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

sensors Article

5 V Compatible Two-Axis PZT Driven MEMS Scanning Mirror with Mechanical Leverage Structure for Miniature LiDAR Application Liangchen Ye, Gaofei Zhang * and Zheng You * State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084, China; [email protected] * Correspondence: [email protected] (G.Z.); [email protected] (Z.Y.); Tel.: +86-10-6277-6000 (G.Z. & Z.Y.) Academic Editors: Michele Magno and Ilker Demirkol Received: 9 February 2017; Accepted: 2 March 2017; Published: 5 March 2017

Abstract: The MEMS (Micro-Electronical Mechanical System) scanning mirror is an optical MEMS device that can scan laser beams across one or two dimensions. MEMS scanning mirrors can be applied in a variety of applications, such as laser display, bio-medical imaging and Light Detection and Ranging (LiDAR). These commercial applications have recently created a great demand for low-driving-voltage and low-power MEMS mirrors. However, no reported two-axis MEMS scanning mirror is available for usage in a universal supplying voltage such as 5 V. In this paper, we present an ultra-low voltage driven two-axis MEMS scanning mirror which is 5 V compatible. In order to realize low voltage and low power, a two-axis MEMS scanning mirror with mechanical leverage driven by PZT (Lead zirconate titanate) ceramic is designed, modeled, fabricated and characterized. To further decrease the power of the MEMS scanning mirror, a new method of impedance matching for PZT ceramic driven by a two-frequency mixed signal is established. As experimental results show, this MEMS scanning mirror reaches a two-axis scanning angle of 41.9◦ × 40.3◦ at a total driving voltage of 4.2 Vpp and total power of 16 mW. The effective diameter of reflection of the mirror is 2 mm and the operating frequencies of two-axis scanning are 947.51 Hz and 1464.66 Hz, respectively. Keywords: 5 V compatible; MEMS scanning mirror; piezoelectric; PZT ceramic; mechanical leverage; impedance matching

1. Introduction MEMS scanning mirrors have been used in many applications including confocal microscopy [1,2], biomedical imaging [3,4], head-up displays [5] and Light Detection and Ranging (LiDAR) [6–9]. Nowadays, these applications, especially LiDAR for unmanned driving or unmanned aerial vehicle (UAV), have recently created a great demand for low-cost, low-dissipation and low-weighted two-axis MEMS scanning mirrors. With the development of MEMS (Micro-Electro-Mechanical System) technology, the MEMS scanning mirror’s advantages make it probably the most suitable laser beam scanner for LiDAR. The MEMS scanner has the advantages of high operating frequency, low-weight and small volume of package. However, most MEMS scanners have a high operating voltage or high power which makes them incapable of being universally integrated in LiDAR. Efforts have been made to decrease MEMS scanning mirrors’ driving voltage and power consumption. A variety of actuations and driving structures have been demonstrated. Electrostatic actuators have the characteristic of extremely high driving voltage but low power. Although using the wafer-level vacuum packaging technology, a total driving voltage of 70 V is still needed to actuate a two-axis MEMS scanning mirror for an electrical MEMS scanner with a FOV (Field of view) of 60◦

Sensors 2017, 17, 521; doi:10.3390/s17030521

www.mdpi.com/journal/sensors

Sensors 2017, 17, 521

2 of 13

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 bending mode is another scan type while the rotational axis is parallel to the X-axis. Figure 2 shows the three DOF (degree of freedom) vibrating model for the MEMS mirror. The terms θ L , θ X and θY in Figure 1a are the rotational displacement of the three sub-structures. The first sub-structure contains two PZT ceramics and a leverage: I1 is the equivalent moment of inertia of the first sub-structure, k11 and c11 are the stiffness and damping coefficients of the two PZT ceramic system, and k12 and c12 are the stiffness and damping coefficients of the leverage. In this model, only the twisting and bending mode of the MEMS mirror are considered. The second sub-structure contains the flexible beam and mirror plate: θY , k2 , c2 and I2 are the mechanical angle, torsional stiffness, damping coefficient and mass moment of inertia of the twisting-mode rotational model. The third sub-structure contains the flexible beam and mirror plate: θ X , k3 , c3 and I3 are the mechanical angle, bending stiffness, damping coefficient and mass moment of inertia of the bending-mode rotational model. The flexible beam can twist and bend at the same time.

