Biomedical Engineering Reference
In-Depth Information
8.6.7 electrostatically Actuated Segmented Facesheet
Microelectromechanical System Mirrors
Segmented. mirrors. have. small. segments. that. can. be. independently. actuated,. typically. in. a. piston.
mode,.with.motion.limited.to.translation.along.the.normal.to.the.mirror.surface,.or.in.piston.tip-tilt.
mode,.where.the.displacement.along.both.the.normal.(piston).and.the.slope.(tip-tilt).within.the.plane.
of.the.segment.are.controlled..Typically.segmented.mirrors.require.higher.order.(i.e.,.more.actuators).
to.make.the.same.amount.of.wavefront.correction.as.a.continuous.facesheet.mirror,.as.shown.by.the.
itting.error.coeicients.in.
Table.8.1
.
(Hardy.1998)..Since.the.mirror.segments.are.independent,.they.do.
not.inluence.their.neighbors.when.they.are.actuated..On.the.other.hand,.since.the.segments.can.move.
independently,.there.can.be.a.large.jump.in.the.phase.between.independent.actuators..An.additional.
challenge.is.the.space.between.the.mirror.segments..Unless.they.are.closely.spaced,.the.ill.factor.can.
be.limited..Typical.ill.factors.of.leading.segmented.mirrors.are.from.98.to.99%.
here. are. currently. two. manufactures. of. segmented. MEMS. mirrors.. Boston. Micromachines.
ofers.a.segmented,.piston-only.MEMS.DM.with.1020.actuators,.1.5.μm.of.stroke.with.actuators.on.
a.pitch.of.300.μm..he.segments.have.a.response.time.that.is.less.than.20.μs..hey.also.ofer.a.low-
latency.driver.for.this.mirror.with.a.34.kHz.frame.rate..Iris.AO.ofers.segmented.mirrors.with.163.
hexagonal.piston-tip.segments,.350.μm.on.a.side,.with.a.pitch.of.600.μm.from.center.to.center..Each.
tip-tilt.segment.has.three.actuators.for.a.total.of.489.actuators..he.stroke.is.5-8.μm..hey.also.ofer.
smaller.DMs.as.described.in.Table.8.4..he.larger.stroke.is.made.possible.by.liting.the.mirror.seg-
ments.out.of.plane.on.actuator.platforms.using.temperature-insensitive.bimorph.lexures,.as.shown.
in.Figure.8.33.
TABlE 8.4
Iris.AO.Segmented.Deformable.Mirror.Speciications
Minimum.
Frequency.
Response.(kHz)
Number.of.
Actuators
Number.of.
Segments
Maximum.
Stroke.(μm)
Maximum.Tilt.
Angle.(mrad)
Aperture.
(mm)
Model
PTT111-5
111
37
5
±5
2
3.5
PTT111-8
111
37
8
±8
2
3.5
PTT489-5
489
163
5
±5
2
7.7
PTT489-8
489
163
8
±8
2
7.7
Each.mirror.segment.has.three.actuators.to.provide.tip-tilt.actuation.
Rigid
high-quality
mirror
segment
Bondsites
Actuator
platform
Electrodes
Temperature
insensitive
bimorph
flexure
FIGuRE 8.33
Segmented.deformable.mirror.from.Iris.AO..(Credit:.Michael.Helmbrecht,.Iris.AO,.CA.)
