AAPPPPLLIICCAATTIIOONN OOFF GGIIAANNTT MMAAGGNNEETTOOIIMMPPEEDDAANNCCEE

AA SSIIMMPPLLEE MMAAGGNNEETTIICC SSEENNSSOORR

A PROJECT REPORT

Submitted in partial fulfillment of the requirements for the award of the degree of

MASTER OF SCIENCE

In

PHYSICS

by

P.SHIJIN BABU

PH08C024

under the guidance of

DR. G. MARKANDEYULU

DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY MADRAS

APRIL 2010

i

CCEERRTTIIFFIICCAATTEE

This is to certify that the work reported in this dissertation entitled Application of Giant

Magneto Impedance Effect- A Simple Magnetic Sensor submitted by Mr. P. Shijin

Babu (PH08C024) was carried out by him under my supervision at the Department of

Physics towards the partial fulfillment for the award of the Master of Science in Physics

at Indian Institute of Technology Madras

April 20, 2010 Dr.G.MARKANDEYULU

Professor

Advanced Magnetic Materials Laboratory

Department of Physics

Indian Institute of Technology Madras

ii

AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS

I sincerely wish to express my deep sense of gratitude to my guide Prof. G.

Markandeyulu for his inspiring guidance, support and persistent encouragement

throughout the project work in this institute.

I take this opportunity to express my gratitude to Ms Sandhya Dwevedi and

Late Mrs. M.R. Sumathi for their incredible help. I like to thank Dr. R Niramala for

helping me in understanding basics of magnetism.

Also I like to thank Mr. M. Dhanapalan for his suggestions and help

regarding electronics and Mr. Manoharan for helping me with electronic components.

P.SHIJIN BABU

iii

CCOONNTTEENNTTSS

Page No.

CCHHAAPPTTEERR 11:: IINNTTRROODDUUCCTTIIOONN 1

1.1 Giant magneto impedance effect 2

1.2 Theory of giant magneto impedance 5

1.3 Application of GMI sensors 7

1.31 Current sensors 7

1.32 Stress sensors 7

1.33 Magnetic sensors 8

1.4 Application of GMI sensors 11

CCHHAAPPTTEERR 22:: EEXXPPEERRIIMMEENNTTAALL TTEECCHHNNIIQQUUEESS 12

2.1 Preparation of the ribbon 12

2.2 Melt spinning 12

2.3 GMI measurements 14

CCHHAAPPTTEERR 33:: DDEESSIIGGNNIINNGG TTHHEE SSEENNSSOORR 15

3.1 Introduction 15

3.2 Ac power supply unit 17

iv

3.3 Battery power supply 18

3.4 Oscillator circuit 19

3.5 Sensing circuit 20

3.51 Schottky diode 21

3.52 Instrumentation Amplifier 22

3.6 Display 24

CCHHAAPPTTEERR 44:: RREESSUULLTTSS AANNDD DDIISSCCUUSSSSIIOONN 26

4.1 GMI studies 27

4.2 Sensor measurements 29

4.21 Measurement of the field produced by Helmholtz coil 29

4.22 Measurement of Earths magnetic field 32

4.23 Measurement of magnetic field variation in campus 34

CCHHAAPPTTEERR 55:: CCOONNCCLLUUSSIIOONN 38

RREEFFEERREENNCCEESS 39

v

LLIISSTT OOFF FFIIGGUURREESS

FFIIGG.. NNOO.. PPAAGGEE NNOO..

1.1 Change in impedance with the field of a 2 magnetic ribbon 1.2 Measurement of GMI 4

1.3 Illustration of dependence of skin depth on field for 4 a magnetic wire and thin film 1.4 A photograph of GMI sensor 8

1.5 Comparison of Different magnetic sensors 9

1.6 Schematic diagram of the sensor 10

2.1 Melt spinning apparatus 13

3.1 Block Diagram of the Sensor 16

3.2 Duel voltage power supply (+12/-12) using 17 7812 and 7912 regulator ICs

3.3 Pin diagram of 7812 and 7912 18

3.4 Battery Power supply Using 7812 and 7912 18

3.5 Circuit Diagram of the Colpitts Oscillator 19

3.6 Circuit diagram of the sensing circuit 20

3.7 Schottky Diode 21

3.8 Simple circuit diagram of an Instrumentation Amplifier 22

vi

3.9 Simplified circuit diagram and pin diagram of 23

AD 620 instrumentation amplifier

3.10 Pin diagram of PM129a DPM 24

3.11 A photograph of the magnetic sensor 25

4.1 XRD pattern of Co68Fe5Si12B15 26

4.2 Variation of GMI of Co68Fe5Si12B15 with field, 28

at 1MHz

4.3 Variation of GMI of Co68Fe5Nb12B15 with field, 28 at 1 MHz 4.4 GMI ratio of Co68Fe5Si12B15 with 29 frequency, at field 6Oe 4.5 Variation of sensor output voltage with magnetic field 31

