core sample used in the sensors. All the layouts (L1, L2, L1d and L2d) used the same sample core (labeled Lx), except for RL. (a) An external, Helmholtz coil produced, H field was used to measure the middle plane magnetic field (B) in the samples. Both Lx and RL curves have the same shape, but for the latter, the demagnetization is slightly stronger. (b) Now the H field is generated by the actual sensors planar coils and B is measured by regular coils placed in both Lx core extremities. The outer region (1) has a lower magnetization compared to the directly excited inner region (2) in the L1 layout. But the curve shape is similar due to the inner core part driving. (c) A much higher magnetization is achieved in the outer region by the L2 layout, almost in pair with the inner region.
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Figure 4: Experimental setup for the sensitivity (left) and noise (right) characterizations. The computer (a) sets the parameters of the lock-in amplifier (b, SR844 DSP) responsible for the excitation signal generation. This signal is amplified by a power amplifier (c, based on an OPA564) directly connected with the excitation coils of the fluxgate (d). The pickup coils are connected to the lock-in which delivery the output back to the computer. In the noise setup, the sensor is surrounded by a three layers mu-metal magnetic shield (light grey). The external field is only required by the sensitivity tests. The computer sets the parameters of a power supply (e, HP 6653A) measured externally by a multimeter (f, Agilent 34401A) linked with the computer. The current flows in a Helmholtz coil (g) responsible for the external magnetic field.
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Figure 5: Sensitivity as a function of the excitation amplitude for all the layouts. The response was taken from the slope of the curves using a linear fit of the ten points measured up to an external field of 100µT .
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Figure 7: Low frequency sensor output fluctuations example taken from L2. a) At Hexc = 1.1 kA/m there are relative low noise levels (the values are shown in the graph header by the Nrms variable) with few sharp level changes (indicated by the arrows). The responsible for these jumps is hypothesize to be a single core volume hard to magnetize. b) Singly increasing the excitation amplitude to 1.15 kA/m makes this region approximately half of the time active in a low frequency pattern giving rise to the sudden increase in the noise levels seen in the figure 6a. c) With Hexc = 1.2 kA/m the system is able drive the volume most of the time, increasing the frequency and reducing the noise levels generated. As the excitation field increases more, this core volume will aling with the drive field consistently no more generating such fluctuations.
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We developed planar fluxgate magnetometers using a single layer of coils. The sensitivity were found to be up to three times higher than a analogous double layer sensor. The noise levels were found to be similar to an analogous double layer sensor. Extra excitation coils and higher excitation amplitudes can further reduce the noise.
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S0924-4247(14)00165-4 http://dx.doi.org/doi:10.1016/j.sna.2014.03.040 SNA 8741
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-6-2013 26-3-2014 31-3-2014
Please cite this article as: Tobias Heimfarth, Marcelo Mulato, Planar fluxgate layouts using a single layer of coils, Sensors & Actuators: A. Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.03.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tobias Heimfartha,∗, Marcelo Mulatoa,∗∗ a
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Planar fluxgate layouts using a single layer of coils
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Department of Physics, Ribeir˜ ao Preto School of Philosophy, Science and Literature, University of S˜ ao Paulo, 14040-901, 3900 Bandeirantes Av., Ribeir˜ ao Preto-SP, Brazil
Abstract
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This work describes the development of planar fluxgate magnetometers consisting of a single coils’ layer. The constructed sensors can achieve analogous noise levels to a similar double layer device, while reducing the manufacture complexity. The tread-off is an early limit in noise reduction by increasing the excitation amplitude, caused by the non-directly excited core part. But placing extra excitation coils parallel to the pick-up ones can avoid this minimum threshold. Fabrication and characterization are presented and discussed.
