•Sensorsare the first elements in the measurement system; they...

1Sensors and Interfacing Resistive Sensor devices

Sensor and Transducers

• Sensors are the first elements in the measurement system; they are in contact with and draw energy from the process or system being measured.

• A transducer converts a physical quantity into an electrical signal.


Sensor and Transducers (cont.)

- position- force- velocity- acceleration- level- flow rate- temperature- pressure

- voltage- current - resistance- capacitance- frequency- inductance

• The quality of the measurement of the variable being controlledsets the bottom line of the overall system performance.

transducerinput output

physical quantity electrical signal


Resistive Sensing Elements• Potentiometers for linear displacement Measurement

Vs Rp

RL VL

wire-woundresolution = 100/n n = number of turns e.g. 0.008%non-linearity ± 0.2%conducting plastic film- zero resolution error- higher temperature coefficient of resistance- non-linearity ± 0.04%hybrid track potentiometer

§ Loading effect non-linear effects


Loading Effect of Potentiometer

• x = d/dT• total resistance: RP Ω

xVER

xRVE ?E sth

p

p

s

thth =⇒==•

Open circuit voltage across the output terminals AB


Loading Effect of Potentiometer


Potentiometers• Basic type of potentiometric displacement transducers


Resistive Temperature Detectors (RTD)

Platinum : -200 ~ +800 °CR0 = 100.0 ΩR100 = 138.5 ΩR200 = 175.83 Ωα = 3.91×10-3 °C-1

β = -5.85×10-7 °C-2

resistance oft coefficien re temperatu:,,0at resistance :

.....)1(

0

320

γβα

γβαCR

TTTRRT

°++++=

0.76% N ,200~0 +=Λ

Co


Temperature Coefficient of Resistance (TCR )

• For metals used as RTD probes, in their respective linear range:

α

α

==

°+=

0

0

0

0at resistance :)1(

RdTdRTCR

CRTRR

T

T


Thermistors§ Thermistor: (Thermally sensitive resistor) resistive temperature elements

made from semiconductor materials

The most commonly used type:oxides of iron group of transition metal elements, such as Cr, Mg, Fe, Co, Ni

N.T.C. (Negative temperature coefficient)the resistance decreases with temperature in a highly non-linear way

constants: K,(Kelvin)K at resistance:R where

exp

βθ

θβ

θ

θ ⎟⎠⎞

⎜⎝⎛= kR

)29825(usually K

re temperatureference resistance:R where

11exp

11

1

11

KC

RR

==

⎥⎦

⎤⎢⎣

⎡−=

oθθ

θθβ

θ

θθor


NTC and PTC

Typical N.T.C. Thermistor• 12KΩ at 25 °C 0.95KΩ at 100 °C• β = 3750K• τair = 19 sec , τoil = 3 sec

P.T.C. (Positive temperature coefficient)When the doping is very heavy, the semiconductor achieves metallic properties and a positive temperature coefficient over a limitedtemperature range.

2θβθα

θ

θ −==R

ddR

Semiconductor’s resistance varies due to the variation in the number of available charge carriers and their mobility. When the temperature increases, the number of charge carriers increase, and the resistance decreases.

Negative temperature coefficient


Semiconductor Junction Temperature Sensor

]kT

qV[expii DSD 1−=

iD : Diode current

q : Electron charge = 1.6*10-19 C

VD : Voltage across diode, in V

k : Boltzmann’s constant = 1.38*10-23 J/K

T : Temperature in Kelvin

iS : Reverse saturation current

]kT

qV[expii DSD ≈

)iiln(

qkTV

S

DD =

)]iiln()

ii[ln(

qkTVV

S

C

S

CBEBE

2121 −=−

)iiln(

qkT

C

C

2

1=


Semiconductor Temperature Sensor AD590

• Difference of two base-emitter voltages of a transistor resulting from two different collector currents Ic1 and Ic2

IT

Q3 Q4

Q2 Q1

R

IC1IC2

+

-VT

Kelvinin re temperatu:101.602 charge electronic elementary :q

J/K 10381.1 / 108.86constant Boltzmann :

ln

19-

23

5-

2

121

θ

θ

C

KeVk

IcIc

qkVVV BEBET

×=

×=

×=

⎟⎟⎠

⎞⎜⎜⎝

⎛=−=

−


Semiconductor Temperature Sensor AD590 (cont.)

