Biomedical Engineering Reference
In-Depth Information
by.using.closed-loop,.feedback.control.of.the.actuator.to.reach.a.command.position,.but.
the.actuator.will.be.slowed.down.by.the.need.to.“hunt”.for.the.command.position.based.
on.the.feedback.signal..his.decreases.the.bandwidth.of.systems.that.use.these.actuators.
•.
Parameter calibration, drit over time.
. For. go-to. operation,. it. would. be. helpful. to. have.
calibrated. parameters. that. describe. the. response. of. the. actuator. to. an. applied. stimulus.
(voltage,.current,.temperature).that.do.not.drit.over.time..Unfortunately,.the.parameters.
that.relate.stimulus.and.response,.such.as.the.piezoelectric.
d
.coeicients,.can.change.over.
time.with.changes.in.temperature.and.depoling.
•.
Feedback.
.To.control.an.actuator.in.a.closed-loop.feedback.system,.an.error.signal.must.
be.generated.that.can.be.fed.back.to.the.control.system..Positions.of.the.actuators.can.be.
measured.with.capacitive.sensors.or.strain.gauges.and.fed.back.to.the.control.system..For.
wavefront.correction,.the.wavefront.error.can.be.measured.with.a.wavefront.sensor.such.
as.the.Shack-Hartmann.sensor.and.the.deviance.of.the.wavefront.from.a.reference.wave-
front.fed.back.to.control.the.actuators.on.a.DM.to.drive.the.error.to.zero.
•.
Linearity.
.A.linear.system.is.desirable.to.avoid.efects.such.as.pull-in.for.electrostatic.actu-
ators,.where.the.nonlinear.electrostatic.force.overwhelms.the.linear.mechanical.restoring.
force,. causing. the. actuator. plates. to. pull. together.. When. the. plates. touch,. they. become.
stuck.together..For.actuators.with.microscopic.dimensions,.surface.adhesion.forces.such.
as. van. der. Waals. attraction. can. dominate. the. mechanical. restoring. force,. leading. to.
“stiction.”
•.
Push-pull (bidirectional actuation).
. Generally,. it. is. helpful. to. have. bidirectional. actua-
tion. that. can. both. push. and. pull. the. mirror. facesheet.. Some. actuation. mechanisms.
such. as. magnetic. forces. can. generate. bidirectional. forces. by. reversing. the. current. in. a.
coil,. whereas. other. actuation. mechanisms. can. generate. only. attractive. forces. such. as.
.electrostatic.actuation.under.voltage.control..When.only.unidirectional.forces.can.be.gen-
erated,. .bidirectional. actuation. can. be. accomplished. by. operation. around. a. bias. condi-
tion..For.electrostatic.actuation,.a.bias.voltage.is.applied.to.the.actuators.and.bidirectional.
actuation.is.achieved.by.increasing.or.decreasing.the.applied.voltage.around.the.bias.point.
•.
Temperature sensitivity.
.Some.actuation.mechanisms,.such.as.piezoelectric.actuation,.are.
sensitive.to.temperature..At.low.temperatures,.the.piezoelectric.gain.
d
.decreases,.requir-
ing.a.higher.voltage.to.achieve.the.same.displacement..At.high.temperatures,.the.piezo-
electric.material.become.depoled,.again.decreasing.the.piezoelectric.gain.coeicient..Once.
a.piezoelectric.material.has.become.depoled.at.a.high.temperature.(i.e.,.above.the.Curie.
temperature),.it.can.be.repoled.by.applying.a.poling.bias.
•.
High voltage or drive current.
.If.the.required.actuation.voltage.for.an.electrostatic.actuator.is.too.
high,.it.can.exceed.the.breakdown.voltage.in.air.or.vacuum,.leading.to.catastrophic.failure.for.the.
actuator..Typically.the.actuation.voltage.will.be.limited.by.the.drive.electronics.to.a.safe.value..
Similarly,.if.the.drive.current.in.a.magnetic.or.thermal.actuator.is.too.high,.it.can.lead.to.failure.
mechanisms.associated.with.high.power.dissipation.
•.
Weight.
.For.DMs.that.are.mounted.on.actuated.mounts.such.as.tip-tilt.stages,.the.weight.of.the.
mirror.must.be.comprehended.in.the.tip-tilt.mount..Inertial.efects.can.limit.the.actuation.band-
width.for.massive.mirrors.
•.
he response time.
.he.response.time.of.the.mirror.determines.the.bandwidth,.the.highest.tempo-
ral.frequency.that.can.be.controlled.
•.
Environment.
.Environmental.efects.such.as.temperature.and.relative.humidity.can.be.important.
to.the.lifetime.of.a.DM..Failure.mechanisms.such.as.electrical.breakdown.can.be.accelerated.under.
extreme. environmental. conditions.. Corrosion. efects. can. also. be. accelerated. by. high. applied.
voltages.in.high.relative.humidity.where.electrochemical.efects.can.occur.at.interfaces.between.
diferent. materials. such. as. gold. and. doped. polysilicon,. which. are. commonly. used. materials. in.
