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Double stage FPCB scanning micromirror for laser line generator
Mechatronics 51 (2018) 75–84
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Double stage FPCB scanning micromirror for laser line generator☆ ⁎
T
Hui Zuo, Siyuan He
Ryerson University, Department of Mechanical and Industrial Engineering, 350 Victoria Street, Toronto M5B 2K3, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Double stage FPCB scanning micromirror Large aperture High flatness Good surface roughness Laser line generator
This paper presents a laser line generator consisting of an electrostatic scanning micromirror utilizing a DoubleStage Flexible Printed Circuit Board (DS-FPCB). It consists of two stages with the rotating electrode in the 1st stage, and the mirror plate in the 2nd stage. Due to both rotating electrode being much closer to the rotational axis and the rotational amplification effect between the two stages, the DS-FPCB micromirror can have a 90% higher maximum rotation angle, in comparison to previously developed single stage FPCB micromirrors, while retaining benefits of low cost (a few dollars) and high surface quality (high flatness, radius of curvature (ROC) > 10 m, good roughness of nanometers). The DS-FPCB micromirror is modeled and prototypes are fabricated and tested, achieving a 3.25° optical rotation at 100 V (sinusoidal wave driving signal) and 10.87° at 150 V (square wave driving signal), both at 117 Hz. A laser line generator based on the DS-FPCB micromirror is developed and tested, whose laser power density along the line is uniform and independent on the projection distance, while the conventional laser line generator's laser power density varies along the laser line and is highly dependent on the projection distance.
1. Introduction MEMS (Microelectromechanical Systems) micromirrors have been developed since 1990s and successfully used in displays [1–5], optical switches [6] and medical devices [7] due to its advantages such as small size, high integration, high reliability and low potential cost. Micromirror based laser scanners have a broad range of applications [8,9], such as barcode reading [10], Light Detection and Ranging (LiDAR) [11], depth sensing using structured light [12], and bio imaging [13,14] etc. A MEMS micromirror could be driven via electrostatic [3,15,16], thermal [17,18], magnetic [19–22] and piezoelectric [23–25] methods. Micromirror based laser scanners require a large aperture (millimeters) and high surface quality of the mirror plate, e.g., high flatness, Radius of Curvature (ROC) > meters with roughness of nanometers. However, this type of MEMS micromirror has a high fabrication cost, e.g., $10's because the fabrication process involves both expensive micromachining and low yield bonding of highly fragile released micro actuators with a mirror plate [26–29]. Consequently, MEMS micromirror based laser scanners are not widely adopted in the market thus far. Flexible printed circuit board (FPCB) micromirrors [30,31] were presented, which can achieve large aperture and high surface quality with very low cost. This technology uses FPCB process to fabricate the flexure and driving parts, which are then bonded with a mirror plate fabricated by dicing a polished silicon wafer coated with
an aluminum film, to achieve large aperture, high surface quality and low cost (e.g., < 1 $ for high volume and few dollars for medium volume). The FPCB micromirror is an electrostatic parallel plate actuator driven by an electrostatic force. Therefore, when comparing with traditional parallel plate electrostatic MEMS micromirrors, there is no difference in terms of power consumption and driving voltage. Because of the material used (normally polyimide) is of low elastic modulus, the resonant frequency is lower. Its outstanding advantages lie in the large aperture (a few millimeters), high flatness (ROC > 10 m) and low cost (a few dollars) while traditional MEMS micromirror is of 10–100's µm in aperture, ROC of centimeters ∼ 1 m (depending on the fabrication technology) and 10s dollars in cost (for mass production). These FPCB micromirrors have small maximum rotation angles, e.g., < 6°, which are enough for the applications of availability indication [30] and as a laser pattern pointer [31]. But it is not enough for some laser scanner applications such as the laser line generator. Usually a scanning laser line is used for measurement, e.g., LiDAR or depth sensing, which requires a larger rotation angle (10's degrees) to cover larger areas with one laser line generator. The laser line generator projects a straight laser line with uniform laser power density at various locations on the line, which could be used for alignment, positioning, barcode scanning, depth sensing, etc. The conventional laser line generator based on cylindrical lens or a Powell lens has a fixed fan angle. Consequently, the length of the laser
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This paper was recommended for publication by Associate Editor Prof Takenori Atsumi. Corresponding author. E-mail addresses: [email protected] (H. Zuo), [email protected] (S. He).
https://doi.org/10.1016/j.mechatronics.2018.03.005 Received 14 November 2017; Received in revised form 12 February 2018; Accepted 8 March 2018 0957-4158/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.
Mechatronics 51 (2018) 75–84
H. Zuo, S. He
line and laser power density are highly dependent on the projection distance. If the distance is far, the line generated becomes very long and the laser power density becomes too low to be useful. Moreover, its laser power density varies along the same line, i.e., higher in the middle of laser line and lower at the edges. An SOIMUMPs micromirror based laser line generator [32] was developed to solve this limitations, which can project a laser line with a constant laser power density and adjustable fan angle. However, the projected laser line becomes too thick when the distance is over 3 m because of the poor mirror surface quality, e.g., ROC = 9.52 cm. The laser line generator based on the FPCB micromirror can overcome this problem due to its high flatness of mirror plate. This paper presents a Double Stage FPCB micromirror (named as DS-FPCB micromirror hereafter). It consists of two stages with the rotating electrode in the 1st stage and the mirror plate in the 2nd stage. As a result of the rotating electrode being much closer to the rotational axis and the rotational amplification effect between two stages, the DS-FPCB micromirror can have a much higher maximum rotation angle, in comparison to previously developed single stage FPCB micromirrors. This is achieved while retaining benefits of a large aperture, low cost and high surface quality. This DS-FPCB micromirror concept was first introduced by the authors in the conference paper [33] with simple and rough modeling, as well as basic prototyping of the micromirror. In this paper, detailed and accurate modeling is performed, such as the coupling-field transient simulation considering the varying electrostatic force instead of assuming a magnitude fixed sinusoidal force. In addition, a laser liner generator based on the DS-FPCB micromirror is built, tested and analyzed. Section 2 introduces the working principle of the DS-FPCB micromirror and laser line generator. Section 3 presents the modeling and simulations. Section 4 describes the prototypes and testing of the micromirror. Section 5 presents the prototype and testing of the laser line generator. The conclusions are summarized in Section 6.
Fig. 1. Structure of DS-FPCB micromirror. (a) Rotating plate. (b) FR4 PCB fixed upper or lower plate. (c) Assembled DS-FPCB micromirror.