4.6 Change in sensor output voltage with rotation in 33 Earths magnetic field 4.7 The chosen places for magnetic field measurement 35

4.8 Magnetic fields at different locations in the campus 37

vii

AABBSSTTRRAACCTT

Giant magnetoimpedance (GMI) has attracted intense research interest

since Panina and Mohri reported the effect in 1994 in Co based amorphous ribbons.

Since the effect is very sensitive to the external magnetic field, it has been exploited for

different types of magnetic sensors. The GMI sensors are highly sensitive, small in size

and consume very low power.

The project presents a simple mobile magnetic sensor based on GMI

effect. Even though the main objective of the project is the design and application of the

magnetic sensor, GMI studies on Melt spun magnetic ribbons are presented for the

better understanding of the phenomena. The sensing elements and the working

frequency of the sensor were chosen from these studies. The sensor can work on both

AC power and batteries. Other than the power supply part the sensor consists of a high

frequency oscillator, a sensing circuit and a digital panel meter for display purpose.

Three different types of field measurements were done using the sensor.

One is the measurement of field produced by the Helmholtz coil, second is the

measurement of Earths magnetic field and finally the most interesting- the magnetic

field variation in the campus. The sensor is found to be very useful in measuring low

magnetic fields. The sensor exhibited a maximum sensitivity of 27.1% (change in

impedance) in Earths magnetic field when finemet was used as the sensing element.

Making use of this sensitivity, a comparative study of variation in magnetic field inside

campus was done by measuring the local magnetic filed in ten different chosen places

in the campus.

1

11 IINNTTRROODDUUCCTTIIOONN

In the current technology-based modern world, magnetic sensors play a

very important role in all our life. Magnetic sensors are used in different of areas like

magnetic storage systems, industrial and military sectors, space research,

geomagnetic research etc [Lenz JE 1990]. Depending on the working situation and the

intensity of the field, a particular magnetic sensor is chosen from the wide range of

sensors. Some examples of magnetic sensors are fluxgate sensors, induction sensors,

hall effect sensors squid magnetic sensors etc. All sensors make use of one or more

field- dependant chemical or physical phenomena.

The recent discovery of the so called Giant Magneto Impedance effect

opened a new door towards high sensitive and low power magnetic sensors. The GMI

effect was first reported by Panina and Mohri in amorphous Co based wires in 1994

[Panina L V, Mohri K. 1994:]. Since then, GMI has obtained an important position in

research because of both scientific research interest and technological applications.

2

11..11 GGIIAANNTT MMAAGGNNEETTOO IIMMPPEEDDAANNCCEE EEFFFFEECCTT::

Giant magneto impedance effect is the large change in

impedance of a ferromagnetic conductor on the application of a steady magnetic field

in the presence of an ac current. The change in impedance of the conductor depends

on the intensity of the magnetic field, frequency of the ac signal, amplitude of the ac

signal and shape and type of the conductor. If Z(Hex=0) and Z(Hex) are the

impedance at without field and with field respectively, then the GMI ratio is defined as

GMI =

(%) =

()( )

() 100% 1.1

.

FIG 1.1 CHANGE IN IMPEDANCE WITH CHANGE IN FIELD OF A MAGNETIC RIBBON

3

At large ac frequencies, GMI ratio up to 360 % has been

observed in zero magnetostrictive amorphous wires and ribbons. After proper thermal

treatment (conventional, joule and magnetic annealing) sensitivity up to 1700% has

been reported. It is because of this high sensitivity, GMI became important topic for

sensor applications. The change in impedance of the conductor is due to the large

change in complex permeability of the conductor even in small magnetic field.

The impedance of the sample is

Z = (Vac/Iac) =R + j X 1.2

Where Vac is the ac voltage applied between the ends of the sample and

Iac is the ac current through the sample.

R is the DC resistance which is due to the scattering of electrons and

X is the reactance which is due to the inductance of the sample.

