Technical Research Sensor Technology

ECE 2799 D04

Example Design – Milestone 1

Measurement System

AcousticBiologicalChemicalElectricalMagneticMechanicalOpticalRadiantThermal

LCDLEDs7-segmentdot-matrixalarmetc…

InputSensor

ModifierOutput

Transducer

Power Supply

Sensor SelectionExample Design: Beer Keg Tap Temperature Sensor

Environmental Conditions Input/Output Range Linearity Offset Operating Life Output Format Overload Characteristics Repeatability/Hysteresis Resolution/Accuracy Sensitivity/Selectivity Size/Cost/Weight Speed of Response Stability (long and short term)

specificgeneral

5 - 7 0C (4 - 8 0C) < 1degree accuracy waterproof durable inexpensive fast low power

<50C 5-70C >70C

Types of temperature sensors

ThermoresistiveRTD (resistive temperature detector) thermistor (thermometer + resistor)

Thermoelectricthermocouple

Semiconductor (IC’s)pn diodebipolar junction transistor

Opticalphosphorescent signal

Acousticpiezoelectric

Thermoresistive sensors

advantages: • temperature range• simplicity of interface circuits• sensitivity• long term stability• inexpensive

disadvantages•not rugged•self-heating

RTD (PTC)

advantages • temperature range• sensitivity• inexpensive

disadvantages: • PTC less sensitive• nonlinear • self-heating

NTC/PTC Thermistor

Thermoelectric sensor

thermocouples

advantages:•temperature range•very ruggedness•inexpensive•fast depending on size

disadvantages:•error is larger than RTD or IC sensor•some types are very sensitive to moister

Semiconductor IC sensors

advantages:•temperature range•highly linear•small•accurate•easy to interface

disadvantages:•sensitive to shock

)2/exp(0 TkqVII B

Optical temperature sensors

Richard Boxfluorescent tubes under high tension wires outside of Bristol England

advantages:•thermally stable•waterproof•good in hostile environments

disadvantages:•expensive•impractical (too big, complicated, etc.)

Acoustic Temperature sensorsadvantages:•thermally stable•waterproof•good in hostile environments

disadvantages:•expensive•complicated circuitry

T

dry air

ultrasound

)/(15.273

5.331 smT

Sensor comparisonsThermoresistors Semiconductor

Temperature IC

RTD Thermistor (NTC)

Analog/Digital

temperature good range good range good range

cost high cost lower cost inexpensive

accuracy most precise accurate very accurate

durability sensitive to strain and shock

rugged sensitive to shock

response time slow fast fast

power problems with self-heating

lower power low power

NTC Thermistor

00

11exp

TTRR TT

Negative Temperature Coefficient

material constant

zero-power resistance at temp T

example

Types of NTC Thermistors

Metallized surface contact slow response times high power dissipations

low cost Bead type fast response times

high stability/reliabilitylow power dissipationmore costly

• bare beads no environmental protection.• glass coated beads not rugged• glass probes easy to handle, durable, stable• glass rods good for mounting on circuit boards

www.thermometrics.com

Selecting a NTC thermistor: glass probe

NTC Thermistor: response time

thermal time constant:

/1 t

as eP

TT

initial ambient temperature

electric power

dissipation constant

Ta=25 0C

P= 0.020 Watts

=0.70 mW/0C

t = 18 – 23 msec

=18 msec

NTC Thermistor: Sensitivity

T (0C) RT/R0

4 2.078

5 2.004

6 1.930

7 1.856

8 1.787

T

R

RT

T

1Temp

Coeff

=-3.7 %/C @ 5 C

NTC Thermistor: Sensitivity

T (0C) RT/R0 RT/R0 min RT/R0 max

4 2.078 2.070 2.112

5 2.004 1.994 2.034

6 1.930 1.920 1.959

7 1.856 1.851 1.888

8 1.787 1.784 1.820

00

11exp)1(

TTXRR TT

X=1%

X=5%

resistor tolerance

RT=(RT/RT0)RT0+/- 0.02RT0

Sensor comparisonsThermoresistors Semiconductor

Temperature IC

RTD Thermistor (NTC) Analog

temperature good range good range

(-80 to 160 0C)

good range

cost high cost lower cost inexpensive

accuracy most precise accurate

(+/- 0.02RT0)

very accurate

durability sensitive to strain and shock

rugged sensitive to shock

response time slow fast

(18-23 msec)

fast

power problems with self-heating

lower power

(max 0.02 W)

low power

Other R=1k-1M

Sensor Classification

what does it measure? what are its specifications? what physical phenomenon is it sensitive to? what material is it fabricated from? what conversion measurement does it use? what are its field of application?

what does it measure (stimulus)?