The rotating plate including the rotating electrode, the mirror holder, beam 1 and beam 2 are fabricated using the standard FPCB process, which are multilayer structures, i.e., a polyimide base layer and a copper layer covered with a thin isolation layer, except for the wire soldering pad area. The fixed plates are made of FR4 PCB. The copper layers of the rotating electrode and fixed electrodes are covered with thin isolation layers to prevent short circuiting in case of touching. The DS-FPCB micromirror has a larger maximum rotation angle in comparison to previously developed FPCB micromirrors [30,31] (referred to as FPCB micromirror 1 and FPCB micromirror 2 in this paper) because:
2. DS-FPCB micromirror and laser line generator 2.1. The DS-FPCB micromirror Fig. 1 shows the structure of DS-FPCB micromirror including the rotating plate and two fixed plates, which is an electrostatic parallel plate actuator driven by electrostatic force. The rotating plate has two stages as shown in Fig. 1(a), i.e., the 1st stage is the rotating electrode with the torsion beam (Beam 1) connected to anchors, the 2nd stage is the mirror plate with beams (Beam 2) connected to the 1st stage. The fixed plates as shown in Fig. 1(b) are above and under the rotating plate, e.g., the upper and lower fixed plates. The rotating electrode is a copper layer in the rotating plate, which can be applied with a driving voltage and pulled up or down by the electrostatic force. The copper layer in the rotating plate is the upper/lower fixed electrode used to apply the driving voltage to generate the electrostatic force. The upper and lower gap spacers are used to form 0.2 mm gaps above and underneath the rotating electrode. The spacers are made of the FR4 PCB stiffener, which are available in the standard FPCB fabrication process. Although a larger gap will result in larger maximum rotation angle, it would require a higher driving voltage. It is easy to build a driving circuit within 200 V driving voltage; otherwise the higher driving cost will diminish the advantage of the low cost micromirror. Based on the experimental and simulation results of the previous two FPCB micromirrors, 0.2 mm gap is selected to keep the driving voltage lower than 200 V. A 2 mm × 2 mm silicon mirror plate is fabricated by dicing a polished 120 µm thick silicon wafer with a 100 nm aluminum coating. It is bonded on top of the mirror holder in the 2nd stage as shown in Fig. 1(c). The reflectivity of the aluminum coated mirror can reach over 85% for a visible laser light. After bonding the diced mirror on top of the mirror holder, it will not cause initial tilting with proper bonding to keep the beams in elastic deformation.
(1) The gap height and distance of the rotating electrode's tip to the rotational axis determine the physical rotation limit, 73.1% of which is the maximum oscillation angle due to the “pull-in” effect [34]. Since the DS-FPCB micromirror has the rotating electrode tip much closer to the rotational axis than that in the previous FPCB micromirrors as shown in Fig. 2, even with the similar rotating electrode length, its maximum rotation angle is much higher. The maximum mechanical rotation angle for only lower direction can be calculated according to Eq. (1).
θRotation = 0.731*arctan (g/d)
(1)
where g is the gap between the rotation and fixed electrode and d is the distance from the fixed electrode's edge to the rotational axis. Considering rotation in both upper and lower directions, the optical rotation angle is double of the mechanical rotation angle, the maximum optical rotation angle will be 4 times of θRotation. For example, gaps are 0.2 mm for all three designs, the maximum rotation angles for the rotating electrode are 5.88° for the previous two designs and 8.13° for the DS-FPCB micromirror according to the dimensions as shown in Fig. 2. (2) In the DS-FPCB micromirror, the 2nd stage (mirror plate)’s rotation is higher than that of the 1st stage (the rotating electrode) due to the rotational amplification effect caused by beams connecting the 1st and 2nd stages. This amplification is about 1.58, as explained in 76
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Fig. 2. FPCB micromirrors. (a) Previous FPCB micromirror 1. (b) Maximum rotation angle of FPCB micromirror 1. (c) Previous FPCB micromirror 2. (d) Maximum rotation angle of FPCB micromirror 2. (e) DS-FPCB micromirror proposed in this paper. (f) Maximum rotation angle of DS-FPCB micromirror.
the next section. Hence the maximum rotation of the mirror plate of the DS-FPCB micromirror is 12.84°, which is much higher than previous designs. In the new design, the mirror plate is further from the rotational axis, which will result in more non-centered scanning. This will cause the laser beam to be off the center of the mirror plate in the laser line generator application. Since the mechanical rotation angle is small ( ± 3°) and the DS-FPCB micromirror has larger aperture size (2 mm × 2 mm), the smaller laser beam (around 1.0 mm) is still on the mirror plate. The rotating electrode will be pulled up when applying a driving voltage to the upper fixed electrode with the rotating electrode grounded. It will be pulled down when driving the lower fixed electrode. The DS-FPCB micromirror works at the resonant mode close to its natural frequency by applying a periodical driving voltage. For an electrostatic micromirror scanner [30,35–38], a periodical driving voltage is usually applied between the moving and fixed electrodes to drive the scanning micromirror working at a forced sinusoidal oscillation. When the frequency of driving voltage approaches its resonant frequency, it will achieve a larger rotation angle. Because the single parallel plate actuator generates one direction force for the moving electrode, it only contributes the oscillation in half of the oscillation cycle. In order to achieve larger rotation angle, the DS-FPCB micromirror has upper and lower electrostatic parallel plate actuators by applying sinusoidal or square wave driving voltage to the upper and lower electrodes with 180° phase difference as shown in Fig. 3. Each of them will contribute torque in half cycle respectively, so the total torque will much higher than that of the conventional single parallelplate actuator with the same driving voltage.
Fig. 3. DS-FPCB micromirror driving signal. (a) Driving voltage to upper fixed electrode. (b) Driving voltage to lower fixed electrode.
2.2. Laser line generator Fig. 4 shows the laser line generator based on the DS-FPCB micromirror. It consists of a laser source, the DS-FPCB micromirror, and a control circuit. The laser line is generated by steering a collimated laser beam through the scanning micromirror. Therefore, at any moment, the laser beam shoots at one location, which results in a constant laser power, not only along the laser line, but also at different projection distances. Furthermore, the laser power density is much higher than that of the conventional lens based laser line generator, in which the laser power is distributed along the laser line.
Fig. 4. The design of laser line generator.
and lower air gaps. As shown in Fig. 5, the rotating electrode consists of a 12 µm thick copper layer which is made of a 1/3rd-ounce copper and 70 µm thick polyimide base layer, which combines the 60 µm thick polyimide base layer and the 10 µm thick cover layer. Air blocks are used to model the air gaps between the rotating electrode and the upper
3. Modeling and simulation Fig. 5 shows the model and its dimensions used in the simulation. The air blocks are used in the transient simulation to model the upper 77
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Fig. 5. Simulation model (unit: mm). (a) Top view. (b) Side view.
and lower fixed electrodes. In the simulation the 10 µm cover layer is added to the 60 µm polyimide base layer. The thickness (except for the mirror plate) of each layer is determined by the standard FPCB process. The mirror plate aperture size (2 × 2 mm) is determined by the laser beam (1–2 mm). Its thickness is determined by the thickness required to maintain a high ROC and available thickness of the silicon wafer. Both the upper and lower gaps are 0.2 mm.
frequency is always purposed required if only consider the resistance to external vibration. However, the higher frequency leads to higher stiffness, which adversely affect the rotation angle with the same driving voltage. The 2nd to 4th mode frequencies are also shown in Figs. 6(b)–(d), which are much greater than the 1st mode. Fig. 6(b) shows that the 2nd natural mode resonant, where the rotating electrode and the mirror plate move out-of-phase, has an amplitude amplification factor significantly larger than that of the 1st mode. But its rotation angle is much smaller than that of 1st mode of resonant which will be explained in the section of transient simulation. So the micromirror will scan in 1-axis as shown in Fig. 6(a). The Young's modulus used for the polyimide and copper layers are 2.5 GPa and 110 GPa, respectively. The density of the silicon mirror plate is 2330 kg/m3.