The impedance of a wire shaped conductor and thin film are given by

Z = Rdckt ()

() wire shaped conductor 1.3

Z = Rdc jka coth(jka) thin film 1.4

Where Rdc is the dc electrical resistance,J0 and J1 are the Bessel functions of the first

kind, t ,the radius of the wire and 2a is the thickness of the thin film. k = (1+j) / , where

j is the imaginary unit and is the electromagnetic skin depth. The skin depth is

4

defined as the distance measured from the surface of the conductor towards its center

to reduce the amplitude of the ac current to 1/e times of its maximum value.

=

in the case of wire 1.5

=

in the case of film 1.6

Where is the conductivity of the material,

f is the frequency of the ac signal,

is the circumferential permeability of the wire and

t is the transverse permeability of the film.

In the presence of magnetic field, the skin depth of the sample changes since it

depends on the permeability of the sample. The skin depth decides the size of cross

sectional area available for the ac current which in turn decides the impedance of the

sample. This idea is illustrated in the picture in the next page

5

FIG.1.2 MEASUREMENT OF GMI

FIG.1.3 ILLUSTRATION OF DEPENDENCE OF SKIN DEPTH (m) ON FIELD FOR

A MAGNETIC WIRE (ABOVE) AND THIN FILM (BELOW)

6

The expected properties for a material to show high GMI effect are the following

High conductivity

High permeability

Low coercivety

Low remnence

High magnetization

Negligible or zero magnetostriction

11..22 TTHHEEOORRYY OOFF GGIIAANNTT MMAAGGNNEETTOO IIMMPPEEDDAANNCCEE EEFFFFEECCTT::

The mechanism and theory of GMI can be roughly divided in to three regimes

depending on the frequency of the ac current

Low frequency regime: In the low frequency region- up to 10 KHz , the change

in impedance of the sample is due to magneto inductive effect. When an ac signal

passes through the sample, it generates a circumferential time varying magnetic field,

which produces an axial electric field Eax .The value of induced voltage across the

sample is decided by the value of the electric field Eax on the surface of the sample and

sample length. The total voltage drop on the sample when an ac current passes

through, can be written as th the sum of the resistive voltage VL and inductive voltage

VR.. i e, V = VL + VR.

7

Resistive Voltage VR =R I 1.7

Inductive voltage VL = Eax(on surface) l =

Lint I 1.8

So, total voltage V = R I

Lint I 1.9

Where R is the dc resistance of the sample,

I is ac current passing through the sample,

is the frequency of the ac current, and

Lint is the internal inductance of the sample.

In the case of a homogeneous magnetic wire, Lint =

. So, it is concluded that at

low frequency, the effect of external magnetic field is to mainly change the

circumferential magnetic permeability of the sample which incidentally changes the

voltage drop across the sample. In low frequency region skin effect has no major part

in GMI since the skin depth is higher than the dimensions of the sample.

Moderate frequency regime: In the moderate frequency regime, which is up to a

few mega hertz it is the skin effect which plays very important role. When an ac current

flows through a conductor, the current tends to concentrate on the surface of the

conductor. Skin depth gives a measure of the skin depth. In moderate frequency

regime also, the problem of explaining change in impedance is solved by

understanding how the permeability of the sample changes due to the external field

and the frequency of the ac current since it is the major factor in deciding the skin

8

depth of the conductor. In the case of wires, it is the effective circumferential

permeability eff which decides the response of magnetic domains towards the

longitudinal field and the frequency of the ac current. The effective circumferential

permeability can be written as

eff =

rot + mov 1.10

Where rot is the circumferential permeability due to the rotation of magnetization

and mov is the circumferential permeability due to domain wall motion.

At frequencies less than 500 KHz. Both terms contributes to the

circumferential magnetization of the sample. At higher frequencies, magnetization

rotation dominates because domain wall movements are suppressed by eddy currents.

High frequency regime: In high frequency region the theory of giant magneto

impedance is not clear as that in moderate and low frequency regimes. But the origin

of the GMI in high frequency regime is believed to be ferromagnetic resonance and it is

proved that both ferromagnetic resonance and exchange interaction plays important

role in it.

11..33 AAPPPPLLIICCAATTIIOONNSS OOFF GGMMII SSEENNSSOORRSS::

The main applications of GMI are the GMI sensors. Since GMI is very

sensitive to applied field, frequency of the ac current and stress, sensors can be

designed to measure the same quantities. Some important sensors are discussed

below.

