Transcript Department of Optical Engineering Zhejiang University

Advanced Sensor Technology
Lecture 3
Jun. QIAN
Department of Optical Engineering
Zhejiang University
A Review of Lecture 2
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Characteristics of sensors
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Transfer function
Sensitivity
Dynamic Range
Hysteresis
Temperature Coefficient
Linearity
Accuracy
Noise
Resolution
Bandwidth
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Lecture 3 Basic Intent
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Review some background on electrical
measurement of sensor outputs
Provide an overview of piezoresistive
devices. Some examples are worked
out using this sensing technique
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Introduction to Sensors Electronics
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The electronics which go along with the
physical sensor element are very important:
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Limit the performance, cost, and range of
applicability
If carried out properly, the design can improve
the characteristics of the entire device,
Focus on basic techniques for processing
the signals most typically produced by a
sensor
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Most sensor act like passive device
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Resistive
Capacitive
Inductive
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Resistive Sensor Circuits
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Resistive sensors obey
Ohm’s law
How to get a voltage signal
out of the sensor?
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Need a constant current
source
The easiest way to build a
current source: voltage
divider
Condition: load R>>sensor R
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Shortcoming: small signal
might need some amplification
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Capacitance measuring circuits
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Many sensors respond to physical signals
by producing a change in capacitance
Impedance:
Very much like a resistor at AC, may
measure capacitance by building voltage
divider circuits, use either resistor or
capacitor as the load resistance
Resistors have much smaller temp coeff.
than caps: 0.3ppm/ºC v.s. 200ppm /ºC
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Capacitance measuring circuits
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A substantial hassle:
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Use clock signal or integrated clock/sampling
circuit
Modulated signal creates an opportunity for use of
some advanced sampling and processing
techniques
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providing an AC bias
Converting the AC for microprocessor interface
Lock-in: bias the sensor and trigger the sampling, get the
low noise signal
Disadvantage: clocked switch inject noise charge into
circuit
Very accurate capacitance measurement still
requires expensive precision circuitry
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Inductance measurement circuits
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Impedance: iL -> essentially resistive
elements
Inductive sensors generally require
expensive techniques for the fabrication of
the sensor mechanical structure: 3D
structure
Inexpensive circuits are not of much use,
expensive anyway!
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Limitations
Limitations to resistance measurement
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Lead resistace -> 4-wire configuration
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Output impedance
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The measuring network resistance places a lower limit on the
value of a resistance which may across the output terminals
An example: 10K thermister+1M load, if connected to an 1K
measuring instrument
-> output voltage would be reduced by ~90%
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Limitations to measurement of capacitance
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Stray Capacitance
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Appear as additional capacitances in the
measuring circuit
Wires moving about with respect to
ground, causing capacitance fluctuations
These effects are due to pressureinduced vibrations in the positions of
objects, referred to as microphonics.
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Piezoresistive devices – an overview
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Silicon-based
 Specific advantages
are:
 High sensitivity,
>0.5mV/mbar
 Good linearity at
constant temperature
 Ability to track
pressure changes
without signal
hysteresis, up to the
destructive limit
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Structure and Assembly
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Principle of Operation
 Deformation by
applied pressure
causes high levels of
mechanical tension at
the edge of
diaphragm
 Semiconductor
resistors on the front
side transduce this
tension into
resistance changes
by means of the
piezoresistive effect. .
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Spec Sheet
Nominal
Pressure Range
(mbar)
100
200
400
1000
Sensitivity
(mV/mb
ar)
0.5
0.25
0.12
0.06
Linearity
(%FSO)
<1
Bridge
Resistance
(k)
5.6
Chip Size
(mm3)
3x3x1
Diaphragm Size
(mm2)
2x2
2.2 x 2.2
x1
1.5 x 1.5 1.1 x 1.1
0.8 x
0.8
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Theoretical background: piezoresistance
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A piezoresistor: a device which
exhibits a change in resistance
when it is strained.
There are two components of
the piezoresistive effect
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the geometric component
the resistive component.
The geometric component of
piezoresistivity:
a strained element undergoes
a change in dimension. These
changes in cross sectional area
and length affect the resistance
of the device.
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Strain: Definition
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Strain is the amount of
deformation of a body
due to applied force
Dimensionless:
mm/mm
strain is often
expressed as
microstrain (), which
is  x 10-6.
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Classical Device: Mercury Tube
An elastic tube filled with a incompressible
conductive fluid, such as mercury (really!)
R = (Resistivity of mercury)(length of tube)/(cross
sectional area of tube)
Gauge factor: K=2 for liquid strain gauge
 What does it mean?
if a liquid strain gauge is stretched by 1%, its
resistance increases by 2%.
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Metal wire based strain gauge
To find K:
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Metal wires:
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stretching of the wire
changes the geometry
of the wire in a way
which acts to increase
the resistance:
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Gauge factor K=2~4
4
= Poisson’s ratio 
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Poisson’s Ratio
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Poisson’s ratio
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When a bar is strained with a
uniaxial force, a phenomenon
known as Poisson Strain causes
the girth of the bar, D, to contract
in the transverse direction.