3.1. Natural frequency of DS-FPCB micromirror Finite Element Analysis (FEA)-ANSYS [39] simulation is performed to find the natural resonant frequency of the DS-FPCB micromirror. The 1st mode of resonant frequency is 117 Hz as shown in Fig. 6(a), which is high enough to form a stable laser pattern without flashing. The criteria for the scanning frequency is > 60 Hz in order to achieve stable laser line without flashing. Beside the > 60 Hz criteria, the 117 Hz for the prototype in this paper is mainly determined by the dimension of the mirror plate and the FPCB plate and beams. A high 1st mode resonant
3.2. Transient response simulation ANSYS transient analysis is performed to determine the rotation angle of DS-PFCB micromirror with a periodical driving voltage. The
Fig. 6. Resonant frequency simulation. (a) 1st mode 117 Hz. (b) 2nd mode 631 Hz. (c) 3rd mode 1382 Hz. (d) 4th mode 1666 Hz.
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Fig. 7. Transient simulation results. (a) Mirror plate's rotation at different driving voltages. (b) 100 V sinusoid driving voltage at 117 Hz.
rotating electrode is applied with 0 V voltage. The top and lower fixed electrodes are applied with sinusoidal or square wave driving voltage with reversed phases. The Multiphysics module in ANSYS is used for simulation. In ANSYS Workbench, the command “et,MATID,226,1001″ is used for the upper and lower air blocks to replace with Solid226 element. It is a coupled-field of 20 nodes brick with five degrees of freedom (DOF) including displacement and voltage, which are suitable for MEMS electrostatic force analysis. The “elastic air” material is assigned to the air blocks with a small Young's modulus 0.001 MPa and zero Poisson's ratio to allow the air area deform. The value of 3.4 is used as the polyimide's relative permittivity. The sinusoidal driving voltages are shown in Eqs. (2) and (3).
Vupper = U (1 + sin(2πft + π )) = U (1 − sin(2πft ))
(2)
Vlower = U (1 + sin(2πft ))
(3)
100 V sinusoidal wave. An optical rotation angle of 1.942° (−0.964°–+0.978°) with the rotating electrode is achieved. An angle of 3.07° (−1.528° to +1.542°) with the mirror plate is achieved after the amplification between two stages, which is 1.58 times. The amplification is about 1.58 times for all the applied voltages with sinusoidal driving and 1.56–1.64 times with square driving voltage from 25 V to 150 V. The difference between the upward and downward rotations is because the rotating electrode is not exactly centered between the upper and lower electrodes, due to the polyimide substrate layer. A transient simulation is also performed to find the rotation angle of the 2nd mode resonant with 631 Hz and 100 V driving voltage. The result shows the optical rotation angle is 0.061° with the rotating electrode and 0.194° with the mirror plate. Although the amplitude amplification factor for 2nd mode resonant is 3.17 which is significantly larger than that of the 1st mode. The rotation angle is much smaller than that of 1st mode resonance. Therefore, the micromirror works best at its 1st mode of resonance.
where U and f are the voltage amplitude and frequency, respectively. For example, if U = 50 V, the absolute voltage is from 0 to 100 V, so there is no negative voltage applied on the fixed electrode. The transient response simulation results are shown in Fig. 7 with a frequency of the driving voltage of 117 Hz, which is the 1st mode natural resonant frequency obtained in Section 3.1. Fig. 7(a) shows DSFPCB micromirror optical rotation angle with the driving voltage from 0 to 150 V. At 150 V square wave driving voltage, the rotation angle of the mirror plate achieves 9.43°, which should not be the maximum rotation since higher voltage (> 150 V) cannot be applied, because of simulation divergence. Simulation divergence caused by high voltage (e.g., > 150 V) also occurred in our simulations of repulsive-force (no “pull-in” effect) actuators/micromirrors using CoventorWare [15,40]. Fig. 7(b) shows an example simulation result with the driving voltage of
4. DS-FPCB micromirror prototype and test 4.1. Prototype of DS-FPCB micromirror Fig. 8 shows the prototype of the DS-FPCB micromirror. The fixed electrode copper area is bigger than the copper area of the rotating electrode for tolerance of misalignment during assembly. The rotating electrode, the double stage beams, the spacer, the wire pad, assemble holes, and mirror holder are fabricated using the standard FPCB fabrication process. The surface quality of the silicon mirror plate is measured using ZYGO 3D optical surface profiler. The radius of curvature 79
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plate can be fabricated in panel and boned together using an adhesive mask layer by machine and then separated later, such as to achieve low cost assembly. The size of assembled DS-FPCB micromirror is 10.3 mm × 8.8 mm × 2.0 mm. The total cost is a few dollars with low quantity.
4.2. Oscillation test The rotation angle of the DS-FPCB micromirror is tested by shooting a collimated laser beam on the mirror plate and measuring the length of the projected laser line. The setup is shown in Fig. 9. A 1.0 mm scale grid paper is placed 1-meter away as the projected screen and used to measure the length of the scanning laser line, and then to calculate the rotation angle. A low voltage driving signal (sinusoidal or square wave) is generated by a signal generator and then converted to 100–150 V by a high voltage amplifier. Two sinusoidal voltages with 180° phase difference based on Eqs. (2) and (3) are applied on the upper and lower fixed electrodes with the middle rotating electrode grounded. Fig. 10(a) shows the rotation results versus the scanning frequency. According to the testing result, its natural resonant frequency is 113 Hz, which is slightly different from the simulation result (117 Hz). The discrepancy should be from the alignment error when assembling the mirror plate. The test results do not show a different rotation angle with 100 V and 120 V driving when the frequency is further away from the natural frequency, e.g., the DC rotation angle, because the measurement cannot reflect very small difference due to the 1.0 mm laser beam size and the 1.0 mm scale grid screen paper. Fig. 10(b) shows rotation angle versus the driving voltage at its resonant frequency. The square wave driving voltage results in 28% more rotation angle than the sinusoidal wave driving voltage. It is much easier to generate a square wave driving voltage for the control circuit. The length of the projected laser line can be controlled by adjusting the amplitude of driving voltage. At 150 V square wave driving voltage, the rotation angle is 10.87° which is 90% larger than the previously developed single stage FPCB micromirrors. Four prototypes were made and tested. Table 1 lists the testing results which shows a 3.25° average rotation angle at 100 V sinusoidal driving. The discrepancy in testing angles and simulation results, as well as the variations in prototypes lie in: (1) The un-flatness of the rotating electrode and its variation, changes the upper and lower electrostatic gaps; (2). The torsion beam (beam1) is not completely flat and parallel to the fixed electrode. Small un-flatness of the torsion beam causes significant tilting of the rotating electrode.
