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Actuators:
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Dielectric
elastomer expands when electric field is applied across
it in thickness. The gray area depicts compliant electrodes. |
Planar
actuators made from thin films of silicone and acrylic elastomers
have demonstrated linear strains of 63% and 215%, respectively,
and energy densities five times that of human muscle [1].
However, certain requirements must be met in order to achieve
this motion. First, very large electric fields around 100
MV/m must be used: With off-the-shelf material layer thickness,
voltages around 5kV are required. Second, the elastomers must
be pre-strained prior to activation. Specifically, when high
pre-strain is applied in one planar direction, expansion occurs
primarily in the other planar direction, and at a much greater
magnitude than without pre-strain. The flexonic context offers
potential solutions to both difficulties
Various
planar actuator frames
The
overall design of the actuator frame must fill two roles.
First, it must have sufficient compliance to allow the elastomer
to expand in the desired direction of motion. Second, the
frame should contain built-in geometry necessary to produce
and maintain a pre-strain perpendicular to the direction
of motion. Possible designs are shown below.
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At the
left, the frame is shown prior to applying pre-strain. Secondary
flexures (oval-shaped) not coupled with the dielectric elastomer
can be "set" into a pre-strained state. The right shows the
secondary flexures extended and locked in place. The middle
portion of the dielectric material (bowtie shaped) would be
stretched parallel with the secondary flexures. In addition
to providing pre-strain, these flexures provide the unidirectional
rigid support perpendicular to the desired direction of motion.
Note that the flexible ends of the bowtie remain in the same
configuration following pre-strain, thereby allowing linear
expansion of the dielectric elastomer.
back
[1]
Pelrine R, Kornbluh R, Pei Q, Joseph J (2000) High-speed electrically
actuated elastomers with strain greater than 100%. Science
287(5454):836-839
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