9

11..3311 CCUURRRREENNTT SSEENNSSOORRSS::

In GMI current sensors, the measurement of the current is

achieved by making use of the magnetic produced by the current which is to be

monitored. A successful model of such a dc current sensor is produced by

R.Valensuela [ Valensuela R,Freijo JJ, Vazquez M Hernado A. 1997] in which the current which

is to be measured is allowed to pass through a solenoid having a sensing element at

the center. When the current flows through the solenoid, the axial magnetic field

produced by the current changes the impedance of the magnetic element. The sensing

element used in the sensor is Co68Fe5Si12B15 amorphous wire. The main advantage of

a GMI current sensor is its reduced size, since the sensing element is just a wire. But

the GMI sensor mentioned above cannot be used to measure ac current. The research

is going on to develop+ ac/dc GMI current sensors and a prototype of such a sensor is

already suggested by Manh-Huong Phan and Hua-Xin Peng [Phan MH, Peng HX, Dung

2008].

11..3322 SSTTRREESSSS SSEENNSSOORRSS::

Stress sensors can be designed by making use of the fact that GMI has

dependence on the applied stress. The advantage of the stress sensor over

conventional stress sensors is its sensitivity. For example a semi conductor strain

gauge shows 15% change in its resistance on the application of maximum strain in the

10

safe limit while a GMI stress sensor shows 315% change in inductance when the

limiting value of strain is applied [Bowles A, Gore J, Tomks G Anew 2005].

11..3333 MMAAGGNNEETTIICC SSEENNSSOORRSS::

Since even a very low field changes the impedance by orders of

magnitude, GMI is the ideal tool for magnetic sensors. A GMI sensor usually uses

ribbons, films or wires as sensing elements. GMI sensors have many important

advantages over conventional magnetic sensors, out of which sensitivity is the most

important of all these. Talking about sensitivity, Yabukami has designed a GMI sensor

which can measure field up to 1.7 10 Oe at 500 KHz [Yabukami S, Mawatari H,

Horikoshi N, Murayama Y, Ozawa T, ishiyama K, 2005]. This resolution is much more than the

resolution possible by normal fluxgate sensors. Aichi Steel Corporation in Japan has

already developed a number of commercial GMI sensors for technological applications.

Another advantage of the GMI sensors are its small size. For example, the head length

of a normal fluxgate sensor is around 20mm while that of a GMI sensor is just around

1mm. Other than this GMI sensors have better thermal stability than conventional

magnetic sensors.

11

Fig.1.4 A photograph of GMI sensor

The comparison of the magnetic GMI sensor with other magnetic sensors is shown

below. Note that the range of GMI sensor is from 10-8 to more than 106 gauss.

FIG. 1.5 COMPARISON OF DIFFERENT MAGNETIC SENSORS

12

A magnetic sensor can be produced in two ways, one is contact

method and the other is non-contact method. In contact method the GMI wire or ribbon

is exposed to the magnetic field and the voltage drop over the wire or ribbon is

measured. In search coil method, the GMI material is placed inside at the center of the

coil and the field is applied along the length of the coil. When an ac current passing

through the coil, emf is induced on the coil which is proportional to change in flux and

the inductance of the coil. In the case of a coil with magnetic core, inductance is given

by

L = n2 V 0 r 1.11

Where n is the number of turns in unit length of the coil,

V, volume occupied by the coil

0, permeability in free space and

r, the relative permeability of the core.

So in the presence of an external field, the permeability of the core changes which

results in change in impedance and voltage drop of the coil. The schematic diagram of

the magnetic sensor is given below

13

Fig. 1.6 Schematic diagram of the sensor

A high frequency ac signal is applied to the search coil with magnetic core.

The voltage drop of the search coil in no field is replicated by a voltage divider and both

are given to the input of a differentia amplifier. The amplifier only amplifies the change

in voltage along the coil and the output is given to the display. The detailed design of

the sensor is given in chapter 3.

11..44 AAPPPPLLIICCAATTIIOONN OOFF GGMMII SSEENNSSOORRSS::

GMI sensors can be used in wide range of fields, some of the areas which

can benefit from GMI sensors are given below

Space research and aerospace applications

Magnetic storage systems

14

Automatic traffic controlling

Bio magnetic measurements

Geo magnetic research

Stress sensing applications

Target detection and non destructive crack detection

15

22

EEXXPPEERRIIMMEENNTTAALL TTEECCHHNNIIQQUUEESS

22..11 PPRREEPPAARRAATTIIOONN OOFF TTHHEE RRIIBBBBOONN::

There are two important steps in making tee magnetic ribbons. First one is

the homogeneous mixing of the constituents using arc melting. Second one is the melt

spinning to produce ribbons. First, the elements are weighed precisely in the desired

proportions and melted several times using arc melting. To ensure the constituents are

mixed well, the melting is repeated several times.