Fig. 8. DS-FPCB micromirror prototype. (a) Upper and lower fixed plate. (b) Rotating plate. (c) Mirror plate on the mirror holder. (d) Assembled DS-FPCB micromirror.
(ROC) of the mirror plate is 15 m with roughness of 3 nm. It is much better than conventional MEMS micromirrors (e.g., ROC < 1 m, if not mirror plate bonding to the released micro actuator). The assembled prototype is shown in Fig. 8(d) which includes the soldered wires connection for the rotating, upper and lower electrodes. The mirror plate is coated with the Epoxy glue first and then bonded on top of the mirror holder. The manual bonding will not cause large misalignment because the mirror holder is only 0.5 mm larger than the mirror plate. Manual bonding relies on the edges alignment with eyes. The rotating plate is sandwiched by the upper and lower fixed plates through a screw nut set. The assembly holes on all three plates were first aligned and then fixed by the screw, which will ensure the alignment of the three electrodes. It takes time to assemble a prototype manually. However, for mass production the FPCB rotating plate and the FR4 PCB fixed
Fig. 9. Setup of the measurement system.
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Fig. 10. Measured DS-FPCB micromirror optical rotation angle. (a) Rotation angle versus frequency with sinusoidal driving voltage. (b) Rotation angle versus driving voltage at its resonant frequency.
ratio is 0.025. The fatigue life of the polyimide beam has been verified in the availability indicator application of FPCB micromirror [30], which uses the same materials as the beams in the DS-FPCB micromirror. The beams in the availability indicator have been bending for 6.39 billion cycles for more than two years’ continuous running without any visible performance deterioration.
Table 1 Prototypes performance. Prototype
1
2
3
4
Average
Resonant frequency/Hz Rotation angle at 100 V sinusoidal driving
113 4.06°
113 3.55°
124 2.58°
119 2.81°
117 3.25°
4.3. Transient response test
5. Laser line generator
A position sensitive detector (PSD) is used to replace the test grid paper at a very close distance to test the DS-FPCB micromirror's dynamic response. The result is shown in Fig. 11 which is recorded by an Oscilloscope and replotted in Excel. The orange line is the control signal when driving only the upper fixed electrode and the blue line is the PSD output signal, which corresponds to the rotation angle of the DS-FPCB micromirror. According to the test result in Fig. 11(a), it takes around 140 ms for the micromirror to enter stable oscillation. This short time will not cause any noticeable delay in a laser line generator. The damping ratio used in simulations is obtained by the free vibration decay. It can be calculated according to Eq. (4) [41].
δ=
1 X ln 1 2πn Xn + 1
5.1. The prototype of laser line generator Fig. 12 shows the prototype of the DS-FPCB micromirror based laser line generator. Fig. 12(a) and (b) shows the bottom and top side of the control circuit. The control circuit includes: (1) A high voltage step up circuit to convert the 5.0 V power supply to 50–150 V range driving voltage for the electrostatic actuation (2) An operational-amplifier based signal generator to generate a 50% duty cycle 5.0 V square wave signal; (3) A transistor based switching circuit to amplify the driving single. The signal frequency can be adjusted from 10 to 700 Hz to fit different natural frequency units through a potentiometer as shown in Fig. 12(b). Another potentiometer is used to adjust the amplitude of the output voltage from 50 V to 150 V for changing the length of scanning laser line. The control circuit is powered by a 5.0 V power supply with a total current of about 60 mA, including the current for the laser module.
(4)
where X1 and Xn+1 are the two peak amplitudes separated by n cycles. According to the Fig. 11(b), X1 = 3.24 and X6 = 1.48, so the damping 81
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Fig. 13. Comparison of the DS-FPCB micromirror laser line generator with conventional one. (a) Projection distance of 1.0 m. (b) Projection distance of 2.0 m. (c) Projection distance of 4.0 m. (d) Projection distance of 6.0 m.
The DS-FPCB micromirror and the laser are mounted on the circuit board. All parts are integrated in a 35 mm × 35 mm × 20 mm box as shown in Fig. 12(c). Fig. 12(d) shows the micromirror based laser line generator and the projected laser line.
Fig. 11. Tested transient response of the DS-FPCB micromirror. (a) Transient response. (b) Damping ratio test. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 12. Laser line generator. (a) Bottom side the driving circuit. (b) Top side the driving circuit. (c) Assembled laser line generator. (d) Projected laser line.
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Fig. 14. Laser power measurement along the laser line. (a) Close to the edge of laser line. (b) The center of laser line. (c) Laser power at projection distance of 1 m. (d) Laser power at projection distance of 0.5 m.
5.2. Comparison with conventional laser line generator
line, in comparison to conventional laser line generator.
Fig. 13 shows the projected laser lines at different distance for two laser line generators, i.e., the conventional lens based and the DS-FPCB micromirror based laser line generators. Both of them have the same laser source power, e.g., about 2.0 mW. The conventional one is FLL52P-635-15 from World Star Technology Inc. with 15° fan angle. In Fig. 13 the upper laser line is generated by the conventional one and the lower line is generated by the DS-FPCB micromirror based laser line generator. The two laser lines have the same length and brightness in 1.0 m distance as shown in Fig. 13(a). The conventional laser line generator has a fixed fan angle, so it will project a longer laser line with further distance. The laser power density will become weaker because the same laser power will be distributed in a longer laser line. However the laser lines generated by the DS-FPCB micromirror line generator can keep the same length and same brightness by adjusting the scanning angle. The laser power at various projection distance is measured by a high-speed photodetector, i.e., DET36A with 3.6 × 3.6 mm active area. Fig. 14(a) and (b) shows measuring spots close to the edge and the center of laser line. The peak laser power is constant at different locations of the line with various distances as shown in Fig. 14(c) and (d), i.e., 2 mW with different pulse width which is the duration when the scanning laser beam passes the photodetector. Fig. 14(c) and 14(d) combine the measurement data for different locations recorded by an Oscilloscope. On the contrary, the laser power on the laser line generated by conventional generator is much lower with significant variation at different projection distances, e.g., 0.13 mW (projection distance of 0.25 m) and 0.04 mW (projection distance of 1.0 m).
