Transcript Department of Optical Engineering Zhejiang University

Advanced Sensor Technology
Lecture 9
Jun. QIAN
Department of Optical Engineering
Zhejiang University
A Review of Lecture 8
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An overview of Modern IR sensors
Quantum, e.g. MCT
 Thermal
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Microbolometer
 Pyro
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An analysis of thermal IR sensor
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Sensitivity
Microbolometer
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Uncooled IR sensor array
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Basic Intent
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This lecture will discuss the basic
principles behind use of electromagnetism
in sensing. Since many established
sensors rely on electromagnetism, this
lecture will cover a broad set of designs
and review several products.
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Electromagnetism and Inductance
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A review of the properties of an inductor as
used in electronic circuits.
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a passive circuit element which resists changes in
current.
The equation which governs its behavior is :
assume (as for all of the differential equations in this
course) that the current and voltage are both
oscillating quantities :
Effective R=-iL
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Analogy: inductor v.s. resistor
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inductance measuring circuits,
exactly like the resistance measuring circuits
the most common approach to inductance
measurement is a bridge.
How big is a typical inductance? a coil with a
1 cm diameter, a 1 cm length, with 1000 loops
of wire: mH
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What is really preferred for an inductor?
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High inductance:
This achieved in a way that is very
much like what is done in capacitors:
material and area
 In an inductor, we can fill the coil with a
material which has a higher magnetic
permittivity.
iron has a relative permittivity of
~300,
permalloy(nickel/iron 80/20)
~ 5000
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The concept of inductance
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If we have a simple coil, and
try to suddenly cause a current
to flow through it,
the initial current flow causes a magnetic
field to begin to form in the coil.
 Since this magnetic field is increasing, there
is a change in the flux through the coil,
 and an opposing voltage appears.
 Eventually, the current rises to its limiting
value, the magnetic field is stable, and the
opposing voltage dies away.
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Magnetic motion detector
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The magnetic field is
confined to the region
between the pole faces,
and is essentially zero
elsewhere. The magnetic
flux through the loops is
simply the area of the
loop that is within the
magnetic field times the
value of the field.
A voltage is induced in
the loop whenever it
moves laterally.
Motion sensor, not a
position sensor
Magnetic Motion Detector and its
Transfer functions
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Geophones
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This approach is the basis of many so-called
`moving coil' detectors, in which a voltage is
generated whenever an external signal causes
a coil to move relative to a permanent magnet.
A good example of a commercial product : the
Geophone, as made by GeoSpace Corp
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In this device, a set of coils measure a differential
voltage whenever a spring-supported magnet moves
generally constructed with a fairly low frequency
resonance - about 1 Hz.
commonly used for detection of seismic signals or
other low-frequency ground vibrations, or in the oil
exploration business with buried explosive charges to
map underground resource deposits.
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Geophone
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Seismometer
The sensitivity of a geophone
is a function of frequency
However, the resolution can be
improved by measuring proof
mass position.
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Existing Geophones/Seismometers
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What is a conventional geophone?
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How does it work? ( Mechanical sensitivity)
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How does it work? (Electrical sensitivity)
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Total Sensitivity
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Resolution
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Conventional Geophone Resolution
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How to improve both sensitivity and resolution?
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Change a low-cost geophone
into a capacitive one
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Capacitance Electrical System
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Control System Design
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Controller Design Concepts
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Predicted and Measured Performance
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A Comparison of Performance
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Experimental Setup at Stanford
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Proximity Sensor
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A pair of coils are wired in a
bridge circuit and biased
with an ac signal. If a
conducting object is
positioned near the end of
the device, it is closer to the
sense coil than the
reference coil.
The presence of a
conductor cause an
additional opposing voltage
in the coil, and the effect is
to increase the inductance
of the coils.
Since the sense coil is
closer to the sheet, its
inductance is increased
more.
Sensing
coil
Reference
coil
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Proximity Sensing: bridge circuit
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The effect of the sense
coil changing more than
the reference coil
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The amplitude of this
difference is proportional to
the inductance difference which is related to the
distance to the metal sheet
in a very complicated way.
