Transcript Department of Optical Engineering Zhejiang University

Advanced Sensor Technology
Lecture 5
Jun. QIAN
Department of Optical Engineering
Zhejiang University
Course project report
(30% toward your grade)
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Topic: High-resolution sensor: force, motion, …
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Issues to address
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Working principle: equations and calculation (MathCAD or
Matlab or Maple), > 5 equation lines
2 Schematic drawings to explain the operation principle: no
engineering drawings required
How to obtain the resolution and/or dynamic range?
noise floor estimation: Johnson noise, op-amp noise, AC pulse
noise…etc. show your work in the form of equations/calculations
Justify material selection in your structure
References
Due: before final exam, hand in a printed report, 4-5
pages (font: 12, single space) in English
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A Review of Lecture 4
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Capacitive sensor
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High sensitivity
Relatively high cost
AC or switched capacitive measurement circuit
Provide an overview of accelerometer.
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Classification by applicable ranges
Exclusively focus on capacitive accelerometer
Useful range <0, Z=a/02
Applications
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Lecture 5: Basic intent
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Provide some general examples of
accelerometers.
A careful attention will be given to a
particular commercial product by the
Analog Devices
Piezoelectricity
Piezoelectric Sensors
Piezoelectric accelerometer
Trends in accelerometer design
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Example: mass on a cantilever
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Configuration:
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1 mg mass located 1 mm
from the support of the
cantilever
The cantilever is 100
microns long and 10
microns thick.
a doped silicon strain
gauge
Resonant freq: 1 kHz
Then a 1 milli-g
acceleration would
cause a change of R by
1ppm – pretty small!
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Silicon Accelerometer
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Silicon accelerometer
with a doped silicon strain
gauge
Minimum resolution: a
few milli-gs, and at cost of
10-50 dollars per device.
more sophisticated
accelerometer technology
relies on the use of
capacitive displacement
transducers within a
micromachined silicon
structure
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Accelerometers for Air Bag safety system
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Initially made available by
Chrysler in the mid 1980s,
now a standard item
Early air-bag systems:
Rolamites
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Frictionless machine from
rollers and bands
two rollers held in a track on
opposite sides of an S-shaped
band of springy metal, the
rollers glide effortlessly in the
track because the band
moves with them as they roll
along. Since the band and
rollers are both moving at the
same speed, there is no slip
or drag between them and
therefore virtually no friction.
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Rolamites Airbag G-gauge
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a little clumsy, but with some packaging,
can perform reliably throughout an automotive lifetime
which might be more than 10 years, and include
exposure to extreme cold and heat for extended
periods of time.
One possible problem has been the tendency to
deploy upon the encounter of a long, deep pothole.
Unnecessary deployment is somewhat dangerous,
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it briefly incapacitates the driver.
it causes some expensive damage to the vehicle dashboard,
and occasionally injures the driver.
numerous reports of hearing loss due to the abrupt change in
pressure caused by deployment in a sealed car.
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Today’s airbag sensor: MEMS
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Airbag Accelerometers
manufacturers: EG&G IC
Sensors , Analog Devices,
Motorola
airbag-like devices for
aeroplanes as early as the
1940s,
the first actual example in a
production car was the 1981
Mercedes-Benz S-Class.
airbag triggering algorithms
are becoming more and more
complex.
The most common MEMS
accelerometer in use is the
ADXL-50 by Analog Devices,
but there are other MEMS
manufacturers as well.
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ADXL50
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ADXL50 measures the difference
between the two capacitors
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ADXL50: Functional Block Diagram
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Principle of Operation
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The detection scheme works by detecting the
difference between two capacitances.
This difference signal is amplified, and compared with a
threshold detector, which sends positive or negative
pulses to the control electrodes
A fully-integrated device
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ADXL50: Performance
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Full-Scale Measurement
Range: 50 g
Self-Test on Digital Command
+5 V Single Supply Operation
Sensitivity Precalibrated to 19
mV/g
Internal Buffer Amplifier for
User Adjustable Sensitivity
and Zero-g Level
Frequency Response: DC to
10 kHz
Post Filtering with External
Passive Components
High Shock Survival: >2000 g
Unpowered
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Test: AD chip for vehicle navigation
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Primary source of info to calculate
position:
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GPS (global positioning by satellite)
24 low earth orbiting satellietes
 Affected by the location of the receiver w/ respect
to environment
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Two types of limitations:
Selective availability: signals deliberately altered
by the military, limiting its accuracy
 Multipath reflections: introducing errors due to
inaccurate distances and times used in the
computation of position.
