Precise Vectorial

Magnetic Sensors

Pavel Ripka

Czech Technical University, Prague

NATO ASI “Smart Sensors and MEMS”Povoa de Varzim, Portugal, 2003

Magnetic Sensors: Overview

• Basics– types, specifications, principles

• Recent trends– miniaturization, high working temperature

• news in AMR and GMR magnetoresistors

• Contactless current sensors

• Applications

Magnetic sensors: basic types

• Magnetic field sensors– semiconductor– ferromagnetic– other (optical, resonant, SQUID…)

• Magnetic sensors of non-magnetic variables– position, force, torque, strain– temperature

Magnetic field sensors

ScalarMeasure the size of B

(“total field B”)

only resonant sensors

VectorMeasure the projection

of B into the sensitive axis• single-axis• tri-axial

most

magnetic sensors

222zyx BBBB

Tri-axial sensors

Wrong waycalculate it from rms values

Many instruments use this

formula, assuming that the field

does not change direction

Right waycalculate it from

instantaneous values

How to get total field B (field magnitude)

222zyx tBtBtBtB

222rmszrmsyrmsxrms BBBB

Tri-axial sensors: compass

X (forward)

Bx

Bz

By

Earth’s Field vector B

Z (down)

Y(right)

Horizontal component

= Azimuth or Heading

Magnetic field sensors: DC and AC

AC

Measure only changing field:

induction coils

Vi .. Induced voltage

.. Magnetic flux

A .. Coil area

N .. Number of turns

DC

Measure DC and AC fields

most

magnetic sensors NAB

dt

d

dt

dVi

Basic sensor specifications

• FS range, linearity, hysteresis• TC (“tempco”) of sensitivity• Offset, offset tempco and long-term stability• Perming (= null change after magnetic shock)

• Crossfield sensitivity• Noise

– PSD , rms or p-p value

• Resistance against environment– temperature, humidity, vibrations

Types of magnetic field sensors

• Semiconductor sensors (Hall, …)• Permalloy magnetoresistors (AMR, GMR, …)• Resonant magnetometers (Proton, Cesium, ...) • SQUIDs (LTS + HTS)• Induction coils, rotating coils• Optical (Fibre optic, ... )• Fluxgate • Other principles (GMI, magnetoelastic, …)

Magnetic position sensors

Analog outputLinear

– Inductive (AC)

LVDT, LCD,Inductosyn– with perm. Magnet (DC)

Angular– DC: Hall, MR– AC:

Encoders

Two-state output(active or switch):

• Induction (speed)• Inductive• Pulse wire (Wiegand)• DC (Hall or MR)

Position sensors with permanent magnet

Linear (NVE)

Rotational (Sentron)

Magnetic field magnitudes

100 T Pulse field

10 T Superconducting magnet

2 T Electromagnet

0.5 T Surface of strong perm. magnet (NdFeB)

0.1 T Surface of cheap magnet (ferrite)

10 mT Power cable

50 T Earth’s field

1 T Vehicle

10 fT Human brain

Basic rules

Dipole field (from small objects)

B 1/r3

Long iron pipe

B 1/r2

Long straight current conductor

B 1/r

Semiconductor magnetic sensors

• Hall – integrated– GaAs, Si, (Ge)

– non-plate: vertical, cylindrical

• Semiconductor magnetoresistors

• ExoticExotic(magnetotransistors, magnetodiodes,(magnetotransistors, magnetodiodes,

rotating current domain, ...)rotating current domain, ...)

Hall sensors: basics

BIt

RV H

H

I t

B

VH

Linear Hall sensor

Asahi Kasei Electronics: InSb Hall element (HW series)

Hall integrated circuit

Analog electronics:

• Delivers constant current

• Amplifies VH

• Flips contacts

• Performs compensations

• May compare with threshold

Honeywell Hall Sensor Using Four Cross-Connected Hall Elements

Programmable Hall sensor

Micronas HAL 800:

only 3 terminals

analog <> digital mode

Vertical Hall sensor

I

V

J

V1

V2

V3

B

B is parallel to the substrate

Advantages:

long-term stability

robustness

Active zone is buried into a mono-crystal, far away from the chip surface.

Permalloy Flux concentrators

Cylindrical Hall device with integrated magnetic flux concentrators

(Sentron AG)

Used for Hall and MR

Increase sensitivity

Possible problems:

• TC of sensitivity

• perming

• linearity

Semiconductor Magnetoresistors

Siemens MR:Needle-shaped low-resistanceprecipitates of NiSb in a matrix

of InSb serve as the shorting bars.

Lorenz force deflects the electrons

electron path is longer

resistance is higher

Short and wide strips are sensitive

Semiconductor MR: disadvantages

• low sensitivity for small fields

• temperature dependent

• no response to field sign

Resistance vs field for an InSb magnetoresistor at 20ºC, 0ºC, 25ºC, 60ºC, 90ºC, and 120ºC (top to

bottom).

AMR: anisotropic magnetoresistance

• Permalloy thin film strip deposited on a silicon wafer magnetized in x direction

• HY rotates magnetisation M R changes by 2%-3%

HY

AMR: linearisation

Bad idea:

Shifting the working point

by bias field

Good idea:

Barber-pole Al bars

deflect the current by 450

(Honeywell)

AMR bridge sensor

Philips KMZ

Full bridge made of meandered resistors with barber-pole strips

AMR: flipping

Unwanted change of strip

permanent magnetization may

distort the sensor characteristic.

Periodical saturation of

the permalloy strips is the cure

Characteristics of Philips KMZ 10

after positive [+] and negative [-] flip

P.F. is characteristics for periodicalflipping

+-

P.F.

