CHARACTERISATION OF MAGNETIC WIRES FOR FLUXGATE CORES

CHARACTERISATION OF MAGNETIC WIRES FOR FLUXGATE CORES
CHARACTERISATION OF MAGNETIC WIRES FOR FLUXGATE CORES
P. Ripka1, M. Butta1, M. Malatek1, S. Atalay2, F. E. Atalay2
1
Czech Technical University, Prague, CZECH REPUBLIC
(Tel : +420-2-000-0000; E-mail: [email protected])
1
Inonu University, Malatya, TURKEY
(Tel : + 422 3410010 (3890-3705); E-mail: [email protected]
Abstract: Orthogonal fluxgate sensors need only a single core and they can be made very small.
Keywords: Selected keywords relevant to the subject.
1. INTRODUCTION
2. THE SAMPLE CORE
Orthogonal fluxgate sensors need only a single
core and they can be made very small; recent
types have cores made of high quality amorphous
soft magnetic wires [1-3].
The operational principle of orthogonal
fluxgates has not been fully understood yet.
Primdahl [4] gives only a basic explanation based
on rotational magnetization, which cannot
adequately describe the influence of hysteresis.
For proper modeling of the sensor it is vital to
measure its characteristics at real operational
conditions.
The main drawback of using magnetic wire
excited by AC current through the wire is that the
magnetic field in the central part or the wire is
small to saturate the material. By increasing the
excitation value this area is reduced, but it is still
present and creates a region causing unwanted
perming effect.
Copper wires with electrodeposited magnetic
layer were proposed to solve this problem: it was
generally believed that the excitation current flows
mainly through the copper wire due to its higher
conductivity and that the magnetic layer is
uniformly magnetized [5]. We will show that this
is true at low frequencies and low magnetic layer
thickness. If the frequency is higher or the
magnetic layer is thick, the current is concentrated
in the surface magnetic layer[6]. The solution of
this problem is an insulation layer between copper
and ferromagnetic layer - unfortunately this is
technological challenge.
The characteristics were measured on 50 µm Cu
wire covered by electrodeposited 10 µm layer of
polycrystalline Co18.97Ni49.60Fe31.43 alloy.
A three-electrode cell was used to carry out
electrochemical experiments. The volume of the
electrochemical bath was approximately 85 ml.
An Ag/AgCl electrode (BAS, 3 M NaCl, and –35
mV vs. SCE at 25 °C) was used as a reference
electrode. The high-density platinum gauze
electrode, which was approximately 500 times
larger than the cathode, was used as an auxiliary
electrode. The bath contained a mixed solution of
5mM Co(SO4)2.6H2O, 0.1M Ni(SO4)2.6H2O,
10mM Fe(SO4)2.6H2O, 0.2 M H3BO3, 35 mM
NaCl, 7 mM C7H4NNaO3S.2H2O and 0.01 g/l
C12H25NaO4S. All solutions were prepared by
dissolving reagent grade chemicals in distilled
water. The bath pH was adjusted to 2.6 by adding
0.1 mM HCl and 0.1 mM NaOH up to the
required value using a Jenway 3520 pH meter. The
plating was carried out at 25 °C in stirred solution
and at a constant potential of –1V vs. Ag/AgCl
(BAS, 3 M NaCl, and –35 mV vs. SCE at 25 °C)
for 180 min.
The sample geometry is shown in Fig. 1.
CoNiFe Film
Copper Wire
H
Figure 1: Geometry of the composite wire
The length of the Py-covered part of the wire was
36 mm. The wire was inserted into the ceramic
tube with only slightly higher internal diameter
and outer diameter of 1 mm. The sensing (pickup) winding was later wound around this tube. We
have used 26 mm long coil wound by 450 turns of
0.09 mm thick Copper wire. Another 300 turns
auxiliary coil of the same length was wound
around the inner coil.
The core B-H loops should be measured for both
tangential and longitudinal directions, as these
loops may be very different due to the anisotropy.
Although the measurement of the longitudinal
hysteresis loop is challenging due to the small
amount of ferromagnetic material, the measuring
procedures are well described in the literature.
This is not the case of tangential hysteresis loop,
which is vital for understanding the transverse
fluxgate.
3. THE INSTRUMENTATION
The current flowing in the wire and the voltage
drop on it have been sampled with NI 5911
digitizer which provides 5 MHz sampling
frequency and 14 bits resolution.
Since the card has only one channel a multiplexer
was used, so that V and Iexc are not sampled
simultaneously. An external trigger is used to
avoid phase delay between the sampled signals.
PC
Function generator
HP33120A
50 Ω
LPT1
Iexc
Ch.1
Data
acquisition
board
V
Ch.2
external
trigger
Sync.
Figure 2: Setup for measuring the circumferential hysteresis loop
4. CIRCUMFERENTIAL HYSTERESIS
LOOPS
Circumferential (tangential) hysteresis loop is
measured for core excited by current flowing
through the core wire: exactly as for the real
operation. The circular flux is derived from the
voltage V, which appears between the wire
terminals. In order to calculate the induced voltage
Vi, correction should be made for voltage drop RI
on the wire resistance R. We found that the DC
resistance value can be used for this correction in
wide frequency range rather than real part of the
impedance. Clear artifacts indicate the wrong
correction: in Fig. 2 the overcompensated curve
has two intersections close to the loop ends. The
circumferential field B is calculated by numerical
integration of the induced voltage.