The magnitude of this transverse
contraction is a material property
indicated by its Poisson's Ratio:
Defined as the negative ratio of
the strain in the transverse
direction (perpendicular to the
force) to the strain in the axial
direction (parallel to the force),
Poisson’s ratio = eT/ eL
Poisson's Ratio for steel, for
example, ranges from 0.25 to 0.3
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Metal wire based strain gauge
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Issues in design
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we would prefer to have a large change in resistance
to simplify the design of the rest of a sensing
instrument,
so we generally try to choose small diameters, small
young's modulus, and large gage factors when
possible.
The elastic limits of most materials are below 1%, so
we are generally talking about resistance changes
which are in the 1% - 0.001% range.
Clearly, the measurement of such resistances is not
trivial, and we often see resistance bridges designed
to produce voltages which can be fed into
amplification circuits.
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Wheatstone bridge
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The Wheatstone bridge is
widely used for precision
measurements of
resistance
How to choose R?
Rx=R+R
R1=R2=R3=R
V=-R*(V/4R)
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Metal Wire Strain Gauge: thin film pattern
• The metallic strain gauge consists of a very fine wire or,
more commonly, metallic foil arranged in a grid pattern.
• The grid pattern maximizes the amount of metallic wire or foil
subject to strain in the parallel direction
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A variety of shapes available
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Strain Gauge Measurement
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In practice, the strain measurements rarely involve
quantities larger than a few millistrain( x 10-3).
To measure the strain requires accurate measurement of
very small changes in resistance
For example, suppose a test specimen undergoes a
strain of 500 . A strain gauge with a gauge factor of 2
will exhibit a change in electrical resistance of only 2
(500 x 10-6) = 0.1%. For a 120  gauge, this is a change
of only 0.12 .
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Foil Strain Gauge
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Gauge factor: a little over 2
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Output
single gauge+3 dummy
resistors
 Area: 2-10 mm2
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Measurement: Quarter-bridge circuit
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If the nominal resistance
of the strain gauge is
designated as RG,
then the strain-induced
change in resistance, R,
can be expressed as
R = RG·K·.
Assuming that R1 = R2 and
R3 = RG,
VO/VEX = - K/4(1+K·/2)
4(1+K·/2) term that
indicates the nonlinearity
of the quarter-bridge
output with respect to
strain
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Increase Sensitivity
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Half-bridge circuit
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Full-bridge
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Tackle Temperature Effect
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Strain gauge material also respond to
changes in temperature
Minimize sensitivity to temperature by
processing the gauge material
Using two strain gauges in the bridge, the
effect of temperature can be further
minimized
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Strain Gauge in Industry
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adf
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Strain Gauge in Industry
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Packaged foil strain gauge
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Signal Conditioning
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Bridge completion
Excitation
Remote sensing
Amplification
Filtering
Offset
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Specifications
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Performance
Hysteresis< 0.02 % Rated Output
(R.O.)
Long Term Stability< 0.04 % Rated
Output (R.O.)
Nonlinearrity< 0.1 % R.O. / Year
NonRepeatability< 0.01 % R.O
Creep/Creep Recovery, 20 minutes<
0.05 % R.O.
Temp. Effect on Zero Balance
 Standard< 0.03 % R.O. / °C
 Optional< 0.004 % R.O. / °C
Temp. Effect on Output
 Standard< 0.025 % Reading /
°C
 Optional< 0.002 % Reading / °C
As oppose to P33. 2007 handbook
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Bridge Completion
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Unless you are using a full-bridge strain gauge
sensor with four active gauges, you will need
to complete the bridge with reference resistors.
Therefore, strain gauge signal conditioners
typically provide half-bridge completion
networks consisting of high-precision
reference resistors.
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Other Issues
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Excitation –typically provide a constant
voltage source to power the bridge. 3 ~
10 V are common.
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While a higher excitation voltage generates
a proportionately higher output voltage, the
higher voltage can also cause larger errors
because of self-heating. .
Remote sensing
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Long lead needs wire compensation
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Other Application: Data Storage
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A 100 micron-long piezoresistive cantilever is
dragged along a polycarbonate disk at 10 mm/s,
bouncing up and down as it passes over sub-micron
indentations in the surface of the disk. This idea is
essentially a high-performance phonograph needle
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Example Calculation: Piezoresistive Cantilever
Here, L is the length, T is the
thickness, and w is the width.
Since F = kZ, we have stiffness
=3x10-6 or 0.03% for K=100
T = 4m, L = 100 m, w = 4 m ,
E = 2 x 1011 N/m2, and F = 10-7 N.
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Optical Strain Gauges?
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Example: Fiber Bragg Grating based
strain sensor
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Cascaded up to 13 sensors, gauge
factor is still low
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For extreme pressure in hostile
environment
Option 1:
single wavelength, observe fringes resulting from
interference
Option 2:
white light source, observe spectrum change due to
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pressure-sensing wavelength dependent
rotator
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Mechanism: it’s all what it matters!
For fused silica
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Summary
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Basic circuits for sensors
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resistance
capacitance
Piezoresistive Device-strain gauge
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strain
Poisson’s ratio
Mercury tube
Metal wire based strain gauge
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Structure
Measurement: various bridge circuits
Temp compensation
Other applications: data storage
Optical strain gauges
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FBG
Polarization
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