Investigation of Electromagnetic Angle Sensor

applied sciences Article

Investigation of Electromagnetic Angle Sensor Integrated in FR4-Based Scanning Micromirror Quan Wen 1,3 , Hongjie Lei 1, *, Fan Yu 1 , Dongling Li 1 , Yin She 1 , Jian Huang 1,2 , Liangkun Huang 1 and Zhiyu Wen 1 1

2 3

*

Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China; [email protected] (Q.W.); [email protected] (F.Y.); [email protected] (D.L.); [email protected] (Y.S.); [email protected] (J.H.); [email protected] (L.H.); [email protected] (Z.W.) College of Information Engineering, Qujing Normal University, Qujing 655000, China Fraunhofer Institute for Electronic Nano Systems (ENAS), 09126 Chemnitz, Germany Correspondence: [email protected]; Tel.: +86-023-6511-1010

Received: 5 November 2018; Accepted: 26 November 2018; Published: 28 November 2018







Abstract: This paper performs a detailed investigation on the electromagnetic angle sensor integrated in the flame retardant 4 (FR4)-based scanning micromirror. An accurate theoretical model is presented, especially considering the coupling effect between the driving and sensing coils. Experimental results agree well with the theoretical results, and show a sensitivity of 55.0 mVp /◦ and a high signal-to-noise ratio (SNR) of 71.9 dB. Moreover, the linearity of the angle sensor can still reach 0.9995, though it is affected slightly by the coupling effect. Finally, the sensor’s good feasibility for feedback control has been further verified through a simple closed-loop control circuit. The micromirror operated with closed-loop control possesses better long-term stability and temperature stability than that operated without closed-loop control. Keywords: integrated angle sensor; electromagnetic; coupling effect; scanning micromirror; closed-loop control; flame retardant 4 (FR4)

1. Introduction Scanning micromirrors are essential core elements in various optical microsystems (e.g., barcode reader, endoscopic probe, and LiDAR) [1–4]. Especially, a large-aperture and low-frequency micromirror is required for a broad range of applications, such as fluorescence microscopes [5], laser projection [6,7], micro-spectrometers [8–10], etc. However, conventional Si-based MEMS (microelectromechanical systems) micromirrors cannot meet the requirements because large-aperture and low-frequency Si micromirrors are fragile, and cannot survive the environmental shocks and vibrations [8,11]. In comparison, flame retardant 4 (FR4)-based micromirrors are more promising, due to the inherent flexibility of FR4 [7,12–14]. Until now, FR4 is the most widely used material for printed circuit boards (PCBs). Thus, commercial PCB technology, with low cost and a short process cycle, can be employed to fabricate the flexure and driving parts. After that, a mirror plate fabricated by dicing an aluminum-coated Si wafer is bonded to achieve large aperture and high surface quality. In general, the micromirror must scan the optical beam at a specific optical scan angle in practical applications. However, the current FR4 micromirror with a simple open-loop control cannot guarantee it, owing to the relatively poor stability of FR4 [12,15]. Therefore, an angle sensor is highly desired to monitor the scan angle in real time, and further form a feedback control. External optical sensors may be a good solution to provide a real-time feedback signal [4,16,17]. However, this requires a complex assembly, and inevitably increases the system’s volume and cost. Appl. Sci. 2018, 8, 2412; doi:10.3390/app8122412

www.mdpi.com/journal/applsci

Appl. Sci. 2018, 8, 2412

2 of 11

Another simple, yet highly effective alternative, is the integrated angle sensor. Several corresponding sensing methods have already been proposed and applied in MEMS micromirrors, such as Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 12 piezoresistive [18,19], capacitive [20], piezoelectric [21,22], and electromagnetic [9,23]. Among these methods, electromagnetic is more suitableistothe be integrated the aforementioned Another simple, yet angle highlysensing effective alternative, integrated in angle sensor. Several FR4 corresponding sensing methodsprocess have already been proposed andprevious applied inwork MEMS micromirrors, micromirror, due to its excellent compatibility. In our [15], we presented such as piezoresistive [18,19], capacitive [20], piezoelectric [21,22], and electromagnetic [9,23]. a FR4-based electromagnetic scanning micromirror monolithically integrated with an electromagnetic Among these methods, electromagnetic angle sensing is more suitable to be integrated in the angle sensor. However, it focused more on basic prototyping, driving performance, and reliability aforementioned FR4 micromirror, due to its excellent process compatibility. In our previous work of the micromirror. As a continuation of previous work, this paper performed accurate modeling [15], we presented a FR4-based electromagnetic scanning micromirror monolithically integrated and detailed tests of the integrated angle sensor. Particularly, we study the coupling effect between with an electromagnetic angle sensor. However, it focused more on basic prototyping, driving the driving coil and sensing coil, which has not been reported by other groups, up to now. Finally, performance, and reliability of the micromirror. As a continuation of previous work, this paper a simple closed-loop control circuit is developed toof further verify the angle sensor’s good feasibility performed accurate modeling and detailed tests the integrated angle sensor. Particularly, we for providing a real-time feedback signal. study the coupling effect between the driving coil and sensing coil, which has not been reported by This as follows. Section 2 control introduces the workingto further principle of the other paper groups, is up organized to now. Finally, a simple closed-loop circuit is developed verify the angle sensor’s feasibility for providing feedback electromagnetic angle good sensor integrated in the FR4a real-time micromirror, and signal. presents the modeling. Section 3 paper is organized as follows. introduces the working the discussesThis the test results and compares it withSection model2results. Furthermore, the principle frequencyofresponse electromagnetic angle sensor integrated in the FR4 micromirror, and presents the modeling. Section and phase-frequency characteristics of the angle sensor, and the closed-loop control, are also described discusses the test results and compares it with model results. Furthermore, the frequency response in this3 section. The conclusion is summarized in Section 4. and phase-frequency characteristics of the angle sensor, and the closed-loop control, are also described in this section. The conclusion is summarized in Section 4. 2. Principle and Modeling