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 2017, is the mode in which the mirror plate rotates along the flexible beam (Y-axis).3 of The Sensors 17, twisting 521 13 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 L , θ X and θ Y in Figure 1a are the rotational displacement of the three sub-structures. The first sub-structure contains two PZT ceramics and a leverage: I 1 is the equivalent moment of inertia of the first sub-structure, k 1 1 and c 1 1 are the stiffness and damping coefficients of the two PZT ceramic system, and k 12 and c 1 2 are the stiffness and damping coefficients of the leverage. In this model, only the twisting and bending mode of the MEMS mirror are considered. The second sub-structure contains the flexible beam and mirror plate: θ Y , k 2 , c 2 and I 2 are the mechanical angle, torsional stiffness, damping coefficient and mass moment of inertia of the twisting-mode rotational model. The (b)θ X , k 3 , c 3 and I 3 are the third sub-structure contains the(a) flexible beam and mirror plate: mechanical bendingthe stiffness, dampingMechanical coefficient and(MEMS) mass scanning momentmirror of inertia of the Figure 1. 1.angle, (a) System withwith two Figure (a) A A sketch sketchof of theMicro-Electronical Micro-Electronical Mechanical System (MEMS) scanning mirror bending-mode rotational model. The flexible beam can twist and bend at the same time. Lead zirconate titanate (PZT) ceramics; (b) Parameters definitions of the device. two Lead zirconate titanate (PZT) ceramics; (b) Parameters definitions of the device.

Figure 2. Model of the 3-DOF two-axis MEMS scanning mirror. Figure 2. Model of the 3-DOF two-axis MEMS scanning mirror.

The Theforce force generated generated by by PZT PZT ceramic ceramic can can be be estimated estimated as: as: AP FP = nd33 EP A U FP = nd33 EPhP P U hP

(1) (1)

where n is the number of the element in the PZT ceramic, U is the applied voltage, and d33 , E P , where n is the number of the element in the PZT ceramic, U is the applied voltage, and d33 , EP , A P , h P AP , the h P piezoelectric are the piezoelectric elastic sectional module, sectional and ofceramic. the PZT ceramic. are constant,constant, elastic module, area and area height of height the PZT FEM FEM (Finite (Finite element element method) method)simulation simulationisisapplied appliedto tosolve solve the the complex complex model model of of the the MEMS MEMS scanning mirror. The parameters of the device are listed in Table 1. Figure 3a shows the scanning mirror. The parameters of the device are listed in Table 1. Figure 3a shows the Z-axis Z-axis displacement bending leverage. TheThe leverage structure can can magnify the displacementatatdifferent differentpositions positionsononthe the bending leverage. leverage structure magnify 0 amplitude of the ceramic. The The parameter y ′ ydefines the amplitude of PZT the PZT ceramic. parameter definesthe thedistance distancebetween betweenthe thepoint pointon on the the leverage and andthe thepivot pivot(O’) (O’)along alongthe theY’-axis. Y’-axis.The Theamplitude amplitudeof ofdisplacement displacementat atthe theend endof ofthe theleverage leverage leverage ) is much largerthan thanthe theamplitude amplitudeofofthe thePZT PZTceramic. ceramic.Figure Figure3b 3bshows showsthe thesimulated simulatedoptical optical ((yy′0 ==LlL1 )l1is much larger scanning angle of the twisting mode and bending mode as a function of driving voltage. The optical scanning angle of the twisting mode and bending mode as a function of driving voltage. The optical scanning angle angle of to to fivefive times larger thanthan the MEMS scanning mirrormirror directly driven scanning of the thedevice deviceisisfour four times larger the MEMS scanning directly by the PZT ceramic (see Figure 3c). From the curve of the single directly driven scanner, the optical driven by the PZT ceramic (see Figure 3c). From the curve of the single directly driven scanner, the scanning angle becomes larger aslarger the driving which is proportional to the amplitude optical scanning angle becomes as the voltage drivingincreases voltage increases which is proportional to the of displacement at the mirror Leverage’s of amplifying amplitude of amplitude displacement amplitude of displacement at base. the mirror base. function Leverage’s function ofthe amplifying the of leads to the decrease in driving voltage. Much lower driving voltage is needed to achieve the same displacement leads to the decrease in driving voltage. Much lower driving voltage is needed to scanning angles. achieve the same scanning angles.

Sensors 2017, 17, 521

4 of 13

Table 1. Parameters of the MEMS scanning mirror.

Sensors 2017, 17, 521

Parameter

Value

Parameter

Value

Ll1 Ll2 Bl hl Lb Bb tb Lf Bf tf

9 mm 2 mm 3 mm 0.5 mm 2 mm 2 mm 0.3 mm 2 mm 0.12 mm 0.105 mm

Lm1 Lm2 Bt Bm0 Bm1 Bm2 tm LP BP hP

1 mm 4.35 mm 1 mm 1.85 mm 4.62 mm 1 mm 0.3 mm 3 mm 3 mm 10 mm

(a)

4 of 12

(b)

Figure bending leverage (the middle PZT is at Figure 3. 3. (a) (a)Z-axis Z-axisdisplacement displacementatatdifferent differentpositions positionsononthe the bending leverage (the middle PZT is ′ ′ the position of and the end of the leverage is at the position of ); (b) The optical y = L y = L 0 0 l 2 l 1 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. PZT.

