5 V Compatible Two-Axis PZT Driven MEMS Scanning Mirror with Mechanical Leverage Structure

13 Pages·2017·8.34 MB·English

sensors
Article
5 V Compatible Two-Axis PZT Driven MEMS
Scanning Mirror with Mechanical Leverage Structure
for Miniature LiDAR Application
LiangchenYe,GaofeiZhang*andZhengYou*
StateKeyLaboratoryofPrecisionMeasurementTechnologyandInstruments,
DepartmentofPrecisionInstrument,TsinghuaUniversity,Beijing100084,China;[email protected]
* Correspondence:[email protected](G.Z.);[email protected](Z.Y.);
Tel.:[phone](G.Z.&Z.Y.)
AcademicEditors:MicheleMagnoandIlkerDemirkol
Received:9February2017;Accepted:2March2017;Published:5March2017
Abstract: TheMEMS(Micro-ElectronicalMechanicalSystem)scanningmirrorisanopticalMEMS
device that can scan laser beams across one or two dimensions. MEMS scanning mirrors can be
appliedinavarietyofapplications,suchaslaserdisplay,bio-medicalimagingandLightDetection
and Ranging (LiDAR). These commercial applications have recently created a great demand for
low-driving-voltageandlow-powerMEMSmirrors. However,noreportedtwo-axisMEMSscanning
mirrorisavailableforusageinauniversalsupplyingvoltagesuchas5V.Inthispaper,wepresentan
ultra-lowvoltagedriventwo-axisMEMSscanningmirrorwhichis5Vcompatible. Inordertorealize
lowvoltageandlowpower,atwo-axisMEMSscanningmirrorwithmechanicalleveragedrivenby
PZT(Leadzirconatetitanate)ceramicisdesigned,modeled,fabricatedandcharacterized. Tofurther
decreasethepoweroftheMEMSscanningmirror,anewmethodofimpedancematchingforPZT
ceramicdrivenbyatwo-frequencymixedsignalisestablished. Asexperimentalresultsshow,this
MEMSscanningmirrorreachesatwo-axisscanningangleof41.9◦ ×40.3◦ atatotaldrivingvoltage
of4.2Vppandtotalpowerof16mW.Theeffectivediameterofreflectionofthemirroris2mmand
theoperatingfrequenciesoftwo-axisscanningare947.51Hzand1464.66Hz,respectively.
Keywords: 5Vcompatible;MEMSscanningmirror;piezoelectric;PZTceramic;mechanicalleverage;
impedancematching
1. Introduction
MEMSscanningmirrorshavebeenusedinmanyapplicationsincludingconfocalmicroscopy[1,2],
biomedical imaging [3,4], head-up displays [5] and Light Detection and Ranging (LiDAR) [6–9].
Nowadays,theseapplications,especiallyLiDARforunmanneddrivingorunmannedaerialvehicle
(UAV),haverecentlycreatedagreatdemandforlow-cost,low-dissipationandlow-weightedtwo-axis
MEMS scanning mirrors. With the development of MEMS (Micro-Electro-Mechanical System)
technology,theMEMSscanningmirror’sadvantagesmakeitprobablythemostsuitablelaserbeam
scannerforLiDAR.TheMEMSscannerhastheadvantagesofhighoperatingfrequency,low-weight
andsmallvolumeofpackage. However,mostMEMSscannershaveahighoperatingvoltageorhigh
powerwhichmakesthemincapableofbeinguniversallyintegratedinLiDAR.
Efforts have been made to decrease MEMS scanning mirrors’ driving voltage and power
consumption. Avarietyofactuationsanddrivingstructureshavebeendemonstrated. Electrostatic
actuatorshavethecharacteristicofextremelyhighdrivingvoltagebutlowpower. Althoughusingthe
wafer-levelvacuumpackagingtechnology,atotaldrivingvoltageof70Visstillneededtoactuatea
two-axisMEMSscanningmirrorforanelectricalMEMSscannerwithaFOV(Fieldofview)of60◦
Sensors2017,17,521;doi:10.3390/s17030521 www.mdpi.com/journal/sensors
Sensors2017,17,521 2of13
and70◦ respectively[10]. Moreover,thepowerofahigh-voltagedrivingcircuitismuchhigherthan
thepowerofthemirror. ElectromagneticMEMSscanningmirrorsareexcitedbycurrentandhavea
lowdrivingvoltage. However,thiskindofactuatorhashigherpowerconsumption,forexample,one
hundredtosomehundredsmilliwatts[11,12].
Piezoelectricactuatorshavetheadvantageofyieldingahighforcewithasmallerdrivingvoltage
comparedtootherdrivingactuators. BulkPZTsareespeciallysuitablefortheMEMSscanningmirror
forasimplefabricationprocessandfastresponse. Chen[13]presentedaMEMSscanningmirrorwith
aY-shapedpiezoelectricactuatorforprojectorapplication. Thepowerconsumptionofthescanner
is13.4mWatadrivingvoltageof10Vppineachaxis,whiletheFOVis27.6◦ and39.9◦ respectively.
The operating frequencies of two-axis scanning are 560 Hz and 25 kHz, but the diameter of the
mirror plate is 1 mm which is too small to reflect the measuring laser beam in long-range LiDAR
applications[14]. TheMEMSscanningmirrorwithPZThasthepotentialforlowervoltageandpower
consumption. However,no5Vcompatible2DMEMSscanningmirrorwithalargeFOVhasbeen
reported yet. Five volts is the most universal voltage in current commercial systems. The boost
DC/DCconverterisnolongerneededwitha5VcompatibleMEMSscanningmirror. Furthermore,
high-voltagecircuitsfordrivingwilloccupymorestaticpowerincircuitthanlow-voltagecircuits
because the static driving current of the operational amplifier (OP) goes higher when the driving
voltageishigher.
Inthiswork,wepresentanewpiezoelectricMEMSscanningmirrorwithmechanicalleverageto
decreasethedrivingvoltageandweobtaineda5VcompatibleMEMSscanningmirrorwithalarge
FOV.AnimpedancematchingmethodofPZTceramicisestablishedtodecreasethereactivepowerof
PZTceramic. Astheresultsshow,inordertoactuateatwo-axisMEMSscanmirrorwithscanning
anglesof41.9◦ ×40.3◦,atotalvoltageof4.2Vppandtotalpowerof16mWareachieved.
2. DesignandModelofTwo-AxisMEMSScanner
Figure1showsthesketchoftheMEMSscanningmirror. ThedevicecomprisesaPZTceramic
actuator,aleveragestructureandaSi-basedMEMSstructure. TheMEMSscanningmirrorconsistsof
amirrorbase,aflexiblebeamandamirrorplatewhichiscoatedwithAufilmtoincreasethereflective
coefficientofthereflector. Thedevices(seeFigure1a)containtwoPZTceramicstowhichadriving
voltageisappliedwitha180◦phaselagwitheachother. ThePZTceramiccanrealizeaZ-axisvibration
atoneendoftheceramicwhenapplyinganACdrivingvoltage. TheamplitudeofPZTceramic’s
vibrationissmall(aboutsomemicrometers)whiletheoutputforce(somehundredNewtons)istoo
largetoactuatethisMEMSdevice,whichhasasmallmomentofinertia. Toimprovetheefficiency
ofthePZTactuator,theleveragestructureisutilizedtomagnifytheamplitudeofthePZTceramic.
ASi-basedMEMSstructureisattachedattheendofleverageandthedisplacementamplitudeofthe
mirrorbaseisincreasedmanymoretimesthantheoutputamplitudeofPZTceramicitself. Twomodes
oftheMEMSscanningmirror’svibrationareusedtoscanthelaserbeam. Thefirstmodeisthetwisting
