Transcript Inductivity sensors

11. Magnetic sensors
Introduction
Principle of work:
• coupling between electric and magnetic circuits is changed
• influence of magnetic field on material parameters of a sensor.
Sensor materials: hard and soft magnetics
semiconductors (structures) sensitive to mag. field.
Electronic
compass
(Honeywell)
Vehicle
detector
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Classification of sensors
Inductive sensors
inductivity (change of L)
transformer – type (change of M – mutual inductance)
electromagnetic - induced EMF
- induced eddy currents
Magnetogalvanic sensors
Hall sensors
magnetoresistors
magnetotransistors
Magnetoelastic sensors
Magnetic field sensors
SQUIDs
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Inductivity sensors
Coil inductance L is proportional to magnetic flux B generated by current I :
N B = L I
It equals ca:
L
 o r N 2 A
2
N
R
l
R 
where: R - reluctance, N- number of turns
l - length
A – cross section
r – relative magnet. permeability of core
l
A o r
L is changed through N or more often through R.
Magnetic coil as an element of a magnetic circuit:
 
B
R

1
NI
R


2
NI
l
l
1 
2
 A   A
o
o r
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Inductivity sensors
With changing air slit
Impedance module
Z  RL2  2L2
RL  RCu  R Z RL  effective resist . of losses
Z  L
For small losses
N2
N2
N2
L


1 2l p
RFe  R p R p
0 A
 Z 
1
lp
R  reluc .
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Inductivity sensors
With changing cross section of a slit
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Inductivity sensors
Differential sensor
Two coils with impedances Z1 i Z2
with common keeper
Slits:
l1  l0  
l2  l0  
Uout  R I  R Î1  Î 2  R( I1  I 2 )
I1,2 
U
kA 2
 2 ( L0 
)  RS2
l0  
Dla 0 < δ < 0.4l0
ΔI ~ δ
Weak dependence of Uout on frequency and
supply voltage
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Inductivity sensors
With changing core position
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Inductivity sensors
Two bridge-connected coils
Shift of a core from the medium position
gives the output voltage Uout
U out 1 L

U in 2 L 0
For a small displacement Δx
L 
dL
x
dx
Hence
U out
1 dL

x
U in 2L 0 dx
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Transformer type sensor
Linear variable differential transformer
(LVDT)
Separation of the power supply and output
circuits, large number of turns of the
output circuit.
Compensation of thermal noise (work range
from criogenic tempwrature till 1500C).
Z1 and Z2
connected
in a push-pull
mode. In the middle
core position
Uout = 0
High sensitivity in the
range of displacements
from 10-7m to 1 m, with
nonlinearity error <3%.
Special construction of
coils is adopted.
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Electromagnetic sensors with induced EMF
EMF is induced by changing magnetic flux (Faraday’s law):
  df/dt
In sensoric solutions one uses generally permanent magnets and
changing flux is obtained by moving electric circuits in a magnetic
field B or on the contrary – by moving source of B or changing
magnetic reluctance for resting electric circuit.
Moving electric circuit in a constant B
Linear velocity sensor
  N` BA 
d dx

v
dt
dt
N
 xB A
l
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Moving electric circuit in a constant B
Angular velocity sensor
  N B A cos   N B A cos  t
d

 N B A sin  t
dt
Some minimal angular velocity is necessary.
For high ω the amplifier is not necessary.
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Change of magnetic reluctance
Electromagnetic tachometer
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Motion of B source
Tachogenerator
Frequency of induced voltage:
f~n·p
p – number of poles
n – number of revolutions
Typical working range:
150 – 3000 rpm
For small angular velocity the number of
magnetic poles is increased.
Generally sensors with induced EMF
are used for:
• measurements of angular velociy
• investigation of vibrations:
x   v dt  k   dt
a
dv
d
k
dt
dt
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Signal conditioniong for a tachometer with gear
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Electromagnetic sensors based on eddy currents
Induced currents and not EMFs are important