1. Current mirror Q3 - Q4

IT Ic1=Ic2Ic1

Ic2

vlots10179

)8(lnqk

IIln

qkVVV .2

6-

2

12BE1BET

θ×=

θ=

⎟⎟⎠

⎞⎜⎜⎝

⎛θ=−=

3. 8 transistors Q2 identical to Q1 connected in parallel



Cin T , TCA1A273

Kin T , TKA1I

A/K 1I 358R if R

210179I

volts10179R2I

Ic2I RIcV

Ic :Rrough current th The .4

out

T

6

T

6T

2T

2T

2

o

o

o

o

⎟⎠⎞

⎜⎝⎛ μ

+μ=

⎟⎠⎞

⎜⎝⎛ μ

=

μ=Ω=

θ××

=

θ×=×

×=×=

−

−



AD590AD592

+4 ~30 V

V73.2

TCmv10Vout

+

⎟⎠⎞

⎜⎝⎛=

o

ΩK1

ΩK .59



AD590AD592

+4 ~30 V

TCmv10Vout ⎟

⎠⎞

⎜⎝⎛=

o

ΩK1

ΩK .59

-VΩ00 1

span

zero

-2.73 V


Temperature Sensor Based on Semiconductor Junctions (AD590 /AD592)

1. Less expensive than RTD2. More linear than thermistors and thermocouples3. Higher output than RTDs or thermocouples4. Reduced operating range (-55ºC ~150 ºC )5. Less linear and accurate than RTDs6. Slower response than bare thermocouples, (1.5s

~10s response time)7. They are commonly used in temperature

controllers, thermostats, thermal protection (e.g. in PCs)


Chemoresistors– a charge in the electrical conductivity of a chemically-sensitive

layer is measured.– Inert substrate: alumina, SiO2

– Active material : metal oxides, organic crystals, conducting polymers

– Geometry and microstructure of the film influence the performance.


Metal Oxide Gas Sensors• ZnO, TiO2, In2O3, …, most commonly used: SnO2

-> n-type semiconductor in an environment containing oxygen.

-> increase in the carrier concentration of n”-> an increase in the electrical conductivity Δσof

the material

– affect the surface conductance in thin single-crystal films.* not suitable for thick metal oxide layers.

−−

−−

+→+

→++

eXOOX

siteOesitevacantOm

sitem

k

sitem

m

k

)()(

)(][2

2

1

2

15.0,][ <<∝′′=Δ rXne rnμσ


Commercial tin oxide sensors


Typical response of a tin oxide gas sensor


Thin film polycrystalline tin oxide gas microsensor

Poor stability -> gas alarms application


Models for Resistive Gas SensorraCGG += 0

Conduction polymer (CP):)1(0 bC

bCaGG+

+=

Dynamics:)bs)(as(

A)s(H++

=11

G0 : base-line conductance of the device in air

a : a sensitivity coefficient

r : power law exponent for oxides 0.5<r<1

b : binding constant for polymer

C : maximum concentration of the gas pulse

Metal Oxide (MOS):


Stress、Strain、Elastic Modulus、Poisson’s Ratio

Stress ≡ Force/ Area =F/AStrain ≡ change in length / original unstressed length e = +Δl / l tensilee = -Δl / l compressive

tensile stress compressive stress


Stress、Strain、Elastic Modulus、Poission’s Ratio (cont.)

Elastic modulus E = stress/strain =(Young’s modulus)

longitudinal tensile strain eL

transverse compressive strain eT

0.4)~ (0.25 ratio spoisson' :νν LT ee −=

ll

AF Δ


Strain Gauges (Metal Resistance)

• a strain gauge is a metal or semiconductor element whose resistance changes when under strain


Strain Gauges (Metal Resistance) (cont.)• The resistance of an element of length l, cross sectional

area A, and resistivity ρ is

T

L

2

e2t

e

RR

AAAR

RAARRR

AR

=Δ

+Δ

=Δ

=Δ

ρρΔ

+Δ

−Δ

=Δ

ρΔ+Δρ

−Δρ

=Δ

ρΔ⎟⎟⎠

⎞⎜⎜⎝

⎛ρ∂

∂+Δ⎟

⎠⎞

⎜⎝⎛∂∂

+Δ⎟⎠⎞

⎜⎝⎛∂∂

=Δ

ρ=

tWW

AA

ll

AA

ll

lAl l

ll

l


Strain Gauges (Metal Resistance) (cont.)

0.2e121G

gauge theof resistance unstrained :R

R RR

G

Gfactor gauge

)21(

)(2

0

0

0

≅Δ

++=

⋅=Δ

→Δ

≡

Δ++=

Δ+−−=

Δ

ρργ

εε

ρρν

ρρν

R

G

Define

e

eeRR

L

LL

≅ 0.3 ≅ 0.4



• metal strain gauge : alloy “advance”(54% Cu) + (44% Ni) + (1% Mg)

• low temperature coefficient of liner expansion, low temperature coefficient of resistance

• bounded type: active axis along the direction of the measured strain


Strain Gauges (Metal Resistance) (cont.)• Typical gauge:

– gauge factor : 2.0 ~ 2.2– unstrained resistance R0 = 120 ± 1Ω– linearity : ≤ ± 0.3%– maximum tensile strain : +2 × 10-2

maximum compressive strain : -1 × 10-2

– maximum operating temperature : 150 °C– change in resistance at maximum tensile strain ΔR = + 4.8 ΩΔR = - 2.4 Ω at maximum compressive strain

– maximum gauge current : 15mA ~ 100mA



• Unbounded strain gauge, see latter• Semiconductor gauges (piezoresistive)