Figure Figure 4a,b 4a,b shows shows the the frequency frequency response response of of the the optical optical scanning scanning angle angle in in the the twisting twisting and and bending mode. The normalized frequency in these two figures can be calculated by bending mode. The normalized frequency in these two figures can be calculated by dividing dividing the the frequency by the resonant frequency. In these simulations, the damping ratios of the twisting mode frequency by the resonant frequency. In these simulations, the damping ratios of the twisting mode and achieved by frequency–domain FSI and bending bending mode mode are are set set as as 0.00042 0.00042 and and 0.0005 0.0005 which which can can be be achieved by frequency–domain FSI (fluid–solid interaction) simulation. The FSI simulation can be conducted using frequency–domain (fluid–solid interaction) simulation. The FSI simulation can be conducted using frequency–domain linearized Solid Mechanics Mechanics module module in in COMSOL COMSOL Multiphysics. Multiphysics. The linearized Navier–Stoke Navier–Stoke and and the the Solid The optical optical scanning angles at the two resonant frequencies increase as the leverage ratio increases scanning angles at the two resonant frequencies increase as the leverage ratio increases (see Figure(see 4c) Figure 4c) and the leverage ratio can be estimated as: and the leverage ratio can be estimated as: L1 k r = Lll1 kr = Ll 2 Ll2

(2) (2)

where Ll1 is the distance between the pivot and the end of the leverage and Ll 2 is the distance where Ll1 is the distance between the pivot and the end of the leverage and Ll2 is the distance between between and the attachment thePZT middle PZTand ceramic and the leverage. the pivotthe andpivot the attachment between between the middle ceramic the leverage. 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

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: Sensors 2017, 17, 521

5 of 13

L k r = l1 (2) Ll 2 mode). When applying a two-frequency mixed driving signal between the two electrodes of the PZT the distance between pivot and thebeam end of the leverage is the distancea where Ll 2time ceramic,Llthe mirror rotates the along the flexible and X-axis at theand same to achieve 1 isreflected between pivot and the attachment between the middle PZT ceramic and the leverage. two-axis the scan.

(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.

Sensors 2017, 17, 521

Parameter Ll 1 Ll 2 Bl h (a) l Lb Bb tb Lf Bf t

f

Value Parameter Lm1 9 mm Lm2 2 mm Bt 3 mm B 0.5 mm m0 Bm1 2 mm Figure 4. Cont. Bm 2 2 mm tm 0.3 mm LP 2 mm BP 0.12 mm hP 0.105 mm

Value 1 mm 4.35 mm 1 mm 1.85 mm (b) 4.62 mm 1 mm 0.3 mm 3 mm 3 mm 10 mm

5 of 12

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 (c) 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 Figure 4. Analysis from FEM FEM simulation: Frequency response of the thethird optical scanning angle 4. Analysis results from (a) Frequency of optical scanning angle utilizedFigure for scanning inresults our device: the simulation: first mode(a) (or twistingresponse mode) and mode (or bending in the twisting mode; (b) Frequency response of the optical scanning angle in the bending mode; the twisting mode; (b) Frequencymixed response of thesignal opticalbetween scanningthe angle the bending mode).inWhen applying a two-frequency driving twoinelectrodes of mode; the PZT (c) The The relationship relationship between between the the optical optical scanning scanning angle angle at at resonant resonantfrequency frequencyand andthe theleverage leverageratio. ratio. (c) ceramic, the reflected mirror rotates along the flexible beam and X-axis at the same time to achieve a two-axis scan. Table 1. Parameters of the MEMS scanning mirror.

Parameter Ll 1 Ll 2 Bl hl Lb Bb (a) tb Lf Bf t

f

Value Parameter Lm1 9 mm Lm2 2 mm Bt 3 mm B 0.5 mm m0 Bm1 2 mm Bm 2 2 mm tm 0.3 mm 2 mm 5. Cont. L P Figure Figure 5. Cont. BP 0.12 mm hP 0.105 mm

Value 1 mm 4.35 mm 1 mm 1.85 mm 4.62 mm 1 mm (b) 0.3 mm 3 mm 3 mm 10 mm

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

Sensors 2017, 17, 521

6 of 13

Sensors 2017, 17, 521

6 of 12

(c)

(d)

Figure Figure5.5.Modal Modalanalysis analysisresults resultsfrom fromfinite-element finite-elementsimulation. simulation.(a)(a)Twisting Twistingmode; mode;(b) (b)In-plane In-plane vibration model; (c) Bending mode; (d) Shifting model. vibration model; (c) Bending mode; (d) Shifting model.

