Sensors 1
Thermal and Actuators: A Survey of Principles and Applications
Chang Liu
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Outline
General knowledge of heat and energy transfer Thermal actuators Thermal sensors
Thermal sensors: sensors for thermal phenomena or sensors that use thermal phenomena Bimetallic cantilevers Thermal resistors Sensors that are based on thermal transfer principles
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Thermal Transfer Principles
Heat will flow between two points of different temperatures. The heat transfer can take one of three forms Conduction Convection
Natural convection Forced convection
Radiation
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Thermal Resistance
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Example: Thermal Resistance of a Suspended Bridge
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Energy Storage
The term sh is the specific heat (J/KgK) The term Cth is heat capacity
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Actuation Methods
Electrostatic Thermal actuation Magnetic actuation Piezoelectric actuation Pneumatic actuation
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Thermal Actuation
Thermal expansion
Liquid Air Solid
Phase change expansion
Vapor Bubble generation Solidification (volume contraction)
Most famous example: ink jet nozzle
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Ink Jet Droplet Injector (TIJ 1.0, 1984)
Bubble formation time: 1 s. Ink ejection time: 15 s. Peak pressure: 14 ATM Upon removal of heat, vapor cools and the bubble retreats. Refill at 24 s, lasts about 25 s. Surface temperature: 90% of critical temperature (vaporization temperature) which is 330 oC. Homogeneous boiling across the surface of the heater, made of tantalum-aluminum (Ta-Al) alloy. The heater has near zero TCR, so zero thermal expansion. HP Ink Jet Printer - Single Drop
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Improvement to TIJ
TIJ 2.0, 1987 - color and improved BW
a layer of Ta is coated on passivation to prevent crack caused by cavitation (collapse of air bubble) more sophisticated firing chamber shape 300 DPI
TIJ 2.5
three mask NMOS process for row and column addressing as interconnects are increasingly difficult to make. 600 DPI
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Pneumatic Thermal Actuators
Valve diaphragm
MRTV silicone elastomer membrane is 2.3 mm on each side
For 20 psi inlet pressure 280 mW was required to close the valve
Thermal expansion fluid
3M PF 5060 achieves 840 m deflection at 0.94 W.
Thermal volume expansion causes the silicone diaphragm to touch the inlet.
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Comparison of Thermal Actuation and Electrostatic Actuation
Electrostatic actuation
Power: low power due to voltage operation. Response speed: high speed. Construction and fabrication: relatively simple range of motion: for parallel plate capacitor, range of motion relatively small.
Thermal actuation
Relatively high power: due to current operation. Lower response speed due to thermal time constant (dissipation and thermal charging) Construction and fabrication: more due complex to material compatibility considerations. Range of motion: relatively large.
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Other Actuation Principles
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Hydraulics Shape memory metal Megnetostrictive effects Osmosis Magnetofluid Internal combustion Laser trapping and light force Surface wetting Ion drive High energy particle drive Magnus effect (a rotating sphere in flow develops lift) Vapor pressure phase change Detonation Biological: Protein, ATP, Myosin-Actin,
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Thermal Sensor Principles
Summary of major principles discussed in class
Thermal bimetallic bending induced by temperature change Thermal resistive transducers
measures change of resistance under temperature variation common materials include polysilicon and metal oxide.
Thermal couples
Seebeck effect
Semiconductor type temperature sensors
diode, transistors
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Other Sensing Principles
Optical fiber sensors Magnetostrictive magnetoelastic sensor Measurement of chemical reaction rate (e.g., oxidation) Frequency of mechanical vibrators Biological temperature receptors Fluorescent sensors (markers) Gas expansion and pressure change Color change Use crickets (or biological systems) to sense temperature. Measure thermal noise
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Basic Principle
Thermal expansion coefficient: dimensional expansion of materials under elevated temperature.
Unit: 1/degC. Or
Volumetric expansion Linear expansion
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Thermal Bimorph Actuator
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Example: T -> E
A thermostat for home use Energy in thermal domain Translates into energy in mechanical domain (bimetallic bending) Translate into position of mercury balls in the tube Translate into electrical trigger for cont...