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A New Compact Fluxgate Current Sensor for AC and DC Application
Article in IEEE Transactions on Magnetics · November 2014
DOI: 10.1109/TMAG.2014.2330373
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015
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Design and Realization of a Novel Compact
Fluxgate Current Sensor
Xiaoguang Yang, Yuanyuan Li, Weidong Zheng, Wei Guo, Youhua Wang, and Rongge Yan
Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus
Reliability, Hebei University of Technology, Tianjin 300130, China
A new closed-loop magnetic current sensor is presented in this paper. The sensor consists of two toroidal magnetic cores. One core
works in fluxgate principle for the measurement of dc and low-frequency ac, and the other one is used as a current transformer for
higher frequency application. Based on the simulation results, a prototype was designed, and the test results have a good agreement
with the simulation results. The closed-loop configuration with a magnetic core and a feedback winding in the sensor improved the
sensitivity of the sensor, eliminated the offset and drift related to temperature, and greatly reduced the error caused by magnetic
hysteresis phenomenon. It can measure currents up to 20 A, with an accuracy of 0.5%, and a 50 kHz small signal bandwidth.
Index Terms— Closed loop, fluxgate principle, magnetic hysteresis.
I. I NTRODUCTION
F
LUXGATE current sensors are always attracting the attention of researchers, because they are more sensitive and
more precise [1]–[4]. Fluxgate current sensors are widely used
in dc or low-frequency ac measurements. They offer a wide
range of new applications and present a high potential for
future applications. The basic working principle of traditional
fluxgate current sensors is based on non-linear property of
ferromagnetic materials, the permeability varying with the
magnetic field around the sensor [5]. More details can be
found in [6].
Various structures of fluxgate current sensor have
been reported. Sharafi [7] presented a new sensor with
low-power consumption. Butta and Ripka [8] proposed a
method for the sensor design to reduce the spurious component
of the output voltage. Velasco-Quesada [9] designed a fluxgate
current sensor with three cores for high current measurement,
and achieved improvement in consumption. Ma [10] presented
a model for self-oscillating fluxgate current sensor, and most
of the parameters that affect the main sensor characteristics
are included in the model. Weiss [11] designed a dc sensor
with improved measurement accuracy, good stability, and
a simple structure based on magnetic potential self-balance
and feedback compensation. Liu et al. [12] introduced
a two-axis miniature fluxgate sensor with Fe-based and
Co-based cores and printed circuit board multilayer
technology. The designed sensor showed new opportunities
for further miniaturization of fluxgate current sensors.
Traditionally, fluxgate current sensors are only suitable
for measurements of dc or low-frequency ac [13]. As the
application of high-frequency power electronics is increasing,
high frequency current measurement is required. To achieve
accurate measurement for dc and high-frequency ac, this paper
proposes a new fluxgate current sensor with a compact structure, which introduces an additional toroidal magnetic core
and a closed-loop configuration into the traditional fluxgate
Manuscript received May 23, 2014; revised July 24, 2014; accepted
September 10, 2014. Date of current version April 22, 2015. Corresponding
author: X. Yang (e-mail: xgyang@hebut.edu.cn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2014.2358671
Fig. 1.
Structure of the proposed current sensor.
Fig. 2.
Profile of the structure of magnetic cores and windings.
current sensor. The proposed current sensor eliminates the
offset and drift related to temperature, reduces the error caused
by magnetic hysteresis, and improves the sensitivity.
II. S TRUCTURE AND P RINCIPLE OF D ESIGNED
F LUXGATE C URRENT S ENSOR
The basic principle of the proposed fluxgate current sensor
is shown in Fig. 1, and the profile of the structure of toroidal
magnetic cores and windings is shown in Fig. 2. As shown in
Fig. 2, the sensor consists of two magnetic cores (T1 and T2 ).
The core T1 is used to measure dc or low-frequency ac
based on fluxgate principle, whereas the other core T2 acts
as a current transformer for medium- or high-frequency ac
measurement.
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4002804
Fig. 3.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015
Fig. 4.
Magnetization curve of nanocrystalline soft magnetic.
Fig. 5.
Magnetic-flux density distribution without compensation.
Fig. 6.
Magnetic-flux density distribution of the dynamic balance state.
Block diagram of closed-loop fluxgate current sensor.
In the structure of the current sensor, W p , W f , We , and Ws
represents the primary winding, feedback winding, excitation
winding, and secondary winding, respectively. N p (usually
N p = 1), N f , Ne , and Ns are their turn numbers, respectively.
The magnetic flux generated by the primary current (I p ), in
the core T1 is Φ p . Similarly, the magnetic flux generated by
feedback current (I f ) is Φ f . Thus, the total magnetic flux in
the core is the sum of Φ p and Φ f . Since the direction of
feedback magnetic field is opposite to the primary magnetic
field generated by I p , the feedback current I f will increase
gradually until the primary magnetic field is balanced by the
feedback magnetic field. When the feedback winding creates
a flux equal in amplitude, but opposite in direction, to the flux
created by I p , the whole system reaches dynamic balance,
namely the zero-flux state. Thus, the relationship between
I p and I f can be expressed as I p = N f I f .