1. Introduction
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Keywords: Fluxgate, NiFe, Magnetic sensor, Planar layout, Electrodeposition, Noise analysis
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Magnetometers enable a wide range of applications such as navigation systems, magnetic-based medical diagnostics, data storage, magnetic ink reading and space exploration [1]. Among various magnetic sensors types, fluxgate is a general use vectorial magnetometer with good trade-off between resolution, power consumption, thermal offset and easy of use. They are able to measure from static magnetic fields up to low frequencies in the 10−3 to 10−11 T range [2]. In order to develop cheaper, smaller and more energy efficient fluxgates, miniaturization using thin films technology is one way to go. However, there ∗
Corresponding author. Corresponding author. Tel.: +55 1636024874. Email addresses: [email protected] (Tobias Heimfarth ), [email protected] (Marcelo Mulato ) ∗∗
Preprint submitted to Sensors and Actuators A
March 26, 2014
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is an important loss in performance, especially in resolution due to the increase of noise [2]. Also, the sensor layout has some new constrains brought by the thin film process: all parts have to be accommodated in parallel layers, that is, a planar layout. A fluxgate sensor uses at least two inductors (one for the core excitation and another for picking up the output signal) but usually three or more [3]. Two main approaches are the most common for manufacturing these elements in planar layouts: planar spiral coils [4] and 3D solenoids [5]. Although the planar spiral coils have worst coupling with the sensor’s core, they are simpler to make, using only a single metal layer. But commonly excitation elements and the pick-up ones are placed in at least two layers, for coupling reasons. That takes away the fewer layers advantage over the 3D solenoids, that intrinsically needs two. In this work some important parameters (sensitivity and noise) of several fluxgates planar layouts were studied. The main idea was to place all the planar coils in a single layer, reducing the building process complexity. To better understand the drawbacks and further consequences of this approach the results are here compared with an already know layout [6]. Additionally, the results for duplicated coil layers, all using the same manufacturing process, are also presented. The tested devices use PCB (printed circuit board) for the coils structures and electroplated NiFe alloy for the sensor’s cores. 2. Layouts Description
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The starting point layout was taken from [6] and labeled reference layout (RL, see figure 1). By simply moving apart the pick-up coils in order to make room for the excitation one in the same metal layer, layout 1 (L1) is introduced. This give raise to a single coil’s layer fluxgate, but with the disadvantage of keeping half of the core length not immersed in an intense excitation field. A second configuration called layout 2 (L2) was also developed. Two extra excitation coils were inserted in the middle of the pick-up ones. They should help the excitation of the outmost core areas. The tradeoff is that both types now have half the spire density when compared to the original ones. By staking two mirrored coil layers the perpendicular excitation field is attenuated and also some excitation amplitude limits due to overheating and equipment power can be overcome, thus this parameter can be studied in a wider rage. This double layer layouts are labeled, double layout 1 (L1d) and double layout 2 (L2d).
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3. Simulations, Fabrication and Experimental set-up configuration
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As can be seen in the finite element simulations (figure 2a), the magnetic field generated by a single planar coils is very inhomogeneous and almost vanish in the regions not directly upon the element. Similar results are found in [4]. In L2 (figure 2c) a significant improvement in the outer cores region is achieved by placing the extra coils. In both cases, the perpendicular component Hz has the same magnitude as the parallel one Hx , not expected for an ideal parallel fluxgate sensor. In the double version layouts the mirrored coils suppress Hz at the same time doubling Hx . Finite element calculations also showed that much higher Hexc to current ratio can be achieved scaling down the layouts. The cores of the sensors were made of electroplated NiFe alloys using the bath described in [7] with the replacement of the acid saccharin by the sodium saccharin: NiSO4 (0.7 mol/l), FeSO4 (0.03 mol/l), NiCl2 (0.002 mol/l), H3 BO3 (0.4 mol/l) and sodium saccharin (0.016 mol/l). The electroplating used a galvanostatic regime with 14 mA/cm2 for 40 minutes [8]. The electroplating process leads to a 7 µm tick, 3 mm wide and 13 mm long NiFe film on top of a 0.3 mm tick cooper sheet substrate. The coils have 18 spires with 220 µm width each and spaced by the same distance. The total coil width is 18 mm that leaves a space in the center where one of the contact pads stands. The cooper layer thickness is about 35 µm. The double coils used in L2 reduce the number of spires by half, but the other dimensions were not changed. The B(H) curves of the cores used are shown in figure 3. B(H) curves were also measured in the extremities of the core when excited by the actual planar coils, in order to investigate the magnetic state in these regions of interest. The sensors were characterized using the following experimental setup (figure 4).The system has an excitation current limit of 1 A. As the H/current ratio varies between layouts, the same occurs to the maximum applied excitation field. The excitation was done by a sine wave with a constant frequency of 100 kHz. 4. Sensitivity Results For each point in the sensitivity data (figure 5) five response curves were taken, containing 10 points in the field range from 0 to 50 µT . The sensitivity is the average of the linear fit slopes and the error bars correspond to the 3