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6. Conclusion A double stage FPCB (DS-FPCB) micromirror and a laser line generator based on it have been developed in this paper. The DS-FPCB micromirror was modeled, simulated, fabricated and tested, which can output a maximum rotation angle of 90% higher than that of previous FPCB micromirrors while retaining advantages of: (1) large aperture and high surface quality (high flatness and good roughness); (2) low cost. A laser line generator based on the DS-FPCB micromirror is built and tested. It shows the benefits of consistent and high laser power density at various projection distance or various locations on the same 83
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Double stage FPCB scanning micromirror for laser line generator☆ ⁎
T
Hui Zuo, Siyuan He
Ryerson University, Department of Mechanical and Industrial Engineering, 350 Victoria Street, Toronto M5B 2K3, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Double stage FPCB scanning micromirror Large aperture High flatness Good surface roughness Laser line generator
This paper presents a laser line generator consisting of an electrostatic scanning micromirror utilizing a DoubleStage Flexible Printed Circuit Board (DS-FPCB). It consists of two stages with the rotating electrode in the 1st stage, and the mirror plate in the 2nd stage. Due to both rotating electrode being much closer to the rotational axis and the rotational amplification effect between the two stages, the DS-FPCB micromirror can have a 90% higher maximum rotation angle, in comparison to previously developed single stage FPCB micromirrors, while retaining benefits of low cost (a few dollars) and high surface quality (high flatness, radius of curvature (ROC) > 10 m, good roughness of nanometers). The DS-FPCB micromirror is modeled and prototypes are fabricated and tested, achieving a 3.25° optical rotation at 100 V (sinusoidal wave driving signal) and 10.87° at 150 V (square wave driving signal), both at 117 Hz. A laser line generator based on the DS-FPCB micromirror is developed and tested, whose laser power density along the line is uniform and independent on the projection distance, while the conventional laser line generator's laser power density varies along the laser line and is highly dependent on the projection distance.
1. Introduction MEMS (Microelectromechanical Systems) micromirrors have been developed since 1990s and successfully used in displays [1–5], optical switches [6] and medical devices [7] due to its advantages such as small size, high integration, high reliability and low potential cost. Micromirror based laser scanners have a broad range of applications [8,9], such as barcode reading [10], Light Detection and Ranging (LiDAR) [11], depth sensing using structured light [12], and bio imaging [13,14] etc. A MEMS micromirror could be driven via electrostatic [3,15,16], thermal [17,18], magnetic [19–22] and piezoelectric [23–25] methods. Micromirror based laser scanners require a large aperture (millimeters) and high surface quality of the mirror plate, e.g., high flatness, Radius of Curvature (ROC) > meters with roughness of nanometers. However, this type of MEMS micromirror has a high fabrication cost, e.g., $10's because the fabrication process involves both expensive micromachining and low yield bonding of highly fragile released micro actuators with a mirror plate [26–29]. Consequently, MEMS micromirror based laser scanners are not widely adopted in the market thus far. Flexible printed circuit board (FPCB) micromirrors [30,31] were presented, which can achieve large aperture and high surface quality with very low cost. This technology uses FPCB process to fabricate the flexure and driving parts, which are then bonded with a mirror plate fabricated by dicing a polished silicon wafer coated with
an aluminum film, to achieve large aperture, high surface quality and low cost (e.g., < 1 $ for high volume and few dollars for medium volume). The FPCB micromirror is an electrostatic parallel plate actuator driven by an electrostatic force. Therefore, when comparing with traditional parallel plate electrostatic MEMS micromirrors, there is no difference in terms of power consumption and driving voltage. Because of the material used (normally polyimide) is of low elastic modulus, the resonant frequency is lower. Its outstanding advantages lie in the large aperture (a few millimeters), high flatness (ROC > 10 m) and low cost (a few dollars) while traditional MEMS micromirror is of 10–100's µm in aperture, ROC of centimeters ∼ 1 m (depending on the fabrication technology) and 10s dollars in cost (for mass production). These FPCB micromirrors have small maximum rotation angles, e.g., < 6°, which are enough for the applications of availability indication [30] and as a laser pattern pointer [31]. But it is not enough for some laser scanner applications such as the laser line generator. Usually a scanning laser line is used for measurement, e.g., LiDAR or depth sensing, which requires a larger rotation angle (10's degrees) to cover larger areas with one laser line generator. The laser line generator projects a straight laser line with uniform laser power density at various locations on the line, which could be used for alignment, positioning, barcode scanning, depth sensing, etc. The conventional laser line generator based on cylindrical lens or a Powell lens has a fixed fan angle. Consequently, the length of the laser
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This paper was recommended for publication by Associate Editor Prof Takenori Atsumi. Corresponding author. E-mail addresses: [email protected] (H. Zuo), [email protected] (S. He).
https://doi.org/10.1016/j.mechatronics.2018.03.005 Received 14 November 2017; Received in revised form 12 February 2018; Accepted 8 March 2018 0957-4158/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.
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line and laser power density are highly dependent on the projection distance. If the distance is far, the line generated becomes very long and the laser power density becomes too low to be useful. Moreover, its laser power density varies along the same line, i.e., higher in the middle of laser line and lower at the edges. An SOIMUMPs micromirror based laser line generator [32] was developed to solve this limitations, which can project a laser line with a constant laser power density and adjustable fan angle. However, the projected laser line becomes too thick when the distance is over 3 m because of the poor mirror surface quality, e.g., ROC = 9.52 cm. The laser line generator based on the FPCB micromirror can overcome this problem due to its high flatness of mirror plate. This paper presents a Double Stage FPCB micromirror (named as DS-FPCB micromirror hereafter). It consists of two stages with the rotating electrode in the 1st stage and the mirror plate in the 2nd stage. As a result of the rotating electrode being much closer to the rotational axis and the rotational amplification effect between two stages, the DS-FPCB micromirror can have a much higher maximum rotation angle, in comparison to previously developed single stage FPCB micromirrors. This is achieved while retaining benefits of a large aperture, low cost and high surface quality. This DS-FPCB micromirror concept was first introduced by the authors in the conference paper [33] with simple and rough modeling, as well as basic prototyping of the micromirror. In this paper, detailed and accurate modeling is performed, such as the coupling-field transient simulation considering the varying electrostatic force instead of assuming a magnitude fixed sinusoidal force. In addition, a laser liner generator based on the DS-FPCB micromirror is built, tested and analyzed. Section 2 introduces the working principle of the DS-FPCB micromirror and laser line generator. Section 3 presents the modeling and simulations. Section 4 describes the prototypes and testing of the micromirror. Section 5 presents the prototype and testing of the laser line generator. The conclusions are summarized in Section 6.
Fig. 1. Structure of DS-FPCB micromirror. (a) Rotating plate. (b) FR4 PCB fixed upper or lower plate. (c) Assembled DS-FPCB micromirror.