The working principle of the proposed current sensor for dc
and low frequency current measurement is shown in Fig. 3.
The hysteresis comparator outputs a symmetric ac voltage
with square waveform. The corresponding current waveform
through the excitation winding is also symmetric when no
external magnetic field is present, namely the primary current
is zero. When an external magnetic field is applied to the
core T1 , the current waveform through the excitation winding
starts to be asymmetric in duration, resulting in a non-zero
signal in the integrator. The non-zero signal drives the
H -bridge drive, which further drives the current into the feedback winding. The current in the feedback winding induces
a magnetic flux in opposite direction to the flux induced by
the primary current. The feedback compensation current (I f )
produces a magnetic flux, which tries to cancel the external
flux, induced by the primary current.
When the feedback winding creates a flux equal in amplitude, but opposite in direction, to the external flux created
by I p , the whole system reaches dynamic balance, namely the
zero-flux state. To obtain a zero-flux condition on the fluxgate
current sensor the feedback winding (W f ) must be excited
with an appropriate current, as shown in Fig. 1. The zero-flux
state in the core T1 is indicated by the value of the integral
current (Ie in Fig. 1), which flows through the sampling
resistor (Rs in Fig. 1). A null value of the integral current (Ie ),
is used to determine the zero-flux condition. When the value
of the integral current (Ie ) is equal to zero, a zero-flux state is
achieved. The current flowing through the feedback winding
also flows through a shunt resistor (Rm in Fig. 3). The current
imposed on the feedback winding is directly proportional to
the primary current.
For medium- or high-frequency ac measurement, an
additional core T2 is included in the current sensor. This
new toroidal core T2 is embraced by the secondary winding,
as shown in Fig. 2. The combination of this new core T2 , the
secondary winding, and the primary current wire operates as
a current transformer for medium and high frequency current
measurement.
III. S IMULATION AND T EST R ESULTS
A. Simulation Analysis
Three dimension finite element analysis (FEA) was
performed on the proposed current sensor. The magnetic
core is made of nanocrystalline soft magnetic material
(Fe73.5 Si15.5 B7 Cu1 Nb3 ). Because this material has high resistivity and the core is made of very thin strip, the eddy current is
very small and not taken into consideration in this FEA. In the
process of the simulation, the BH curve of nanocrystalline
soft magnetic material was used, as shown in Fig. 4. The
internal and external diameter of the core is 12.5 and 20 mm,
respectively. The height of the cores is 8 mm. The turn number
of primary winding, that of feedback winding, and that of
secondary winding is 1, 200, and 300, respectively.
When the primary current is 15 A, simulation results were
obtained without and with a feedback compensation winding,
as shown in Figs. 5 and 6, respectively. Initial magnetization
curve shown in Fig. 4 is used in this simulation process. The
simulation result in Fig. 5 shows that the flux is symmetrically concentrated in the toroidal magnetic core, gradually
becomes larger as the radius of the core decreases, and the
flux through the magnetic core leads to a non-zero-flux state
without a feedback compensation. In contrast, Fig. 6 shows
that with feedback compensation, the magnetic-flux density
is approximately zero in the magnetic core and reaches to a
zero-flux state. There is small magnetic flux density in part of
the magnetic core shown in Fig. 6, because there is leakage
flux produced by the feedback winding. When a zero-flux
YANG et al.: DESIGN AND REALIZATION OF A NOVEL COMPACT FLUXGATE CURRENT SENSOR
Fig. 7.
Voltage waveform of the sampling resistor when I p = 0 A.
Fig. 8.
Voltage waveform of the sampling resistor when I p = 2 A.
state is achieved by the feedback compensation winding, the
whole system reaches a dynamic balance state.
In the process of simulation, the excitation winding is
excited by square voltage with the frequency of 8 kHz and
amplitude of 6 V. The choice of the exciting frequency effects
on the properties of sensor. Too higher frequency will cause
too much noise, and too lower frequency will decrease the
sensitivity of the sensors. In general, the exciting frequency
is several kilohertz for a fluxgate sensor. In the design, the
exciting frequency is finally determined to be 8 kHz by test.
When the primary current I p = 0 A, the voltage waveform
across the sampling resistor (Rs ) is obtained, as shown in
Fig. 7. When the primary current is not zero, simulation results
show that the voltage waveform across sampling resistor Rs
is asymmetric in duration. As an example, when the primary
current I p = 2 A, the asymmetric voltage waveform across
Rs is shown in Fig. 8. Comparing Fig. 8 with Fig. 7, it shows
that the primary current affects the excitation current: 1) when
the primary current is zero, the voltage waveform across Rs is
symmetric, i.e., the average value of the excitation current is
zero and 2) however, when the primary current is not zero, the
voltage waveform across Rs is asymmetric, and the average
value of the excitation current is not zero. The value and sign
of the excitation current depend on the value and direction
of I p . In the simulation process, the hysteresis property shown
in Fig. 4 was taken into consideration.