22..22 MMEELLTT SSPPIINNNNIINNGG::

After homogeneous mixing of the elements, the alloy is passed to melt

spinning setup to produce amorphous ribbons. The main parts of the melt spinning

setup are a commercial radio frequency furnace and a copper drum which is connected

to a high speed motor. The material is placed inside a quartz tube whose top end is

connected to an argon cylinder and having a small orifice (about 6mm) at the bottom.

The duty of argon gas is to prevent the material from oxidation and to push the material

out of the quartz tube. A high frequency high voltage induction coil is placed around

16

coil to melt the material inside the sample. The sample melts because of eddy current

heating. The quartz tube is placed 2-3 mm above the copper drum having a radius 18

cm and rotating at 2800rpm. The quartz tube is placed at angle 70 to the vertical for the

easy output of the ribbons. When the material is melted inside the tube, argon pressure

ejects the melt which falls on the rotating copper drum. The thickness of the ribbon is

inversely proportional to the rotation of the motor. The quality of the ribbon depends on

the pressure of the argon gas inside the quartz tube, the distance between quartz tube

and the copper drum and the quality of the surface of the copper drum.

17

FIG. 2.1 MELT SPINNING APPARATUS

22..33 GGMMII MMEEAASSUURREEMMEENNTTSS::

The GMI measurements are made using HP 4192A LF IMPEDANCE

ANALYSER, Helmholtz coil and a constant current source for the Helmholtz coil. The

measurements are made in frequency range 100 KHz to 10 MHz by the application of

18

a constant current 10mA through the sample. The field is applied by the help of

Helmholtz coil in the range -6 Oe to +6 Oe in the longitudinal direction of the ribbon.

The field is varied by varying the current through the Helmholtz coil.

19

33

DDEESSIIGGNN IINNGG TTHHEE SSEENNSSOORR

33..11 IINNTTRROODDUUCCTTIIOONN::

Over the last few years there has been many models of magnetic

sensors based on GMI effect. Over all those sensors, there are only very few which are

mobile and handy. The sensor discussed here belongs to this category. It is handy

and works on battery so that it can be taken anywhere to take measurement just like a

multimeter. This section explains the basic idea and fabrication of such a sensor.

Other than the battery power source, its always good to have a an AC

power source for the sensor so that battery power can be saved in case if there is any

AC plug around. So the power supply part of the sensor includes a duel regulated

power supply which uses the AC mains along with a back up battery power supply .

The schematic diagram of the sensor is given below. Other than the

power supply part, the sensor consists of three parts. The high frequency Oscillator

part, the sensing part and the display part. The high frequency oscillator part is a

colpitts oscillator, which produces an alternating current of frequency 10 mega hertz

20

approximately. The ac signal is passed over to the next part in which the signal is

applied to a search coil which actually senses the magnetic field. The search coil is

followed by a schottky barrier diode which rectifies the voltage drop on the search coil

followed by a instrumentation amplifier. The duty of the instrumentation amplifier is to

cut off the unwanted voltage and to amplify it accordingly for the display part to display

it.

FIG. 3.1 BLOCK DIAGRAM OF THE SENSOR

The following paragraphs will discuss each section in detail with appropriate circuit and

figures.

21

33..22 AACC PPOOWWEERR SSUUPPPPLLYY UUNNIITT::

The duty of the power supply is to supply constant voltage to

the sensing circuit, oscillator and the digital panel meter. The Oscillator demands a

constant +12 volt, The sensing circuit demands +12 and -12 volt and the digital panel

meter needs voltage between 7 and 11. The circuit diagram of the power supply used

in the sensor is given below. The AC 230 volt is given to the center tapped step down

transformer whose output voltage is 15-0-15. After the transformer, follows the usual

rectifier circuit. The diodes D1,D2,D3,D4 rectifies the ac voltage and the capacitors

C1,C2,C3,C4 filters the ac components and gives a smooth dc voltage to the input of

the voltage regulator IC s. IC 1 is 7812 positive regulator ic and IC 2 (7912) is negative

regulator IC. The terminals of the regulator IC s can be identified from the picture given

below. Since the digital panel meter cannot take 12 volt, an appropriate voltage divider

is used to produce a voltage between 7 and 11 out of +12 volt.

22

FIG. 3.2 DUEL VOLTAGE POWER SUPPLY (+12/-12) USING 7812 AND 7912 REGULATOR ICS

7812 7912

FIG 3.3 PIN DIAGRAM OF 7812 AND 7912

33..33 BBAATTTTEERRYY PPOOWWEERR SSUUPPPPLLYY::

The battery power supply is made by connecting four nine volt

batteries in series. The end terminals gives +18 and -18 which are given to the input

23

terminals of regulator IC s, 7812 and 7912.The regulator IC give a constant +/- 12 volt

for the oscillator and the sensing circuit. Since the DPM cannot afford 12 volt, a

connection is taken from positive terminal of the one of the batteries to produce a nine

volt terminal.