B
rot E  
t
Changing magnetic field induces rotational
electric field
Eddy current proximity sensors
Magnetic coil is a part of resonsnce circuit of LC generator
Dynamics 1 – 50 mm
Resolution 0.1 mm
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Magnetogalvanic sensors
Hall sensors
Lorentz force:
F=qvxB
UH = (RH/d) I B = γ I B
for elongated sample
In general it is necessary to introduce the geometrical
factor G and an offset voltage:
UH = γ G I B + Ur
Ur –offset voltage, constant or varying slowly
for B=0.
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Hall sensors
Hall constant for sample with carriers of one type is equal:
RH = ± r/ nq
r – coeff. dependent on the mechanism
of carriers scattering
n – carriers concentration
q – elementary charge
Large UH signal is obtained for samples of high carriers mobility µ ( InSb, GaAs ):
EH/Ex = µBz
EH – Hall field
Ex – field generating current
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Hall sensors shapes
Hall sensor
labelling
CC/HC –interchangeable
contacts
CC – curent contacts
HC – Hall contacts
IC technology
(vertical),
B field - tangent
to the sample surface
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Hall sensors technology
- samples from bulk material
- thin film structures
- integrated structures:
- MOS structures
- epitaxial GaAs structures
- superlattices in MBE (Molecular Beam Epitaxy)
technology
- bipolar IC structures
Most of recently produced commercial Hall sensors are manufactured
using bipolar integrated circuits technology (IC).
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Hall sensors technology
Isolation of the Hall structure is
accomplished by the reverse polarization
of p/n junctions
Active part of the structure is epitaxial n layer with diffused isolation p
areas and n+ contacts.
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Hall sensor parameters
•
absolute sensitivity:
SA = ∂ UH / ∂ B dla I = const
•
sensitivity vs. supply current:
SI = SA / I
•
sensitivity vs. supply voltage:
SU = SA / U
•
offset: equivalent field Bo giving
offfset voltage Uo :
Bo = Uo / S.A.
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Applications of hall sensors
Contactless measurement of position
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Contactless measurement of position
differential connection
of Hall sensors
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Contactless measurement of current
Around the current lead (linear) concentric magnetic field is generated
B = μoI/(2πr),
then B ~ I for r = const
Simple construction,
good linearity
High sensitivity is obtained applying the magnetic core with a slit δ ~ 1 mm,
in which a Hall sensor is placed.
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Contacless measurement of current
Flux of field B:
B 
R 
NI
R  R
l Fe
0 r A
R 

0 A
N – number of turns
Rµ, Rδ – reluctances of a core
and a slit
B field in a slit:
B
ΦB μ0 N I μ0 N I


 UH  I
A l Fe  δ
δ
μr
Current measurement range: 10A – several kA
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Contactless measurement of power
Multiplication feature of the Hall sensor is utilized
Load current iL produces field B
which is measured as described before:
B ~ iL
Voltage uL is transformed
giving current iin supplying the Hall
sensor:
iin ~ uL
Voltage indicated by the sensor:
u H   iin B  ur ( t )  k u L iL  ur ( t )  k pL  ur ( t )
u L ( t )  U 0 L cos  t
For the
resistance
load:
iL ( t )  I 0 L cos  t
ur ( t )  U 0 r cos  t offset voltage
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u H ( t )  k  U 0 L I 0 L ( 1  cos 2 t )  U 0 r cos  t
2
Using low-pass filter one obtains
signal prop. to the average power
1
u H  k  U 0 L I 0 L
2
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Brushless DC motor
Rotor has a built-in permanent magnet.
Driving coils, being a part of a stator, are controlled by two Hall sensors.
Hall sensors register relative positions of a rotor and with the help of transistors
control the coils currents.
Currents in a stator change very smooth.
Advantages of the motor:
• prolonged lifetime (only bearings are
weared)
• low noise
• lack of sparking
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Magnetoresistors
Demonstrate the dependence of resistance on magnetic field.
Early years of magnetoresistors development were based on utilization of semiconductors,
eg. InSb for B > 2kGs.
Recently magnetoresistors are manufactured using:
• ferromagnetic metals (Thomson effect) also known as AMR (anisotropic
magnetoresistance) effect,
• layer magnetic structures (GMR – giant magnetoresistance)
• magnetic tunnel junctions (MTJ)
AMR elements come into use with the development of thin film technologies.
The following alloys are used:
Ni Fe
Ni Co
Ni Fe Co
Change of resistance as a function of magnetic field depends on the angle between the
current and the axis of magnetic anisotropy (axis of easy magnetization).
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AMR magnetoresistor
For ε = ± 45°
the dependence
is quasilinear
Practical solution
Metallic stripes
force the direction
of current flow
BARBER-POLE sensor
manufactured by PHILIPS.
Characteristics nearly linear.
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Giant magnetoresistance (GMR)
Sudden drop of resistance in the presence of a magnetic field for the multilayer
structure, where magnetic layers ( Fe, Co ) are separated by nonmagnetic
layers ( Cu, Ag ) ( Baibich 1988).
Scattering of electrons depending on the direction of spin vs. magnetization vector M
a – spin up, b – spin down
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GMR
In practise the superlattices are manufactured
Chracteristics of GMR superlattice
[Co (1.1nm) Cu(0.9nm)] · 100
Sensitivities for small magnetic fields are obtained when the structure
has a form of so called magnetic valve.
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GMR
IBM Almaden Res. Center
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Magnetic Tunnel Junction (MTJ)
Two ferromagnetic electrodes CoFeB
are separated by the tunnel layer of MgO insulator.
The current flows perpendicularly to the junction.
For antiparallel orientations of a free layer (upper)
and fixed (lower) one gets high resistance (IrMn – the
layer causing fixation).
Change of magnetization of upper layer for parallel
orientation gives drop of the resistance.
CR Magnetics Inc.
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Magnetic Tunnel Junction (MTJ)
The spin-up electrons are those with spin orientation parallel to the external magnetic field, whereas
the spin-down electrons have anti-parallel alignment with the external field.
If no voltage is applied to the junction, electrons tunnel in both directions with equal rates. With a bias
voltage U, electrons tunnel preferentially to the positive electrode. With the assumption that spin is
conserved during tunneling, the current can be described in a two-current model. The total current is
split in two partial currents, one for the spin-up electrons and another for the spin-down electrons.
These vary depending on the magnetic state of the junctions.
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MTJ sensor
Micro Magnetics
The STJ-001 low-field magnetic
microsensor in die form.
Active areas as small as 1x2 microns
The die is 1.9 mm square and 300 microns thick.
It has four gold wirebonding pads which allow four-point
measurement of the device resistance.
The field sensitivity of the STJ-001 is 5 nT, which is ten
thousand times smaller than the magnetic field of the Earth.
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Applications of MR sensors
Read head in a disc drive
Principle of operation of
magnetoresistive read head.
Important are only changes of
flux in the direction
perpendicular to the magnetic
medium.
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Disc drive
IBM Almaden Res. Center
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Applications of MR sensors
Read head in a disc drive
MR read head in comparison to induction heads:
• signal is independent of the tape speed
• higher sensitivity, then higher writing density
These heads cannot be however at the same time the write heads (inductive)
• First head with MR sensor – 1970r.
• Heads for reading tapes, IBM –1985r.
• Recently all hard disc drives utilize MR elements for reading.
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HD Tunnel Reader
Industry first 120 GB 2.5-in Seagate Momentus II
high capacity mobile drive with MTJ reading element.
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Applications of MR sensors
• Contactless measurements of DC and AC currents,
transformation of DC curents
• Registration of position and revolution of magnetic materials
• Digital compass
• Credit card readers
• Checking of failed wirebonding and defects in semiconductor structures
• MRAM memories
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Galvanic isolation
DC current transformer
Summation of two currents
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Identification of coins
When the coin is moved in the field of a coil, eddy currents are induced.
Phase shift between the coil signal and magnetoresistor signal are measured, what is
characteristic for a given coin, independently of the speed of coin moving..
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ABS system with MR sensors
At sliding tendency electronic and hydraulic systems control respective
breaks.
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Application of MTJ sensors
Defects in
microstructure
connections
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Application of MTJ sensors
Failed
wirebonding
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Magnetoelastic sensors
Change of magnetic properties under the influence of mechanical interactions
Stress σ rotates magnetization MS by an angle θ
in respect to magnetic field H.
From the condition of minimum energy (for θ<< θ0)
one obtains:
sin 2 0