The rotating plate including the rotating electrode, the mirror holder, beam 1 and beam 2 are fabricated using the standard FPCB process, which are multilayer structures, i.e., a polyimide base layer and a copper layer covered with a thin isolation layer, except for the wire soldering pad area. The fixed plates are made of FR4 PCB. The copper layers of the rotating electrode and fixed electrodes are covered with thin isolation layers to prevent short circuiting in case of touching. The DS-FPCB micromirror has a larger maximum rotation angle in comparison to previously developed FPCB micromirrors [30,31] (referred to as FPCB micromirror 1 and FPCB micromirror 2 in this paper) because:
2. DS-FPCB micromirror and laser line generator 2.1. The DS-FPCB micromirror Fig. 1 shows the structure of DS-FPCB micromirror including the rotating plate and two fixed plates, which is an electrostatic parallel plate actuator driven by electrostatic force. The rotating plate has two stages as shown in Fig. 1(a), i.e., the 1st stage is the rotating electrode with the torsion beam (Beam 1) connected to anchors, the 2nd stage is the mirror plate with beams (Beam 2) connected to the 1st stage. The fixed plates as shown in Fig. 1(b) are above and under the rotating plate, e.g., the upper and lower fixed plates. The rotating electrode is a copper layer in the rotating plate, which can be applied with a driving voltage and pulled up or down by the electrostatic force. The copper layer in the rotating plate is the upper/lower fixed electrode used to apply the driving voltage to generate the electrostatic force. The upper and lower gap spacers are used to form 0.2 mm gaps above and underneath the rotating electrode. The spacers are made of the FR4 PCB stiffener, which are available in the standard FPCB fabrication process. Although a larger gap will result in larger maximum rotation angle, it would require a higher driving voltage. It is easy to build a driving circuit within 200 V driving voltage; otherwise the higher driving cost will diminish the advantage of the low cost micromirror. Based on the experimental and simulation results of the previous two FPCB micromirrors, 0.2 mm gap is selected to keep the driving voltage lower than 200 V. A 2 mm × 2 mm silicon mirror plate is fabricated by dicing a polished 120 µm thick silicon wafer with a 100 nm aluminum coating. It is bonded on top of the mirror holder in the 2nd stage as shown in Fig. 1(c). The reflectivity of the aluminum coated mirror can reach over 85% for a visible laser light. After bonding the diced mirror on top of the mirror holder, it will not cause initial tilting with proper bonding to keep the beams in elastic deformation.
(1) The gap height and distance of the rotating electrode's tip to the rotational axis determine the physical rotation limit, 73.1% of which is the maximum oscillation angle due to the “pull-in” effect [34]. Since the DS-FPCB micromirror has the rotating electrode tip much closer to the rotational axis than that in the previous FPCB micromirrors as shown in Fig. 2, even with the similar rotating electrode length, its maximum rotation angle is much higher. The maximum mechanical rotation angle for only lower direction can be calculated according to Eq. (1).
θRotation = 0.731*arctan (g/d)
(1)
where g is the gap between the rotation and fixed electrode and d is the distance from the fixed electrode's edge to the rotational axis. Considering rotation in both upper and lower directions, the optical rotation angle is double of the mechanical rotation angle, the maximum optical rotation angle will be 4 times of θRotation. For example, gaps are 0.2 mm for all three designs, the maximum rotation angles for the rotating electrode are 5.88° for the previous two designs and 8.13° for the DS-FPCB micromirror according to the dimensions as shown in Fig. 2. (2) In the DS-FPCB micromirror, the 2nd stage (mirror plate)’s rotation is higher than that of the 1st stage (the rotating electrode) due to the rotational amplification effect caused by beams connecting the 1st and 2nd stages. This amplification is about 1.58, as explained in 76
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Fig. 2. FPCB micromirrors. (a) Previous FPCB micromirror 1. (b) Maximum rotation angle of FPCB micromirror 1. (c) Previous FPCB micromirror 2. (d) Maximum rotation angle of FPCB micromirror 2. (e) DS-FPCB micromirror proposed in this paper. (f) Maximum rotation angle of DS-FPCB micromirror.
the next section. Hence the maximum rotation of the mirror plate of the DS-FPCB micromirror is 12.84°, which is much higher than previous designs. In the new design, the mirror plate is further from the rotational axis, which will result in more non-centered scanning. This will cause the laser beam to be off the center of the mirror plate in the laser line generator application. Since the mechanical rotation angle is small ( ± 3°) and the DS-FPCB micromirror has larger aperture size (2 mm × 2 mm), the smaller laser beam (around 1.0 mm) is still on the mirror plate. The rotating electrode will be pulled up when applying a driving voltage to the upper fixed electrode with the rotating electrode grounded. It will be pulled down when driving the lower fixed electrode. The DS-FPCB micromirror works at the resonant mode close to its natural frequency by applying a periodical driving voltage. For an electrostatic micromirror scanner [30,35–38], a periodical driving voltage is usually applied between the moving and fixed electrodes to drive the scanning micromirror working at a forced sinusoidal oscillation. When the frequency of driving voltage approaches its resonant frequency, it will achieve a larger rotation angle. Because the single parallel plate actuator generates one direction force for the moving electrode, it only contributes the oscillation in half of the oscillation cycle. In order to achieve larger rotation angle, the DS-FPCB micromirror has upper and lower electrostatic parallel plate actuators by applying sinusoidal or square wave driving voltage to the upper and lower electrodes with 180° phase difference as shown in Fig. 3. Each of them will contribute torque in half cycle respectively, so the total torque will much higher than that of the conventional single parallelplate actuator with the same driving voltage.
Fig. 3. DS-FPCB micromirror driving signal. (a) Driving voltage to upper fixed electrode. (b) Driving voltage to lower fixed electrode.
2.2. Laser line generator Fig. 4 shows the laser line generator based on the DS-FPCB micromirror. It consists of a laser source, the DS-FPCB micromirror, and a control circuit. The laser line is generated by steering a collimated laser beam through the scanning micromirror. Therefore, at any moment, the laser beam shoots at one location, which results in a constant laser power, not only along the laser line, but also at different projection distances. Furthermore, the laser power density is much higher than that of the conventional lens based laser line generator, in which the laser power is distributed along the laser line.
Fig. 4. The design of laser line generator.
and lower air gaps. As shown in Fig. 5, the rotating electrode consists of a 12 µm thick copper layer which is made of a 1/3rd-ounce copper and 70 µm thick polyimide base layer, which combines the 60 µm thick polyimide base layer and the 10 µm thick cover layer. Air blocks are used to model the air gaps between the rotating electrode and the upper
3. Modeling and simulation Fig. 5 shows the model and its dimensions used in the simulation. The air blocks are used in the transient simulation to model the upper 77
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Fig. 5. Simulation model (unit: mm). (a) Top view. (b) Side view.
and lower fixed electrodes. In the simulation the 10 µm cover layer is added to the 60 µm polyimide base layer. The thickness (except for the mirror plate) of each layer is determined by the standard FPCB process. The mirror plate aperture size (2 × 2 mm) is determined by the laser beam (1–2 mm). Its thickness is determined by the thickness required to maintain a high ROC and available thickness of the silicon wafer. Both the upper and lower gaps are 0.2 mm.
frequency is always purposed required if only consider the resistance to external vibration. However, the higher frequency leads to higher stiffness, which adversely affect the rotation angle with the same driving voltage. The 2nd to 4th mode frequencies are also shown in Figs. 6(b)–(d), which are much greater than the 1st mode. Fig. 6(b) shows that the 2nd natural mode resonant, where the rotating electrode and the mirror plate move out-of-phase, has an amplitude amplification factor significantly larger than that of the 1st mode. But its rotation angle is much smaller than that of 1st mode of resonant which will be explained in the section of transient simulation. So the micromirror will scan in 1-axis as shown in Fig. 6(a). The Young's modulus used for the polyimide and copper layers are 2.5 GPa and 110 GPa, respectively. The density of the silicon mirror plate is 2330 kg/m3.