FIG. 3.4 BATTERY POWER SUPPLY USING 7812 AND 7912

33..44 OOSSCCIILLLLAATTOORR CCIIRRCCUUIITT::

The high frequency ac signal needed for the sensing circuit is

produced by the colpitts oscillator. The reason to choose colpitts oscillator from other

oscillators is that it is very simple and sturdy. The circuit diagram of the oscillator is

give below. The resistors R1, R2 and R3 are part of the voltage divider and the

inductor L1 along with the capacitors C1 and C2 determines the frequency of the

24

oscillator. The voltage needed for feedback is taken from the voltage divider made by

the two capacitors in serial. The frequency of the oscillator is given by

2.1

FIG 3.5 CIRCUIT DIAGRAM OF THE COLPITTS OSCILLATOR

33..55 SSEENNSSIINNGG CCIIRRCCUUIITT::

Sensing circuit is the main part of the magnetic sensor. The circuit

diagram of the sensor is given below. The output of the oscillator is given to the search

coil which is connected in series with a resistor. The purpose of the resistor is to keep

25

the voltage drop across the coil to an optimum value. The sensing coil is a simple coil

inside which a magnetic material is placed as core. In this case it is melt spun magnetic

ribbon. Since the ribbon shows GMI effect, when an axial magnetic field is applied, the

voltage drop across the coil changes according to the intensity of the field present. This

is the basic idea of the sensor. Since the voltage drop over the coil without the field is

always added to the voltage drop in the presence of field, it should be subtracted from

the actual voltage drop to increase precision.

FIG. 3.6 CIRCUIT DIAGRAM OF THE SENSING CIRCUIT. THE VALUES OF THE RESISTORS ARE

R1=10K, R2=30K (THREE 10K IN SERIES), R3= 0 - 0.5K, R4=1K AND R5=10K.

To subtract the unwanted voltage drop and to amplify only the change in voltage

drop produced by the field, An instrumentation amplifier is used. The voltage drop over

26

the coil, which is rectified using a schottky diode is given as one input of the amplifier.

The second input is from a variable resistor which works as a voltage divider and

replicates the voltage drop of the coil when no field is present. The schottky diode and

the instrumentation amplifier used are discussed below.

33..5511 SSCCHHOOTTTTKKYY DDIIOODDEE::

Schottky diode is a special kind of semi conductor diode with very low forward

voltage drop and very fast switching action. The main reason to choose schottky diode

over the normal diode is its very low barrier voltage. The normal silicon diode has a

voltage drop of 0.7 volt, which means in some field, if the coil produces a voltage drop

less than 0.7 volt, the sensor will not be able to measure the field. The voltage drop of

the schottky diode used in the circuit (RA 69), has a voltage drop of 0.2 volt which is

why it is preferred for the sensor circuit.

Fig. 3.7 Schottky Diode Schematic Symbol

27

Another reason to choose schottky diode over normal diode is its very

low reverse recovery time. A normal p-n junction diode take hundreds of nano seconds

to change from conducting state to non conducting state while a schottky diode takes

practically no time for it which makes it ideal for very high frequency circuits.

33..5522 IINNSSTTRRUUMMEENNTTAATTIIOONN AAMMPPLLIIFFIIEERR::

An instrumentation amplifier is an advanced form of the differential amplifier.

An instrumentation amplifier usually consist of a differential amplifier with one or more

voltage followers which makes it suitable for the construction of weighing machines,

Temperature sensors, field sensors etc. The additional benefits over simple differential

amplifier are high common mode rejection ratio, high input impedance, low noise and

low dc offset voltage. A simple circuit diagram of an instrumentation amplifier is given

below

FIG. 3.8 SIMPLE CIRCUIT DIAGRAM OF AN INSTRUMENTATION AMPLIFIER

28

As shown in the diagram, the above instrumentation amplifier consists of three

op-amps out f which one is a differential amplifier with a gain equal to R3/R2 and

differential input resistance 2 R2.The other two op-amps are arranged in a such a way

that it buffers the input voltages V1 and V2.The voltage gain of the setup is given by