3. Impedance Matching for Power Supply of MEMS Scanner 3. Impedance Matching for Power Supply of MEMS Scanner 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 result in high wattless power. Impedance matching of the ultrasonic transducer working in ultrasonic resonant frequency has been investigated [15]. In this chapter, impedance matching for PZT ceramic resonant frequency has been investigated [15]. In this chapter, impedance matching for PZT ceramic operating in two low mixed frequencies is modeled for a two-axis MEMS scanner. operating in two low mixed frequencies is modeled for a two-axis MEMS scanner. 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 0 represents the capacitance of the piezoelectric material and represents (see Figure 6a). In this model, C0 represents the capacitance of the piezoelectric material and represents dielectric dielectriclosses losseswithin withinthe theceramics ceramicsand andcan canbebeneglected. neglected.The Thedynamic dynamicvibration vibrationofofPZT PZTceramic ceramicisis described dynamiccapacity capacity(C( C) 1and ) and mechanical dissipation resistance describedby bydynamic dynamicinductor inductor ((LL11),),dynamic mechanical dissipation resistance (R1 ). 1 ( Acoustic radiation of ceramic PZT ceramic is represented by resistance R 1 ). Acoustic radiation of PZT is represented by resistance (R L ). ( RL ). Themodel modelof ofPZT PZT ceramic ceramic can be asas shown in in Figure 6b. 6b. The ”model, model, shown Figure The be simplified simplifiedby bythe the“C “ PC− −RRP ” P P parallel capacity (CP()Cand parallel resistance (R P )( can by solving the equation: The parallel capacity ) and parallel resistance cancalculated be calculated by solving the equation: R ) be P

P

1 1 1 1 1 1 1 11 1 = = ++ + = = + Z Z1/(1iωC R L1 / (i ω1/ RP 0 )C0 ) iωL 1 1+ / (i ω / (i(iωC R 1R+1R+ ) RPP) + 11/ iω L ω C1 1) )++ CP(iωC L

(3)(3)

where Z Z isisthe 6a.6a. where theimpedance impedanceofofthe thecircuit circuitininFigure Figure We measured the impedance of PZT ceramic usingaaprecision precisionimpedance impedanceanalyzer analyzer(Agilent (Agilent We measured the impedance of PZT ceramic using E4980,Agilent AgilentTechnologies, Technologies,Santa SantaClara, Clara,CA, CA,USA). USA).As Asthe theimpedance impedanceresults resultsshow show(see (seeFigure Figure6c), 6c), E4980, the parallel capacity (C ) of PZT ceramic only changes about 1.5% from 950 Hz to 1450 Hz. The change the parallel capacity ( C P P) of PZT ceramic only changes about 1.5% from 950 Hz to 1450 Hz. The change of parallel resistance (R ) is much larger and much more sensitive to frequency because mechanical of parallel resistance ( R P P) is much larger and much more sensitive to frequency because mechanical vibration-based mechanical dissipation resistance (R1 ) changes a lot as the frequency changes. vibration-based mechanical dissipation resistance ( R 1 ) changes a lot as the frequency changes. 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 (CP ). A matching parallel inductor (Ls ) is introduced in parallel with PZT ceramic (see Figure 6d). In this application, two mixed frequency excitations are applied and the amplitude of the two excitations is U1 and U2 while the frequency is f 1 and f 2 . The total power consumption before impedance matching is: S = Ue f f Ie f f

1 = 2

q

s U21 + U22

(2πU1 f 1 CP )2 +

U12 U22 2+ + ( 2πU f C ) 2 2 P R2P1 R2P2

where R P1 and R P2 are parallel resistances at f 1 and f 2 respectively.

(a)

(b) Figure 6. Cont.

(4)

(c)

(d)

Figure 5. Modal analysis results from finite-element simulation. (a) Twisting mode; (b) In-plane 7 of 13 (c) Bending mode; (d) Shifting model.

Sensors 2017,model; 17, 521 vibration

3. Impedance Matching for Power Supply of MEMS Scanner The total power consumption after compensation can be calculated as: s There are some other in the MEMS scanning mirror. The q problems when applying PZT ceramics 2 2 U U U U 1 2 1 0 0 0 2 1 2 2+ power Sof=this is U high in2 some in)relation its large which result +to(2πU − (2πUapplications Ue factuator )2 + 2will(5) 2 f 2 CP capacity 1 f 1 CP − 1 + U2 f Ie f f = 2 2π f 1 Ls 2π f 2 Ls R2P1 R P2 in high wattless power. Impedance matching of the ultrasonic transducer working in ultrasonic resonant The frequency has been investigated In this inductor chapter, (L impedance matching for PZT ceramic best optimization inductance of[15]. the parallel C ) can be generated by calculating operating in two low mixed frequencies is modeled for a two-axis MEMS scanner. the minimum value of Equation (4). To find the local extremum, we take one differential of the above equation (∂S0/∂L) andofsetPZT this differential equation to zero (seeusing Equation (6)). Equivalent circuit (see The circuit model ceramic can be established Mason’s Figure 6a). In this model, q C 0 represents the capacitance of the piezoelectric material and represents 2

U1

U1

2

U2

U2

0 f 2 C p − 2π fvibration ) f2 ] of PZT ceramic is U1 + U2 [(2πU1 f 1 CP − 2π f Ls ) f + (2πU dielectric losses∂Swithin The2dynamic 2 Ls 1 1 = therceramics and can be neglected. =0 (6) ∂L0 U12 U22 U U described by dynamic inductor ( ), dynamic capacity ( ) and mechanical dissipation resistance L C 2 1 2 2 2 1 4πL (2πU f1 C − ) + ) + + (2πU f C − 0