3.1. Natural frequency of DS-FPCB micromirror Finite Element Analysis (FEA)-ANSYS [39] simulation is performed to find the natural resonant frequency of the DS-FPCB micromirror. The 1st mode of resonant frequency is 117 Hz as shown in Fig. 6(a), which is high enough to form a stable laser pattern without flashing. The criteria for the scanning frequency is > 60 Hz in order to achieve stable laser line without flashing. Beside the > 60 Hz criteria, the 117 Hz for the prototype in this paper is mainly determined by the dimension of the mirror plate and the FPCB plate and beams. A high 1st mode resonant
3.2. Transient response simulation ANSYS transient analysis is performed to determine the rotation angle of DS-PFCB micromirror with a periodical driving voltage. The
Fig. 6. Resonant frequency simulation. (a) 1st mode 117 Hz. (b) 2nd mode 631 Hz. (c) 3rd mode 1382 Hz. (d) 4th mode 1666 Hz.
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Fig. 7. Transient simulation results. (a) Mirror plate's rotation at different driving voltages. (b) 100 V sinusoid driving voltage at 117 Hz.
rotating electrode is applied with 0 V voltage. The top and lower fixed electrodes are applied with sinusoidal or square wave driving voltage with reversed phases. The Multiphysics module in ANSYS is used for simulation. In ANSYS Workbench, the command “et,MATID,226,1001″ is used for the upper and lower air blocks to replace with Solid226 element. It is a coupled-field of 20 nodes brick with five degrees of freedom (DOF) including displacement and voltage, which are suitable for MEMS electrostatic force analysis. The “elastic air” material is assigned to the air blocks with a small Young's modulus 0.001 MPa and zero Poisson's ratio to allow the air area deform. The value of 3.4 is used as the polyimide's relative permittivity. The sinusoidal driving voltages are shown in Eqs. (2) and (3).
Vupper = U (1 + sin(2πft + π )) = U (1 − sin(2πft ))
(2)
Vlower = U (1 + sin(2πft ))
(3)
100 V sinusoidal wave. An optical rotation angle of 1.942° (−0.964°–+0.978°) with the rotating electrode is achieved. An angle of 3.07° (−1.528° to +1.542°) with the mirror plate is achieved after the amplification between two stages, which is 1.58 times. The amplification is about 1.58 times for all the applied voltages with sinusoidal driving and 1.56–1.64 times with square driving voltage from 25 V to 150 V. The difference between the upward and downward rotations is because the rotating electrode is not exactly centered between the upper and lower electrodes, due to the polyimide substrate layer. A transient simulation is also performed to find the rotation angle of the 2nd mode resonant with 631 Hz and 100 V driving voltage. The result shows the optical rotation angle is 0.061° with the rotating electrode and 0.194° with the mirror plate. Although the amplitude amplification factor for 2nd mode resonant is 3.17 which is significantly larger than that of the 1st mode. The rotation angle is much smaller than that of 1st mode resonance. Therefore, the micromirror works best at its 1st mode of resonance.
where U and f are the voltage amplitude and frequency, respectively. For example, if U = 50 V, the absolute voltage is from 0 to 100 V, so there is no negative voltage applied on the fixed electrode. The transient response simulation results are shown in Fig. 7 with a frequency of the driving voltage of 117 Hz, which is the 1st mode natural resonant frequency obtained in Section 3.1. Fig. 7(a) shows DSFPCB micromirror optical rotation angle with the driving voltage from 0 to 150 V. At 150 V square wave driving voltage, the rotation angle of the mirror plate achieves 9.43°, which should not be the maximum rotation since higher voltage (> 150 V) cannot be applied, because of simulation divergence. Simulation divergence caused by high voltage (e.g., > 150 V) also occurred in our simulations of repulsive-force (no “pull-in” effect) actuators/micromirrors using CoventorWare [15,40]. Fig. 7(b) shows an example simulation result with the driving voltage of
4. DS-FPCB micromirror prototype and test 4.1. Prototype of DS-FPCB micromirror Fig. 8 shows the prototype of the DS-FPCB micromirror. The fixed electrode copper area is bigger than the copper area of the rotating electrode for tolerance of misalignment during assembly. The rotating electrode, the double stage beams, the spacer, the wire pad, assemble holes, and mirror holder are fabricated using the standard FPCB fabrication process. The surface quality of the silicon mirror plate is measured using ZYGO 3D optical surface profiler. The radius of curvature 79
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plate can be fabricated in panel and boned together using an adhesive mask layer by machine and then separated later, such as to achieve low cost assembly. The size of assembled DS-FPCB micromirror is 10.3 mm × 8.8 mm × 2.0 mm. The total cost is a few dollars with low quantity.
4.2. Oscillation test The rotation angle of the DS-FPCB micromirror is tested by shooting a collimated laser beam on the mirror plate and measuring the length of the projected laser line. The setup is shown in Fig. 9. A 1.0 mm scale grid paper is placed 1-meter away as the projected screen and used to measure the length of the scanning laser line, and then to calculate the rotation angle. A low voltage driving signal (sinusoidal or square wave) is generated by a signal generator and then converted to 100–150 V by a high voltage amplifier. Two sinusoidal voltages with 180° phase difference based on Eqs. (2) and (3) are applied on the upper and lower fixed electrodes with the middle rotating electrode grounded. Fig. 10(a) shows the rotation results versus the scanning frequency. According to the testing result, its natural resonant frequency is 113 Hz, which is slightly different from the simulation result (117 Hz). The discrepancy should be from the alignment error when assembling the mirror plate. The test results do not show a different rotation angle with 100 V and 120 V driving when the frequency is further away from the natural frequency, e.g., the DC rotation angle, because the measurement cannot reflect very small difference due to the 1.0 mm laser beam size and the 1.0 mm scale grid screen paper. Fig. 10(b) shows rotation angle versus the driving voltage at its resonant frequency. The square wave driving voltage results in 28% more rotation angle than the sinusoidal wave driving voltage. It is much easier to generate a square wave driving voltage for the control circuit. The length of the projected laser line can be controlled by adjusting the amplitude of driving voltage. At 150 V square wave driving voltage, the rotation angle is 10.87° which is 90% larger than the previously developed single stage FPCB micromirrors. Four prototypes were made and tested. Table 1 lists the testing results which shows a 3.25° average rotation angle at 100 V sinusoidal driving. The discrepancy in testing angles and simulation results, as well as the variations in prototypes lie in: (1) The un-flatness of the rotating electrode and its variation, changes the upper and lower electrostatic gaps; (2). The torsion beam (beam1) is not completely flat and parallel to the fixed electrode. Small un-flatness of the torsion beam causes significant tilting of the rotating electrode.