modeinwhichthemirrorplaterotatesalongtheflexiblebeam(Y-axis). Thebendingmodeisanother
scantypewhiletherotationalaxisisparalleltotheX-axis.
Figure 2 shows the three DOF (degree of freedom) vibrating model for the MEMS mirror.
The terms θ , θ and θ in Figure 1a are the rotational displacement of the three sub-structures.
L X Y
The first sub-structure contains two PZT ceramics and a leverage: I is the equivalent moment of
1
inertiaofthefirstsub-structure,k andc arethestiffnessanddampingcoefficientsofthetwoPZT
11 11
ceramicsystem,andk andc arethestiffnessanddampingcoefficientsoftheleverage.Inthismodel,
12 12
onlythetwistingandbendingmodeoftheMEMSmirrorareconsidered. Thesecondsub-structure
containstheflexiblebeamandmirrorplate:θ ,k ,c andI arethemechanicalangle,torsionalstiffness,
Y 2 2 2
dampingcoefficientandmassmomentofinertiaofthetwisting-moderotationalmodel. Thethird
sub-structurecontainstheflexiblebeamandmirrorplate: θ ,k ,c and I arethemechanicalangle,
X 3 3 3
bendingstiffness,dampingcoefficientandmassmomentofinertiaofthebending-moderotational
model. Theflexiblebeamcantwistandbendatthesametime.
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and 70° respectively [10]. Moreover, the power of a high-voltage driving circuit is much higher than
the power of the mirror. Electromagnetic MEMS scanning mirrors are excited by current and have a
low driving voltage. However, this kind of actuator has higher power consumption, for example, one
hundred to some hundreds milliwatts [11,12].
Piezoelectric actuators have the advantage of yielding a high force with a smaller driving voltage
compared to other driving actuators. Bulk PZTs are especially suitable for the MEMS scanning mirror
for a simple fabrication process and fast response. Chen [13] presented a MEMS scanning mirror with
a Y-shaped piezoelectric actuator for projector application. The power consumption of the scanner is
13.4 mW at a driving voltage of 10 Vpp in each axis, while the FOV is 27.6° and 39.9° respectively.
The operating frequencies of two-axis scanning are 560 Hz and 25 kHz, but the diameter of the mirror
plate is 1 mm which is too small to reflect the measuring laser beam in long-range LiDAR
applications [14]. The MEMS scanning mirror with PZT has the potential for lower voltage and power
consumption. However, no 5 V compatible 2D MEMS scanning mirror with a large FOV has been
reported yet. Five volts is the most universal voltage in current commercial systems. The boost
DC/DC converter is no longer needed with a 5 V compatible MEMS scanning mirror. Furthermore,
high-voltage circuits for driving will occupy more static power in circuit than low-voltage circuits
because the static driving current of the operational amplifier (OP) goes higher when the driving
voltage is higher.
In this work, we present a new piezoelectric MEMS scanning mirror with mechanical leverage
to decrease the driving voltage and we obtained a 5 V compatible MEMS scanning mirror with a large
FOV. An impedance matching method of PZT ceramic is established to decrease the reactive power
of PZT ceramic. As the results show, in order to actuate a two-axis MEMS scan mirror with scanning
angles of 41.9° × 40.3°, a total voltage of 4.2 Vpp and total power of 16 mW are achieved.
2. Design and Model of Two-Axis MEMS Scanner
Figure 1 shows the sketch of the MEMS scanning mirror. The device comprises a PZT ceramic
actuator, a leverage structure and a Si-based MEMS structure. The MEMS scanning mirror consists
of a mirror base, a flexible beam and a mirror plate which is coated with Au film to increase the
reflective coefficient of the reflector. The devices (see Figure 1a) contain two PZT ceramics to which
a driving voltage is applied with a 180° phase lag with each other. The PZT ceramic can realize a
Z-axis vibration at one end of the ceramic when applying an AC driving voltage. The amplitude of
PZT ceramic’s vibration is small (about some micrometers) while the output force (some hundred
Newtons) is too large to actuate this MEMS device, which has a small moment of inertia. To improve
the efficiency of the PZT actuator, the leverage structure is utilized to magnify the amplitude of the
PZT ceramic. A Si-based MEMS structure is attached at the end of leverage and the displacement
amplitude of the mirror base is increased many more times than the output amplitude of PZT ceramic
itself. Two modes of the MEMS scanning mirror’s vibration are used to scan the laser beam. The first
mode is the twisting mode in which the mirror plate rotates along the flexible beam (Y-axis). The
Sensors2017,17,521 3of13
bending mode is another scan type while the rotational axis is parallel to the X-axis.
Sensors 2017, 17, 521 3 of 12
Figure 2 shows the three DOF (degree of freedom) vibrating model for the MEMS mirror. The
terms θ , θ and θ in Figure 1a are the rotational displacement of the three sub-structures. The
L X Y
first sub-structure contains two PZT ceramics and a leverage: I is the equivalent moment of inertia
1
of the first sub-structure, k and c are the stiffness and damping coefficients of the two PZT ceramic
11 11
system, and k and c are the stiffness and damping coefficients of the leverage. In this model, only
12 12
the twisting and bending mode of the MEMS mirror are considered. The second sub-structure contains
the flexible beam and mirror plate: θ , k , c and I are the mechanical angle, torsional
Y 2 2 2
stiffness, damping coefficient and mass moment of inertia of the twisting-mode rota tional model. The
third sub-structure contains the( afl)e xible beam and mirror plate(b: )θ , k , c and I are the
X 3 3 3
mechaFnigiucarel 1a. n(ag)l eA, sbkeetncdh ionfg t hset iMffincreos-sE,l edctaromnpicianl gM eccoheafnfiiccaile Snyts teamnd (M mEMasSs) smcaonmnienngt m oirfr oirn werittiha twoof the
Figure1. (a)AsketchoftheMicro-ElectronicalMechanicalSystem(MEMS)scanningmirrorwith
bendiLnega-dm zoirdceo nraottea ttiitoannaalt em (PoZdTe)l .c Terhaem filcesx; i(bbl)e P bareaamme tcearns dtwefiinsitt iaonnds obfe tnhde daet vthicee .s ame time.
twoLeadzirconatetitanate(PZT)ceramics;(b)Parametersdefinitionsofthedevice.