1 1 P

2 2 0

R2

2π f 1 Ls

R2P2

2π f 2 Ls

P1 ( R 1 ). Acoustic radiation of PZT ceramic is represented by resistance ( RL ).

can achieve optimal inductance by solving the“above equation. Theas optimal ” model, showninductance in Figure 6b. The We model of PZT the ceramic can be simplified by the CP − R P when minimum power consumption can be achieved is: The parallel capacity ( C P ) and parallel resistance ( R P ) can be calculated by solving the equation: 2

2

2

2

U1 /1f 1 + U2 / f 2 1 1 1 1 = + LC = 4π 2 CP (U 2 + U 2 ) = + Z 1 / (i ω C0 ) iω L1 + 1 / (i ω C1 ) +1 R 1 + 2RL 1 / (i ω CP ) RP

(7)

(3)

In fact, inductors are not ideal components and have internal resistance. Their circuit model

Z is the impedance of the circuit in Figure 6a. where contains an inductor and a series-wound resistor (see Figure 7). Figure 7 illustrates the simulated power We measured impedancematching of PZT circuit ceramic using a precision impedance analyzer (Agilent consumption of the the impedance with a variable series-wound resistor. The power E4980, Agilent Technologies, Santa Clara, USA). theminimum impedance results show (see Figure consumption of the PZT ceramic alone CA, is 29.7 mW.As The power is 12.2 mW, 13.9 mW, 6c), the parallel capacity ) of PZTthe ceramic only changes Hz to 50 1450 Hz. The change 16.5 mW and 21.9( C mW when series-wound resistorabout (Rs ) is1.5% 0 Ω, from 10 Ω, 950 20 Ω, and Ω respectively. P In comparison with of the MEMS scanning mirrorsensitive without impedance matching, themechanical power of parallel resistance ( Rthe )power is much larger and much more to frequency because P with impedance matching can be reduced to 12.2 mW which is about three times less than PZT ceramic alone.

vibration-based mechanical dissipation resistance ( R 1 ) changes a lot as the frequency changes.

(a)

(b) Figure Cont. Figure 6. 6. Cont.

Sensors 2017, 17, 521

8 of 13

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, Figure 6.and (a) Mason’s equivalent circuit ceramic; circuit of PZT 16.5 mW mW when the series-wound resistor ( R s (b) )(b) is Simplified 0Simplified Ω, 10 Ω, equivalent 20equivalent Ω, and 50 Ω respectively. Figure 6. 21.9 (a) Mason’s equivalent circuitof of PZT PZT ceramic; circuit of PZT ceramic; (c) Measured parallel capacity atdifferent different frequencies; (d) Schematic diagram of In comparison with the power of the and MEMS scanning mirror without(d) impedance matching, ceramic; (c) Measured parallel capacity andresistor resistor at frequencies; Schematic diagram of the the impedance matching method. power impedance matching thewith impedance matching method.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 P ). A matching parallel inductor ( L s ) is introduced in parallel with PZT ceramic (see Figure 6d). In this application, two mixed frequency excitations are applied and the amplitude of the two excitations is U 1 and U 2 while the frequency is f 1 and f 2 . The total power consumption before impedance matching is: S

2

1

= U eff I eff =

U 12 + U 22 （ 2 π U 1 f1 C P）2 +

2

U1 2

R P1

2

U2

+ （ 2π U 2 f 2 C P）2 +

(4)

2

RP 2

where RP1 and RP 2 are parallel resistances at f 1 and f 2 respectively. The total power consumption after compensation can be calculated as: 2

2

U U U inductance. U 1simulated power consumptions Figure 7.UVariable Variable versus compensational ′ ′ Figure simulated inductance. U + U power + （compensational 2π U f C S ′ =7. fC （ 2 π U consumptions ） +versus ）+ eff I eff = 2

2

2

1

2

2

1

1

P

1

1

2π f1 L s

2

2

2

2

RP1

P

2

2π f 2 L s

2

2

RP 2

(5)

4. Fabrication and Assemble

The best optimization inductance of the parallel inductor ( LC ) can be generated by calculating

MEMS scanning mirror was produced using thebulk bulk MEMS fabrication techniques. The MEMS mirror produced using the MEMS techniques. the minimum value scanning of Equation (4). was To find the local extremum, we take fabrication one differential of the A above A piezoelectric angle position sensor is integrated flexible beam Si-based structure piezoelectric angle position sensor is integrated on on thethe flexible beam on on thethe Si-based structure to equation ( ∂S '/ ∂L ) and set this differential equation to zero (see Equation (6)).

