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Mechatronics > Smart Actuation > Flashcards

Smart Actuation Flashcards

1
Q

Smart materials - definition

A

Designed with one or more properties that can be changed in response to an external stimuli

Part of group of materials broadly known as functional materials

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2
Q

Why use smart material sensors/actuators

A
  • real time response
  • high accuracy
  • exploit functional properties
  • highly embeddable
  • minimal effect on structural properties
  • reduction in weight
  • less power consumption
  • better reliability
  • recent tech advancements allow better integration
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3
Q

Traditional vs smart materials

A

Traditional
- designed for certain performance requirements e.g speed
- unable to change specifications if change in environment

Smart
- can accommodate unpredictable environments & meet exact performance requirements
- offer more efficient solutions for wide range of applications

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4
Q

Smart material: types

A

Based on input parameters can categorize smart materials:

  1. Electric field: piezo electric, electro active polymers (EAP), IPMC, electrostrictive, electrostatic MEMS
  2. Magnetic field: magnetostrictive, magneto-rheological (MR) fluid
  3. Chemical: Mechano-chemical
  4. Heat: Shape memory alloy, shape memory polymer
    5: Light: photostrictive

Most of these have direct & reverse effects. e.g piezoelectric materials can be used as an actuator or at the same time a sensor

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5
Q

Active polymer: definition & types

A

Polymers that respond to external stimuli by changing shape/size.

Two main type:
1. Active polymers - respond to input stimuli e.g pH, magnetic field, light
2. Electro-active - respond to change of electrical input

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6
Q

Active polymer - light driven

A

Light polymers
Contract - under Visible light
Expand - under UV light

Responds to light due to azobenzene group containing N=N group.

Visible light - N=N bonds have cis formation, polymer and bent
UV - N=N bonds are trans and polymer flattens

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7
Q

Electro-active polymer (EAP) types

A

Two types

  1. Electronic (EEAP): need high activation voltage (>150V/μm). These materials have high density energy & rapid response time (mili s)
  2. Ionic (IEAP) - small driving voltage (1-5V). Slow response time and performs better under wet conditions.
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8
Q

Types of IEAP & EEAP

A

EEAP
1. Dielectric EAP
2. Electrostrictive paper
3. Ferroelectric polymer
4. Liquid crystal elastomer

IEAP
1. Ionic polymer gels (IPG)
2. Ionic polymer metal composite (IMPC)
3. Conducting polymers
4. Carbon nanotubes (CNT)

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9
Q

IPMC structure

A

IMPCs consist for polymer matrix (needs to be kept moist) sandwiched between 2 metal layers

Polymer = a fixed network with - charge balanced by mobile positive ions. Selemion/Nafion = 2 popular ion exchange membranes

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10
Q

IPMC stimuli response

A

When subjected to DC voltage

Accumulation of cations near cathode —> water molecules move to this side —> hyrdophilic expansion

Polymer bends to anode side, with time = back diffusion off water molecules —> slow relaxation towards cathode

Extent of actuation depends on polymer type, counter ion type, presence of moisture, quality of metalization

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11
Q

IPMC pros & cons

A

Pros
- large deformation
- low actuation voltage
- fast response

Cons
- low force
- electrode delamination

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12
Q

Active polymer - future

A

Higher responsiveness - larger actuation for smaller stimulation

Agility - faster response (increasing bandwidth of existing smart materials)

Higher order functionality -
self sensing/ self actuating,
self healing,
autophagous - energy harvesting/scavenging

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13
Q

Shape memory alloys (SMA) - definition & types

A

SMAs are metals that “remember” their initial shape.

Types that are currently popular
- copper zinc aluminum (Cu Zn Al)
- copper alum nickel (Cu Al Ni)
- iron manganese silicon (Fe Mn Si)
- Nickel titanium (Ni Ti)

Generic name for family of Ni Ti alloys = NiTinol

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14
Q

SMA stimuli response

A

SMA has multiple solid phases
- low temp (martensite)
- high temp (austenite)

  • to set the shape of the SMAs, it must be held in austenite phase at high temp (~500c)
  • when cools transfers to martensite, but without changing shape now if deformed —> material doesn’t change phase but alignment pattern changes slightly.
  • so if heat wire —> returns
    
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15
Q

Piezoelectrics history

A

Piezo originate from Greek word piezin = squeeze/press/pressure

Piezoelectric effect discovered by curie brothers (1880) —> pressure generates electrical charge.

Reverse piezoelectric effect discovered by Gabriel lippmann —> electric fields can deform piezoelectric materials

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16
Q

Piezoelectric materials history II

A

Industrial breakthrough —> with piezoelectric ceramics —> discovered that barium titanate (BaTiO3) adopts pe properties when electric field applied.

BaTiO3 largely replaced by intermetallic compound lead zirconate titanate known as PZT

Used in many applications —> lighters, speakers, signal transducers

17
Q

ferroelectric polarization definition

A

Ferroelectric = materials with built in electrical polarization when electric field applied (inline regular electric fields which don’t have this property)

Polarization = orientation of electric dipoles I.e the alignment of electric charges within material

18
Q

PZT - ferroelectric polarization (fep)

A

To make a ceramic material piezoelectric, a process called ferroelectric polarization is needed.