FiguFrige u2r. eM2o.dMeol doef lthoef t3h-eD3O-DF OtwFot-waxoi-sa MxisEMMES MscSanscnainngn imngirmroirr.r or.
The force generated by PZT ceramic can be estimated as:
TheforcegeneratedbyPZTceramiccanbeestimatedas:
A
FFP==nndd33EEP hAPPUU (1()1 )
P 33 P hP
P
where n is the number of the element in the PZT ceramic, U is the applied voltage, and d , E ,
wherenisthenumberoftheelementinthePZTceramic,Uistheappliedvoltage,andd ,E 3,3A ,Ph
33 P P P
A , h are the piezoelectric constant, elastic module, sectional area and height of the PZT ceramic.
arPethePpiezoelectricconstant,elasticmodule,sectionalareaandheightofthePZTceramic.
FFEEMM ((FFiinniittee eelleemmeenntt mmeetthhoodd)) ssiimmuulalattioionn isis aapppplileiedd ttoo ssoollvvee tthhee ccoommpplleexx mmooddeell ooff tthhee MMEEMMSS
ssccaannnniinngg mmiirrrroorr.. TThhee ppaararammeetetersr soof fththee ddeevvicicee aarree lliisstteedd iinn TTaabbllee 11.. FFigiguurree 33aa sshhoowwss tthhee ZZ--aaxxiiss
ddiissppllaacceemmeennt taat tddififfefreernent tpposoistiiotinosn sono nthteh beebnednindgin lgevleervaegrea.g Te.heT lheevelreavgeer asgtreuscttruurcet ucarne cmaangmniafyg nthifey
atmhepalimtupdleit uodf ethoef PthZeTP cZeTracmerica.m Tihc.e Tphaerapmareatemr etyer′ yd(cid:48)edfeinfiense sthteh eddisitsatnancec ebbetewtweeenen ththee ppooinint toonn tthhee
lleevveerraaggee aanndd tthhee ppiivvoott ((OO’’)) aalloonngg tthhee YY’’--aaxxiiss.. TThhee aammpplliittuuddee ooff ddiissppllaacceemmeenntt aatt tthhee eenndd ooff tthhee lelevveerraaggee
((yy′(cid:48)==LL) i)s imsmucuhc hlalragregre trhtahna nthteh eamamplpitliutudde eofo fththe ePPZZTT cecrearammici.c .FFigiguurere 33bb shshoowwss ththee ssimimuulalatetedd ooppttiiccaall
l1l1
ssccaannnniinngg aannggllee ooff tthhee ttwwiissttiinngg mmooddee aanndd bbeennddiinngg mmooddee aass aa ffuunnccttiioonn ooff ddrriivviinngg vvoollttaaggee.. TThhee ooppttiiccaall
ssccaannnniinngg aannggllee ooff ththeed devevicieceis ifso fuorutro tfiov efivtiem teismleasrg learrgthearn ththaen MthEeM MSEscManSn sicnagnmniinrrgo rmdiirrreocrt ldyidrericvtleyn
dbryivtehne bPyZ Tthcee PraZmT icce(rsaemeiFci g(sueree F3icg)u.rFer o3mc).t Fhreocmu rtvhee ocfutrhvee soinf gthlee dsiinregcltel ydidrreicvtelyn dsrciavnenne sr,cathnenoerp, ttihcael
ospcatincnali nsgcaannngilnegb eacnogmlee bselacorgmeresa slatrhgeedr raisv itnhge vdorlitvaignegi nvcorletaagsees iwnchriecahseiss pwrohpicohr tiiso nparlotpootrhtieoanmalp tloit uthdee
aomfdpilsitpuldacee omf ednitspatlatcheemmeinrtr oart btahsee .mLierrvoerr abgaes’es. fLuenvcetiroangeo’fs afmunpcltifioynin goft haemapmlipfyliitnugd tehoef admispplliatcuedme eonft
dleisapdlsactoemtheentd elecaredass etoi nthder idveincgrevaoselt aing ed.rMivuincgh lvoowlteargde.r iMviuncghv ololtwagere disrniveiendge dvotlotaagceh iiesv neetehdeesda mtoe
ascchainenvien tghaen sgalmese. scanning angles.
Sensors2017,17,521 4of13
Table1.ParametersoftheMEMSscanningmirror.
Parameter Value Parameter Value
L 9mm L 1mm
l1 m1
Ll2 2mm Lm2 4.35mm
Bl 3mm Bt 1mm
hl 0.5mm Bm0 1.85mm
L 2mm B 4.62mm
b m1
Bb 2mm Bm2 1mm
tb 0.3mm tm 0.3mm
Lf 2mm LP 3mm
Bf 0.12mm BP 3mm
Sensors 2017, 17, 521 tf 0.105mm hP 10mm 4 of 12
(a) (b)
Figure 3. (a) Z-axis displacement at different positions on the bending leverage (the middle PZT is at
Figure3. (a)Z-axisdisplacementatdifferentpositionsonthebendingleverage(themiddlePZTis
tahtet hpeopsiotisoitni oonf ofy′y=(cid:48) L=l2L al2nadn tdheth eenedn dofo tfhteh elelveevreargaeg eisi saat ttthhee ppoossiittiioonn ooff yy(cid:48)′==LLl1l1);) ;((bb)) TThhee ooppttiiccaall
ssccaannnniinngg aannggllee rreessppoonnssee ooff tthhee ddeevviiccee iinn ccoommppaarriissoonn wwiitthh tthhee ddeevviiccee ddiirreeccttllyy ddrriivveenn bbyy PPZZTT..
Figure 4a,b shows the frequency response of the optical scanning angle in the twisting and
Figure 4a,b shows the frequency response of the optical scanning angle in the twisting and
bending mode. The normalized frequency in these two figures can be calculated by dividing the
bending mode. The normalized frequency in these two figures can be calculated by dividing the
frequency by the resonant frequency. In these simulations, the damping ratios of the twisting mode
frequencybytheresonantfrequency. Inthesesimulations,thedampingratiosofthetwistingmode
and bending mode are set as 0.00042 and 0.0005 which can be achieved by frequency–domain FSI
andbendingmodearesetas0.00042and0.0005whichcanbeachievedbyfrequency–domainFSI
(fluid–solid interaction) simulation. The FSI simulation can be conducted using frequency–domain
(fluid–solidinteraction)simulation. TheFSIsimulationcanbeconductedusingfrequency–domain
linearized Navier–Stoke and the Solid Mechanics module in COMSOL Multiphysics. The optical
linearized Navier–Stoke and the Solid Mechanics module in COMSOL Multiphysics. The optical
scanning angles at the two resonant frequencies increase as the leverage ratio increases (see
scanninganglesatthetworesonantfrequenciesincreaseastheleverageratioincreases(seeFigure4c)
Figure 4c) and the leverage ratio can be estimated as:
andtheleverageratiocanbeestimatedas:
L
kkr== LLll11 (2()2 )
r Ll2
l2
where L is the distance between the pivot and the end of the leverage and L is the distance
whereL l1isthedistancebetweenthepivotandtheendoftheleverageandL isthel2distancebetween
l1 l2
between the pivot and the attachment between the middle PZT ceramic and the leverage.
thepivotandtheattachmentbetweenthemiddlePZTceramicandtheleverage.
A finite-element modal analysis is performed; the results of the MEMS scanning mirror are
showninFigure5. Thefirstmodalfrequencyis997.2Hz(seeFigure5a)atwhichthescanningmirror
vibratesalongtheflexiblebeam(calledthetwistingmode). Thesecondmodalisin-plainvibration
(seeFigure5b). Atthethirdmodalfrequencyof1408.4Hz(seeFigure5c),thescannerrotatesalongthe
X-axis(calledthebendingmode). Thefourthmodalismuchhigherthanthefrequencyofbending
modevibrationatwhichthemirrorshiftsalongtheZ-axis(seeFigure5d). Twovibrationmodesare
utilizedforscanninginourdevice: thefirstmode(ortwistingmode)andthirdmode(orbending
(a) (b)
Figure 4. Cont.
Sensors 2017, 17, 521 4 of 12
(a) (b)
Figure 3. (a) Z-axis displacement at different positions on the bending leverage (the middle PZT is at
the position of y′=L and the end of the leverage is at the position of y′=L ); (b) The optical
l2 l1
scanning angle response of the device in comparison with the device directly driven by PZT.
Figure 4a,b shows the frequency response of the optical scanning angle in the twisting and
Sebnseonrsd 2i0n1g7, 1m7,o 5d21e . The normalized frequency in these two figures can be calculated by dividi5n ogf 1t2h e
frequency by the resonant frequency. In these simulations, the damping ratios of the twisting mode
and bending mode are set as 0.00042 and 0.0005 which can be achieved by frequency–domain FSI
(fluid–solid interaction) simulation. The FSI simulation can be conducted using frequency–domain
linearized Navier–Stoke and the Solid Mechanics module in COMSOL Multiphysics. The optical
scanning angles at the two resonant frequencies increase as the leverage ratio increases (see
Figure 4c) and the leverage ratio can be estimated as:
Sensors2017,17,521 5of13
L
k = l1 (2)
r L
l2
mode). Whenapplyingatwo-frequencymixeddrivingsignalbetweenthetwoelectrodesofthePZT
wcehrearme icL,lt1h eisr etflhee cdteisdtamnicrer obrertowtaeteens tahloen pgivthoet flanexdi btlheeb eenadm oafn dthXe -laexviesraatgteh aensdam Lel2ti mise thtoe adcihsiteavnecea
btwetow-aexenis tshcea np.ivot and the attachment between the middle PZT ceramic and the leverage.
(c)
Figure 4. Analysis results from FEM simulation: (a) Frequency response of the optical scanning angle
in the twisting mode; (b) Frequency response of the optical scanning angle in the bending mode;
(c) The relationship between the optical scanning angle at resonant frequency and the leverage ratio.
Table 1. Parameters of the MEMS scanning mirror.
Parameter Value Parameter Value
L 9 mm L 1 mm
l1 m1
L 2 mm L 4.35 mm
l2 m2
B 3 mm B 1 mm
l t
Sensors 2017, 17, 521 h 0.5 mm B 1.85 mm 5 of 12
(a) l m0 (b)
L 2 mm B 4.62 mm
b m1
B 2 mFmig ure 4. CoBnt. 1 mm
b m2
t 0.3 mm t 0.3 mm
b m
L 2 mm L 3 mm
f P
B 0.12 mm B 3 mm
f P
t 0.105 mm h 10 mm
f P
A finite-element modal analysis is performed; the results of the MEMS scanning mirror are
shown in Figure 5. The first modal frequency is 997.2 Hz (see Figure 5a) at which the scanning mirror