B. Test Results
A prototype of the fluxgate current sensor was designed
based on the analytical and simulation results. T1 and T2
are identical. The internal and external diameter of the core
are 12.5 and 20 mm, respectively, and the height of the
cores is 8 mm. The magnetic core is made of nanocrystalline
soft magnetic material and the magnetization curve is shown
in Fig. 4. The cores, T60006-L2020-W409, are from the
company of VAC in Germany.
The excitation winding is wound around T1 with 100 turns,
and the feedback compensation winding is wound around both
T1 and T2 with 200 turns. For the designed current sensor,
the threshold value of the exciting current is calculated to be
∼20.4 mA, which guarantees that core T1 can reach saturation
state. Secondary winding is wound around T2 with 300 turns.
In the test process, the excitation winding is excited
by square voltage with the frequency of 8 kHz and the
Fig. 9.
4002804
Voltage waveform of the sampling resistor when I p = 0 A.
Fig. 10.
Voltage waveform of the sampling resistor when I p = 2 A.
Fig. 11.
Conic fitting curves of dc measurement of the current sensor.
amplitude of 6 V. When the primary current I p = 0 A, the
voltage waveform of the sampling resistor (Rs ) is obtained,
as shown in Fig. 9. When the primary current I p = 2 A, the
voltage waveform of the sampling resistor is obtained,
as shown in Fig. 10. The experimental results, shown
in Figs. 9 and 10, have good agreements with the simulation
results shown in Figs. 7 and 8, respectively.
When the primary current was changed from 0 to 20 A,
measurements were performed in the following two cases:
1) the fluxgate probe consisting of a toroidal magnetic core
and no feedback winding and 2) the probe consisting of a
toroidal magnetic core and a feedback winding wound around
core T1 , in a closed-loop configuration.
For the two cases, the tested input and output characteristics
of the designed sensor are shown in Fig. 11. The output
voltage is the voltage across shunt resistor Rm . The figure
shows that, with a closed-loop configuration, the sensitivity
and measuring precision of the sensor are greatly improved,
and the measurement range is also extended. In addition, with
open-loop configuration, the sensor is easy to saturate and has
poor linearity.
To analyze the hysteresis effect for the sensor, the test
procedure was arranged for both closed-loop configuration
and open-loop configuration as follows. In the first step, the
primary current was increased from 0 to 20 A, and the outputs
of sensor were measured. In the second step, the primary
current was decreased from 20 to 0 A, and the outputs of
the sensor were measured again. The input and output characteristic curves are shown in Fig. 12(a). The corresponding
output errors caused by hysteresis phenomenon of the
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015
Fig. 14.
Frequency response of the sensor.
then the gain is defined as 20 lg (V( f ) /V0 ), which is a function
of the frequency. Tested frequency response of the designed
current sensor is shown in Fig. 14. When the gain is equal
to −3 dB, the cutoff frequency can be determined from Fig. 14.
The resulting cutoff frequency, representing the bandwidth of
sensor, is 50 kHz.
Fig. 12. (a) Input and output characteristic curves of the proposed current
sensor. (b) Output errors caused by hysteresis phenomenon.
IV. C ONCLUSION
The proposed fluxgate current sensor works in a closedloop configuration, which greatly improves the sensitivity and
precision of the sensor, significantly reduces hysteresis effect,
limits the relative error to ∼0.5%, and the bandwidth of the
sensor is ∼50 kHz.
Compared with three cores fluxgate sensors available on
market and reported in the literatures, the designed sensor has
a more compact structure.
As an advance in power electronic products development,
the need for high precision current sensors in high-frequency
application keeps increasing. Therefore, the designed current
sensor in this paper has wide application prospects.
R EFERENCES
Fig. 13.
Relative error of the current sensor.
proposed current sensor are shown in Fig. 12(b). Both figures
demonstrate that the closed-loop configuration can significantly reduce hysteresis effect.
The relative error of tested results of dc measurement
ranging from 0 to 20 A with the designed sensor is shown
in Fig. 13, which is limited to 0.5%. The figure shows that
introducing closed-loop configuration into the sensor greatly
reduces the relative error.
To verify the temperature stability of the designed sensor,
tests were performed with its operating temperature changed
from 25 °C to 70 °C. Tested results showed that the output
voltage offset and drift related to temperature is less than
±0.21 mV (the full scale is 6 V). From the above, it can be
seen that the offset and drift related to temperature is almost
eliminated in the designed sensor. The main reason for that
is as follows. A closed-loop configuration is helpful to reduce
any disturbance to the sensor, and the magnetic core material
has good temperature characteristics, so the magnetic and
electrical properties change very small when the temperature
changes.
Let V( f ) represent tested output voltage when the amplitude
of I p is fixed and its frequency is swept from 0 to 100 kHz.
In addition, let V0 represent the tested result when I p is dc,
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