2. Principle and Modeling Figure 1a shows the schematic drawing of the assembled FR4 micromirror integrated with angle sensor. The device a FR4 platform, a 11.7 mm ×FR4 10.3micromirror mm aluminum-coated silicon Figure 1a shows comprises the schematic drawing of the assembled integrated with mirror plate on it, a pair of permanent magnets assembled in parallel, and the package structure angle sensor. The device comprises a FR4 platform, a 11.7 mm × 10.3 mm aluminum-coated silicon (i.e., the coverplate and baseplate). The magnets thicknesses of theinFR4 platform and siliconstructure mirror plate mirror plate on it, a pair of permanent assembled parallel, and the package (i.e., are theand coverplate and baseplate). The of the platform and silicon in mirror plate 0.4inner 0.4 mm 0.5 mm, respectively. Thethicknesses layout of the FR4FR4 platform is illustrated Figure 1b.are The mm and 0.5 mm, respectively. The layout of the FR4 platform is illustrated in Figure 1b. The inner sensing coil (double-layer planar Cu coil) and outer driving coil (single-layer planar Cu coil) are sensing coil integrated (double-layer planar coil) and outer adriving coilsignal (single-layer planar Cu coil) are simultaneously into the Cu platform. When driving with the resonant frequency simultaneously integrated into the platform. When a driving signal with the resonant frequency (361.8 Hz) of the micromirror is applied in the driving coil, the generated Lorentz force will actuate the (361.8 Hz) of the micromirror is applied in the driving coil, the generated Lorentz force will actuate mirror to oscillate around the torsion bars at resonance. At the same time, an electromotive force is the mirror to oscillate around the torsion bars at resonance. At the same time, an electromotive force induced in the sensing coil. Therefore, the induced electromotive can be read out as a sensor signal for is induced in the sensing coil. Therefore, the induced electromotive can be read out as a sensor signal monitoring the variation of the of deflection angle. for monitoring the variation the deflection angle.

Figure (a) Schematic drawing the assembled FR4-based FR4-based electromagnetic scanning micromirror Figure 1. (a)1.Schematic drawing of of the assembled electromagnetic scanning micromirror integrated angle sensor. Layoutofofthe theintegrated integrated sensing FR4 platform. integrated withwith angle sensor. (b)(b) Layout sensingcoil coilininthe the FR4 platform.

The motion (mechanical half deflectionangle angle θ) θ) of of the micromirror cancan be expressed The motion (mechanical half deflection theoscillating oscillating micromirror be expressed by the following dynamical equation: by the following dynamical equation:

 J..mθ + C . θ + Kθ = T ,

Jm θ + C θ + Kθ = T,

(1)

(1)

Appl. Sci. 2018, 8, 2412

3 of 11

where Jm , C, K, and T represent the moment of inertia of the platform with Si mirror plate, the damping coefficient, the torsion stiffness of torsion bars, and the driving torque, respectively. As a sinusoidal driving voltage signal, Ud = Ud0 sinωt is applied in the driving coil, and the corresponding driving current Id is Id = q

Ud Rd 2 + (ωLd )

2

= q

Ud0 Rd 2 + (ωLd )2

sin ωt,

(2)

where Rd and Ld represent the resistance and inductance of the driving coil, respectively. Thus, the driving torque can be described as T = Id BMd ,

(3)

where B is the magnetic flux density produced by the magnets; Md is identified as the area sum of all the driving coil, and it can be calculated from  Nd  −b − n − 1 a + b Md = ∑ w−2s ( )( ) · ((2l − 4s − 3a − 5b) − 4(n − 1)( a + b)) 2  n =1  −b − N − 1 a + b · ((l − 2s − 2a − 3b) − 2( Nd − 1)( a + b)), − w−2s ( )( ) d 2

(4)

where Nd is the number of the driving coil turns, l and w are the length and width of the FR4 platform, respectively, a is the spacing of adjacent coils, b is the width of coil, and s is the width of the exterior border zone of the platform. Therefore, by solving Equation (1), the mechanical half deflection angle θ at resonant frequency ω n is  π θ = θ0 sin ωn t − , (5) 2 where θ 0 is the amplitude of deflection angle and is calculated as θ0 =

U BMd Q q d0 . K Rd 2 + (ωLd )2

(6)

Meanwhile, the moving induced electromotive force εs in the sensing coil can be described as .

