Download Thermal Actuation

Thermal Sensors and Actuators:
A Survey of Principles and Applications
Chang Liu
Chang Liu
MASS
UIUC
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
Chang Liu
MASS
UIUC
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
Chang Liu
MASS
UIUC
Chang Liu
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UIUC
Thermal Resistance
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Example: Thermal Resistance of a Suspended
Bridge
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Chang Liu
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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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UIUC
Actuation Methods
•
•
•
•
•
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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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MASS
UIUC
Ink Jet Droplet Injector (TIJ 1.0, 1984)
•
•
•
•
•
•
•
•
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Bubble formation time: 1 ms.
Ink ejection time: 15 ms.
Peak pressure: 14 ATM
Upon removal of heat, vapor
cools and the bubble retreats.
Refill at 24 ms, lasts about 25
ms.
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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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.
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• 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 complex due to material
compatibility considerations.
– Range of motion: relatively
large.
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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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UIUC
Basic Principle
• Thermal expansion coefficient: dimensional expansion of
materials under elevated temperature.
– Unit: 1/degC. Or
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L
L
T
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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 controlling the AC
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Scanning Probe Microscopy Probe for Nano
Lithography
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Thermal Bimetallic Actuation
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Chang Liu
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Material Properties
• Rules for designing
efficient thermal
bimetallic actuator:
• maximize
difference in a
• ease of fabrication
• material thermal
stability
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Material
Aluminum
Thermal
conductivity
(W/cmK)
2.37
Temperature
coefficient of
expansion (ppm/K)
25
Aluminum oxide (polycrystalline)
0.36
8.7
Aluminum oxide (saphire)
Carbon
Carbon (diamond)
Cr
Cu
GaAs
Ge
Au
Si
0.46
0.016
23
0.94
4.01
0.56
0.6
3.18
1.49
6
16.5
5.4
6.1
14.2
2.6
SiO2 (thermal oxide)
0.0138
0.35
SiN (silicon nitride)
0.16
1.6
Polyimide (Dupont PI 2611 D)
Poly silicon (LPCVD)
0.34
3
2.33
Ni
Ti
0.91
0.219
13
8.6
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Micro Ciliary Motion System
- biomimetic micro motion system
• Biomimetic ciliary transport
system
– utilizing large number of
distributed actuators to achieve
macroscopic motion.
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Thermal Bimetallic Actuator
• Composite layer: polyimide
(organic polymer) + metal
(heater)
• Resistance of heater: 30-50 ohm
• current input: 25 mA (above
which polyimide might be
damaged)
• Cutoff frequency is 10 Hz.
• Beam bends upward due to
intrinsic stress (tensile) in
polyimide;
• Upon heating, the thermal
expansion in the polyimide is
more extensive - the beam
therefore bends downwards.
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Thermal Resistive Transducers
• As temperature increases, the following variables change
– electrical resistivity
– dimensions, since R   l
A
• The value of the resistance as a function of the temperature is
generally referred to as
RT  R0 (1  a R [T  T0 ])
– where R0 is the value of resistance at room temperature To
 aR is called the TCR, or temperature coefficient of resistance, with
unit being oC-1.
– Note the equation is true for moderate temperature excursions.
Chang Liu
MASS
UIUC
Common Materials for Thermal Resistor
• Doped silicon or polysilicon
– Most commonly used in silicon micromachining for simplicity of
fabrication
– doping on the neighborhood of 1-2x1019 cm-3 gives zero TCR
– higher doping, TCR approximately 0.2-0.5%/oC
– lower doping, TCR approximately - 2%/oC (1018 cm-3 doping) or 6%/oC (2x1016 cm-3 doping)
• Pure metal
– the value of aR is on the order of 5000 ppm/oC, which is around
0.5%/oC.
– Usually positive (resistance increase with temperature)
• Semiconducting oxides of metal
– oxide of Li, Cu, Co, Ti, Mn, Fe, Ni etc
– value of is around negative 4-6%/oC, or 4-6%/K.
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MASS
UIUC
Known TCR of Polysilicon (Doped with Boron)
• High TCR at low concentration; but value is less stable over
long term.
1
0
TCR (%/C)
-1
-2
-3
-4
-5
-6
-7
1.00E+16
1.00E+17
1.00E+18
1.00E+19
1.00E+20
Boron Doping (cm^-3)
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A resistive temperature sensor may also serve as a
ohmic heater
• Ohmic heating power
• P=I2R
• The heating power is partially used to raise the temperature of a
resistor and partially lost to surroundings through
– Conduction
– Convection
– Radiation
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IV-Characteristics
• The Current-Voltage Relationship for a resistor is obtained
when the voltage is systematically varied and the current is
recorded.
– Automated machine (semiconductor curve tracer)
– Manual data collection
• What is the IV characteristic of a thermal resistor?
• Why cann’t we just use an ohm-meter (multimeter) to measure
the resistance?
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Thermal Insulation if Characterized by Thermal
Resistance, RT
Electrical vs. thermal analogy
Voltage – temperature difference
Current – thermal heat flux
Resistance – thermal resistance
l
V
R   ;R 
A
I
l
T
Rth   th ; Rth 
A
P
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I, or P
For electrical
R[Ohm]=V[Volt]/I[Amp]
=electrical resistivity*length/area
For thermal
RT=(T1-T2)/power[W]
=thermal resistivity*length/area
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Conclusions
• Know how to measure the TCR of a thermal resistor;
• Know how to measure the current-voltage characteristics (IV
curve) of a thermal resistor;
• Know how to obtain resistance-power relationship based on the
IV curve;
• Know how to obtain the temperature-power relationship based
on the IV curve;
• Know how to calculate the thermal resistance based on the IV
curve.
Chang Liu
MASS
UIUC
Thermal Couples - Seebeck Effect
• Thermal electric effect refers to the generation of electrical
potential when a temperature differential exist across a piece of
material. At the high temperature end, more electron will be
excited into the conduction band and starts diffusion into the
colder region.
• The Seebeck effect (nameed after Seebeck), is commonly
characterized by the Seeback coefficient which is expressed in
the following form, for a single piece of metal:
a
V
T
• A working thermal couple with two different Seeback
coefficients develop a voltage difference when subject to a
temperature change of T.
High T
ΔV
a ab  a a  a b
V  (a a  a b )T
Low T
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Why “thermal couple”?
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Chang Liu
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UIUC
Seebeck Coefficient of Common Thermal Couple
Materials
• The rule of thumb is to find materials with maximum different
of Seebeck coefficients.
Parameters
Type J
Type K
Type N
Type T
Type P
Composition 1
(as, mV/K)
Iron
(17.7)
Platinum
(0 TO -5)
45% Ni
55% Al
(?)
-40
Constantan
(55% Cu
45% Ni)
(?)
-250
87% Pt
13% Rh
(?)
Minimum temperature
Nicrosil
(71-86% Ni
14% Cr
0-15% Fe) (?)
Nisil
(95% Ni
4.5% Si)
(?)
-230
Copper
(?)
Composition 2
(as, mV/K)
Chromel
(90% Ni
10% Cr)
(29.8)
Alumel (95%
Ni 2% Al
2% Mn 1%
Si) (-10.85)
-200
Maximum temperature
850
1100
1230
400
1350
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Exercise Problem
• First calculate the radius
k
1
6 w1w2 E1 E2t1t 2 (t1  t 2 )(a1  a 2 )T


