Technical Research Sensor Technology

Originally Prepared by:Prof. Shela Aboud

Modified by:Prof. Bitar

Example Design – Milestone 1

Measurement System

AcousticBiologicalChemicalElectricalMagneticMechanicalOpticalRadiantThermal

LCDLEDs7-segmentdot-matrixalarmetc…

InputSensor

ProcessingOutput

Transducer

Input Sensor Technology

• What types of sensors are needed on your project?

• What sensor specifications do you need to consider?

Sensor SelectionExample Design: 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

Thermoelectricthermocouple

Semiconductor (IC’s)pn diodebipolar junction transistor

Opticalinfrared detector

Acousticpiezoelectric

Thermoresistive sensors

advantages: • temperature range• simple 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 rugged• inexpensive• fast depending on size

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

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

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

disadvantages:• limited temperature range

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