CE520 Lec #6 Sensor

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CE 520: Introduction to Smart Structures Technologies

Sensing Technologies

Hoon SohnDepartment of Civil and Environmental Engineering

Korea Advanced Institute of Science and Technology

Daejeon, Korea(Lecture #6)

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2Civil and Environmental Engineering Department CE520: Smart Structures Technologies

Definition of Sensor

Definition of Sensor: – An device that receives and responses to a signal or stimulus. – An electronic device that responds to some kind of an physical input

property and convert it into an electrical signal which is compatible withelectronic circuit.

– Sensor outputs can be in a form of voltage, current, or charge.Sensor vs. Transducer:

– A transducer converts one type of energy into another form of energy: Ex: aspeaker that converts an electrical signal into variable magnetic field, andinto acoustic waves. Actuators are other examples.

– Sensor converts any type of energy into electrical signals.Passive vs. Active: – Passive sensor: passive sensors directly generate an electric signal in

response to an external stimulus. That is, the input stimulus energy isconverted by the sensor into output energy without the need of an additionalpower source. (Ex: a thermocouple and piezoelectric sensors).

– Active Sensor: The active sensors require external power for theiroperation. The change of output signals is related to the external stimulus.

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Sensor Terminologies I

Here, a sensor is regarded as a black box where we onlyconcern with the input and output relationship.Transfer function : An ideal or theoretical input-outputrelationship for each sensor. It can be a linear or nonlinearfunction.Input and output full scales: A dynamic range of a stimulusor output that is covered by a sensorError : The difference between the ideal value and themeasurement.Accuracy : The maximum error between the ideal value andthe measured one. They are often represented in terms of

(1) measured input value, (2) percent of input span, and (3)output voltage (current) value.Calibration error : The inaccuracy permitted by amanufacturer during the fabrication process. Calibrationerror is often provided with the sensor spec.

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Sensor Terminologies

Input Quantity (g)

O u

t p u

t Q u a n t i t y

( V )

Ideal Line

InputOutput

ySensitivit

Zero offset

2d

Upper & Low Accuracy Bounds

Accuracy (%) =VoltageScaleFull

d100

True Curve

Linear Range

D y n a m

i c R a n g e

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Sensor Technologies II

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Sensor Terminologies III

Hysteresis : A sensor displays a hysteresis behavior when

the sensor response has a directionality. (Primary due tofriction and structural change of the sensor materials.Nonlinearity : A nonlinearity is the maximum deviation of thereal transfer function from the approximated straight line.

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http://slidepdf.com/reader/full/ce520-lec-6-sensor 7/557Civil and Environmental Engineering Department CE520: Smart Structures Technologies

Sensor Terminologies IV

Saturation : At a certain level of the input stimulus, the

output signal become no longer responsive.Repeatability : The maximum difference between twoconsecutive readings.Resolution : the smallest variation of the stimulus that canbe sensed at the output level. (There are several differentdefinitions of resolutions, which will be described later on.)

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General Classification of Instruments based onDynamic Characteristics

Zero order instrument

First order instrument

Second order instrument

)()( 00 t xbt ya

)()()( 001 t xbt yadt

t dya

)()()()(

0012

2

2 t xbt yadt

t dya

dt t yd

a

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Dynamic Characteristics of Sensor

Dynamic Characteristics: When an input stimulus varies, a

sensor response generally does not follow exactly. This isbecause both the sensor and the interface circuit have theirown dynamic characteristics .Many sensors can be often modeled as a single degree-of-freedom system.To understand the dynamic characteristics of a sensor, let’sreview structural dynamics briefly.

Seismic MassEquivalent

Piezoelectric Transducer

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Review of Structural Dynamics I

Seismic Mass)(t x

0)()(:motionof Equation t kxt xm

t Bt At x sincos)(:Solution

frequencyresonance:wherem

k

(1) Free vibration of an undamped system

(2) Free vibration of a damped system

Do it yourself (Refer to any structural dynamics or vibration books)

k

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Review of Structural Dynamics II

(3) Response of an undamped system to harmonic loading

)(t x

t F t f o sin)(

t F t kxt xmo sin)()(:motionof Equation

t k

F t x o

sin

)(11

)(:Solution 2

(4) Response of a damped system to harmonic loading

Do it yourself (Refer to any structural dynamics or vibration books)

k

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Review of Structural Dynamics III

Linear System

)(t f Ky yC y M

t F o sin

km

c

c

cr

r

r

r r k

F A

cr

o

2and

12

tanand)2()1(

1where 2222

)sin()( t At x

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Back to Dynamic Characteristics of Sensor

