rtd fluxgate magnetometers - unict · rtd fluxgate magnetometers ... a typical induction coil...

RTD Fluxgate MagnetometersSalvatore La Malfa

10/11/2010

RTD Fluxgate MagnetometersSalvatore La Malfa

A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.

en.wikipedia.com

Magnetometers can be divided into two basic types:• Scalar magnetometers measure the total strength of the magnetic field to

which they are subjected, and• Vector magnetometers have the capability to measure the component of the

magnetic field in a particular direction, relative to the spatial orientation of the device.

Introduction

Applications

Geomagnetic

Space Research

Security

MilitaryBiomagnetic Measurements

Navigation

UXO

Nondestructive Testing

Antitheft SystemsAutomotive

Magnetic Marking and labeling

Magnetic trackers (immunoassay)

0.1 degree precision compass

Metal detectorsTraffic monitoring

Material state monitoring

Magnetic bar codes

Mapping and monitoring

Submarines

RTD Fluxgate MagnetometersSalvatore La MalfaMagnetometers: specifications

• Induction sensors• Hall Effect• Magnetoresistive (AMR, GMR)• Resonance Magnetometers• Magneto-optic• Proton-precession (NMR)• Fluxgate• SQUID• …

Resolution

Power consumption

AccuracyDimensions

Field rangeBandwidth

Operating temperature

Cost

Sensitivity+noise

SPECIFICATIONS

Required specifications are application-dependent

RTD Fluxgate MagnetometersSalvatore La MalfaInduction Sensors: Air Coils

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The basic description of induction sensors starts from the Faraday law:

• Vi is the voltage induced in a coil having N turns

• Φ is the magnetic flux in the coil, Φ = B A

• A is the core cross-sectional area• H the magnetic field in the sensor

core• µr is the sensor core relative

permeability

Search coil Rotary coil Fluxgate

A typical induction coil magnetometer consists of a multilayer solenoid

Air Coils (or loop antennas) contain no nonlinear magnetic materials, so they have linear amplitude characteristics and their parameters are very stable in time

• Parasitic capacitances• Current output • 45 cm diameter, 2.5x2.5 cm cross-section

• BW: 20Hz to 10kHz• Noise: 0.3 pT/Hz0.5 @ 20 Hz• Sensitivity: 25mV/nT• Accuracy: 1%

Commecial example: ACM-1 (Meda)

RTD Fluxgate MagnetometersSalvatore La MalfaHall Effect

Magnetic field sensors based on the Hall effect are probably the most widelyused magnetic sensors. Interestingly, Hall magnetic sensors are relatively rarelyused to measure just a magnetic field. P. Ripka, Magnetic sensors and magnetometers, 2000

When a current-carrying conducor isplaced into a magnetic field, a voltagewill be generated perpendicular to boththe current and the field. This principleis known as the Hall effect

Commecial example: SS495A1 (Honeywell)

• 35 mW power consuption• Field range: +/- 60 mT• Temperature Range: -40 to +150 °C• Sensitivity: 30 mV/mT

Hall effect elements are much more used as a key component in contactless sensors for linear position, angular position, velocity, rotation and electrical current.

RTD Fluxgate MagnetometersSalvatore La MalfaAnisotropic Magneto Resistance

Magnetoresistive (MR) sensors make use of themagnetoresistive effect, the property of a current-carrying magnetic material (tipically Permalloy) to change its resistivity in the presence of an external magnetic field

It is obvious from this quadratic equation, that the resistance/magnetic field characteristic is non-linear and inaddition, each value of R is not necessarily associated with a unique value of H. However, Themagnetoresistive effect can be linearized by depositing aluminium stripes (Barber poles), on top of thepermalloy strip at an angle of 45° to the strip axis. As alumin ium has a much higher conductivity thanpermalloy, the effect of the Barber poles is to rotate the current direction through 45° (the current flowassumes a ‘saw-tooth’ shape), effectively changing the rotation angle of the magnetization relative to thecurrent from α to α − 45°.

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Commecial example: HMC1043 (Honeywell) – 3 axis• Resolution = 12 nT• Field range: +/- 0.6 mT• BW = DC to 5 MHz• Sensitivity: 1 mV/V/G• 90 mW power consuption

Dimensions = 3mm x3mmCost = 1 @ € 34,00, >1000 @ € 9

RTD Fluxgate MagnetometersSalvatore La MalfaSuperconducting QUantum Interference Device

A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions.

The Josephson effect is the phenomenon of electric current across two weakly coupled superconductors, separated by a very thin insulating barrier. This arrangement—two

superconductors linked by a non-conducting barrier—is known as a Josephson junction; the current that crosses the barrier is the Josephson current.

SQUIDs are sensitive enough to measure fields as low as 5 aT(5×10−18 T) within a few days of averaged measurements. Their noise levels are as low as 3 fT·Hz-½.