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standard deviation. The maximum measured sensitivity, extracted from the curves, are (in V /T ): 65 at 1.1 kA/m for RL; 158 at 1.15 kA/m for L1; 115 at 0.8 kA/m for L2; 263 at 1.0 kA/m for L1d; 185 at 1.25 kA/m for L2d. RL presented the lowest sensitivity due to the half core length, with higher demagnetization (that can be seen in the lower Bsat in figure 3a). Furthermore the pick-up coils are slightly more distant than in the case of L1 and L2. The same occurs for the second layer of the doubled layouts, what is due to the core cooper substrate (300 µm thick). The coupling of these coils with the core is worse, both in pick-up and excitation ones. This later effect was compensated by decreasing the H/current ratio in L1d and L2d. The poorer couplings explain why doubling the pick-up coils does not double the sensitivity. Also, the sensitivity maximum occurs for higher Hexc in the double layer sensors, which can be explained by the attenuation of the Hz component. Let alone, sensitivity values are relevant in case of signal amplification limitations, but does not say much about the sensor resolution and other important parameters. Especially in the case of fluxgate sensors, these values can vary orders of magnitude. It depends on lots of factors like number of pick-up coil spires, core materials, pick-up coils/cores coupling, cores dimensions and excitation frequency. The devices were manufactured keeping these factors the same, as much as possible, so the values can be better directly compared. But the major role of these measurements are to normalize the noise values as seen in the next section. 5. Noise Results
For each Hexc , five 60 seconds long data stream were collected with a 2 milliseconds interval between points. The power spectral density were estimated by the Welch’s method using 10000 points per segment, followed by averaging the five resulting spectra. Figure 6 shows the rms noise present in the frequency range of 0.1 to 10 Hz. All layouts presented about the same noise level and behavior in the sub-saturation excitation region, rising an order of magnitude for excitations up to 0.5 kA/m. From 0.5 to 1 kA/m, the saturation region, the highest levels were found. Some strong low frequency fluctuations occur for higher excitations, particulary for RL (at 1 kA/m) and L2 (at 1.15 kA/m). It can be explained by hard to magnetize core volumes that do not consistently change its magnetization each excitation cycle. This kind of behavior was already described 4
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by [9], but for much higher H values. The instabilities seems to be related to single or few clusters because of their statistical properties: they are strongly related to a specific Hexc . Figure 7 shows a section of the time series measured in the L2 case, which is consistent with the few domain instability. One can expect to find this phenomena in the L2d at similar Hexc as the same cores were used in both sensors. In fact, at Hexc =1.2 kA/m, L2d presented a noise peak caused by the same two levels oscilation seen in its single counterpart. Likewise these patterns are related to the big noise increase in the RL sensor at about 1 kA/m. For excitation amplitudes >1 kA/m, while the noise for L1d stabilize at about 7 µVrms , L2d shows a slight decreasing trend. This finding is compatible with the expected noise level of the underexcited core part: once the fully excited part is saturated, a further increase in Hexc will not help in this outer region. This effect is mitigated in L2(d) because of the inserted excitation coils. As for the single layers layouts, due to the current limit of the experimental setup, it was not possible to scan this high excitation amplitude region. It can only be inferred that the behavior stands. For RL, the same experimental limitation persists, but in this case there are enough evidence in the literature to say that the noise decreases [1], despite the large fluctuations found this particular study. The sensitivity-normalized noise levels, relevant for the determination of the resolution of the devices, are shown in figure 8. They are the result of dividing the values of figure 5 by the ones in figure 6. A simple empirical 1/Hexc behavior describe fairly well the proposed layouts overall trends, but not RL. It has exponential-like decay but is much affected by the already decribed fluctuations. RL has the best case excitation profile, with no underexcited regions, therefore it is expected that the noise decays with a higher ratio. But its shorter core results in higher demagnetizing field, enough to keep its noise levels higher than the other layouts in the tested range. Note that doubling the core length results in a fourfold sensor area increase. Both L1 and L2 have the same overall behavior as their double counterparts in the measured region and do not differ significantly from each other also. Although L2 present some particular noise levels lower than L1 it also fluctuate to much higher ones. In the excitation amplitude region Hexc >1.2 kA/m (not achievable by RL, L1 and L2), L1d remains roughly constant with a sight increasing trend, deviating from the 1/Hexc reference. Its negative sensitivity’s slope (fig. 5b) in association with constant raw noise in the high excitation amplitudes region (fig. 6c) resulting in the slowly increasing normalized noise 5
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(fig. 8c). L2d, on the other hand, presented a constant saturated sensitivity for Hexc > 1 kA/m (fig. 5c), that allied with the decreasing raw noise level in this region (fig. 6e), leads to a decreasing normalized noise (fig. 8e) despite some small fluctuations. This seems to be a much better result. According to what was presented above, and considering the general trends, the bests measured noise characteristics were obtained for the L2 based sensors.