The actual relation
between distance and
inductance change is too
complicated to derive in
general, since it relies on
the geometry of the
situation, so this
approach is not generally
used for accurate position
sensing.
Effective R=-iL
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Commercial Inductive Proximity Sensors
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Applications
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Automation
Robotics
……
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Case Study: Giant magnetoresistance
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GMR discovered in 1988 independently by Baibich et al.
in Paris and Binasch et al.in Jülich.
It is the phenomenon where the resistance of certain
materials drops dramatically as a magnetic field is
applied.
It is described as Giant since it is a much larger effect
than had ever been previously seen in metals.
It has generated interest from both physicists & device
engineers, as there is both new physics to be
investigated and huge technological applications in
magnetic recording and sensors.
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Outline
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Introduction
Science of GMR
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Discovery of GMR
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Fert’s and Grünberg’s original papers
Further research by IBM
Application of GMR
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Anisotropic magnetoresistance
Giant magnetoresistance
GMR-based spin valves in hard drives
Impact of GMR on the storage media industry
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GMR – why is it useful?
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Discovery and application of the GMR
phenomenon is responsible for the ubiquitous
availability of economical, high density
information storage in our society.
Compact 160 GB Mp3 players and 1 TB hard
drives, now widely available, owe their existence
to GMR and subsequent related advances.
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Science of GMR
Anisotropic Magnetoresistance
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Anisotropic Magnetoresistance – Reported in 1857 by
British physicist Lord Kelvin.
When a current is passed through a magnetic
conductor, resistance changes based on the relative
angle between the current and the conductor’s
magnetization.
Resistance increases when current is parallel to
magnetization and decreases when current is
perpendicular to magnetization.
Cause: electron spin-orbit coupling
Used as the basis of hard drive reading before GMR
was discovered.
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Science of GMR
Giant magnetoresistance
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System:
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a thin layer of
nonmagnetic material
sandwiched between
two layers of magnetic
material.
Right: a Fe-Cr-Fe
trilayer used in
Grünberg’s original
experiment.
[3]
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Science of GMR: Mott Model
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The electrical conductivity in metals can be described in
terms of two largely independent conducting channels,
corresponding to the up-spin and down-spin electrons,
and electrical conduction occurs in parallel for the two
channels.
In ferromagnetic metals the scattering rates of the upspin and down-spin electrons are different.
(We will assume that the scattering is strong for
electrons with spin antiparallel to the magnetization
direction and weak for electrons with spin parallel to the
magnetization direction.)
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Science of GMR
Giant magnetoresistance
(zero magnetic field)
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Antiparallel magnetization
Both electron spins
experience small
resistance in one layer
and large resistance in
the other.
Total resistance is
ferromagnetic
non-ferromagnetic
ferromagnetic
Rantpara  12 ( R  R )
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Science of GMR
Giant magnetoresistance
(magnetic field applied)
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Parallel magnetization
Up-spin electrons
experience small
resistance, down-spin
electrons experience
large resistance.
ferromagnetic
non-ferromagnetic
ferromagnetic
Total resistance is
2 R R
R para 
R  R
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Science of GMR
Giant Magnetoresistance
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Difference in resistance is given by:
1 ( R  R )
R  Rpara  Rantipara  
2 ( R  R )
2
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Discovery of GMR
Fert and Grünberg
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Discovered independently by Professor
Albert Fert of Université Paris-Sud in
France and Professor Peter Grünberg of
Forschungszentrum in Jülich, Germany.
Both groups submitted papers to Physical
Review in the summer of 1988.
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Discovery of GMR
Fert
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60-bilayered Fe-Cr
structure at 4.2 K
Nearly 50% drop in
resistance observed!!
[5]
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Discovery of GMR
Grünberg
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Fe-Cr-Fe trilayer at
room temperature
1.5% drop in
resistance reported
[3]
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Discovery of GMR
IBM
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Stuart Parkin of IBM attempted to reproduce the effect
using the sputtering technique
Fert and Grünberg used molecular beam epitaxy, a more
precise but slower and more expensive method.
Parkin’s group succeeded, observing GMR in the first
multilayer sample’s produced.