to measure angle position of both ceramics, a circuit a Titanium measure thethe angle position of both axisaxis [16].[16]. TwoTwo PZT PZT ceramics, a circuit boardboard and aand Titanium alloy alloy beam are glued using instant adhesive glue CA40H (Minnesota Mining and beam are glued using 3M 3M instant adhesive glue CA40H (Minnesota Mining and Manufacturing U U U U U + U （2 [ πU f C ） +（2 π U f C ） ] of the 8a). The Si-based Company, St. Paul, USA) to form the base ′ f MEMS scanner L Figure f 2π f L 2 π f(see ∂SMN, = =0 the actuator assembled mirror is then glued to using CA40H. The entire structure is into an (6) ∂L U U U U L U f C U f C 4 π （2 π ） + + （2 π ） + Aluminum-alloy package package with with aa glass glass window window on on it to protect the structure from damage and Aluminum-alloy 2π f L 2π f L R R disruption of ofairflow. airflow.Driving Driving voltage is applied to the drive onPZT the ceramic. PZT ceramic. 8b disruption voltage is applied to the drive line line on the FigureFigure 8b shows the completed assembly of the scanning device. shows the completed assembly of the scanning device. We can achieve the optimal inductance by solving the above equation. The optimal inductance 2

1

2

1

2

1

1

1

2

P

2

1

s

2

p

2

1

s

2

2

2

0

2

0

2

1

1

1

2

1

P

2

2

1

s

2

2

P1

2

0

2

2

2

s

P2

when minimum power consumption can be achieved is: 2

LC =

2

2

U 1 / f1 + U 2 / f 2

2

4 π C P (U 1 + U 2 ) 2

2

2

(7)

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

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 Sensors 2017, 17, 521 of airflow. Driving voltage is applied to the drive line on the PZT ceramic. Figure 8b shows disruption the completed assembly of the scanning device.

(a)

9 of 13

(b)

Figure 8. (a) Assembly of the MEMS scanning mirror and (b) Package of the MEMS scanning mirror.

Figure 8. (a) Assembly of the MEMS scanning mirror and (b) Package of the MEMS scanning mirror.

5. Measurement Sensors 2017, 17, 521

9 of 12

In chapter, features of the MEMS scanning mirror were experimented. Resonant Frequency 5. this Measurement and optical scan angle were the key characteristics of the MEMS scanning mirror. These two parameters In this chapter, features of the MEMS scanning mirror were experimented. Resonant Frequency were measured thisangle section. impedanceofmatching was also realized thistwo chapter and and optical in scan wereMethod the key of characteristics the MEMS scanning mirror. in These the power of the MEMS scanning mirror was measured. parameters were measured in this section. Method of impedance matching was also realized in this chapter and the power of the MEMS scanning mirror was measured.

5.1. Frequency and Optical Scanning Angle Test 5.1. Frequency and Optical Scanning Angle Test

Figure 9 shows the experimental setup for measuring the scanning angles of the MEMS scanning Figure 9 shows the experimental setup for measuring the scanning angles of the MEMS scanning mirror. A red laser emitted towards the MEMS scanning mirror was reflected by the resonant mirror mirror. A red laser emitted towards the MEMS scanning mirror was reflected by the resonant mirror and then the laser beam was reflected to the screen. The optical scanning angle could be calculated and then the laser beam was reflected to the screen. The optical scanning angle could be calculated from the length of the laser line screenand and the distance between the MEMS from the length of the laser lineon onthe the screen the distance between the MEMS scanningscanning mirror mirror and the TheThe driving appliedbetween between poles theceramic PZT ceramic was a mixture andscreen. the screen. drivingvoltage voltage applied twotwo poles of theofPZT was a mixture frequency signal frequenciesofoftwo twosignal signal were were ff1 and and f 2f respectively. respectively. The The driving driving voltage frequency signal andand thethe frequencies voltage was by a data acquisition (NI USB-6216, National Instruments, Austin, TX,USA) and was generated bygenerated a data acquisition card (NIcard USB-6216, National Instruments, Austin, TX, USA) and then amplified by an operational amplifier (OP). then amplified by an operational amplifier (OP). 1

(a)

2

(b)

Figure 9. (a) Diagram of the experimental setup; (b) The experimental setup.

Figure 9. (a) Diagram of the experimental setup; (b) The experimental setup. Figure 10a illustrates the relationship between the optical scanning angles and driving frequency measured Laser Doppler Velocimetry between (LDV). The amplitude of vibration at theand margin of thefrequency Figure 10aby illustrates the relationship the optical scanning angles driving mirror plate was transmitted to the scanning angle using the geometrical relationship. The driving measured by Laser Doppler Velocimetry (LDV). The amplitude of vibration at the margin of the frequencies of the two rotational axes were 947.51 Hz and 1464.66 Hz and the Q values were 1289 mirror plate was transmitted to the scanning angle using the geometrical relationship. The driving and 1046 respectively. The frequency band widths of the two-axis MEMS scanning mirror were so frequencies thethe twocross-coupling rotational axes were 947.51 Hz and Hzmode and the Q values were 1289 and narrowofthat between the twisting and1464.66 bending could be neglected. 1046 respectively. Thethe frequency band widths of the scanning two-axisangle MEMS mirror were Figure 10b shows relationship between the optical andscanning driving voltage in the twoso narrow 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.

