Magnetic Sensors: An Overview

Magnetic Sensors: An OverviewMagnetic Sensors: An Overview

Evangelos HristoforouLaboratory of Physical Metallurgy

National Technical University of Athens

IEEE Sensors 2003 Conference - Toronto, Canada

Outline

•Part A: Engineering Theory of Magnetism•A short introduction to magnetics•The magnetization process•Magnetic effects for engineering applications

•Part B: Sensors Based on Magnetic Materials•Sensing principles based on magnetic effects•Applications of sensors based on magnetic materials•Developing a sensor


Part A:

Engineering Theory of Magnetism


A short introduction to magnetics

• Paramagnetism


∆H

M

• Diamagnetism

M

H

H=0 A/m

M

H>>0 A/m

MH=M - ∆M

χ>0

χ<0

Ferromagnetism

• Short range or exchangeinteraction results in spin-spin alignment of neighboring spins in some TM materials

• The spin-spin alignment results in the formation of the so called ferromagnetic domains


Ferromagnetism

Magnetic domains cannot be extended to infinityDomain-domain interaction results in magnetic domains separated bymagnetic domain walls


M=0

Orientation of dipoles in domain walls changes gradually

The Basic Mechanisms of Magnetization Process

• Domain wall motion:• Reversible• Irreversible

• Domain rotation:• Barkhausen jumps: irreversible effect• Small angle rotation: reversible effect


Reversible Magnetic Domain MotionReversible Magnetic Domain Motion

M=0H=0

M>0

Msat

H


H

M

Hs

Ms

Irreversible Magnetic Domain MotionIrreversible Magnetic Domain Motion


M=0H=0

M>0

Msat

H

Defect

H

M

Hs

Ms

Irreversible domain rotation:Irreversible domain rotation:BarkhausenBarkhausen JumpsJumps


H

Easy axis A Easy axis B

Reversible Rotation of Domains


Reversible magnetic domain rotation after the irreversible magnetisation process

H ∆H

The Magnetization Process


H

M

Hs

Ms

• Virgin curve– Intrinsic magnetization

changes under external field– Main curve: three parts

• Domain wall motion• Barkhausen jumps• Rotation of magnetization

– Minor loops

• Saturation Ms– Completely oriented dipoles

H

Mo

MB

Ms

The Magnetization Process

• B-H loop– Saturation & back– Irreversible process – remanence– Coercive force (field) & coercive slope– Properties & dynamic properties: f, T, σ– Single phase systems have symmetric B-H loops– Minor loops: Inside the B-H loop– Size of B-H loops: hard - soft - semi– Non-single-phase materials may result in asymmetric B-H loop– Large Barkhausen Effect: an interesting property


-1,1

0

1,1

-30 0 30

H

M/Ms


B-H loopMain parameters: Temperature - Stress - Frequency

Magnetic effects

Magnetostriction:– Magnetic domain

rotation (reversible and irreversible)

– +λ(H): zero, classic,unhysteretic, giant, colossal

– Dynamic properties, hysteresis


-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

-30 0 30

H

λ/λs


Magnetic effects

Magnetostrictive Delay Lines:an application of the magnetostriction effect

Ie

Ie

MDL

MDL

Vo

a

a

Detectable change of flux density

Propagating elastic pulse

Generated microstrains

Magnetic effects

Z-H loops

Magnetoresistance(MR, AMR, GMR, CMR)

Magnetoimpedance(MI, SI, GMI)


0

100

200

300

400

-80 -40 0 40 80

UPDOWN

∆Z/

Z (%

)

H (Oe)

1.5 mA

b)200

300

400

-4 -2 0 2 4H (Oe)

-15 -10 -5 0 5 10 15-4

-3

-2

-1

0

GMR%

H (kOe)

AgPt (2.8 nm)-Co(0.5 nm) AgCu (2.8 nm)-Co(0.5 nm) AgSi (2.8 nm)-Co(0.5 nm)

Magnetic effects

Inductive effects

Wiedeman effect

Domain wall nucleation& propagation

Classic set-ups(LVDT, fluxgates)


I (t)exc

Vind

µ(t) B(t) B0

H0

iEXC

DA Vout

PSD

INT

2f

GEN

C1

C2

f

Icomp

vi

Magnetic effects

Magneto-optic effectsBitter, Kerr, Faraday effects

Use of M-O effectsCase study:displacement sensor


H = Hs

H = 0

H = -Hs

Magnetic effects

Spintronics

Spin valves

Spin tunneling


Pinning ofReference layer

ShapeAnisotropy(geometric)

