EXPERIMENTAL STUDY OF THE PERMEABILITY OF A
FERROMAGNETIC COMPOSITE
Zheng Hui and Prof. Li XP
Electrical and Computer Engineering
National University of Singapore
9 Engineering Drive 1 Singapore 117576
ABSTACT
Experimental study on the permeability of ferromagnetic composites was performed. Results were
obtained under conditions with different environment parameters, e.g. the turns of pick up coils, the
value of surrounding magnetic field, the input voltages and frequencies. Reasons of permeability
changes are discussed.
INTRODUCTION
Numerous studies have been dedicated to the electrodynamics of metal-dielectric composites,
particularly to the permeability change due to environment variations. In present work, a traditional
way of experiment was adopted. An amorphous wire with thickness of 30-micron was used as sample.
The purpose is to study the relation between variations of key external parameter and the changes in
permeability as well as the induced emf in the coil.
MATERIALS AND METHODS
Before the experiment was carried out, coils with different number of turns were made by using wires
of 0.8 mm radius. The numbers of turns are 25, 50, 75 and 100. However, after some trials, it was
found that the number of turns of the pick up coil did not contribute much to the induced emf. Thus, we
chose the number of turns of the pick up coil to be 50.
The amorphous wire (sensor) was put inside the pick up coil. Square wave inputs were supplied to the
sample wire and the induced emf which can be read from the oscilloscope were picked up by the coil.
The whole circuit was surrounded by external magnetic field. The experiment circuit for testing the
microwire composite was set up as figure 1.
There were mainly three key parameters which can determined the permeability changes, namely the
input voltage (V), driving frequency (F) and the strength of the external magnetic field (M). Therefore,
the experiments were carried out in three ways shown in table 1.
Figure 1: Experiment circuit
Frequency
Magnetic field
Input voltage
Fixed
Vary
Vary (AC)
Vary
Vary
Fixed (AC)
Vary
Vary
Fixed (TTL)
Table 1: Ways of experiments
The sensitivity of the sensor is reflected by how its inductive output signals respond to varying external
magnetic field, Hext. Since this external field is generated in air, and taking permability of air to be
similar to that of vacuum, we have Bext = µ0 Hext, where µ0 = permeability of vacuum. The magnetic flux
density, Bext, is related to the induced emf by V = -d(BextS)/dt, where magnetic flux, Φ, is given by Φ =
BextS, where S = surface area of coil.
The induced emf is in turn proportional to the change in the magnetic flux density within the sensor
core (Bcore), where Bcore = µ0 µcore Hcore. In order to have a large change in induced emf for a small
change in external field (i.e. higher sensitivity), we would require ∆Bcore to be large. This is possible
only if there are large changes in the core’s permeability, hence amorphous wires and Iron-Nickel
composite wires, which are excellent soft magnetic materials with high permeability, are being tested
to be used as the sensor cores.
RESULTS AND DISSCUSSION
The test was conducted by comparing the sensitivity results between the sensors which is AC driven
core and DC driven core. In the test, we observed that for an AC driven core, the output response is as
such: one positive impulse output at the ‘falling edge’ of the input square-wave AC signal, and one
negative impulse at the ‘rising edge’ of the input. These output responses were measured against the
applied external magnetic field when the driving frequency was varied for a fixed input voltage, as well
as when the input voltage was varied at a fixed frequency. In general, the sensor gave better response
when it was subjected to a driving frequency of 100kHz and above. The inductive signals from the
sensor also gave a more linear relationship with the external magnetic field when smaller AC input
voltages are applied.
For the DC configuration, a TTL (transistor-transistor logic) input signal was applied to the sensor core.
Unlike in the case of the AC driven core, the output response of the DC driven core was mostly
unaffected when the driving frequency was varied. The AC configuration was chosen to be used in
later tests, as the inductive signals would be most sensitive, and hence most easily detectable, at the
sharp rising and falling edges of the AC square-wave input signals. In addition, we concluded that it
would be more favorable to conduct the sensitivity tests on the sensors using lower input voltages since
the ‘magnetic noises’ from within the set-up (from equipment, cables etc.) are more significant at such
voltages, indicating that the sensor is likely to be more sensitive.
Following charts are summary from the experiments
Magnetic Field vs Output emf Response( Summary for AC driven core with input voltage 2.5VP-P)
220
200
180
Output emf Response/mV
160
10kHz,rising edge
10kHz, falling edge
100kHz, rising edge
100ikHz, falling edge
Poly. (10kHz,rising edge)
Poly. (10kHz, falling edge)
Poly. (100kHz, rising edge)
Poly. (100ikHz, falling edge)
140
120
100
80
60
40
20
0
500
1000
1500
2000
Hext/milliGauss
Output emf Response vs Magnetic Field ( Summary for AC driven core at 100kHz)
190
170
900mV,rising edge
900mV,falling edge
1.4V,rising edge
1.4V,falling edge
2.5V,rising edge
2.5V,falling edge
Poly. (900mV,rising edge)
Poly. (900mV,falling edge)
Poly. (1.4V,rising edge)
Poly. (1.4V,falling edge)
Poly. (2.5V,rising edge)
Poly. (2.5V,falling edge)
Output emf Response/mV
150
130
110
90
70
50
30
10
0
50
100
150
200
250
300
350
Hext/milliGauss
400
450
500
550
600
Magnetic Field vs Output emf Response( Summary for DC driven core with input voltage 1.25V)
1kHz,10kHz
rising edge
500
1kHz,10kHz
falling edge
450
100kHz rising
edge
100kHz falling
edge
Output emf Response/mV
400
1MHz rising
edge
350
1MHz falling
edge
300
Poly.
(1kHz,10kHz
rising edge)
Poly.
(1kHz,10kHz
falling edge)
Poly. (100kHz
rising edge)
250
200
Poly. (100kHz
falling edge)
Poly. (1MHz
rising edge)
150
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Hext/milliGauss
Poly. (1MHz
falling edge)
Magnetic Field vs Output emf Response ( Summary for DC driven core, TTL 1.25V)
500
450
Outout emf Response/mV
400
1kHz,10kHz rising edge
1kHz,10kHz falling edge
100kHz, rising edge
100kHz, falling edge
Poly. (100kHz, rising edge)
Poly. (1kHz,10kHz rising edge)
Poly. (1kHz,10kHz falling edge)
Poly. (100kHz, falling edge)
350
300
250
200
150
0
50
100
150
200
250
300
350
400
450
500
550
600
Hext/milliGauss
CONCLUSION
After the whole working period on the UROP program, it had summarize some of the results obtained
throughout the testing as well as solving some of the problems encountered. Moreover, the relations
between the variation in permeability and the changes in some key environment factors were studied.
However, some of the problems raised up during the testing were not completely solved for example
the poor shielding factor and noisy environment. More improvements in both experiment environment
and experimenter’s knowledge are needed for further researches.