•	First, a strong electric field is applied to the ceramic, causing an asymmetry in its structure.
•	This electric field makes the tiny electric charges inside the ceramic align in a certain way, creating polarization.
•	During this process, areas with a good alignment to the electric field grow, while those with a poor alignment shrink.
•	This alignment is mostly maintained even after removing the electric field, but some parts may revert due to internal stresses.
•	It’s important to note that above a certain temperature, called the Curie temperature, the ceramic loses its piezoelectric properties.
•	When an electric field weaker than the original polarization is applied, the ceramic expands due to a piezoelectric effect, where ions in its structure shift.
•	This expansion is partly due to the intrinsic effect of the ceramic’s structure.
19
Q

PZT - properties

A
  • Piezo actuators convert electric energy directly to mech energy
  • fast response , allow response time of few micro s. Acceleration rates of >10000g.
  • high force, high load piezo actuators can move several tons
  • high resolution, allow motions in sub nano m range. No friction elements that limit res
  • low energy consumption, static operation/ holding heavy loads (even for long) = almost no power used. Piezo actuator like capacitor = when rest, no heat
20
Q

PZT - properties II

A
  • No magnetic field, piezoelectric effect related to only electric field. Magnetic doesn’t affect nor produced.
  • no wear/tear as no moving parts
  • vaccum/clean room compatible, don’t cause abrasion or need lubrication
  • operational at cryogenic temp, can continue to work at temp close to 0K
21
Q

PZT - properties III

A
  • motion range, PZT actuators can be designed to create diff motion.
  • travel range, is between few 10- few 100 μm for linear actuators. Bending actuators achieve a few mm
  • PZT stack, by stacking PZT discs can increase overall stroke w no voltage increase
  • limitations, PZT polarization can be lost mechanically, thermally, electrically. Hysteris can be seen in statin voltage curve.

Hysteris - where relationship between strain/voltage not symmetric when field increased then decreased

22
Q

PZT - equations/relationship

A

Polarized piezoelectric Marietta’s have several coefficients/relationships but simplified —>

D = d x σ x P^T x e
D = electric flux density
d = piezoelectric charge/ (strain coefficient)
σ = mechanical stress
P^T = permittivity
e = electric field

ε = S^e x σ + d x e
ε = mechanical strain
S^e = compliance or elasticity coefficient

23
Q

PZT - equations/relationship II

A

These relationships only work with small amounts of electricity (small signal values)

Material response to electric field is straight line (linear)

Strain = ε —> ε = ΔL/L - pzt length
Stress = σ —> σ = F/A
Compliance = S^e —> S^e = 1/E
E = young modulus
Electric field = e —> e = V/L
V = activation voltage

24
Q

PZT - matrix equations

A

If stress only applied paralysis to field —>

ΔL3 = L3/EL1xL2 x F3 + d33 x V3

If stress only applied perpendicular to field —>

ΔL1 = L1/EL2xL3 x F1 + d13xL1/L3 x V3

25
Q

PZT - biomorphs

A

Two PZT strips can be bonded together and connected (horizontally) so that on extends whilst other contracts.

26
Q

PZT - bimorph equations

A

1/R = M/EI (curvature equation)
M = moment created by PZT
EI = 2nd moment of area
R = radius

No external forces —> M = F x h
In terms of stress —> M = σ x b x h^2

σ = stress of one strip
b = width, h = height

If bimorph had width of b & height of 2h

I = 2bh^3/3

Curvature equation then rearranged so insert ^ I

1/R = σbh^3/E(2bh^3/3) —>

1/R = 3σ/2hE

27
Q

PZT - bimorph equations II

A

Blocked stress related to voltage by —>

ε = S x σ + d x e
ε = 0 —> σ/E = -d13 x e

Can be subbed until curvature equation —>

1/R = 3σ/2hE —> 1/R = - 3d13e/2h
—> = 3d13V/2h^2

Deflection & curvature related =

δ = R - Rcosθ = R (1 - cosθ)
Cosθ ≈ 1 - θ^2/2 ( for small θ )

—> δ = Rθ^2/2

From diagram (on slides) θ = L/R

—> δ = L^2/2R

28
Q

PZT - bimorph equations III

A

Deflection due to inverse piezo electric effect

1/R = 3d13V/2h^2
δ = L^2/2R

—> δ = - 3L^2d13V/4h^2

Other forces also cause deflection (W = external forces)

δ = - 3L^2d13V/4h^2 + L^3/3EI x W
—> δ = - 3L^2d13V/4h^2 + L^3/2Ebh^3 x W

29
Q

PZT uses

A

PZT extremely popular in several mechatronics uses.

  • precision mechanics: micro pumps, valves, active vibration, cancelling, micro/nano robotics
    -optics and measurement systems: rapid positioning, scanning of mirrors, image stabilization, active auto focus
  • Medicine: micro manipulation, microsurgery, cell penetration

Mechatronics (4 decks)

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