Design Realization lecture 15

John Canny / Jeremy Risner

10/9/03

Last Time

Composites: Fiberglass, carbon fiber and kevlar.

Hierarchical materials.

Cellular materials, honeycomb and foam.

This time

Polymers for actuation

“Wet” versus “Dry” actuation

“Wet” – Ionic actuators. Utilize mobility or diffusion of ions. polymer-metal composites conductive polymers others . . .

“Dry” – Electronic actuators. Utilize Coulomb forces. dielectric elastomers electrostrictive polymers others . . .

Polymer-Metal Composites

Ionic Polymer Metal Composites (IPMC) ion exchange polymer membrane – selectively

pass ions of a single charge - Dupont Nafion gold plated electrodes on either side applied voltage induces movement of ions and

water – causes expansion on one side bending movement

Polymer-Metal Composites

performance of IPMC strain: 3% energy density: 0.01-0.1 J/cm3

speed: 100 Hz output pressure: 10-30 MPa drive voltage: 1-2 V

Polymer-Metal Composites

work best in aqueous environments robot fish in tank

EAMEX, Japan

Conductive Polymers

Polypyrrole (PPy)– conductive polymer oxidation-reduction reaction when voltage is

applied redox induces ion flow into or out of polymer flow in = expansion requires electrolyte

Conductive Polymers

performance for PPy bilayer actuator strain: 12.4% energy density: 0.040 J/g speed: <1Hz output pressure: 22 MPa drive voltage: +/- 1V

Conductive Polymers

attach polymer to a unstretchable film (gold) to create unimorph actuator

Electrostricted Polymers

Electrostricted graft elastomers motion achieved through electrostriction applied electric field induces a change from one

polarized direction to another, or one phase to another.

flexible backbone

polarized chain

Electrostricted Polymers

performance strain: 4% energy density: 0.245 J/g speed: 10 kHz output pressure: 22 MPa drive voltage: 2 – 3 KV

Dielectric Elastomers

elastomer film is sandwiched between compliant electrodes

apply electric field: E = V/m Maxwell pressure: p = ee0E2

electrodes squeeze elastomer in thickness

V+apply voltage

Dielectric Elastomers

materials available off-the-shelf 3M VHB acrylic tape various silicone elastomers

desired features high dielectric constant and breakdown strength low elastic modulus – high % elongation thin film

Dielectric Elastomers increase performance through prestrain

stretch elastomer film in one planar direction fix motion in prestrained direction allow expansion in other planar direction during

activation

V+

1 2 3 4

dielectric elastomer

electrode

rigid constraints

Dielectric Elastomers

performance strain: >200% energy density: 0.75 – 3.4 J/cm3

speed: 10Hz - 20kHz output pressure: 3.0 – 7.2 MPa drive voltage: 5kV

Dielectric Elastomers

morphologies planar actuators

• butterfly/bowtie unimorphs/bimorphs rolls bellows/speakers

Actuator Comparison

Actuator Work

Actuator Power