Frequency band of operation : describes the frequencyrange that a sensor operates. Frequency limits aredescribed by lower and higher cut-off frequency.Frequency response describes how fast and how slowly asensor can respond to a change in the input stimulus.Time constant : The time required to charge a capacitor to63.2 % of full charge or to discharge it to 36.8% of its initialvoltage. This is another indicator that describes how fat asensor reacts to a stimulus.Cut-off frequency : The frequency either above which orbelow which the power output is reduced to 1/2 or avoltage reduction to 70.7%. This happens to be close to −3decibels, and the cutoff frequency is frequently referred toas the −3 dB point.Resonant frequency : Some sensors have a second-orderresponse, and the resonance frequency is related tomechanical, thermal, or electrical properties of the sensor.

)( RC

)21( RC f c

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Time Constant and Cut-off Frequency

)1(:ICase(b) t

so eV V

t

so eV V

:IICase(c)

Case I

VsR

C

Case II

Vo

(a) RC Circuit (d) Cut-off frequency

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Sensor Applications

Accelerometer

Fiber optic sensorThermocoupleStrain gauge (during the signal conditioning lecture)

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Typical Accelerometer Data Sheet

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Typical Accelerometer Data Sheet

Dynamic Performance –

Voltage sensitivity: 100 mV/g – Resolution: 0.001 g – Frequency range: 1 to 2,000 Hz – Range (- 4 V to + 4 V output) : - 4 g to + 4 g – Transverse sensitivity: less than 3 % – Resonant frequency: greater than 7 kHz

Electrical – Excitation voltage: 24 to 28 VDC – Constant Current: 2 to 4 mA

– Discharge time constant: 0.5 secEnvironmental – Vibration: 8 g, Shock: 5,000 g – Temperature Range: 0 – 150 oF

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Accelerometer Applications

Air Bag

Navigation SystemEstimation of velocity and displacementVibration measurementsMany smart devices including IPhone, IPad, …

Control systems And more…

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Accelerometer Basics

Definition: Accelerometers are electromechanical transducers used formeasuring absolute acceleration.Purpose : Proper selection of accelerometers requires a fundamentalunderstanding of an accelerometers’ basic principles of operation.Types : two basic types of accelerometers are – Seismic accelerometers (a.k.a.: accelerometers)

– Servo accelerometers (a.k.a.: force-balanced, inertial accelerometers)

Excitation

Surface of Primary Structure

Seismic mass

k c

Displacement Transducer

Piezoelectric transducerPiezoresistance transducerCapacitance transducer

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Piezoelectricity (Piezo means “squeeze” in Greek)

In piezoelectric materials such as quartz, the generated

charge ( Q) is proportional to the applied force ( F ):

C dF

C QV

dF Q

/

Artificially polarized materials such as ceramics and somepolymers can also be polarized to have the piezoelectriceffect.

Crystalline materialshas randomly oriented

dipoles

High temperature produces stronger agitation of dipoles and when theyare subjected to electric field, they are aligned along the field lines

below the Curietemperature, its

polarization remainspermanent

+

-

Piezoelectric coefficient

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Capacitance

C (=Q/V) is a measure of the amount of electric charge

stored for a given electric potential.Q=CV where Q is charge in Coulomb, V is voltage in Volt,and C is capacitance in Farad.Electron has a charge of e=1.602x10 -19 C.

Helium atom (schematic)Showing two protons (red),

two neutrons (green) and twoelectrons (yellow).

Permitivity constant=8.8542x10 -12 C2 /Nm 2

d

AC

0

Electrodes (conductive plates)

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Resistance

A material has electrical resistance when it has the ability

to pass electric current. (R=V/i)The resistance of the material is related to the crosssection (a), length of the conductor (l) and the resistivity ( r )of the materialElectrical resistance changes when the material ismechanically deformed.

The resistance is also related to temperature.

dl v

l dR

v

l

a

l R r r r 2

2

)](1[ 00 T T R R

Material resistivity

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Different Displacement Transducers

Piezoelectric Transducer : Charge is proportional to applied force anddeformation. Good for a high frequency range. Most commonly used forexperimental modal analysis.

Piezoelectric Transducer

Piezoceramic Resistance

Seismic Mass

Piezoresistance or CapacitanceTransducer

Piezoresistance Transducer : A change in resistance is proportional toimposed deformation. This type of accelerometer has excellent lowfrequency response down to DC level.

Piezoceramic Capacitance

Capacitance Transducer : The motion of the mass changes thecapacitance of the capacitor.