vibrates along the flexible beam (called the twisting mode). The second modal is in-plain vibration
(see Figure 5b). At the third modal frequency of 1408.4 Hz (see Figure 5c), the scanner rotates along
the X-axis (called the bending mode). The fourth modal is much higher than the frequency of bending
(c)
mode vibration at which the mirror shifts along the Z-axis (see Figure 5d). Two vibration modes are
utilizedFF iifggouurrr ees c44a.. nAAnnniaanllgyyss iiissn r roeessuuurll ttdss effrrvooimmce FF: EEthMMe ssfiiimmrsuutll aamttiiooonnd::e (( aa())o FFrr rteewqquuiseetnniccnyyg rr eemssppooodnness)ee aoonff dtthh eeth ooipprdttiicc maall ossccdaaenn nn(oiinnrgg b aaennnggdlleei ng
mode).ii nnW tthhheeen tt wwapiisspttiilnnyggi nmmgoo add etew;; ((obb-))f rFFerrqeeqquuueeennncccyyy mrreeissxppeoodnn dsseer ioovffi ntthhgee s ooigppnttiiaccaal llb ssecctaawnnenneiinnngg t haanneg gtllwee oiinn e ttlhheceet rbboeednnddeisinn oggf mmthooedd Pee;;Z T
cerami(c(cc,) )t ThThhee er rereeflllaaettciiotoenndssh hmiippi rbbreeottwrw reeoeentna ttthheese oaopplottiinccagall tsshccaean nfnnleiinnxgigb aalneng gblleeea aamtt rre eassnoondna aXnnt-ta ffrxreeiqsqu uaeetn ntcchyyea ansnaddmt thehe etl ilemevveeer rataogg eaer crahatitioeiov.. e a
two-axis scan.
Table 1. Parameters of the MEMS scanning mirror.
Parameter Value Parameter Value
L 9 mm L 1 mm
l1 m1
L 2 mm L 4.35 mm
l2 m2
B 3 mm B 1 mm
l t
h 0.5 mm B 1.85 mm
l m0
L 2 mm B 4.62 mm
b m1
B 2 mm B 1 mm
b m2
(a) t 0.3 mm t 0.(3b m) m
b m
Lf F2i gFmuigmrue r5e. 5C.oCnto.n tL. P 3 mm
B 0.12 mm B 3 mm
f P
t 0.105 mm h 10 mm
f P
A finite-element modal analysis is performed; the results of the MEMS scanning mirror are
shown in Figure 5. The first modal frequency is 997.2 Hz (see Figure 5a) at which the scanning mirror
vibrates along the flexible beam (called the twisting mode). The second modal is in-plain vibration
(see Figure 5b). At the third modal frequency of 1408.4 Hz (see Figure 5c), the scanner rotates along
the X-axis (called the bending mode). The fourth modal is much higher than the frequency of bending
mode vibration at which the mirror shifts along the Z-axis (see Figure 5d). Two vibration modes are
utilized for scanning in our device: the first mode (or twisting mode) and third mode (or bending
mode). When applying a two-frequency mixed driving signal between the two electrodes of the PZT
ceramic, the reflected mirror rotates along the flexible beam and X-axis at the same time to achieve a
two-axis scan.