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Dead reckoning:
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starting coordinates, direction, speed
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Test: AD chips for vehicle navigation
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Direction: gyroscope
Speed: accelerometer
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Test ADXL202：2g
3 dB bandwidth set around 10 Hz,
RMS noise figure of 1.9 mg, ~resolution 10-3
Zero drift over 24hrs: 2.4mg
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Test: AD chips for vehicle navigation
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Field test
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GPS on and off for
correlation
30m away from ref.
Without GPS:
1 2 1
at  (2.4 10 3 10)(1 3600) 2  155,520m
2
2
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Piezoelectricity
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a phenomenon in which forces applied to a segment of
material lead to the appearance of electrical charge on
the surfaces of the segment.
This polarization of the crystal leads to an
accumulation of charge :
Q (charge) (1x3 matrix)
= d (piezoelectric coefficient 3x3 matrix ) F (Force1x3 matrix)
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Piezoelectric Materials
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Many polymers, ceramics, and molecules
such as water are permanently polarized:
A permanently-polarized material such as
quartz (SiO2) or barium titanate (BaTiO3)
will produce an electric field when the
material changes dimensions as a result of
an imposed mechanical force: piezoelectric
effect.
Conversely, an applied electric field can
cause a piezoelectric material to change
dimensions. This phenomenon is known as
electrostriction, or the reverse piezoelectric
effect.
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Piezoelectricity
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Typical values of the piezoelectric
charge coefficients are
1-100 pico-coulombs/N.
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a 1 cm x 1cm slab of 1 mm thick
PZT ( lead zirconium titanate ). A 1
N force is applied along the z axis,
which is the 1mm dimension. What
voltage appears across electrodes
on the large surfaces?
to produce a larger voltage, we
need to reduce the capacitance of
this structure. The easiest way to
do this is to reduce the area.
It is interesting that the dimensions
of the object completely cancel out
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Two types of devices: resonant and non-resonant
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Non-resonant: generator-like
Low sensitivity
 Simple design
 Powerless device
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Resonant: looking for change in
resonance frequency
Research started as early as 1930s
 High sensitivity
 Quartz: <20% change of resonance freq.
 Silicon: >200% change allowed
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 Q Factor
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Quartz: cut related properties
Y-cut: large temp coefficient of
resonant freq: 80-100ppm/ C,
can measure 10-6 C;
 LC-cut: 10-3 C measurement
from 80-230 C;
 All-quartz structure pressure
sensor:
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Resolves 10-7 of full scale,
Record-maker!
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Compression Mode
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The original compression designs for accelerometers
combined an X cut quartz crystal (or ceramic element)
preloaded with a seismic mass.
The benefits of the simple, reliable design were good
sensitivity and a high natural frequency providing a
broad useable frequency range.
Drawbacks included sensitivity to base strain effects
and a thermal transient sensitivity also manifested
through strain effects.
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Shear Mode
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Most common construction for
piezoelectric accelerometer is
shear mode.
fashioned in configurations:
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tri-shear,
planar shear
annular shear.
Significant benefits over original
compression designs in that the
crystal sensing element is better
isolated from base strain inputs and
thermal transients by mounting the
crystal on an internal post. Shear
designs also typically exhibit lower
transverse sensitivity and base
strain sensitivity, and have better
thermal stability.
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Flexure Beam
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Flexure beam is another
geometric variation of sensing
element structure. While it has
one of the highest outputs for
low profile, low cost and
lightweight designs, it is less
used due to challenges with
shock survivability and thermal
transients. In applications
where extremely low profile
elements prevail, some flexure
beam designs are still the only
available solution.
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Quartz based sensors
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Various Designs:from quartz to silicon
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USpatent 5396798
132,138,140:silicon arms, can experience
frequency variations on the order of 300% of
the base frequency
148:piezoelectric resistor, senses the vibration
of the silicon arms
141:drive capacitor, provides sufficient energy
to sustain the vibration of the silicon arms
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Piezo leakage
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piezoelectrics are not generally very good dielectrics.
In particular, piezoelectric materials are somewhat
leaky
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a charge placed on a pair of electrodes gradually leaks
away.
there is a time constant for the retention of a voltage on the
piezoelectric after the application of a force.
This time constant depends on the capacitance of the
element, and the leakage resistance.
Typical time constants are of order 1 sec.