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Piezoelectric Sensor

Seismic Mass

Base Mass

k c)t(x

m )t(y

Structure

)()()()( t xmt kz t z ct z m

)()()( t xt yt z Define

Quart (transducer)

Using Laplace Transformation

222

2

2)()(

)(

)()()()(

s

s x

k csms

s xm s Z

s xm skZ scsZ s Z ms

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Piezocapacitive and Piezoresisitve Accelerometers

Capacitive Accelerometer

– Analog Devices ADXL210 – Bosch SMB110 – Crossbow CXL01LF1

(low “g” accelerometer)

Piezoresistive Accelerometer –

High Performance Planar Accelerometer – (Patridge et al. 2000)

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Fabrication of MEMS (MicroElectro-Mechanical Systems)Accelerometers: Surface Micromachining

Direct extension of IC manufacturing technology.

2.5D structures are formed by alternate deposition andetching of sacrifical and structural thin films (the key is theuse of a sacrificial layer).Enables integration of microelectronic andmicromechanical components.Can manufacture devices in the order of 10-100 m.

Common sacrificial materials Common structural materials

Silicon OxidesPhotoresist

Phosphosilicate glass (PSG)

Polysilicon Aluminum

Silicon Nitride

MEMS Cantilever Beam Design using Surface

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MEMS Cantilever Beam Design using SurfaceMicromachining

Silicon Oxide

STEP 1 Deposition Process : form a silicon oxide sacrificial layer.(using oxidation or chemical deposition)

STEP 2 Photolithography : pattern the thin-film layer using chrome mask.

Chrome Mask

Photolithgraphy process using UV light

STEP 3 Deposition Process : add polysilicon structural and photoresist layers.

Polysilicon

STEP 4 Photolithography : pattern the thin-film layer using chrome mask.

PhotoresistChrome Mask

Photolithgraphy process using UV light

STEP 5 Etching : selectively remove the silicon oxide and photoresist layers.(using dry plasma or wet chemical etching)

Silicon Substrate

Side View

Polysilicon

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MEMS Fabrication Process

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Example: Comb-Drive Accelerometer

Benefit from smaller gaps because force is inversely

proportional to gap size.Generated charge is linearly proportional to the overlappedarea.

Proof mass

Sensor electrodes Sensor electrodes

Anchor to substrate

Folded spring

Safety bump

Anchor to substrate

Movement

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Where are MEMS Devices Used?

Inertial SensingTire Pressure Sensor Temperature Sensor Digital Micromirror Display

Accelerometer SensingMicro-FluidicsRF-MEMS

Nanotechnology MEMS Accelerometer

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Accelerometer Calibration

Shaker

Top

Add a known massm

Accelerometer

Base

)t(am)t(F

aSVaa

FSV f f

a

a

f

f

S

VmS

Vf

f

aa SV

VmS

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Selection of Accelerometer for Your Application

What is the frequency range of your application?

What is the anticipated magnitude of acceleration?What is the operating temperature range?What level of resolution is required?What is the maximum tolerable size?

Is there intense acoustic electromagnetic or electrostaticfield present?What kind of power supply is available?What is an anticipated overshock?

What kind of data acquisition system will be used with youraccelerometer?Is it compatible with your data acquisition system?Is it cost effective?

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Fiber Optic Sensing Outline

Optical fiber and photonics basicsfibers (optical waveguides)photodetectors, couplers, filters, connections

Fiber sensor approachesintensity modulationinterferometry (phase difference modulation)Bragg gratings (wavelength modulation)

Fiber sensor advantages/disadvantagesCourtesy of Michael Todd at UCSD

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Some Fundamental Optics Ideas

• Optical radiation is an electromagnetic phenomenon and may be described by electromagneticfield equations (electromagnetic waves)

• A waveguide is a dielectric (electrically non- conducting) material that is used to “guide” or propagate these waves

• Optical propagation features:

• The refraction angle depends on the relative light wave speeds in the two materials; therefractive index (n ) of a material is the ratio of light speed in a vacuum to light speed in thematerial (so always greater than 1)

medium 1

medium 2

incident ray reflected ray

transmitted or refracted

ray

i r

t

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Total Internal Reflection

n 2

< n 1

n 1

refracted rays

reflected rays

total internal

reflection

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Fiber: Cylindrical Optical Waveguide

• If medium 1 index is larger than medium 2 index, and the incident angle is large enough,then total internal reflection occurs: wave will not transmit into medium 2, and this isthe basis for how an optical waveguide works• Optical fibers are cylindrical dielectric waveguides:

core• glass-based (silica,fluoride, chalcogenide)

• n~1.44 (1.31-1.55 m)• 8-980 m in diameter

• glass-based or plastic-based• n<1.44• 125-1000 m in diameter

cladding

coating/jacketing• plastic (acrylate, polyimide)• for protection, mechanical strength