(a) (b)
Figure 5. Cont.
Sensors2017,17,521 6of13
Sensors 2017, 17, 521 6 of 12

(c) (d)
Figure 5. Modal analysis results from finite-element simulation. (a) Twisting mode; (b) In-plane
Figure 5. Modal analysis results from finite-element simulation. (a) Twisting mode; (b) In-plane
vibration model; (c) Bending mode; (d) Shifting model.
vibrationmodel;(c)Bendingmode;(d)Shiftingmodel.
3. Impedance Matching for Power Supply of MEMS Scanner
3. ImpedanceMatchingforPowerSupplyofMEMSScanner
There are some other problems when applying PZT ceramics in the MEMS scanning mirror. The
There are some other problems when applying PZT ceramics in the MEMS scanning mirror.
power of this actuator is high in some applications in relation to its large capacity which will result
The power of this actuator is high in some applications in relation to its large capacity which will
in high wattless power. Impedance matching of the ultrasonic transducer working in ultrasonic
resultinhighwattlesspower. Impedancematchingoftheultrasonictransducerworkinginultrasonic
resonant frequency has been investigated [15]. In this chapter, impedance matching for PZT ceramic
resonantfrequencyhasbeeninvestigated[15]. Inthischapter,impedancematchingforPZTceramic
operating in two low mixed frequencies is modeled for a two-axis MEMS scanner.
operatingintwolowmixedfrequenciesismodeledforatwo-axisMEMSscanner.
The circuit model of PZT ceramic can be established using Mason’s Equivalent circuit (see
The circuit model of PZT ceramic can be established using Mason’s Equivalent circuit
Figure 6a). In this model, C represents the capacitance of the piezoelectric material and represents
(seeFigure6a). Inthismode0l,C0representsthecapacitanceofthepiezoelectricmaterialandrepresents
dielectric losses within the ceramics and can be neglected. The dynamic vibration of PZT ceramic is
dielectriclosseswithintheceramicsandcanbeneglected. ThedynamicvibrationofPZTceramicis
ddesecsrcirbibeded bbyy ddyynnaammicic iinndduuccttoorr ((LL11)),,d dyynnaammicicc acappacaictiyty( C(1C)1a)n adnmd emcheacnhiacnailcdails dsiipsasitpioantiorens irsetasinsctaen(cRe1 ) .
(ARco).u Astciocursatdicia rtaiodniaotifoPnZ oTf cPeZrTam ceicraismriecp irse rseepnrteesdenbyterde sbiys traensciset(aRnce). (R ).
1 L L
TThhe emmooddeel looff PPZZTT cceerraammiicc ccaann bbee ssiimmppllifiifeieddb byyt hthee“ C“PC−−RRP””m modoedle,la,s assh sohwonwinn iFnig Fuirgeu6rbe .6Tbh. e
P P
Tphaer paallreallclealp caacpitayci(tCyP ()Cand) apnadr apllaerlarlelesli srteasnisctean(RceP )(cRan) bceanca blceu claaltceudlabtyeds oblyv isnoglvthinege tqhuea etiqouna:tion:
P P
1 1 1 1 11 1 1 1 1
Z =Z1/=(1iω/(Ciω0)C+)+iωiωLL1++11//((iiωωCC1))++RR+1R+R=L1/=(iω1C/(i)ω+CRP) + RP (3)( 3)
0 1 1 1 L P P
whereZZistheimpedanceofthecircuitinFigure6a.
where is the impedance of the circuit in Figure 6a.
We measured the impedance of PZT ceramic using a precision impedance analyzer (Agilent
We measured the impedance of PZT ceramic using a precision impedance analyzer (Agilent
E4980,AgilentTechnologies,SantaClara,CA,USA).Astheimpedanceresultsshow(seeFigure6c),
E4980, Agilent Technologies, Santa Clara, CA, USA). As the impedance results show (see Figure 6c),
theparallelcapacity(C )ofPZTceramiconlychangesabout1.5%from950Hzto1450Hz. Thechange
the parallel capacity (C P) of PZT ceramic only changes about 1.5% from 950 Hz to 1450 Hz. The change
ofparallelresistance(PR )ismuchlargerandmuchmoresensitivetofrequencybecausemechanical
of parallel resistance (R P) is much larger and much more sensitive to frequency because mechanical
vibration-basedmechaPnicaldissipationresistance(R )changesalotasthefrequencychanges.
1
vibration-based mechanical dissipation resistance (R ) changes a lot as the frequency changes.
PZTceramicofNAC2002-H12isusedinourM1 EMSscanningmirror. ThisPZTceramichasabout
1.8micro-faradscapacity(impedanceof88.4ohmsatafrequencyof1kHz)andsomehundredsohms
parallelresistor. Reactivepowerismorethan25timeslargerthanvalidpower. Toacquiremorevalid
powerconsumptionfromthepowersupplement,impedancematchingmustbeconsidered. Thegoal
istolowerthereactivepowerconsumptionwhichiscausedbyparallelcapacity(C ). Amatching
P
parallelinductor(L )isintroducedinparallelwithPZTceramic(seeFigure6d).
s
Inthisapplication,twomixedfrequencyexcitationsareappliedandtheamplitudeofthetwo
excitations is U and U while the frequency is f andf . The total power consumption before
1 2 1 2
impedancematchingis:
(cid:115)
1(cid:113) U2 U2
S =U I = U2+U2 (2πU f C )2+ 1 +(2πU f C )2+ 2 (4)
eff eff 2 1 2 1 1 P R2 2 2 P R2
P1 P2
whereR andR areparallelresistancesat f and f respectively.
P1 P2 1 2

