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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
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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
RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers: Behavioral 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: Behavioral Model
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.
RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers
RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers
RTD Fluxgate MagnetometersSalvatore La MalfaRTD Fluxgate Magnetometers
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.
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