• Optical fibers are characterized by the normalized frequency V :

V 2 a

ncore

2ncladding

2 V < 2.405 single modeV > 2.405 multi-mode

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Optical Sources: Light-Emitting Diodes

Surface-emitting LED (SLED) Edge-emitting LED (ELED)

• LEDs are semiconductor devices that emit incoherent light, through spontaneous emission,when electrical current is passed through them• Fabrication materials are typically GaAs and AlGaAs (850 nm) and InGaAsP (1330-1550 nm)• SLEDs used for short-distance (0-3 km), lower bit rate (<250 Mb/s) systems, ELEDs for large distance, higher bit rate systems• ELEDs more sensitive to temperature fluctuations than SLEDs• optical bandwidth typically 30-70 nm FWHM, Gaussian profile• max power typically 15 W - 20 mW (superluminescent)

1550 SLED

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Photodetector: Light to Volts

• photodetectors are devices through which optical power is converted to an electrical signal

via an absorption process

• photons are converted to electric charge carriers, and an electric field is applied to the photodetection region to measure their effect

• most common types: PIN and avalanche

photodiodes

• APD has higher responsivity (internalgain) and higher shot noise than PIN

• PIN is cheaper, doesn’t require thermalcompensation

• typical InGaAs performance:

950-1650 nm operation, 1 A/W, 5 ns response time, 0.2 pW/Hz 0.5 noise

3-4 cm

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Component Integration: General Sensing System

opticalsource

sensingmechanism

photodetection

interferometry

intensitymodulation

Bragg gratings electronicprocessing

(non-optical)

~30 cm

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Optical Intensity Changes: Microbend Sensors

measurand

fiber

light modulatedlight

• measurand directly excites corrugated fiber clamp• localized bending in the fiber causes transmission power loss• for good sensitivity, typically requires multimode fiber • sensitivities 10 -10 m/Hz 1/2 reported• advantages: low insertion loss (light stays in fiber), fail-safe (total failure = no light),• disadvantages: requires compensation scheme (multiple sources of intensity fluctuations),

behavior highly dependent on modal properties (need optical source andinsertion control)

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Optical Intensity Changes: Evanescent Field Interaction

• with cladding removed, evanescent wave can interact directly with measurand

• typically, measurands are chemical or biological species or moisture (absorb light)• can be enhanced with specialized polishes, films, or layers• disadvantage: weak interaction with measurand due to small optical field penetration

into the cladding

evanescent waveinteraction

measurand

cladding


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• extrinsic architecture: light exits fiber to interact

with measurand (almost always mechanical)• fiber separation physically modulated by measurand,leading to transmission or reflection power loss• very simple devices (low cost), but suffer from nonlinearity, poor coupling efficiency,and high sensitivity to extraneous (undesirable) measurands

Optical Intensity Changes: Waveguide Coupling

measurand


measurand

light

modulatedlight

transmission configuration

reflection configuration

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Primary Interferometer Configurations

light in photodetectioncoupler coupler

reference fiber

signal fiber Mach-Zehnder

Michelson

light in

photodetection

coupler

signal fiber

reference fiber

reflectors

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Interferometric Sensing

• An interferometer is a device in which two (or more) optical pathways are compared

• A sensor may be realized by coupling one of the optical paths to the measurand (signalarm) and isolating the other path (reference arm)

• If the measurand physically changes the length of the signal arm, then the relativedifference ∆L between the path lengths creates an optical phase change ∆ø between thetwo signals when they are recombined:

I I 0[1 M cos ] I

0[1 M cos(2 n

L )]

• When this recombined signal is photodetected, its intensity is given by

2 n

L

where I 0 is the mean signal level, M is the visibility of the interferometer, n is the corerefractive index, and is the wavelength of the light.

The detector signal directly encodes the measurand changes.

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Intrinsic Local Sensor: Bragg Grating

• A fiber Bragg grating is region of periodic refractive index perturbation inscribed in the coreof an optical fiber such that it diffracts the propagating optical signal at specific wavelengths.

fibercore

refractive index modulation period, T

• Each time the forward-propagating light encounters a stripe (index mismatch), some isscattered (diffracted)

• Scattered light accrues in certain directions if a phase-matching condition is satisfied:in particular, at the resonant wavelength given by r nT , light is reflected backward in

phase with previous back-reflections such that a strong reflection mode at wavelengthr is generated