(a) (b)
Figure 6. Cont.
Sensors 2017, 17, 521 6 of 12

(c) (d)
Figure 5. Modal analysis results from finite-element simulation. (a) Twisting mode; (b) In-plane
Sveinbsrorast2io01n7 ,m17o,d52e1l; (c) Bending mode; (d) Shifting model. 7of13
3. Impedance Matching for Power Supply of MEMS Scanner
Thetotalpowerconsumptionaftercompensationcanbecalculatedas:
There are some other proble(cid:115)ms when applying PZT ceramics in the MEMS scanning mirror. The
1(cid:113) U U2 U U2
power So(cid:48)f= thUis(cid:48) acI(cid:48)tua=tor is Uhi2g+h Uin2 so(m2πeU apfpClic−ations1 in) r2e+latio1n+ to( 2itπsU lafrgCe c−apaci2ty w)2h+ich 2will( 5r)esult
in high wattelfefsse fpf ow2er. Im1 ped2ance m1at1chPing 2oπff 1tLhse ultrRa2Ps1onic tran2s2duPcer 2wπfo2rLksing iRn 2Pu2ltrasonic
resonant Tfrheeqbueesntcoyp thimasi zbaeteionn iinnvdeuscttiagnacteedo f[1th5e].p Ianr athlleisl icnhdaupctteorr, (iLmp)ecadnanbceeg menaetrcahteindgb yfocra PlcZulTa tcinergamic
C
operathtienmg iinnim twumo lvoawlu emoifxEedqu farteiqonue(4n)c.iTeos fiisn mdtohdeelolecdal feoxrt rae mtwuom-,awxies tMakEeMonSe sdciaffnenreenrt.i aloftheabove
eTqhuea tcioirnc(u∂iSt (cid:48)/m∂oLd)aenl dosf ePttZhTis dceifrfaermenicti aclaenq ubaeti oensttaobzleisrhoe(sde euEsqinuga tiMona(s6o)n).’s Equivalent circuit (see
Figure 6a). In this model, C represents the capacitance of the piezoelectric material and represents
dielectric losses∂ Sw(cid:48)it=hin (cid:113)thUe(cid:114) 12c0e+raUm22[i(c2sπ aUn1df 1cCaPn− be2π nUf11eLgsl)eUfc11te+d(. 2TπhUe2 df2yCnpa−m2icπU fv22Libs)rUaf22ti]on =of0 PZT cera(6m)ic is
d(Resc)r.i bAecdo ubsyt idc y∂rLna0damiatiic4o πinnL do20fu Pc(tZ2oπTr U (c1eLfr11aC)m,P di−cy in2sπa Urfm1e1Lpiscr) e2cs+aepnRaUt2Pc1e21idt+y b ((y2C πre1Us)2i saf2tnaCdn0 c−me e(2πcRUhf22aL)ns.) i2c a+l dRU2Pi222ssipation resistance
1 L
The Wmeocdaenl aocfh PieZveT tcheeroapmtiimc aclainn dbuec tsaimncpelbifyiesdol vbiyn gththee “aCbov−eRequ”a tmioond.eTlh, eaos psthimowalnin idnu cFtiagnucree 6b.
P P
whenminimumpowerconsumptioncanbeachievedis:
The parallel capacity (C ) and parallel resistance (R ) can be calculated by solving the equation:
P P
U2/f2+U2/f2
Z1 =1/(i1ωC )+iωLLC =+14/π(1i2ωC1CP1()U+12R+2U+22R2) =1/(iω1C )+ R1 (7) (3)
0 1 1 1 L P P
In fact, inductors are not ideal components and have internal resistance. Their circuit model
Z
where is the impedance of the circuit in Figure 6a.
containsaninductorandaseries-woundresistor(seeFigure7).Figure7illustratesthesimulatedpower
We measured the impedance of PZT ceramic using a precision impedance analyzer (Agilent
consumptionoftheimpedancematchingcircuitwithavariableseries-woundresistor. Thepower
E4980, Agilent Technologies, Santa Clara, CA, USA). As the impedance results show (see Figure 6c),
consumption of the PZT ceramic alone is 29.7 mW. The minimum power is 12.2 mW, 13.9 mW,
the p1a6r.a5llmelW caapnadci2ty1. 9(CmW) owf hPeZnTt hceersaemrieisc- wonoluyn cdhraensgisetso ra(bRou)ti s10.5Ω%, f1r0omΩ, 92500Ω H,azn tdo 51045Ω0 rHeszp. eTchtiev eclhy.ange
P s
of paIrnaclloeml rpeasriisstoannwcei t(hRthe) pios wmeruochft hlaerMgeErM aSndsc amnunicnhg mmoirrreo rswenitshitoiuvtei mtop ferdeaqnuceenmcyat cbheicnagu,sthe empoecwhearnical
P
with impedance matching can be reduced to 12.2 mW which is about three times less than PZT
vibration-based mechanical dissipation resistance (R ) changes a lot as the frequency changes.
ceramicalone. 1

(a) (b)
Figure 6. Cont.
Figure6.Cont.
Sensors2017,17,521 8of13
Sensors 2017, 17, 521 7 of 12
Sensors 2017, 17, 521 8 of 12

(c) (d)
power consumption of the PZT ceramic alone is 29.7 mW. The minimum power is 12.2 mW, 13.9 mW,
1F6i.g5u mreW 6 .a n(ad) 2M1.a9s monW’s weqhueinv tahleen ste rciiercs-uwito oufn dP ZreTs icsetorarm (Ric; )( bis) 0S Ωim, 1p0li fΩie, d2 0e Ωqu, iavnadle 5n0t Ωci rrceuspite cotfi vPeZlyT.
Figure 6. (a) Mason’s equivalent circuit of PZT ceramic;s(b) Simplified equivalent circuit of PZT
Icne rcaomcmeirpca;ma (rciic)s ;oM(nce) aMwseuitarhseu dtrh epdea prpaaolrlwaellle ecrla cpoafap catichtiyety aManndEd MrreeSssi isssttcooarrn aantt iddniifgffef ermerneirntrftor ferrq ewuqeuintehcnioecusi;et( sdi;m) (Sdpc)he Sedcmahnaetcmiec admtiiacag tdrcaihamignorgaf,m t hoef
pthoew iemthr epweimidthpane idcmaen pmceeadmtacnahtcicneh gimn mgatemcthhetiohndog.d .can be reduced to 12.2 mW which is about three times less than
PZT ceramic alone.
PZT ceramic of NAC2002-H12 is used in our MEMS scanning mirror. This PZT ceramic has
about 1.8 micro-farads capacity (impedance of 88.4 ohms at a frequency of 1 kHz) and some hundreds
ohms parallel resistor. Reactive power is more than 25 times larger than valid power. To acquire more
valid power consumption from the power supplement, impedance matching must be considered.
The goal is to lower the reactive power consumption which is caused by parallel capacity (C ). A
P
matching parallel inductor (L ) is introduced in parallel with PZT ceramic (see Figure 6d).
s
In this application, two mixed frequency excitations are applied and the amplitude of the two
excitations is U and U while the frequency is f and f . The total power consumption before
1 2 1 2
impedance matching is:
S =U I = 1 U2+U2 （2πU fC ）2+U12 +（2πU f C ）2+ U22 (4)
eff eff 2 1 2 1 1 P R2 2 2 P R2
P1 P2
where R and R are parallel resistances at f and f respectively.
P1 P2 1 2
The total power consumption after compensation can be calculated as:

FFiigguuSrre′e =77..U VV′aarrIii′aabb=llee1 ssiimmUuu2ll+aattUeedd2 pp（oo2wwπeeUrr ccfooCnnss-uummUpp1ttiioo）nn2ss+ vvUeerr12ssuu+ss（ cc2ooπmmUppeefnnCssaa-ttiioonnUaa2ll iinn）dd2+uuccUttaa22nncc ee.. (5)
eff eff 2 1 2 1 1 P 2πf L R2 2 2 P 2πf L R2
1 s P1 2 s P2
44.. FFaabbrriiccaattiioonn aanndd AAsssseemmbbllee
The best optimization inductance of the parallel inductor (L ) can be generated by calculating
C
TThhee MMEEMMSS ssccaannnniningg mmirirroror rwwasa sprpordoudcuecde dusuinsign gthteh ebublku lkMEMMESM fSabfraibcaritcioanti otnechtencihqnuieqsu. eAs.
the minimum value of Equation (4). To find the local extremum, we take one differential of the above
equapAtiieopznioe e(zl∂oeeSclter'/icc∂t rLaicn)ga alneng dple osspeiotti stoihntii oss nednissfefonersr oeisrn itinisateiln getrqeaguteraadtt ieoodnn ottnhoe zt hfelereoxfl ie(bsxleeibe bl eEeaqbmueaa omtino ontnh (e6t h)S)ei.- bS ai-sbeads esdtrustcrtuucrteu troe
mtoemaseuarseu trheet ahnegalne gploespitoiosnit ioofn boofthb oatxhisa [x1i6s].[ 1T6w].oT PwZoT PcZerTamceircasm, ai ccsir,cauicti rbcouairtdb oanardd aa TnidtaaniTuimta nailulomy
balelaomy baeraem galureedg luuesdinugs i3nMg 3iMnstiannstt aandthaedshivesei vgUelugelu CeUAC4A04H0 H(M(MininnenseostoUat aMMiniUniningg aanndd MMaannuuffaaccttuurriinngg
U2 +U2（[ 2πU fC - 1 ） 1 +（2πU f C - 2 ） 2 ]
CCoommppaannyy,, SStt.. PPaauul∂l,,S MM′ NN,, UUSS1AA)) tt2oo ffoorrmm1 tt1hheeP bb2aaπsseef Looff tthhfee MMEEMMSS2 ss2ccaapnnnn2eeπrr f((ssLeeee FFfiigguurree 88aa)).. TThhee SSii--bbaasseedd
mmiirrrroorr iiss tthheenn gglluu∂Leedd= ttoo tthhee aaccttuuaattoorr uussiUinngg CC1AAs44U002HH1.. TThhee eennttiirree ssUttrruu2ccttsuurreeU 2ii2ss =aass0ss eemmbblleedd iinnttoo aann (6)
AAlluummiinnuumm--aallllooyy ppaa0cckkaag4gπee Lw2w（iitthh2 πaaU ggfllCaasss-s wwiinn1ddoo）ww2+ oonn1 +iitt（ tt2ooπ UpprroofttCeecc-tt tthhee2 sstt）rruu2+ccttuu2rree ffrroomm ddaammaaggee aanndd
0 1 1 P 2πfL R2 2 2 0 2πf L R2
ddiissrruuppttiioonn ooff aairirflflooww. .DDrirviivnign gvovltoalgtaeg ies aisppapl1iepsdli etod tthPo1e tdhreivder liivnee loinn ethoen 2PtZshTe cPeZrPaT2mciecr. aFmigiucr.eF 8ibg ushreow8bs
tshheo wcosmthpelectoedm apslesetemdbalsys oemf tbhley socfanthneinsgca dnenvinicge.d evice.
We can achieve the optimal inductance by solving the above equation. The optimal inductance
when minimum power consumption can be achieved is:
U2 / f 2 +U2 / f 2
L = 1 1 2 2 (7)
C 4π2C (U2 +U2)
P 1 2
In fact, inductors are not ideal components and have internal resistance. Their circuit model
contains an inductor and a series-wound resistor (see Figure 7). Figure 7 illustrates the simulated
power consumption of the impedance matching circuit with a variable series-wound resistor. The

(a) (b)
Figure 8. (a) Assembly of the MEMS scanning mirror and (b) Package of the MEMS scanning mirror.
Sensors 2017, 17, 521 8 of 12
power consumption of the PZT ceramic alone is 29.7 mW. The minimum power is 12.2 mW, 13.9 mW,
16.5 mW and 21.9 mW when the series-wound resistor (R ) is 0 Ω, 10 Ω, 20 Ω, and 50 Ω respectively.
s
In comparison with the power of the MEMS scanning mirror without impedance matching, the
power with impedance matching can be reduced to 12.2 mW which is about three times less than
PZT ceramic alone.

Figure 7. Variable simulated power consumptions versus compensational inductance.
4. Fabrication and Assemble
The MEMS scanning mirror was produced using the bulk MEMS fabrication techniques. A
piezoelectric angle position sensor is integrated on the flexible beam on the Si-based structure to
measure the angle position of both axis [16]. Two PZT ceramics, a circuit board and a Titanium alloy
beam are glued using 3M instant adhesive glue CA40H (Minnesota Mining and Manufacturing
Company, St. Paul, MN, USA) to form the base of the MEMS scanner (see Figure 8a). The Si-based
mirror is then glued to the actuator using CA40H. The entire structure is assembled into an
Aluminum-alloy package with a glass window on it to protect the structure from damage and
Sensors20d1is7r,u1p7,ti5o2n1 of airflow. Driving voltage is applied to the drive line on the PZT ceramic. Figure 8b shows 9of13
the completed assembly of the scanning device.
(a) (b)
Figure 8. (a) Assembly of the MEMS scanning mirror and (b) Package of the MEMS scanning mirror.
Figure8.(a)AssemblyoftheMEMSscanningmirrorand(b)PackageoftheMEMSscanningmirror.

