Biomedical Engineering Reference
In-Depth Information
Alternatively,. the. edge. of. the. mirror. can. be. supported. by. actuators.. he. actuators. can.
either.be.lexible,.like.springs,.or.rigid,.like.posts..he.spring.support.geometry.is.similar.
to.box.springs.on.a.mattress.
•.
Actuator type: .here.are.diferent.means.for.actuating.the.mirror.surface.
•.
Piezoelectric actuators (lead zirconium titanate, PbTiO 3 [PZT]) ..An.applied.voltage.causes.a.
mechanical.strain.that.causes.a.linear.change.in.the.length.of.the.crystal..he.piezoelectric.
d .coeicients.relate.the.strain.produced.to.the.electric.ield.that.is.applied..A.typical.value.
would.be. d .=.390.×.10 −12 .m/V.to. d .=.650.×.10 −12 .m/V,.or.approximately.a.few.angstroms/
volt..Since.the.motion.is.so.small,.piezoelectric.crystals.are.typically.connected.electrically.
in.parallel.and.stacked.together.so.that.their.individual.displacements.accumulate..In.this.
manner,.it.is.possible.to.get.±10.μm.for.an.applied.voltage.of.±100-200.V..hese.materials.
are.temperature.sensitive..he.piezoelectric. d .coeicients.decrease.at.low.temperature.and.
they.can.become.“depoled”.above.the.Curie.temperature.
•.
Electrostrictive actuators (lead magnesium niobate [PMN]) .. An. applied. voltage. causes. a.
quadratic.increase.in.the.length.of.the.crystal..If.the.voltage.is.reversed,.the.crystal.still.
lengthens.. To. increase. and. decrease. the. length. of. the. crystal—to. push. and. pull. on. a.
facesheet—the.actuator.can.be.operated.around.a.bias.voltage.
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Electrostatic actuators. .An.applied.voltage.induces.charge.between.two.conducting.plates,.
giving.rise.to.attractive.Coulomb.force.between.the.negative.charges.on.one.plate.and.the.
positive.charges.on.the.other.plate..If.one.plate.is.ixed.and.the.other.one.is.free.to.move,.
the.released.plate.will.be.drawn.toward.the.ixed.plate..he.attractive.force.between.the.
plates.is.nonlinear..It.varies.with.the.square.of.the.applied.voltage.and.inversely.with.the.
square.of.the.plate.separation..To.keep.the.plates.from.being.drawn.together.and.touch-
ing,.a.linear.spring.mechanical.restoring.force.is.typically.used..Nonetheless,.the.nonlin-
ear.attractive.electrostatic.force.will.eventually.overcome.the.linear.mechanical.restoring.
force,. and. the. plates. will. be. pulled. in. or. drawn. together.. he. typical. operating. range. is.
one-third.of.the.original.gap.before.pull-in.occurs..he.range.can.be.increased.by.using.
nonlinear.springs.or.leveraging.efects.
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Magnetic actuators .. A. magnetic. ield. from. a. current. passing. through. a. coil. (i.e.,. a. voice.
coil). interacts. with. the. magnetic. ield. of. a. permanent. magnet,. generating. an. attractive.
force.when.the.poles.are.opposite.and.a.repulsive.force.when.they.are.aligned..he.align-
ment.of.the.electromagnetically.generated.ield.can.be.reversed.by.reversing.the.direction.
of.the.current.low..Since.current.must.low.through.the.coil.to.generate.the.electromag-
netic.ield,.there.is.constant.power.dissipation.when.actuated.
•.
hermal actuators ..hermal.expansion.of.a.material.when.it.is.heated.is.used.for.actuation..
Typically. thermal. actuators. are. able. to. generate. a. large. force,. but. they. dissipate. a. lot. of.
power.owing.to.the.heating.
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Bimorph (piezo, thermal). . A. bimorph. actuator. uses. the. diferential. expansion. between.
two.materials..he.diferential.expansion.can.be.caused.by.heating,.where.the.diference.
in.thermal.expansion.coeicients.of.the.two.materials.causes.them.to.expand.at.diferent.
rates..he.diferential.expansion.can.also.be.caused.by.the.piezoelectric.efect,.where.one.
piezoelectric.material.is.caused.to.expand.and.the.other.piezoelectric.material,.bonded.to.
the.irst,.is.caused.to.contract.with.an.applied.voltage.
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Actuator characteristics ..here.are.a.number.of.characteristics.required.by.the.actuators.
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Repeatability (hysteresis, go-to command). .Piezoelectric.actuators.exhibit.a.hysteresis.efect..
Where.they.go.depends.on.the.history.of.where.they.have.been..Typically.they.exhibit.a.
hysteresis.loop.that.describes.the.displacement.as.a.function.of.applied.voltage..Hysteresis.
makes. open-loop,. go-to. command. positioning. diicult,. since. the. position. the. actuator.
goes.to.depends.on.where.the.actuator.has.been.previously..his.problem.can.be.overcome.
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