InducedAnisotropy

Orange PeelCoupling

Stray FieldCoupling

(geometric)

Sensors Iso lation Coil

Magnetic materials

BulkThin films

RQ (ribbons & wires)Particulate media

Composites


Stress reliefInducing anisotropyTailoring B-λ-Z(H)Nanocrystallization

Micromechanics

Applications of magnetic materials

(Fe Co Ni)-(ETM)-(RE)-(NMM)

Applications of magnetic materials

Powertransformers

motorsactuators

Recordingrecording mediarecording heads

technology

Sensors

&


Magnetic Sensors: An OverviewMagnetic Sensors: An Overview

Part B: SensorsPart B: Sensors


Sensors

• Position

• Stress

• Field

• Position (tapes, digitizers, 3-d)• Dilatometry: LVDT, MDL• Speed & vibration: accelaration, …

• Load cells & torque meters• Pressure gauges and stress sensors• Force/pressure digitizers

• NDT applications• Recording• Compass


Data from EMSA 2002, Athens, Greece

Position Sensors

• Position sensor based on the magnetostrictive delay line technique– Sensitivity: 5 µm; Uncertainty: + 10 µm– Drawbacks: ambient field sensitive– Characteristics are optimised by using amorphous alloys


Ie

Pulse voltage output train of the in series connected search coils

Horizontally movable permanent Nd-Fe-B magnetMDL

-10

-5

0

5

10

0 10 20

Measuring length (mm)U

ncer

tain

ty (m

icro

ns)

Position Sensors(MDL technique cont.)


ECT-MDL digitizerECT-MDL digitizer

Dilatometer and stress sensor Moving magnet sensor

Position Sensors

• Position sensor based on the linear variable differential transformer (LVDT) set-up. (1) Soft magnetic material, (2) Excitation coil, (3) In series opposition search coils.– Sensitivity: 100 µm– Uncertainty: + 500 µm– Repeatable response from -70 C to 150 C– Drawbacks: ambient field sensitive– Improved characteristics by material tailoring


1 2 3

02040

6080

100

0 10 20

Magnet-piston position (mm)S

enso

r res

pons

e (m

V)

Position SensorsPosition sensor based on an inductive principle


Position Sensors

• Position sensor based on permanent magnet tape. (1) Permanent magnetic thin films, (2) Searching magnetic head.

– Sensitivity: 100 µm– Uncertainty: + 500 µm– Repeatable response from -70 C to 150 C– Drawbacks: ambient field sensitive– Characteristics can be improved by material tailoring

1 2


Position Sensors

• Angular positioning sensor. (1) Radial permanent magnets in tooth arrangement, (2) Searching field sensor.

– Repeatable response from -70 C to 150 C– Great reproducibility allowed use in automobile industry (ABS)– Drawbacks: ambient field sensitive– Advantages: no dust sensitive


1 2

Stress sensors

• Stress sensors based on inductive effects. (1) Soft magnetic element, (2) Inductive excitation coil.– More sensitive than strain gauges, if amorphous alloys are used– Good noise level after heat annealing– Good reproducibility after field annealing– Ambient field sensitive


FF

1 2

0

20

40

60

80

0 50 100 150

Tensile stress (N/sq.mm)V

o (m

Vol

ts)

Stress sensors

Pressure, stress & torque sensors based on MDLs


Ie Vo

MDL

Ie Vo

MDL0

1020

3040

5060

70

0 20 40 60

Applied force (gr)

Vo

(mV

olts)

01020304050607080

0 20 40 60

Applied force (gr)V

o (m

Vol

ts)

Stress Sensors

Thickness sensor based on a coilless MDL set-up


0

2

4

6

8

10

10 30 50 70 90 110 130Pulsed current (mA)

Out

put V

olta

ge (m

V)

0

2

4

6

8

10

0 2 4 6 8 10 12 14 16 18 20Thickness (X100nm)

Out

put (

mV

)

Stress Sensors

• Magneto-inductive (MI) stress & torque sensors. (1) Sinusoidal excitation circuit, (2) MI element.

