1. Unit-2

2. Working Principles for Microactuators
Power
Supply
Micro
Action Element Unit
Power supply: Electrical current or voltage
Transduction unit: To covert the appropriate form of power supply into the desired form of actions of the actuating element
Actuating element: A material or component that moves with power supply
Output action: Usually in a prescribed motion
Actuating Transduction
Output

3. Actuation Using Thermal Forces
● Solids deform when they are subjected to a temperature change (∆T)
● A solid rod with a length L will extend its length by ∆L = α∆T, in which
α = coefficient of thermal expansion (CTE) – a material property.
● When two materials with distinct CTE bond together and is subjected to a temperature change, the compound material will change its geometry
as illustrated below with a compound beam:
Heat α1 α2
● These compound beams are commonly used as microswitches and relays in MEMS products.
α1 > α2

4. Actuation Using Shape Memory Alloys (SMA)
● SMA are the materials that have a “memory” of their original geometry (shape)
at a typically elevated temperature of production.
● These alloys are deformed into different geometry at typically room temperature.
● The deformed SMA structures will return to their original shapes when they are heated to the elevated temperature at their productions.
● Ti-Ni is a common SMA.
● A microswitch actuated with SMA:
Shape Memory Alloy Strip e.g. TiNi or Nitinolor
Resistance Heating Strip
Silicon Cantilever Beam
Constraint Base

5. Actuation Using Piezoelectric Crystals
● A certain crystals, e.g., quartz exhibit an interesting behavior when subjected to a mechanical deformation or an electric voltage.
● This behavior may be illustrated as follows:
Induced Mechanical
Forces V
Mechanical force induced Electric voltage induced electric voltage mechanical deformation
● This peculiar behavior makes piezoelectric crystals an ideal candidate for microactuation as illustrated in the following case:
Mechanical
Deformation
Applied Voltage, V

6. Silicon Cantilever Beam
Actuation Using Piezoelectric Crystals-Cont’d
A micro relay or microelectrical switch
Electrodes V
Silicon Cantilever Beam
Constraint Base
Piezoelectric

7. r 2 A B 4πε Actuation Using Electrostatic Forces
● Electrostatic Force between Two Particles – The Coulomb’s Law: A
(with charge q) B
The attraction or repulsive force: F = qq'
where ε = permittivity of the medium between the two particles
= x C2/N-m2 or 8.85 pF/m in vacuum (= εo) r
Distance,
Attraction F F
Repulsion
(with charge q’)
4πε
r 2
r = Distance between the particles (m)

8. F = − 1 ε r ε o WL V 2 d 2 C = ε r ε o = ε r ε o
Actuation Using Electrostatic Forces-Cont’d
● Electrostatic Force Normal to Two Electrically Charged Plates:
Length, L V
Gap, d
A WL
d d
● The induced normal force, Fd is:
F = − 1 ε r ε o WL V 2
in which εr = relative permittivity of the dielectric material between the two plates
Width, W
C = ε r ε o
= ε r ε o
● The induced capacitance, C is: d 2
d 2
(see Table 2.2 for values of εr for common dielectric materials).

9. ε r ε o L F = − 1 ε r ε o W V 2 L FL 1 F = − V 2 d
Actuation Using Electrostatic Forces-Cont’d
● Electrostatic Force Parallel to Two Misaligned Electrically Charged Plates:
Fd F
L FL
● Force in the “Width” direction:
w d
● Force in the “Length” direction:
F = − 1 ε r ε o W V 2 w V d W 1
ε r ε o L F = − V 2 L 2 d

10. Applications of Microactuations
Microgrippers An essential component in microrobots in assembly microassemblies
Two gripping methods:
Gripping Arms
V Electrodes the gap,d V Not practical b/c requiring more space.
The sliding plate electrodes
V V - Popular method. Can have
actuators
electrodes,∆L
and surgery
The normal plate electrodes
Closing
many sets to make “Comb drive”
Aligning the

11. Number of Electrode Pairs
A Typical Microgripper with “Comb drive” Actuators:
400 µm
100 µm
Drive Arm
Arrangement of electrodes:
10 µm
Closure Arm
Drastic reduction in required
120
100 80 60 40 20
Number of Electrode Pairs V
Extension Arm
140
actuation voltage with increase
of number of pairs of electrodes:
R equired Voltage, v

12. ∆C where P = Air pressure (Pa)
Applications of Microactuations
Miniature Microphones A niche market in mobile telecommunications and
Acoustic Wave Input dB = unit of noise level:
dB→ MPa ⎛ P ⎞
Diaphragm: ⎝ Po ⎠
∆C where P = Air pressure (Pa)
at threshold sound level
Pressure equalization hole
Most microphones are designed for dB in the frequency range of Hz
A major challenge in MEMS microphone design and manufacture is the packaging and integration of MEMS and CMOS integrated circuits for signal conditioning
and processing
intelligent hearing aides
(air pressure wave)
dB = 20 log10 ⎜ ⎟
Electrical signal output: ≈ 1µ
1µm thick
Backplate (≈2 µm)
Air gap (≈2 µm)
Acoustic holes
Po = Reference air pressure