Sensors & Actuators

96 Pages·2015·2.16 MB·English

by  Helena Sarmento

Sensors & Actuators
Temperature
2014-2015 Sensors & Actuators -H.Sarmento
Outline
• Non-electric devices.
• Contact:
– RTD - Resistance temperature detectors.
– Silicon resistive sensors
– Thermistors.
– Thermocouples.
– Semiconductor pn junction.
– Piezoelectric temperature sensors.
• Non contact
– Infrared thermometer.
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Non-electric devices
• Read visually, do not produce electrical signals.
– Molecular change-of-state devices.
– Fluid expansion devices.
– Bimetallic devices.
• Disadvantages: less accurate than electrical sensors, and
temperature values not easily recorded.
• Advantages: portability and independence from a power
supply.
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Molecular change-of-state devices
• Devices whose appearance changes once a certain temperature
is reached: labels, crayons, lacquers and liquid crystals.
[Source: Omega]
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Fluid expansion devices
• the volume of a liquid or a gas changes as a function of
temperature.
[Source: Labon]
[Source: Omega]
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Bimetallic devices (1)
• Two metals with dissimilar thermal expansion coefficients.
• Manufactured in different forms for cooking, refrigerators and
freezes, industrial applications.
[Source: globalspec]
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Bimetallic devices (2)
• Two metals with similar moduli of elasticity and thicknesses.
• The radius of curvature r changer with temperature.
T
2
T
1
T  T
t 2 1
a a
A B
T  T
1 2
2t
T T 
2 1  
3r a a
A B
thermal expansion coefficients a ,a
A B
• Complex sensor: use of a displacement sensor to measure r.
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Thermo resistive sensors (1)
• RTD (Resistance Temperature Detector), silicon resistive sensors
(KTY) and NTC (Negative Temperature Coefficients) thermistors.
RTD: Metal (Pt)
KTY: doped silicon
NTC: doped ceramic
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Thermo resistive sensors (2)
• Power supply requirements: active (modulating).
• Stimulus perception: contact.
• Stimulus detection: absolute.
• Complexity: direct sensor.
• Type of stimulus: thermal.
• Transduction principle: resistive.
• Energy conversion: thermal electrical.
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Resistance temperature detectors
• Metallic: almost platinum (linear, predictable response, long-
term stability, and durability) used, but also nickel, copper,
tungsten (rare) and alloys.
• Resistivity increases with temperature.
RTD
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