MSH
Optimal case: θ0 = 450, H - small

A measure of torque FR
for a rod fixed at one side
is a torsion angle θ
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Measurement of torque with a magnetoelastic sensor of cylindrical
shape
With no stress (torque t = 0),
there is no coupling between coils
After application of torque t the output
voltage is generated
U out = kt
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SQUID Magnetometer
SQUID (Superconducting Quantum Interference Device)
is a very sensitive magnetometer used to measure extremely subtle magnetic fields,
based on superconducting loops containing Josephson junctions. SQUIDs are sensitive
enough to measure fields as low as 5 aT (5×10−18 T). Animals produce very small
magnetic fields in the range 10-9T to 10-6T.
Two Josephson junctions are connected in parallel
in a superconducting loop. In the absence of any
external magnetic field, the input current I splits
into the two branches equally. If a small external
magnetic field is applied to the superconducting
loop, a screening current IS, begins circulating in
the loop that generates a magnetic field canceling
the applied external flux. The induced current is in
the same direction as I in one of the branches of the
superconducting loop, and is opposite to I in the
other branch; the total current becomes I/2 + IS in
one branch and I/2 - IS in the other. As soon as the
current in either branch exceeds the critical current
IC, of the Josephson junction, a voltage appears
across the junction.
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If the input current is more than Ic, then the SQUID always operates in the resistive
mode. The voltage in this case is thus a function of the applied magnetic field and
the period equal to Φ0 . The screening current is the applied flux divided by the selfinductance of the ring. Thus ∆Φ can be estimated as the function of ∆V (flux to
voltage converter) as follows:
∆V = R ∆I
2I = 2 ∆Φ/L, where L is the self inductance of the superconducting ring
∆V = (R/L) ∆Φ
Left: Plot of current vs. voltage for a SQUID. Upper and lower curves
correspond to nΦ0 and (n+1/2)Φ0 respectively. Right: Periodic voltage
response due to flux through a SQUID. The periodicity is equal to one
flux quantum, Φ0.
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