Parkin’s group began experimenting with various sample
compositions and layer thicknesses to better understand
GMR and how to integrate it into magnetic storage.
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GMR in practice
Spin Valve
[8]
[7].
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GMR’s effect on hard drive industry
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First GMR hard drive deployed:
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16.8GB, IBM, 1997
Current largest hard drive:
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1 TB by Hitachi,2007
4 TB, 2012
[11]
[12]
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Case Study: Giant magnetoresistance
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GMR has been the
subject of a huge
international research
effort due to the
numerous technological
applications.
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The largest is in the data
storage industry: IBM
The effect is most usually
seen in magnetic
multilayered structures,
where two magnetic layers
are closely separated by a
thin spacer layer a few nm
thick.
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Case Study: GMR
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It is analogous to a polarisation experiment, where
aligned polarisers allow light to pass through, but
crossed polarisers do not.
The first magnetic layer allows electrons in only one
spin state to pass through easily - if the second
magnetic layer is aligned then that spin channel can
easily pass through the structure, and the resistance is
low.
If the second magnetic layer is misaligned then neither
spin channel can get through the structure easily and
the electrical resistance is high.
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GMR: Spin Valve
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A spin valve is in general a sample consisting
essentially of a GMR trilayer:
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One layer is very magnetically soft - meaning it is very
sensitive to small fields.
The other is made magnetically 'hard' by various schemes meaning it is insensitive to fields of moderate size. As the soft
'free' layer moves about due to applied fields, the resistance
of the whole structure will vary.
The central part of the sample consists of two magnetic
layers (in our case usually permalloy with a thin covering of
Co), separated by a Cu spcer layer. One magnetic layer is
pinned or exchange biased by an antiferromagnetic material using FeMn and IrMn,
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Case Study: Giant Magnetoresistance
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Brain-computer interface
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By the end of 2012, San Francisco–
based Emotiv's sensor-laden EPOC
headset
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enable gamers to use their own brain
activity to interact with the virtual worlds
where they play.
headset's 14 strategically placed head
sensors are at the ends of what look like
stretched, plastic fingers that detect
patterns produced by the brain's electrical
activity.
detects brain activity noninvasively using
electroencephalography (EEG), a measure
of brain waves, via external sensors along
the scalp that pick up the electrical bustle
in various parts of the furrowed surface of
the brain's cortex, a region that handles
higher order thoughts.
These neural signals are then narrowed
down and interpreted in 30 possible ways
as real-time intentions, emotions or facial
expressions that are reflected in virtual
world characters and actions
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EEG and MEG
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Electroencephalography (EEG) is
the measurement of electrical activity
produced by the brain as recorded
from electrodes placed on the scalp.
MEG Sensor Array
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Field distribution by 510 coils at distinct
locations. Coils configured into 306 MEG
channels.
The coil configuration optimally
combines the focal sensitivity of 204
planar gradiometers and the widespread
sensitivity of 102 magnetometers.
Accurate geometry and excellent
balance of the sensors provided by thinfilm technology.
Interference compensation by on-line
Signal Space Projection (SSP) using the
sensor array as a reference; involves no
increase of white noise.
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EEG
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Mind controlled helicopter
Mind controlled wheel chair
……
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MEG: Megnetoencephalography
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MEG
Left – Stimulus induced
afferent electrical current
(green) conducted along
neuronal pathway (blue)
reaches cortical surface.
Right – Current flow at
neuronal level (Pyramid
cells) induces surrounding
magnetic field (red).
Orientation of the
current flow and
induced magnetic field
in relation to cortical
surface and one MEG
sensor element.
Left – Magnetic isofield map
and time course of the
somatosensory response at
20 ms, Right – topological
display of MEG channels.
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Summary
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We have reviewed the basic principles of
induction, and examined several examples of
devices which use this principle to measure
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We have also looked at
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Velocity (Geophone)
position or presence of objects (Proximity sensor)
magnetoresistance magnetometers,
In general, a wide variety of magnetic sensorbased instruments are available.
The emergence of thin-film sensors is important
for the disk-drive industry, and other applications
of thin film magnetometers can be expected.
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