2 2
2 2
2
2
r ( w1 E1t1 )  ( w2 E2t 2 )  2 w1w2 E1 E2t1t 2 (2t1  3t1t 2  2t 2 )
6  (10  10  6 ) 2  57  150  1018  0.5  1.5  10 12  2  10  6  11.67  10  6  20
(10  10  6  150  109  2.25  10 12 ) 2  ( w2 E2t 22 ) 2  2 w1w2 E1 E2t1t 2 (2t12  3t1t 2  2t 22 )
1.796013  10 9

1.1390625  10 11  (10  10  6  57  109  0.25  10 12 ) 2  ...
1.796013  10 9

1.1390625  10 11  2.030625  10 14  2  (10  10  6 ) 2  57  150  1018  0.5  1.5  10 12 (2t12  3t1t 2  2t 22 )
1.796013  10 9

1.141093125  10 11  1.2825  (2  2.25  10 12  3  0.625  10 12  2  0.25  10 12 )
1.796013  10 9

 88.78794
1.141093125  10 11  1.2825  6.875  10 12
r  0.01126m
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MASS
UIUC
Calculate Vertical Displacement
 q=l/r=0.08881 radian=5.0884 o
• d=r-rxcosq44.3758 mm.
q
r
r
d
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MASS
UIUC
MEMS Infra Red Sensor – Hybrid Sensing
• Thermal bimetallic
material as sensing
• IR infrared beam
heat the top plate
• temperature rise
causes the zigzagged beam to
bend
• The distance
between the parallel
plate capacitor
changes
• thermal isolation
allows maximum
temperature rise
given the absorbed
energy.
Chang Liu
MASS
UIUC