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Bragg Grating Fabrication

optical fiberouter cladding

fiber core(Ge-doped)

reflection =

n UV

sin /22 n T =

coherent ultraviolet beamat wavelength UV=244 nm

modulation of refraction index(Bragg grating)grating period T

• This photosensitivity occurs because electronic absorptions in silica materials are in this UVregime; this effect is enhanced with Ge-doping through Ge sub-oxide defect production• Defects leads to refraction index change (Kramers-Kronig relations)

l h l

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Bragg Gratings Act as Optical Notch Filters

t r a n s m

i s s i o n

i n t e n s i t y

wavelength

nT

broadband light inserted here

cladding core grating

typical LED sourcespectrum (input)

r e f l e c t i o n

i n t e n s i t y

wavelength

• light at wavelength is reflected

• FWHM of the reflection peak istypically 0.1-0.3 nm

• if the fiber is locally stretched or

compressed, T changes, meaningchanges

• gratings may be multiplexed in thewavelength domain by initiallywriting each grating to reflect at a

unique wavelength

• sensor system must track individualwavelength shifts

Fib O i C T bl Fil

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Fiber Optic Components: Tunable Filters

broad-band light

enters the filter...

A stepped voltagedrives a piezoelectricdevice which controlsthe mirror spacing

…but only a narrowwavelength band gets

passed throughthe filter

• produced for wavelength operation 360-1600 nm• free spectral ranges between 40-60 nm• passband of ~0.1 nm (at 1550 nm)• losses below 3 dB

6-7 cm

QuickTime™ and aBMP decompressor

are needed to see this picture.

G i I i T bl Fil

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Grating Interrogation: Tunable Filters

1550 SLED

tunable fiber Fabry-Perot

filter

tunableacousto-optic

filter

photodetector

O i d G i C i

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Optics Module

1550 SLED

tunable fiber Fabry-Perot

sensing array

2x2 Mach-Zehnder interferometer

2x2 3x3

photodetectors

compensationarray

Compensation Module

G = i + gG

T

A = i + gA T

• two gratings on thermallymismatched substrates and

placed in sealed package• may be interrogated serially

with sensing FBGs by placing them at spectrum edge

• interferometer drift andthermal shifts are detectedin this way

thermaldrift

interferometer drift

Optics and Grating Compensation

Fib S Ad t

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Fiber is ~125 microns,adding negligible weight and space to application

Built-in telemetryeliminates invasivewiring

Fiber Sensor Advantages

Fib S Ad t

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Bragg grating rosette

Resistive gage rosette

composite

hull

Fiber Sensor Advantages

Fiber sensors are immune to electromagnetic interference and won’t create a spark source.

Fib S Di d t g

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Fiber Sensor Disadvantages

•Lack of commercialization, particularly at the system level(a “stand -alone” box that’s “plug -and- play”)

• Cost per sensor is high for FBGs (~$100 per sensor),BUT cost per channel is competitive

• Fiber size (128 micron or even 80 micron) may lead topossible delamination sites for embedded applications

-56 micron single mode fiber now available!

•For FBGs, severe strain gradients over gage length maycause chirping leading to loss of signal

• Serialization causes risk: loss of one FBG sensor in anarray leads to loss of all “downstream” sensors: can bepartially compensated for in design

Summary

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Summary

Accelerometer – Reliable: has been around for a long time.

– In a sense, it is still invasive sensing. – Wiring is an issue. – Relatively expensive.Strain Gauge – Less Invasive than accelerometers. – Very local measurement. – High noise level. – Relatively cheap, but not reusable.Fiber Optic Sensor – Immune to electromagnetic interference. – Multiplexing is possible. – Cost per sensor is about $100. – Does not create spark source.Wireless Sensing – Remove labor and cost intensive wiring – Easier installation – Power supply and interference are still a big issue

Additional Information

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Additional Information

New technology development in sensing: http://www.sensorsmag.comBuyer’s Guide in Society for Experimental Mechanics:http://www.sem.org/ Accelerometers and commercial sensors: – http://www.pcb.com/ – http://www.endevco.com/ – http://www.kistler.com/

– http://www.wilcoxon.com/ – http://www.bksv.com/ Strain gauges and accessories: – www.vishaymg.comFiber Optic Sensors – Jose Miguel Lopez-Higuera (ed.), Handbook of Optical Fibre Sensing

Technology , John Wiley and Sons Ltd. (Chichester, UK), 2002. – Eric Udd (ed.), Fiber Optic Sensors: An Introduction for Scientists and

Engineers , Wiley Interscience (New York), – T. S. Yu and S. Yin (eds.), Fiber Optic Sensors , Marcel Dekker Inc. (New

York), 2002.

http://www.sensorsmag.com/

http://www.bksv.com/


http://www.vishaymg.com/




http://www.sensorsmag.com/

CE520 Lec #6 Sensor

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