2.2

In the sensor discussed her, the instrumentation amplifier used is AD 620. The

simplified actual circuit diagram and pin diagram of AD 620 is given below. Another

advantage of instrumentation amplifier including AD 620 is that the gain can be varied

by changing just one resistance value. In the case of AD 620, the voltage gain is given

by

Av = 1+(49.4k/RG) 2.3

29

FIG. 3.9 SIMPLIFIED CIRCUIT DIAGRAM AND PIN DIAGRAM OF AD 620 INSTRUMENTATION

AMPLIFIER

In the circuit used in the sensor, voltage drop of the coil is given to the

non inverting input terminal while the voltage drop from the variable resistor is given to

the inverting terminal. In this way the variable resistor can be used as a zero correction

knob. This arrangement keeps the output voltage positive always. The gain of the

30

amplifier can be selected between 6 and 60 by using the two way switch. This is

achieved by connecting two resistors of resistance value 1k and 10 k between

terminals 1 and 8 through a two way switch. The output is taken from terminal 6 is

passed to the display section.

33..66 DDIISSPPLLAAYY

To display the output voltage of the amplifier, a 3 digital panel meter

is used. The DPM is cheap and available in all electronic shops. A picture of PM 129

3 LED DPM is given below. The newly bought PM129A DPM can only measure

voltages up to 200mV. But by changing the specific resistor values on the board, it can

be changed to 20V, 200V and even 1000 V.

b

FIG. 3.10 PIN DIAGRAM OF PM129A DPM

31

A complete picture of the sensor designed is given below .

FIG 3.11PHOTOGRAPH OF THE SENSOR

1.Battery power supply

2.display

3.Main circuit (Sensing circuit and Oscillator)

4.Sensing coil

5. Regulated power supply

32

44

RREESSUULLTTSS AANNDD DDIISSCCUUSSSSIIOONN::

Even though the main objective of the project is to design a magnetic sensor and

to find practical application by it, a basic study of GMI has been done to understand the

concept and to be familiar with. The X-ray pattern of the Fe68 Co5 Si12B15 melt spun

ribbon at different annealing temperature is given below

FIG 4.1 XRD PATTERN OF Fe68 Co5 Si12B15 MELT SPUN RIBBON AT DIFFERENT ANNEALING

TEMPERATURES

33

44..11 GGMMII SSTTUUDDIIEESS:: The dependence of GMI on the applied magnetic field

and ac current frequency are studied using an impedance analyzer and Helmholtzs

coil. The frequency has been changed from 100 kHz to 10 MHz and the magnetic

field from -6 Oe to 6 Oe. For field variation measurements, 5 cm long melt spun

Co68Fe5Si12B15 and Co68Fe5Nb12B15 ribbons are used (the samples are made by

Ms. Sandhya Dwevedi). Co68Fe5Si12B15 ribbon alone is used for freequency

measurements.From the graph given below it is clear that the impedance changes

rapidly up to a few mega hertz, and after that the slop decreases. This can be

explained by the effect of skin effect on impedance of the sample. As explained earlier,

in moderate freequency regime, the skin effect is relatively large and the relation

between freequency and skin depth is given by

=

4.1

Where is the resistivity of the ribbon,

the permeability and f the frequency of the ribbon.

To study the dependence of impedance on field, constant current of

amplitude 10 mA and frequency 1 MHz is given to the ribbon and the field is varied

using Helmholtz coil. Since the frequency is 1MHz, the variation in impedance still can

be explained by skin effect. The applied field filed decreases the permeability of the

ribbon, which increases the skin depth which results the decrease in impedance. The

variation of GMI ratio with field is shown in the graph below

34

FIG 4.2 THE VARIATION OF GMI OF CO68 Fe5 Si12 B15 WITH FIELD AT 1MHZ

FIG 4.3 THE VARIATION OF GMI OF CO68 Fe5 Nb12 B15 WITH FIELD AT 1 MHZ

35

FIG 4.4 THE CHANGE IN GMI RATIO OF CO68 Fe5 Si12 B15 WITH FREQUENCY AT FIELD 6 Oe

44..22 SSEENNSSOORR MMEEAASSUURREEMMEENNTTSS::

The sensor whose design has been explained in the report is found to

be very useful for low field measurements. Three different types of field measurements

were taken using the sensor.