Primary coil Secondary coil

µWire (d = 100µm) Fe-Si-B Core

Excitation Magnetic Field

Core Magnetization

Output Voltage

Hx = 0, RTD = T+-T- = 0 Hx > 0, RTD = T+-T- > 0

T+ T- T+ T-

Hx+Hc

-Hc

The target magnetic field intensity is

transduced into time interval changes

This is the main difference between RTD

and 2nd harmonic Fluxgate

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Introduction

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Sensitivity

Triangular excitation ensures theoretical

linearity of the in-out characteristic

cexcex HtHHHtHHt =+⇒=+ )sin()(: 111 ω

cexcex HtHHHtHHt −=−−⇒−=+ ))2

(sin()(: 222

τω

τ+= 13 tt

−=e

xc

H

HHt arcsin

11 ω

2arcsin

12

τω

+

+=e

xc

H

HHt

τω

+

−=e

xc

H

HHt arcsin

13

ββ xc

cx

HHtHtHt

−=⇒=+ 111 :

β

τβτβ 22

: 222

++=⇒−=

−−xc

cx

HHtHtHt

τβ

τ +−=+= xc HHtt 13

)( 2312 ttttTTRTD −−−=−= −+

)( 2312 ttttTTRTD −−−=−= −+

)sin( tH(t)H ee ω=

+≤≤+

−−

+≤≤−=

τττττβ

τττβ

NtNt

NtkkkttH e

4

3

4..

2

44)(

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: PCB

• Simple and cheap circuit• It can be designed to be low power• Digital output, TTL-compatible.

Voltage to current converter with floating load

Instrumentationdifferential amplifier

Level shifter

Schmitt Trigger

Periodic driving current(8mApp @ 300 Hz typical)

The duty cycle of the output digital signal is related to the intensity of

the target magnetic field

V�I converter

InstrumentationAmplifier

Level Shifter

Schmitt Trigger

RTD FG Primary Coil

RTD FG Secondary Coil

From function generator To digital counters

Proper magnetic shielding is crucial when dealing with magnetic measurements!

• High permeability materials, usually Ni-Fe alloys, are used for magnetic shielding.

• Two or more concentric thin cylinders, separated by air-gaps, are more effective that one thick cylinder.

In our lab…“Custom-made” magnetic shield• Metglas 2714A (ribbon)

µr=1e6 (DC)• Two concentric cylinder

At CePTIT (Misterbianco)Anechoic Chamber

The ideal scenario for magnetic measurement

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Magnetic Shielding

5m shielded cables for both the magnetometerand the calibrationsolenoid

PC+LabVIEW

NI USB-6251With 32-bits, 80 MHz counters

PCB

GPIB-controlledInstrumentation

Anechoic Chamber

The RTD Fluxgate isplaced along the axis of the cylindrical calibrationsolenoid

In order to minimize the uncertainty associated with the open-loop applied

magnetic field intensity a FEM analysis was performed

RTD Fluxgate Magnetometers

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Sensitivity

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Calibration Curve

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Resolution

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Modeling

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Primary CoilFerromagnetic

CoreSecondary Coil��

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An analytical description of magnetic hysteresis is needed in order to obtain a complete RTD Fluxgate behavioral model!

In the past years, many mathematical hysteresis models have been developed, but unfortunately, few of them can be implemented in SPICE or

SPICE-like simulators

A NEW MAGNETIC HYSTERESIS BEHAVIORAL MODEL FOR SYSTEM-LEVEL SPICE SIMULATIONS

B. Andò, S. Baglio, A. R. Bulsara, S. La Malfa - Eurosensors 2010 (special issue)

RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Behavioral Model

Block diagram schematization of the proposed M-H core model

Block diagram schematization of the complete RTD Fluxgate model


Increasing Amplitude Freq = 325 Hz

Flow chart of the parameters extraction procedure

The 2000 measurements refer to the following excitation parameters :�Amplitude = [0,6:0,6:30] mApk | Frequency = [50:25:1025] Hz

Starting from the two measured waveforms (excitation current and output voltage)

a pre-processing algorithm produces the following quantities: �He(t) – Excitation Magnetic Field (A/m) | B(t) – Magnetic Flux Density (T)� M(t) – Core Magnetization (A/m) | HC – Coercitive Field Value (A/m) | MR – Remanent Magnetization (A/m)

15.5( / )

368.5( / )

426.2( / )

C

R

S

H A m

M A m

M A m

= = =

8.4( )

350( )e

e

I mApk

f Hz

= =


RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers

Interpolation of the ‘a’ parameter through a 2nd order, bivariate polynomial function. The average error on the considered domain is 1.1%. The other three parameters behaves similarly and are thus not shown.


Multisim simulation results for the operating condition of 4.5mApk @ 350 Hz witha linear ramp applied target field (0uT at 0ms, 70 µT at 20 ms). The blackwaveform is the output voltage of the magnetometer, after the amplification andlevel-shift stages. The square waveform is the output of the Schmitt trigger whoseduty cycle increases as the target field grows.

rtd fluxgate magnetometers - unict · rtd fluxgate magnetometers ... a typical induction coil...

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