– Very sensitive, especially in high frequencies


1

2

Vout

Vout

0

10

20

30

40

50

0 0,4 0,8 1,2 1,6

Applied force (X10N) or applied torsion (turns/m)

Vol

tage

out

put (

mV

) Torsion

Stress

Stress Sensors: Torque Meters


Air-flux (A)

C3 C4

C1 C2

RibbonSignal coil

Cantilever

C3-C4

AC1-C2

Air-flux (A)

Air-flux Sensors Based on the Inverse Wiedeman Effect


Ambient field dependence

• A serious drawback in mechanical sensors based on magnetics

• There are three solutions:– Two axes shielding by nanocrystalline thin

ribbons– Active compensation by induced field in some

cases (MDL and MI set-ups)– Smart field sensing


Field Sensors

Fluxgates


IEX

Vout

B0

c)

noise PSD (pTrms/√Hz)

frequency (Hz)

1 100.10.1

1000

100

10

1

1.03760 Hz Y = 2.919 (pTrms/√Hz)

time (s)

0 5 10 15 20 25

sensor noise (pT)40

20

0

-20

-40


Field Sensors

Magnetoresistive heads


Field Sensors

MDLs

IeVo

MDL

-60

-40

-20

0

20

40

60

-160 -120 -80 -40 0 40 80 120 160

Applied field (A/m)

Vol

tage

out

put (

mV

)


Field Sensors

GMR & Spintronics

SubstrateSi

Hard Layer

Spacing Layer

current strap

Isolation

Sensing Layer

(Co)(FeMn)

current direction for lin. characeristic

Cu

Cu

Co/Py

hyst.

Smart Sensors

Multifunctional system: combination of measurements• - Magnetostrictive delay line• - Magnetoimpedance set-up• - Domain wall nucleation and propagation set-up

Using three different types of response to determine three different inputs


IeVo

Magnetic Element

Sensor Applications

• Industrial– Sensors in production (piston position, stress monitoring, else)– Sensing systems for industry– NDT&E using field detection techniques


Ie

MDL

Voltage pulses due to cracks on the magnetic surface

Under test magnetic surface

GMR Sensor Response

Shielded GMR Sensor Response

Hall Sensor Response

A Comparison Between Hall and GMR Sensors for NDT&E Inspection


A Micro-Fluxgate Arrangement for NDT&E Inspections


The 4 m-Fluxgate Sensors The Sensor

The Under Inspection Magnetic Anomaly

Sensor Applications

BiomedicalField monitoring, diabetes & blood monitoring, stress & strain monitoring,

cardiac measurements, artificial sphincters


B (Tesla)

BiomagneticFields

Lung particles

Human Heart Skeletal muscles Fetal heartHuman eyeHuman Brain

Human Brain (evoked response)

SQUID System Noise level

Earth’s Field

Urban Noise

Car @ 50m

Screwdriver@ 5m

Transistor, IC chip @ 2m

Transistordie @ 1m

Electromagnetic Measurements in Bio-Engineering

GMI Measurements


Sensor Applications• Military applications

– Gyroscopes (field)– Magnetic signatures (field)

• Automotive– Positioning & tracking (field)– Torque sensors (stress)– On-off- ABS rotation (positioning)

• Other applications– Environmental: E/M low frequency– Laboratory : characterization techniques

– Domestic : current path searcher


Sensor Applications


Induced m agnetic f ie ld

D C cu rrent

r ibbon

Current source

Fu n ction gen era tor

Lock - in Am p lif ie r

O scillo scope

G PIB In te rfa c e

Induced m agnetic f ie ld

D C cu rrent

r ibbon

Current source

Fu n ction gen era tor

Lock - in Am p lif ie r

O scillo scope

G PIB In te rfa c e

Electric Current Meter


AMR Bridge Micro-coils

Micromagnetic arrangements

Micromagnetic arrangements


Developing a sensor

• Facing-up the problem– measurement– range – sensitivity– Field of use (application)– Parameters affecting

• Choice of sensing principle (effect)– Taking into account: range – sensitivity – parameters– Preliminary experiments: indicative results– Scheduling the experiment & and the sensor


Sensor Development

• Sensing element : thin film, RQ, powder, else• Sensing device: the first prototype• Sensor electronics &calibration set-up• Measurements• Parametric effects: field, stress, temperature• Design of the sensor housing• Development of the sensor prototype• Calibration & Corrective actions• Final prototype & technical envelop


Designing the future

Is miniaturization a future challenge?– Nanoscale resolution in writing & reading– Sensors with secondary standard uncertainty (nanotechnology?)– Smart multipurpose sensors– Parametric control (field-temperature- stress)– Silicon-chip magnetic sensors

Is there any key-parameter in designing magnetic sensors?– Hysteresis: the key point– Stability of B-λ-Z(H)– Parametric effects: field- stress-temperature


Magnetic Sensors: An Overview

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