Fig. 8. DS-FPCB micromirror prototype. (a) Upper and lower fixed plate. (b) Rotating plate. (c) Mirror plate on the mirror holder. (d) Assembled DS-FPCB micromirror.
(ROC) of the mirror plate is 15 m with roughness of 3 nm. It is much better than conventional MEMS micromirrors (e.g., ROC < 1 m, if not mirror plate bonding to the released micro actuator). The assembled prototype is shown in Fig. 8(d) which includes the soldered wires connection for the rotating, upper and lower electrodes. The mirror plate is coated with the Epoxy glue first and then bonded on top of the mirror holder. The manual bonding will not cause large misalignment because the mirror holder is only 0.5 mm larger than the mirror plate. Manual bonding relies on the edges alignment with eyes. The rotating plate is sandwiched by the upper and lower fixed plates through a screw nut set. The assembly holes on all three plates were first aligned and then fixed by the screw, which will ensure the alignment of the three electrodes. It takes time to assemble a prototype manually. However, for mass production the FPCB rotating plate and the FR4 PCB fixed
Fig. 9. Setup of the measurement system.
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Fig. 10. Measured DS-FPCB micromirror optical rotation angle. (a) Rotation angle versus frequency with sinusoidal driving voltage. (b) Rotation angle versus driving voltage at its resonant frequency.
ratio is 0.025. The fatigue life of the polyimide beam has been verified in the availability indicator application of FPCB micromirror [30], which uses the same materials as the beams in the DS-FPCB micromirror. The beams in the availability indicator have been bending for 6.39 billion cycles for more than two years’ continuous running without any visible performance deterioration.
Table 1 Prototypes performance. Prototype
1
2
3
4
Average
Resonant frequency/Hz Rotation angle at 100 V sinusoidal driving
113 4.06°
113 3.55°
124 2.58°
119 2.81°
117 3.25°
4.3. Transient response test
5. Laser line generator
A position sensitive detector (PSD) is used to replace the test grid paper at a very close distance to test the DS-FPCB micromirror's dynamic response. The result is shown in Fig. 11 which is recorded by an Oscilloscope and replotted in Excel. The orange line is the control signal when driving only the upper fixed electrode and the blue line is the PSD output signal, which corresponds to the rotation angle of the DS-FPCB micromirror. According to the test result in Fig. 11(a), it takes around 140 ms for the micromirror to enter stable oscillation. This short time will not cause any noticeable delay in a laser line generator. The damping ratio used in simulations is obtained by the free vibration decay. It can be calculated according to Eq. (4) [41].
δ=
1 X ln 1 2πn Xn + 1
5.1. The prototype of laser line generator Fig. 12 shows the prototype of the DS-FPCB micromirror based laser line generator. Fig. 12(a) and (b) shows the bottom and top side of the control circuit. The control circuit includes: (1) A high voltage step up circuit to convert the 5.0 V power supply to 50–150 V range driving voltage for the electrostatic actuation (2) An operational-amplifier based signal generator to generate a 50% duty cycle 5.0 V square wave signal; (3) A transistor based switching circuit to amplify the driving single. The signal frequency can be adjusted from 10 to 700 Hz to fit different natural frequency units through a potentiometer as shown in Fig. 12(b). Another potentiometer is used to adjust the amplitude of the output voltage from 50 V to 150 V for changing the length of scanning laser line. The control circuit is powered by a 5.0 V power supply with a total current of about 60 mA, including the current for the laser module.
(4)
where X1 and Xn+1 are the two peak amplitudes separated by n cycles. According to the Fig. 11(b), X1 = 3.24 and X6 = 1.48, so the damping 81
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Fig. 13. Comparison of the DS-FPCB micromirror laser line generator with conventional one. (a) Projection distance of 1.0 m. (b) Projection distance of 2.0 m. (c) Projection distance of 4.0 m. (d) Projection distance of 6.0 m.
The DS-FPCB micromirror and the laser are mounted on the circuit board. All parts are integrated in a 35 mm × 35 mm × 20 mm box as shown in Fig. 12(c). Fig. 12(d) shows the micromirror based laser line generator and the projected laser line.
Fig. 11. Tested transient response of the DS-FPCB micromirror. (a) Transient response. (b) Damping ratio test. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 12. Laser line generator. (a) Bottom side the driving circuit. (b) Top side the driving circuit. (c) Assembled laser line generator. (d) Projected laser line.
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Fig. 14. Laser power measurement along the laser line. (a) Close to the edge of laser line. (b) The center of laser line. (c) Laser power at projection distance of 1 m. (d) Laser power at projection distance of 0.5 m.
5.2. Comparison with conventional laser line generator
line, in comparison to conventional laser line generator.
Fig. 13 shows the projected laser lines at different distance for two laser line generators, i.e., the conventional lens based and the DS-FPCB micromirror based laser line generators. Both of them have the same laser source power, e.g., about 2.0 mW. The conventional one is FLL52P-635-15 from World Star Technology Inc. with 15° fan angle. In Fig. 13 the upper laser line is generated by the conventional one and the lower line is generated by the DS-FPCB micromirror based laser line generator. The two laser lines have the same length and brightness in 1.0 m distance as shown in Fig. 13(a). The conventional laser line generator has a fixed fan angle, so it will project a longer laser line with further distance. The laser power density will become weaker because the same laser power will be distributed in a longer laser line. However the laser lines generated by the DS-FPCB micromirror line generator can keep the same length and same brightness by adjusting the scanning angle. The laser power at various projection distance is measured by a high-speed photodetector, i.e., DET36A with 3.6 × 3.6 mm active area. Fig. 14(a) and (b) shows measuring spots close to the edge and the center of laser line. The peak laser power is constant at different locations of the line with various distances as shown in Fig. 14(c) and (d), i.e., 2 mW with different pulse width which is the duration when the scanning laser beam passes the photodetector. Fig. 14(c) and 14(d) combine the measurement data for different locations recorded by an Oscilloscope. On the contrary, the laser power on the laser line generated by conventional generator is much lower with significant variation at different projection distances, e.g., 0.13 mW (projection distance of 0.25 m) and 0.04 mW (projection distance of 1.0 m).
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6. Conclusion A double stage FPCB (DS-FPCB) micromirror and a laser line generator based on it have been developed in this paper. The DS-FPCB micromirror was modeled, simulated, fabricated and tested, which can output a maximum rotation angle of 90% higher than that of previous FPCB micromirrors while retaining advantages of: (1) large aperture and high surface quality (high flatness and good roughness); (2) low cost. A laser line generator based on the DS-FPCB micromirror is built and tested. It shows the benefits of consistent and high laser power density at various projection distance or various locations on the same 83
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