5. Measurement
Sensors 2017, 17, 521 9 of 12
Inthischapter,featuresoftheMEMSscanningmirrorwereexperimented. ResonantFrequency
5. Measurement
andopticalscananglewerethekeycharacteristicsoftheMEMSscanningmirror.Thesetwoparameters
In this chapter, features of the MEMS scanning mirror were experimented. Resonant Frequency
weremeasuredinthissection. Methodofimpedancematchingwasalsorealizedinthischapterand
and optical scan angle were the key characteristics of the MEMS scanning mirror. These two
thepopwarearmoeftetrhse wMerEe MmeSassucraendn iinn tghims siercrtoiornw. Masetmhoeda osuf irmedp.edance matching was also realized in this
chapter and the power of the MEMS scanning mirror was measured.
5.1. FrequencyandOpticalScanningAngleTest
5.1. Frequency and Optical Scanning Angle Test
Figure9showstheexperimentalsetupformeasuringthescanninganglesoftheMEMSscanning
Figure 9 shows the experimental setup for measuring the scanning angles of the MEMS scanning
mirror. AredlaseremittedtowardstheMEMSscanningmirrorwasreflectedbytheresonantmirror
mirror. A red laser emitted towards the MEMS scanning mirror was reflected by the resonant mirror
andthenthelaserbeamwasreflectedtothescreen. Theopticalscanninganglecouldbecalculated
and then the laser beam was reflected to the screen. The optical scanning angle could be calculated
fromthelengthofthelaserlineonthescreenandthedistancebetweentheMEMSscanningmirror
from the length of the laser line on the screen and the distance between the MEMS scanning mirror
andthaends cthree esncr.eeTnh. eThder idvriinvigngv vooltltaaggee aapppplileiedd bebtewtewene etwnot wpooleps oolfe tsheo fPZthTe cPerZamTicc ewraasm ai cmwixtausrea mixture
frequefrnecqyuesnicgyn asilgannald atnhde tfhree qfrueqeunecnieciseso foft wtwoos sigignnaall wweerree ff a anndd f fre srepsepcetcitviveellyy.. TThhee ddrriivviningg voltage
11 22
wasgevnoletargaete wdabs ygeanderaattaeda bcqy uai dsiattiao naccqaurisditi(oNn IcUarSdB (N-6I2 U16S,BN-62a1t6io, nNaaltiIonnsatlr Iunmstreunmtse,nAts,u Astuisnti,nT, XTX,U, SA)and
USA) and then amplified by an operational amplifier (OP).
thenamplifiedbyanoperationalamplifier(OP).
(a) (b)
Figure 9. (a) Diagram of the experimental setup; (b) The experimental setup.
Figure9.(a)Diagramoftheexperimentalsetup;(b)Theexperimentalsetup.
Figure 10a illustrates the relationship between the optical scanning angles and driving frequency
Fmigeuarsuer1e0da biyll uLasstrear tDesopthpleerr eVlaeltoiocinmsehtirpy b(LeDtwV)e. eTnhteh aemopplittiucdael socfa vninbriantgioann agt ltehse amndargdirni voifn tghef requency
measumrierdrorb pylaLtea swears DtraonpspmliettredV etol othciem sceatnrnyin(gL DanVgl)e. uTshineg athme pgleiotumdetericoafl vreilbartiaotnisohnipa. tThthe edrmivainrgg in of the
frequencies of the two rotational axes were 947.51 Hz and 1464.66 Hz and the Q values were 1289
mirrorplatewastransmittedtothescanningangleusingthegeometricalrelationship. Thedriving
and 1046 respectively. The frequency band widths of the two-axis MEMS scanning mirror were so
frequenciesofthetworotationalaxeswere947.51Hzand1464.66HzandtheQvalueswere1289and
narrow that the cross-coupling between the twisting and bending mode could be neglected.
1046rFeisgpuercet 1iv0be lsyh.oTwhse thfer erqeluateinoncsyhbipa bnedtwweeidn tthhse oopfttihcael stwcanon-ainxgis anMgEleM anSd sdcraivninnign vgolmtaigrer oinr twhee trweos onarrow
rotational axes. As introduced in chapter 2, the device contained two PZT ceramics to which two
different driving voltages were applied with a 180° phase lag with each other. When measuring the
optical scanning angle of one rotational axis, the driving voltage at the resonant frequency of this
mode was applied. The twisting mode of the device was able to achieve a 41.9° optical angle with a
driving voltage of 2 Vpp. To achieve a 41.2° optical angle in the bending mode, the driving voltage was
2.2 Vpp.
When driving the device using a two-frequency mixed signal, the MEMS scanning mirror could
project a two-dimensional pattern (see Figure 11). The driving frequencies of the two rotational axes
were 947.51 Hz and 1464.66 Hz, and the optical scanning angles were 41.9° and 40.3°. It was
unavoidable that the rotation of the two axes affect each other and a decrease in the optical scanning
angle was observed in comparison with the result in Figure 10b.
Sensors2017,17,521 10of13
thatthecross-couplingbetweenthetwistingandbendingmodecouldbeneglected. Figure10bshows
therelationshipbetweentheopticalscanningangleanddrivingvoltageinthetworotationalaxes.
Asintroducedinchapter2,thedevicecontainedtwoPZTceramicstowhichtwodifferentdriving
voltageswereappliedwitha180◦ phaselagwitheachother. Whenmeasuringtheopticalscanning
angleofonerotationalaxis,thedrivingvoltageattheresonantfrequencyofthismodewasapplied.
Thetwistingmodeofthedevicewasabletoachievea41.9◦ opticalanglewithadrivingvoltageof
Senso2rsV 2p01p7., T17o, 5a2c1h i evea41.2◦ opticalangleinthebendingmode,thedrivingvoltagewas2.2Vpp. 10 of 12
Sensors 2017, 17, 521 10 of 12
(a) (b)
Figure 10. (a) Frequency spectrum of the optical scanning angles of the MEMS scanning mirror;
Figure10.(a)FrequencyspectrumoftheopticalscanninganglesoftheMEMSscanningmirror;(b)the
(b) the optical scanning angles versus the applied variable voltage in the two rotation axes.
opticalscanninganglesversustheappliedvariablevoltageinthetworotationaxes.
Whendrivingthedeviceusingatwo-frequencymixedsignal,theMEMSscanningmirrorcould
project a two-dimensional pattern (see Figure 11). The driving frequencies of the two rotational
axeswere947.51Hzand1(4a6)4 .66Hz,andtheopticalscanningangles(wb)ere41.9◦ and40.3◦. Itwas
unavoidablethattherotationofthetwoaxesaffecteachotherandadecreaseintheopticalscanning
Figure 10. (a) Frequency spectrum of the optical scanning angles of the MEMS scanning mirror;
anglewasobservedincomparisonwiththeresultinFigure10b.
(b) the optical scanning angles versus the applied variable voltage in the two rotation axes.

Figure 11. A two-dimensional pattern scanned by the MEMS scanning mirror.
5.2. Results of Impedance Matching

The efficiency of impedance matching was validated in this section. A power measurement
Figure 11. A two-dimensional pattern scanned by the MEMS scanning mirror.
circuit (see Figure F1i2gau)r we1a1s. aAptpwloie-ddi mtoe nmsieoansaulrpea tttheren psocawnnere dobf yththe eMMEEMMSS ssccaannnnininggm mirriorrr.or alone or with
the impedance matching circuit. The driving current was measured by measuring the voltage of a
5.2. Results of Impedance Matching
sample resistor when the device operated at its resonant frequency. The driving voltage and current
The efficiency of impedance matching was validated in this section. A power measurement
were monitored by a data acquisition device (NI USB-6216) and the power consumption was
circuit (see Figure 12a) was applied to measure the power of the MEMS scanning mirror alone or with
calculated by multiplying the effective value of the voltage and the effective value of the current in a
the impedance matching circuit. The driving current was measured by measuring the voltage of a
LabVIEW programme according to Equation (4) (see Figure 12b).
sample resistor when the device operated at its resonant frequency. The driving voltage and current
were monitored by a data acquisition device (NI USB-6216) and the power consumption was
calculated by multiplying the effective value of the voltage and the effective value of the current in a
LabVIEW programme according to Equation (4) (see Figure 12b).