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The planar fluxgates with a single layer of coils presented similar to lower noise levels compared to a double layer reference with the same coils geometrical parameters in the low excitation amplitude region studied. Although the levels decreases with a higher ratio in the reference layout it was more affected by low frequency fluctuations caused by hard to magnetize core volumes. The simpler single layer layout proposed reaches a minimum noise level that can not be reduced just by increasing the excitation amplitude. But by inserting auxiliary excitation coils parallel to the pick-up ones this problem can be mitigated. These coils helps in the saturation of the outer parts of the magnetic core reducing the noise levels. 7. Acknowledgements
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This work was supported by FAPESP, CAPES and CNPq brazilian agencies. References
[1] P. Ripka, Magnetic Sensors and Magnetometers, Artech House Publishers, 2001. [2] P. Ripka, Advances in fluxgate sensors, Sensors and Actuators A: Physical 106 (1-3) (2003) 8–14. doi:10.1016/S0924-4247(03)00094-3. [3] P. Ripka, Review of fluxgate sensors, Sensors and Actuators A: Physical 33 (3) (1992) 129–141. doi:10.1016/0924-4247(92)80159-Z. [4] I. Vincueria, M. Tudanca, C. Aroca, E. Lopez, M. Sanchez, P. Sanchez, Flux-gate sensor based on planar technology, IEEE Transactions on Magnetics 30 (6) (1994) 5042 –5045. doi:10.1109/20.334293. 6
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[5] S. Kawahito, H. Satoh, M. Sutoh, Y. Todokoro, High-resolution microfluxgate sensing elements using closely coupled coil structures, Sensors and Actuators A: Physical 54 (1–3) (1996) 612–617. doi:10.1016/S09244247(97)80024-6.
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[6] S. Choi, S. Kawahito, Y. Matsumoto, M. Ishida, Y. Tadokoro, An integrated micro fluxgate magnetic sensor, Sensors and Actuators A: Physical 55 (2–3) (1996) 121–126. doi:10.1016/S0924-4247(97)80066-0.
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[7] J.-M. Quemper, S. Nicolas, J. Gilles, J. Grandchamp, A. Bosseboeuf, T. Bourouina, E. Dufour-Gergam, Permalloy electroplating through photoresist molds, Sensors and Actuators A: Physical 74 (1-3) (1999) 1–4. doi:10.1016/S0924-4247(98)00323-9.
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[8] T. C. dos Santos, M. Mulato, Analysis of electrodeposited NiFe thin films for the development of planar fluxgate magnetic sensors, MRS Online Proceedings Library 998. doi:10.1557/PROC-998-J05-21.
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[9] D. Scouten, Sensor noise in low-level flux-gate magnetometers, IEEE Transactions on Magnetics 8 (2) (1972) 223 – 231. doi:10.1109/TMAG.1972.1067284.
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Figure 4: Experimental setup for the sensitivity (left) and noise (right) characterizations. The computer (a) sets the parameters of the lock-in amplifier (b, SR844 DSP) responsible for the excitation signal generation. This signal is amplified by a power amplifier (c, based on an OPA564) directly connected with the excitation coils of the fluxgate (d). The pickup coils are connected to the lock-in which delivery the output back to the computer. In the noise setup, the sensor is surrounded by a three layers mu-metal magnetic shield (light grey). The external field is only required by the sensitivity tests. The computer sets the parameters of a power supply (e, HP 6653A) measured externally by a multimeter (f, Agilent 34401A) linked with the computer. The current flows in a Helmholtz coil (g) responsible for the external magnetic field.
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Figure 7: Low frequency sensor output fluctuations example taken from L2. a) At Hexc = 1.1 kA/m there are relative low noise levels (the values are shown in the graph header by the Nrms variable) with few sharp level changes (indicated by the arrows). The responsible for these jumps is hypothesize to be a single core volume hard to magnetize. b) Singly increasing the excitation amplitude to 1.15 kA/m makes this region approximately half of the time active in a low frequency pattern giving rise to the sudden increase in the noise levels seen in the figure 6a. c) With Hexc = 1.2 kA/m the system is able drive the volume most of the time, increasing the frequency and reducing the noise levels generated. As the excitation field increases more, this core volume will aling with the drive field consistently no more generating such fluctuations.
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We developed planar fluxgate magnetometers using a single layer of coils. The sensitivity were found to be up to three times higher than a analogous double layer sensor. The noise levels were found to be similar to an analogous double layer sensor. Extra excitation coils and higher excitation amplitudes can further reduce the noise.
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