Sensors 2017, 17, 521

10 of 13

that the cross-coupling between the twisting and bending mode could be neglected. Figure 10b shows the relationship between the optical scanning angle and driving voltage in the two 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 Sensors 2017, 17, 521 10 of 12 2 Vpp. To achieve a 41.2◦ optical angle in the bending mode, the driving voltage was 2.2 Vpp.

Sensors 2017, 17, 521

10 of 12

(a)

(b)

Figure 10. 10. (a)(a) Frequency of the theoptical optical scanning angles ofMEMS the MEMS scanning Figure Frequencyspectrum spectrum of scanning angles of the scanning mirror; mirror; (b) the (b) the optical scanning angles versus the applied variable voltage in the two rotation axes. optical scanning angles versus the applied variable voltage in the two rotation axes.

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 (b) were 41.9◦ and 40.3◦ . It was (a) unavoidable that the rotation of the two axes affect each other and a decrease in the optical scanning Figure 10. (a) Frequency spectrum of the optical scanning angles of the MEMS scanning mirror; angle was observed in comparison with the result in Figure 10b. (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 Figure 12a) was to measure the power scanningmirror. mirror alone or with 11. applied A two-dimensional pattern scannedofbythe theMEMS MEMS scanning 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 Figure 12b). The driving voltage and current sample resistor when the device operated at(4) its(see resonant frequency.

Sensors 2017, 17, 521

11 of 13

5.2. Results of Impedance Matching The efficiency of impedance matching was validated in this section. A power measurement circuit (see Figure 12a) was applied to measure the power of the MEMS scanning mirror alone or with the impedance matching circuit. The driving current was measured by measuring the voltage of a 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 to Equation (4) (see Figure 12b). Sensors 2017, 17,according 521 11 of 12 Sensors 2017, 17, 521

11 of 12

(a) (a)

(b) (b)

Figure (a) system; (b) devices of the Figure 12. 12. (a)Diagram Diagram of of the the measuring measuring power power circuit (b) Hardware 12. (a) Diagram of the measuring power circuit system; (b) Hardware devices of the measurement system. measurement system. system.

Inductors Inductors with with variable variable values values were were applied applied to to the the impedance impedance matching matching circuit circuit of of the the MEMS MEMS Inductors with variable values were applied to the impedance matching circuit of the MEMS scanning scanningmirror. mirror. The Theabove above measuring measuringsystem system was was used used to tomeasure measure the the power power of of the theMEMS MEMS scanner scanner scanning mirror. The above measuring system was used to measure the power of the MEMS scanner with with or or without without the the impedance impedance matching matching circuit. circuit. The The driving driving voltage voltage of of the the MEMS MEMS scanning scanning mirror mirror with or without the impedance matching circuit. The driving voltage of the MEMS scanning mirror was 2 Vpp at 947.51 Hz, increased by 2.2 Vpp at 1464.66 Hz. The minimum power of the MEMS was 22 Vpp Vpp at at 947.51 947.51 Hz, Hz, increased increased by by 2.2 2.2 Vpp Vpp at at 1464.66 1464.66 Hz. Hz. The The minimum minimum power power of of the the MEMS MEMS was scanning mirror with the impedance matching circuit was 16 mW while itit was 30.4 mW without the scanning mirror with the impedance matching circuit was 16 mW while was 30.4 mW without the scanning mirror with the impedance matching circuit was 16 mW while it was 30.4 mW without the impedance matching circuit (see Figure 13). A total of 47.4% power was saved using the impedance impedance matching circuit (see Figure 13). A total of 47.4% power was saved using the impedance impedance matching circuit (see Figure 13). A total of 47.4% power was saved using the impedance matching matching method. method. matching method.

Figure 13. Power of the MEMS scanning mirror with the impedance matching circuit in comparison Figure Figure 13. 13. Power Power of of the the MEMS MEMS scanning scanning mirror mirror with with the the impedance impedance matching matching circuit circuit in in comparison comparison with simulation results. with simulation with simulation results.

6. 6. Conclusions Conclusions A A 55 V V compatible compatible two-axis two-axis MEMS MEMS scanning scanning mirror mirror with with aa large large FOV FOV is is presented presented in in this this paper paper for the first time. The mechanical leverage was designed to decrease the driving voltage of the for the first time. The mechanical leverage was designed to decrease the driving voltage of theMEMS MEMS scanning scanningmirror. mirror.An Animpedance impedancematching matchingmethod methodfor forPZT PZTceramic ceramicdriven drivenby byaatwo-frequency two-frequencymixed mixed