AMR: flipping

Flipping:

+ decreases offset+ reduces perming+ increases sensitivity

- increases power

consumption Honeywell AMR sensor

with integrated flat flipping coil

GMR: Giant Magnetoresistance

Spin - dependent scaterring:

Resistance of two thin ferromagnetic layers separated by a thin nonmagnetic conducting layer can be altered by changing the moments of the ferromagnetic layers

from parallel to antiparallel.

B

I

Common GMR structures

20 m

2 m 10 nm

A)C)

B)

AF Pinning Layer

Magnetic Layer

Non-magnetic Layer

A: Spin valve

B: Sandwich

C: Multilayer

technology developed

for reading heads

GMR sandwich

Sensitive, but not good for linear sensors

0.7

0.705

0.71

0.715

0.72

0.725

0.73

0.735

0.74

-50 -40 -30 -20 -10 0 10 20 30 40 50applied field (Oe)

vo

ltag

e (

V)

GMR: spin valve

2600

2650

2700

2750

-30 -20 -10 0 10 20 30

Applied Field (mTesla)

Res

ista

nce

(

)

8

9

10

11

12

13

-200 -100 0 100 200

Angle (Degrees)

Re

sis

tan

ce

(O

hm

s)

0

Large field response

hard layer may be demagnetized

Angular response

Unpinned soft layer rotates with external field

If saturated, responds only to field direction, not value

GMR bridge sensor

GMR resistors configured as a Wheatstone bridge sensor (NVE)

• R2, R3 are shielded

• R1, R4: field is concentrated by approx. D1/D2

Still has nonlinear

response

unlike AMR bridge

(NVE) 0

50

100

150

200

250

-2.5 -1.5 -0.5 0.5 1.5 2.5

Applied Field (mTesla)

Vo

ltag

e (m

V)

0

10

20

30

40

50

Ou

tpu

t (m

V/V

)

Integrated digital GMR sensor

Very sensitive:

target may be – small– weak magnet– far away

Schematic and logic output of NVE digital sensor

Advantages of magnetoresistors

compared to Hall sensors:

• high sensitivity– for position sensors: magnet may be

cheaper or smaller or airgap higher– for magnetic field sensors: higher accuracy

• no piezo effect

• higher operational temperatures

Fluxgate sensors

Classical fluxgates:

precise, but expensive (CTU Prague)

Most sensitive room-temperature magnetic sensors

Based on non-linear magnetization characteristics of ferromagnetic core.Measure up to 1 mTwith 100 pT resolution

Fluxgate principle

• Ferromagnetic core- non-linear B-H

• Excitation and sensing coil

• Core is periodically saturated by Iexc, drops to 1twice each period

• Measured B0 causes 2nd harmonics in Vind

• In absence of external field, magnetisation is symmetrical

• External measured field causes assymetry – detected in induced voltage

Basic types of fluxgate

• Double core suppresses first harmonics

Fluxgate magnetometer

Micro-fluxgate sensors

(in development)

• flat coils• electrodeposited core or

amorphous strips• electronics on chip

• cheap• resolution still higher than

MR

Shizuoka University

Contactless current sensors

NVE GMR sensor measures current in close wire

Long straight current conductor

B 1/r

PCB - integrated current sensor

1 – the current lead

2 – the ferromagnetic yoke

3 – Vertical Hall sensor or MR

Sentron

Current sensing: Sensor Array

Array of six sensors:

• increased sensitivity

• resistant against external currents and fields

Sentron Hall sensors with field concentrators

measure current flowing through the hole

Applications: geophysics

Geometrics

cesium magnetometer

for field mapping

Applications: Biomagnetism

Magnetopneumography

= mapping of the ferromagnetic dust deposited in human lungs

SQUID or fluxgate magnetometer

Possible medical applications

1. Tracking devices for monitoring the 3-D position and orientation sensing potential applications in

virtual and augmented reality systems orthopaedics and biokinematics. 2. Sensor field for magnetopneumography: mapping the distribution of ferromagnetic particles in the lungs after they are magnetised by strong DC field. 2. Monitoring the position of magnetic markers,

e.g. “magnetic biscuits”

Applications: Compass

Honeywell 3-axis AMR

magnetometer with digital output

X (forward)

Bx

Bz

By

Hearth

Z (down)

Y(right)

Hnorth

= Azimuth or Heading

ForwardLevel

roll

pitch

Com

pass

RightLevel

x

y

z

Magnetic compass + inclinometers

= backup for GPS

Applications: Identification of Vehicles

Vehicles can be identified usig magnetic signature

The same technology is used for detection of ferromagnetic bodies

Magnetic field of Skoda carmeasured by 3-axis CTU fluxgateunder the road surface

Fluxgate object locator

DIMADS

Schiebel Austria,sensors from Czech Technical

University

Applications: Security and Military

Conclusions

Magnetic sensors are• contactless• robust

– vibrations, temperature, dirt

• cheap• low power

New types include• Hall: integrated, vertical• Ferromagnetic Magnetoresistors: AMR, GMR

Resources

• www.nve.com (GMR)• www.Sentron.ch (vertical Hall)• www.ssec.honeywell.com/magnetic/ (AMR)

• www. Micronas.com (Hall)• www.Infineon.com (Siemens: Hall, GMR)• www.semiconductors.Philips.com/automotive/sensors_discretes (AMR)

• www.Geometrics.com (resonant magnetometers)• measure.feld.cvut.cz/groups/maglab (fluxgate)• Magnetic sensors and Magnetometers (book)

Artech, 2001,www.artechouse.com