Because of this effect, piezoelectrics are not very useful for
the detection of static quantities, such as the weight of an
object.
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Equivalent Circuit
Equivalent electrical circuit for
piezomeasurement circuit.
Rs = Resistance of piezo
Cs = Capacitance of piezo
Cc = Capacitance of cable
Ca = Capacitance of amplifier circuit
Ra = Resistance of amplifier circuit
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Advantages and disadvantages
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Advantages over other sensing mechanism
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the device generates its own voltage. Because of this, the
sensor element does not need to have power applied to it in
order to function.
For applications where power consumption is a significant
constraint, piezoelectric devices can be very valuable.
some interesting scaling laws which suggest it is useful in small
devices.
The primary disadvantages
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inherently sensitive only to time varying signals.
the Curie temperature .
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If the crystal is ever heated to near the Curie temperature, it can
become `de-poled' which can result in a loss of piezoelectric
sensitivity.
For various materials, this Curie temperature can be as high as
600C or as low as 50C. The need to stay below this temperature
can impose serious constraints on the applicability of these sensors.
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Useful Frequency Range
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As the natural frequency is approached, the
frequency response of the sensor increases
logarithmically. It is a rising response, rather
than a decreasing response that limits the
frequency response at the upper limit.
Low frequency response is limited by the
interaction of the electronics in the coupler or
signal conditioner in typical measurement
systems.
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When to use piezoelectric sensor?
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Nevertheless, if you have a fast timevarying signal, you should give serious
thought to the use of piezoelectric sensing
elements.
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Example: Piezoelectric Accelerometer
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10 g mass resting on a slab of PZT. The
piezo slab has dimensions of 1 square cm in
area, and 1mm thick.
Find: V as a function of force and V(1mili-g)
V(1milli-g)~10-5V
Q1: measurable?
Q2: improvements?
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Improvements
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The piezoelectric element can have a
smaller area and a larger thickness.
A preamp
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Commercial Piezoelectric Accelerometer
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Piezoelectric
accelerometers are
on the market,
Primarily offered for
vibration
measurement.
For moderate signals
(milli-gs), fairly small
devices with simple
circuits are quite
sufficient
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Trends in Accelerometer Design
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Trends in accelerometer requirements for
military and aerospace
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Increased High- and Low-Frequency Response
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Range
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The current limitation at the National Institute of Standards and
Technology (NIST) is 15,000 Hz, but work is in progress that is
expected to raise this limit to 20,000 Hz .
Upper Limit
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Pyroshock: short-duration, high-amplitude, high-frequency,
transient structural responses in aerospace vehicles.
Pyroshock on rocket or missile systems: explosive bolts and nuts,
pin pullers, separation of spent rocket booster stages, linear
cutting of the structure, and other actions that produce a nearinstantaneous release of strain energy.
To support structural analysis of typical military and aerospace
systems,
However, the higher resonant frequencies possible with MEMS
technology often increase the accelerometer's ability to survive a
pyrotechnic event.
commercial piezoelectric accelerometers typically have a
maximum resonant frequency 1/5 that currently achievable with
MEMS technology.
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Increased High- and Low-Frequency Response
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Lower limit
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Experimental modal analysis, or the ability to
characterize structures empirically in terms of their
damping, resonant frequencies, and vibratory mode
shapes,
The first vibratory mode of a transport aircraft such as
the Boeing 737 or McDonnell-Douglas DC9 occurs at
<1 Hz. This is the wing "butterfly" mode.
The space station and other large space structures
have even lower structural frequencies.
Experimental characterization of these structures is
now possible using variable capacitance MEMS
accelerometers as well as piezoelectric
accelerometers.
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Piezoelectric types are not capable of frequency response to
0 Hz. Improved housing isolation from thermal and acoustic
stimuli, however, results in designs with
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responses to <0.1 Hz
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MOEMS accelerometer
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Performance Comparison
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MZI-type Design
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MZI-type Design
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Phase shift introduced:
P0: input power
P: output power
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Sensitivity
: phase shift
Photo current v.s. acceleration
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Minimal detectable acceleration:
I0: induced photo
current
Id:dark current
*Photon noise and
thermal noise limited
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Accelerometer 2012
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Optomechanical
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Photonic crystal cavity
nano-tethered test mass of
high mechanical Q-factor
Resolution=10g/Hz
Dynamic range 50dB
Bandwidth>20kHz
Nature Photonics, 6, 768-772, (2012).
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