Sensors 2017, 17, 521

12 of 13

6. Conclusions A 5 V compatible two-axis MEMS scanning mirror with a large FOV is presented in this paper for the first time. The mechanical leverage was designed to decrease the driving voltage of the MEMS scanning mirror. An impedance matching method for PZT ceramic driven by a two-frequency mixed signal was built to decrease the reactive power and total power of the MEMS scanning mirror. Full optical scanning angles of 41.9◦ and 40.3◦ were achieved at a total voltage of 4.2 Vpp for the twisting axis and bending axis respectively. The driving frequencies of the two rotational axes were 947.51 Hz and 1464.66 Hz with a mirror size of 2 mm. The power consumption of the MEMS scanning mirror can be decreased to 16 mW when applying the method of impedance matching. The 5 V compatible MEMS scanning mirror with very low power consumption can broaden the usage of the MEMS scanning mirror in miniature applications. Future work will focus on its application in LiDAR systems. Acknowledgments: The work was supported by the Research Foundation of Tsinghua University. Author Contributions: Liangchen Ye is responsible for the research. Gaofei Zhang and Zheng You gave advice on the design and the experiment. Liangchen Ye performed the experiments and analyzed the data. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2. 3. 4.

5. 6. 7. 8. 9.

10. 11. 12. 13. 14.

Piyawattanametha, W.; Ra, H.; Mandella, M.J.; Loewke, K.; Wang, T.D.; Kino, G.S.; Solgaard, O.; Contag, C.H. 3-D Near-Infrared Fluorescence Imaging Using an MEMS-Based Miniature Dual-Axis Confocal Microscope. IEEE J. Sel. Top. Quant. 2009, 15, 1344–1350. [CrossRef] Gilchrist, K.H.; Dausch, D.E.; Grego, S. Electromechanical performance of piezoelectric scanning mirrors for medical endoscopy. Sens. Actuators A Phys. 2012, 178, 193–201. [CrossRef] [PubMed] Ataman, C.; Urey, H.; Wolter, A. A Fourier transform spectrometer using resonant vertical comb actuators. J. Micromech. Microeng. 2006, 16, 2517. [CrossRef] Vuong, B.; Sun, C.R.; Harduar, M.K.; Mariampillai, A.; Isamoto, K.; Chong, C.H.; Standish, B.A.; Yang, V. 23 kHz MEMS based swept source for optical coherence tomography imaging. In Proceedings of the 2011 Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Boston, MA, USA, 20 August–3 September 2011; pp. 6134–6137. Hedili, M.K.; Freeman, M.O.; Urey, H. In Microstructured head-up display screen for automotive applications. Proc. SPIE 2012, 8428, 84280X. Kasturi, A.; Milanovic, V.; Atwood, B.H.; Yang, J. UAV-Borne LiDAR with MEMS Mirror-Based Scanning Capability. Proc. SPIE 2016, 9832, 98320M. Lee, X.; Wang, C. Optical design for uniform scanning in MEMS-based 3D imaging lidar. Appl. Opt. 2015, 54, 2219–2223. [CrossRef] [PubMed] Zhang, X.; Koppal, S.J.; Zhang, R.; Zhou, L.; Butler, E.; Xie, H. Wide-angle structured light with a scanning MEMS mirror in liquid. Opt. Express 2016, 24, 3479–3487. [CrossRef] [PubMed] Ikeda, M.; Goto, H.; Sakata, M.; Wakabayashi, S.; Imanaka, K.; Takeuchi, M.; Yada, T. Two Dimensional Silicon Micromachined Optical Scanner Integrated With Photo Detector and Piezoresistor. In Proceedings of the Solid-State Sensors and Actuators 1995, Stockholm, Sweden, 25–29 June 1995; pp. 293–296. Hofmann, U.; Janes, J.; Quenzer, H. High-Q MEMS Resonators for Laser Beam Scanning Displays. Micromachines 2012, 3, 509–528. [CrossRef] Yalcinkaya, A.D.; Urey, H.; Holmstrom, S. NiFe Plated Biaxial MEMS Scanner for 2-D Imaging. IEEE Photonics Technol. Lett. 2007, 19, 330–332. [CrossRef] Cho, I.; Yoon, E. A low-voltage three-axis electromagnetically actuated micromirror for fine alignment among optical devices. J. Micromech. Microeng. 2009, 19, 85007. [CrossRef] Chen, C.D.; Wang, Y.J.; Chang, P. A novel two-axis MEMS scanning mirror with a PZT actuator for laser scanning projection. Opt. Express 2012, 20, 27003–27017. [CrossRef] [PubMed] Wolter, A.; Shu-Ting, H.; Schenk, H.; Lakner, H.K. Applications and requirements for MEMS scanner mirrors. Proc. SPIE 2004, 5719, 64–75.

Sensors 2017, 17, 521

15. 16.

13 of 13

Guo, L.; Lin, S.; Xu, L. Study on inductance capacitance matching features of piezoelectric ceramic transducer. J. Shaanxi Norm. Univ. 2010, 38, 39–42. Zhang, C.; Zhang, G.; You, Z. A Two-Dimensional Micro Scanner Integrated with a Piezoelectric Actuator and Piezoresistors. Sensors 2009, 9, 631–644. [CrossRef] [PubMed] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).