B=
Φ 1
1
= ∫ Vi dt = ∫ (V − Ri )dt
A A
A
where A is the circular area: A = l · t = l · (r2-r1)
t is the thickness of the ferromagnetic layer
r1 is the copper wire diameter
r2 is the external diameter of the core
1
0.8
0.6
0.4
0.2
0
-300
-200
-100
0.2-
0
100
200
300
0.40.60.81-
Figure 3. Overcompensated
hysteresis loop
circumferential
The corrected set of hysteresis loops is shown in
Fig. 4: each curve belongs to a certain amplitude
of the excitation current.
1.5
1
0.5
0
-300
-200
-100
0
100
200
300
-0.5
-1
-1.5
Figure 4. Set of B[T]-H[A/m] circumferential
loops at 10 kHz for current amplitude from 10 mA
to 50 mA (span 5 mA)
From Fig.4 one can see that a minimum rms
current of 40 mA is required to fully saturate the
core. This minimum current is increasing with
frequency due to the eddy currents and other
losses. This is clearly demonstrated in Fig. 5,
which shows the sensor characteristics: second
harmonics output voltage versus DC measured
field.
For 30 mA excitation current this
characteristics has gross hysteresis and the sensor
is useless. This is dramatically improved for 40
mA excitation.
Figure 5. Second harmonic of induced voltage
[V] vs Bext [µT]; Iexc=30 mA and 40 mA, 10 kHz.
3. LONGITUDINAL CURVES
Axial (longitudinal) hysteresis loops were
measured by using the pick-up coil described in
Part 2.
The sensor characteristics may be different from
the core wire material characteristics: the sensor is
characterized by the total flux through the pick-up
coil, which contains only a small core area and
large area of air. At higher excitation intensities
the core permeability is low and the air flux
becomes important. It should be noted that
numerical correction is only possible because of
high-resolution A/D converter we use. Older
systems use analog compensation by subtracting
voltage from the compensation coil; this requires
nulling before each measurement.
The sensor axial loops are shown together with
gating curves in the next section.
5. GATING CURVES
Gating curves are the most important internal
characteristics of fluxgate sensors. They show
pick-up coil flux Фa versus the excitation field H
[4]. Gating curve has two peaks which distance is
equal to the coercivity of the longitudinal
hysteresis curve. The height of the peaks depends
on the axial (measured) external dc field Bmeas .
An example of gating curves for two values of the
measured field is shown together with axial sensor
curve in Fig. 8 and 9. (Odd harmonics caused bz
crosstalk were removed by calculation to meke the
figures more clear.)
estimates (2) and (3), while that estimate (1),
which was based on linearized Landau-Lifshitz
equation, should be corrected.
Table 1. Estimates of circular permeability
Measured values
Figure 6. Axial sensor curve and gating curve for
Bmeas= 370 µT
Freq
kHz
1
10
20
50
100
1000
Ls
uH
8.91
5
4.13
2.99
2.272
0.727
|Z|
Ohm
0.524
0.916
1.145
1.667
2.186
6.498
Est.
from
GMI
(1)
Est.
Est.
from from
Ls (2) Ls (3)
µø
16420
6880
5430
4840
4260
3970
µø
6703
3761
3106
2249
1709
547
µø
8825
4951
4089
2961
2250
720
REFERENCES
Figure 7. Axial sensor curve and gating curve for
Bmeas= 1 mT
6. CIRCULAR PERMEABILITY
In order to estimate the current profile by Finite
Element
method,
the
conductivity
and
permeability of both wire regions should be
known. We calculated the permeability using three
different formulas: either from GMI (Giant
Magneto-Impedance) data (1) [7] or from the
measured wire induction assuming that all the
current flows through the copper only (2) or the
current is uniformly distributed uniformly in both,
the copper and magnetic layer (3).
The results for Ni79.1Fe20.9/Cu wire are shown in
Tab. 1. We assume that the real value is between
[1]
Magnetic Sensors and Magnetometers,
Pavel Ripka (ed), Artech House, 2001, pp 40-41
[2]
I.
Sasada,
“Orthogonal
fluxgate
mechanism operated with dc biased excitation”, J.
Appl. Phys., 91 (2002), pp. 7789-7791.
[3]
Z.J. Zhao, X.P. Li, H.L. Seet, X.B. Qian, P.
Ripka: Comparative study of the sensing
performance of orthogonal fluxgate sensors with
different amorphous sensing elements, to appear
in Sens. Act. A
[4]
F. Primdahl, “The Fluxgate Mechanism,
Part I: The Gating Curves of Parallel and
Orthogonal Fluxgates”, IEEE Trans. Magn., 6
(1970), pp. 376-383.
[5]
Fan J, Li XP, Ripka P: Low power
orthogonal fluxgate sensor with electroplated
Ni80Fe20/Cu wire, Journal of Applied Physics 99
(8): Art. No. 08B311 APR 15 2006
[6] J. P. Sinnecker,et al.: Journal of Magnetism
and Magnetic Materials, Volume 249, Issues 1-2,
August 2002, Pages 16-21
[7] L.D. Landau, E.M. Lifshitz,: Electrodynamics
of Continuous media, Pergamon press, oxford
1975
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