1. Measurement of the field produced by Helmholtz coil

2. Measurement of Earths magnetic field using different samples

3. Measurement of Magnetic field variation in IIT campus

36

44..2211 MMEEAASSUURREEMMEENNTT OOFF TTHHEE FFIIEELLDD PPRROODDUUCCEEDD BBYY HHEELLMMHHOOLLTTZZ

CCOOIILL:: In this measurement, the ribbon is placed inside the sensing coil and

both are placed between the Helmholtz coils with the length of the ribbon parallel to the

field. Two different readings are taken using different ribbons, Finemet and

CO68Fe5Si12B15. The field is increased in steps of 0.06Oe from zero to 2Oe and the

corresponding sensor output voltage is noted. The graph representing the

measurement is shown below. The one advantage of the sensor is that the ribbon or

wire which acts as the core can be changed for appropriate field measurements. Even

though the graphs are not linear, with an appropriate calibration table the sensor can

be used to measure low magnetic fields.

37

FIG 4.5 VARIATION OF SENSOR OUTPUT VOLTAGE WITH MAGNETIC FIELD

38

44..2222 MMEEAASSUURREEMMEENNTT OOFF EEAARRTTHHSS MMAAGGNNEETTIICC FFIIEELLDD::

To measure the Earths magnetic field, the coil carrying ribbon is rotated in

Earths magnetic field and the change in sensor output voltage is noted. When the coil

is perpendicular to the Earths magnetic field (E-W direction), the field is perpendicular

to the easy magnetization axis of the magnetic ribbon (for a fresh ribbon easy axis is

decided by the shape and demagnetizing effects). So Earths magnetic field has no or

negligible effect on the coil when it is placed perpendicular to it. But when the coil is

rotated, the parallel component of the Earths magnetic field acts on the ribbon and the

impedance of the coil starts changing. When it is exactly parallel to the direction of the

field (N-S direction), the whole field is acting on the ribbon and the change in

impedance of the coil is maximum. So the difference in sensor reading is proportional

to the Earths magnetic field. The first graph shows relatively small changes in sensor

reading while the second one bigger change. From the graphs, Co68Fe5Si12B15 ribbon

shows 9.11%(B) change in impedance CO68Fe5Nb12B15(B) ribbon shows 5.49%

Finemet shows 27.12%(D) and CO68Mn5Si12B15 shows 25.79%(E) in earths magnetic

field.

39

FIG 4.6 CHANGE IN SENSOR OUTPUT VOLTAGE WITH ROTATION IN EARTHS MAGNETIC FIELD

40

44..2233 MMEEAASSUURREEMMEENNTT OOFF MMAAGGNNEETTIICC FFIIEELLDD VVAARRIIAATTIIOONN IINN

CCAAMMPPUUSS:: An interesting study of the magnetic field variation in IITM campus has

been done by taking the sensor to different places and measuring the filed in the local

area .The sensor recorded considerable amount of change in magnetic field in

campus. But it is important to note that the changes in field mentioned here are the

relative change in magnetic field, not the exact values.

41

FIG 4.7 THE CHOSEN PLACES FOR MEASUREMENT OF MAGNETIC FIELD

42

The places numbered in the picture are these

1. Near main entrance

2. Construction area

3. Lake side

4. Peliamman temple

5. Stadium

6. Gajendra circle

7. OAT

8. Sangam ground

9. Near biotechnology department

10. Velachery gate

The variation in the field in the campus is measured using CoFeSB magnetic ribbon.

The relative change in magnetic filed over the campus is shown in the graph below.

From collected values, stadium has the smallest and biotechnology department has

the highest values of magnetic field.

43

FIG 4.8 MAGNETIC FIELD AT DIFFERENT LOCATIONS IN CAMPUS

1. Near main entrance

2. Construction area

3. Lake side

4. Peliamman temple

5. Stadium

6. Gajendra circle

7. OAT

8. Sangam ground

9. Near biotechnology department

10. Velachery gate

44

55

CCOONNCCLLUUSSIIOONNSS

The GMI studies on ribbons explain the basic idea and mechanism of GMI.

The Co68Fe5Nb12B15 ribbon shows a maximum of 11.2% and Co68Fe5Si12B15 ribbon

shows maximum of 26.7% change in Impedance at frequency 1MHz.The magnetic

sensor developed is found to be very useful in low magnetic fields. Using

Co68Fe5Si12B15 and Co68Fe5Nb12B15 magnetic ribbons as sensing elements, the field

produced by a Helmholtz coil was measured and the corresponding graphs are given.

Using four different ribbons CO68Fe5Nb12B15, Co68Fe5Si12B15, CO68Mn5Si12B15 and

finemet the Earths magnetic field was measured. The corresponding percentage

change impedance are 5.49%, 25.79%,, 9.11% and 27.12%. Also study of variation in

magnetic field in campus has been done and a graph was drown showing the relative

change in magnetic field with different locations.

45

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