–

– P type silicon : G : +100 ~ +175N type silicon : G : -100 ~ -140

greater sensitivity– Temperature coefficient of resistance is larger

↑⇒↑⎟⎟⎠

⎞⎜⎜⎝

⎛ρρΔ G

e1



• The bounded strain gauge as a transducer element

⎪⎩

⎪⎨

⎧

terpotentiomelinear -encoder optical -

LVDT -choise possibleOther

Favorable Factors1. Small size and very low mass2. Fully bonded to basic spring structure (shock-resistance)3. Excellent linearity over wide range of strains



4. Low and predictable thermal effect5. Highly stable with time6. Relatively low in cost7. Circuit output is a resistance change

Limiting Factor1. Thermal degradation (66 °C ~ 260 °C)2. Output signals are relatively low3. Careful installation procedure require (material procedure)4. Moisture effects


Temperature Compensation

• Use a dummy gage at the transverse direction

F

active gage

F

dummy gage

• Active gages : 2(one for tension, one for compression)


Temperature Compensation (cont.)

0 )(22

1

)()(

variationtureby tempera caused is R Suppose44

VE 24

24V

)2(22VR)(2R

221

)(

Sth

S

S

32

3

41

4

=⎭⎬⎫

⎩⎨⎧

Δ+Δ+

−=

⎭⎬⎫

⎩⎨⎧

Δ++Δ+Δ+

−+

=

Δ

=⋅Δ

=Δ>>

Δ+⋅Δ

=

Δ+⋅−⋅Δ+

=Δ+⋅

−=

⎭⎬⎫

⎩⎨⎧

Δ++−

+=

⎭⎬⎫

⎩⎨⎧

+−

+=

RRRRV

RRRRRR

RRRVE

GeVR

RRR

RRR

RRRV

RRRVV

RRRR

RRRV

ZZZ

ZZZVE

S

Sth

S

SSS

sSth

RRR

RR

Δ+

Z4Z3Z2

Z1Eth

VS


Temperature Compensation (cont.)

• Temperature – induced effects are eliminated

RR Δ−

RR Δ+

RRVV s

out 2Δ

=

Vs


Temperature compensation (cont.)

• Four-active-strain-gauge bridge

-

+Vout

RR Δ−

RR Δ+

RR Δ+

RR Δ−

Vs

RRVV s

outΔ

=


Conductor Resistance Compensation

• strain gage connection to a remote bridgethree wire connection

Amp display

RR Δ+

RR

R

Rwire

Rwire

The same resistance


Load Cell

• Load cell: a transducer designed to measure force

• Elastic sensing element using strain gage(load cell)

elasticelement

dislacement sensor

Force X V

potentiometerstrain gage

LVDT

electricalsignal

• Elastic sensing element using strain gauges– General features:

fairly stiff high k and small K (steady state sensitivity)

small xstrain gauges are used as secondary displacement sensor

nω


Force Control of Robot Fingers Using Strain Gage

From: Budi Santosa, KU Leuven


From: BudiSantosa, Ph.D Thesis, KU Leuven

Application of Strain Gage in Finger Design


Cantilever Load Cell• top surface : tensile strain +e• bottom surface : compressive strain –e

e0

2

GRΔR : gauge strain

FEwtx)6(le

=

−=

• strain gauges 1 and 3 : +e

• strain gauges 2 and 4 : -e

deflection bridge voltage

)Ge1(RRRRR 0031 +=Δ+==

)Ge1(RRRRR 0042 −=Δ−==


Pillar Load Cell

• compressive stress: -F/A

• strain gauges

deflection bridge voltage

AEFee

AEFe

LT

L

νν +=−=

−=

)1(

)1(

00042

00031

AEGFRGeRRRR

AEFGRGeRRRR

L

T

−=+==

+=+==ν


Torque Sensor• Applied torque shear strain φ

linear strain on the shaft surface• Maximum strain

+e : max. tensile strain : +45 ° to the shaft axis -e : max. compressive strain : -45 ° to the shaft axis

• strain gauges

modulusshear :S aS

Te 3⋅⋅π=

)Ge1(RRR)Ge1(RRR

042

031

−==+==


Piezoresistive Sensing Elements

• Piezoresistive effect: the change in resistivity ρ of a material with applied mechanical strain e →

• Piezoresistive effect can be obtained by silicon doped with N or P type material• Example: Strain gage with high gage factors• Applications: piezoresistive pressure sensors -silicon diaphragm used as elastic element -four piezoresistive strain elements form a deflection

bridge

ρρΔ

e1


Piezoresistive Coefficient Πl

• Gage factor

EstraineyresistivitratioPoissons

eG

σρν

ρρν

=

Δ++=

: ,: , :

121

/Nml

l2

00

t coefficien tivePiezoresis:

)1(

∏

∏+=Δ+= σρρρρ


fE

Ee

eEG

eEeEeE

l

ll

ll

ll

∏

∏++=∏++=

∏≈∏

=Δ

∏=∏=Δ

νν

ρρ

ρρ

ρσρρ

21121

0

00

ν21+

Gage Factor


Micromachined Silicon Pressure Sensor


Pressure Sensor

From:N. Maluf

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