Acoustic Biological Chemical Electrical Magnetic Mechanical (pressure) Optical Radiant Thermal

Wave amplitude, phase, polarizationSpectrumWave velocityOther

Wave amplitude, phase, polarizationSpectrumWave velocityRefraction indexReflectivity, absorptionOther

MagnitudeDifferenceRate of changeOther

what are its specifications?

Environmental Conditions Cost Input/Output Range Linearity Offset Operating Life Output Format Overload Characteristics Repeatability/Hysteresis Resolution/Accuracy Sensitivity/Selectivity Size/Weight Speed of Response Stability (long and short term)

what are its specifications?

Environmental Conditions Cost Input/Output Range Linearity Offset Operating Life Output Format Overload Characteristics Repeatability/Hysteresis Resolution/Accuracy Sensitivity/Selectivity Size/Weight Speed of Response Stability (long and short term)

temperatureaccelerationvibrationshockambient pressuremoisturecorrosive materialselectromagnetic fields

what are its specifications?

Environmental Conditions Cost Input/Output Range Linearity Offset Operating Life Output Format Overload Characteristics Repeatability/Hysteresis Resolution/Accuracy Sensitivity/Selectivity Size/Weight Speed of Response Stability (long and short term)

what physical phenomenon is it sensitive to?

Biological Chemical Electric, Magnetic or EM wave Heat Mechanical displacement or wave Radioactivity, radiation Other

what conversion measurement does it use?

Thermoelectric Photoelectric Photomagnetic Thermoelastic Electroelastic Piezoelectric Other

What are its field of application?

Agriculture Civil Engineering Energy, Power Health, Medicine Manufacturing Military Scientific Measurements Transportation, automotive Recreation, toys Space Other

Temperature Scaletemperature scales1664 Hooke: zero scale at freezing point of distilled water (zero point)1694 Renaldi: 2 points linear (melting point ice and boiling point water) divide by 12 1701 netwon: 2 points linear (zero point and armpit temperature of healthy englishman (point 12) water boils: point 341706 Fahrenheit: zero point mixture of water, ice, salt…..96 degrees (found in the blood of a healthy man)…ice melts 32, boils 212degrees1742 celsius: ice melts 0 and water boils at 100

kelvin…..triple point…at 273.16degree kelvin, at 4.58 mm Hg pressue..water vapor, liquid and ice can coexist. (approximately 0 degrees C)….linear, zero point is temp where kinetic energy of all moving particle is zero….absolute zero…not possilbe!

Sensor

Advantages of electrical measurement systems

many microelectronic circuits already exist (applications: amplification, filtering, modulation)

many options for information display or recording electrical signal is well suited for transmission

Sensor

AcousticChemicalElectricalMagneticMechanicalOpticalRadiantThermal

Electrical

Temperature Measurement

sensor type:contact sensor or thermal radiation sensor?

conduction, convection, radiation

Contact Temperature Sensorcontact sensing….measurement is complete when no more thermal gradient between surface (beer) and sensor probe.

dQ=aA(T1-T) dt Q=absorbed heat,a=thermal conductivity of sensor-beer interfaceA=heat transmitting surface areaT=temperature

sensor, specific heat c and mass m

dQ=mcdT

aA(T1-T) dt=mcdT

T=T1-Ke-t/tT thermal time constant

measurement after 5 -10 time constants

tT=mc/a/A

(short time constant)

sensing element

1.sensing element: low specific heat, high thermal conductivity, strong and predictable temp sensitivity.2.contacts: interface between sensing element and electrical circuit….low thermal conductivity, low electrical resistance (sometimes used to support sensor)3. protective element: physically protects sening element from environment: low thermal resistance, high elctrical isolation properties……impermeable to moisture or other factes that spuriously affect sensing element