ε s = BMs θ,

(7)

where Ms is identified as the area sum of all the sensing coil, and it can be calculated from Ns



a + 32 b + (n − 1)( a + b) · ((5a + 7b) + 4(n − 1)( a + b)) n =1

− a + 32 b + ( Ns − 1)( a + b) · ((3a + 4b) + 2( N  s − 1)( a + b))

w 3 l + 2 − s · a + 2 b + ( Ns − 1)( a + b) + 2 − s ,

Ms = 2 ∑

(8)

where Ns is the number of the sensing coil turns. All the aforementioned parameters are listed in Table 1. Table 1. Main parameters of the FR4 micromirror integrated with angle sensor. Parameters

l

w

Nd

Ns

a

b

s

Q

K

B

Value Unit

12 mm

12 mm

11 -

9 -

4 mil

4 mil

0.5 mm

59 -

0.016 N·m/rad

100 mT

Table 1. Main parameters of the FR4 micromirror integrated with angle sensor.

Parameters Value Appl. Sci. 2018,Unit 8, 2412

l 12 mm

w 12 mm

Nd 11 -

Ns 9 -

a 4 mil

b 4 mil

s 0.5 mm

Q 59 -

K 0.016 N·m/rad

B 100 mT

4 of 11

By substituting Equation (5) to Equation (7), the moving induced electromotive force εs in the By substituting Equation (5) to Equation (7), the moving induced electromotive force εs in the sensing coil can be expressed as sensing coil can be expressed as ωt . ε εs s==θθ ωt. (9) n BM s sin 0ω 0ω n BM s sin According According to to Equation Equation (9), (9), we we can can see see that that the the amplitude amplitude of of the the moving moving induced induced electromotive electromotive force force is proportional to that of the deflection angle. Moreover, it has a phase difference of π/2 with is proportional to that of the deflection angle. Moreover, it has a phase difference of π/2 with the the deflection deflection angle angle of of the the micromirror, micromirror,and andisisin inphase phasewith withthe thedriving drivingvoltage. voltage. However, sensor output voltagevoltage consists consists of not only moving-induced electromotive However, the theangle angle sensor output of the not only the moving-induced force but, also, the coupling voltage coupled from the driving signal. The coupling effect thecoupling driving electromotive force but, also, the coupling voltage coupled from the driving signal. of The and can be described a coupled model, its equivalent is effectsensing of thecoils driving and sensing as coils can beinductor described as a and coupled inductor circuit model,model and its shown in Figure 2a. The resistances of the driving coil and sensing coil are 3.7 Ω and 2.4 Ω, respectively. equivalent circuit model is shown in Figure 2a. The resistances of the driving coil and sensing coil In to better analyze the couplingIneffect andtoobtain corresponding parameters, areorder 3.7 Ω and 2.4 Ω, respectively. order betterthe analyze the coupling effect electromagnetic and obtain the field finite element analysis (FEA) of the devicefield is performed by using the Ansoft Maxwell corresponding parameters, electromagnetic finite element analysis (FEA) of thesimulation device is tool. All the material properties used in the simulation are from the material library of the software. performed by using the Ansoft Maxwell simulation tool. All the material properties used in the The finite element model the mirror consisting the FR4 The substrate, sensing coils, and simulation are from the of material library of the of software. finite driving elementand model of the mirror an aluminum-coated mirror plate,and is shown in coils, Figureand 2b,an as aluminum-coated well as the parameters obtained consisting of the FR4 silicon substrate, driving sensing silicon mirror through magnetostatic analysis. We can see parameters that the coupling effect is weak, due to the small coupling plate, is shown in Figure 2b, as well as the obtained through magnetostatic analysis. We coefficient (Kc the = 0.23606). to our previous work, thecoupling current incoefficient the driving is much c = 0.23606). can see that couplingAccording effect is weak, due to the small (Kcoil larger than to that in previous the sensing coil. the Thus, the coupling effect ofcoil theissensing onthan the driving According our work, current in the driving much coil larger that in coil the can be neglected. We only need to analyze the coupling voltage in the sensing coil coupled from We the sensing coil. Thus, the coupling effect of the sensing coil on the driving coil can be neglected. driving signal. only need to analyze the coupling voltage in the sensing coil coupled from the driving signal.

2. (a) (a) An Anequivalent equivalentcoupled coupledinductor inductorcircuit circuit model describe coupling effect between Figure 2. model to to describe thethe coupling effect between the the driving sensing coils. Electromagnetic field finite element model of the mirror consisting driving andand sensing coils. (b) (b) Electromagnetic field finite element model of the mirror consisting of the FR4 substrate, driving and sensing of the FR4 substrate, driving and sensingcoils, coils,and andan analuminum-coated aluminum-coatedsilicon siliconmirror mirrorplate, plate, and and the parameters obtained by by magnetostatic magnetostatic analysis. analysis. Rdd and Rss are the resistances, and LLdd and Lss are the inductances of the driving coil and sensing coil, respectively. M is the mutual inductance between the driving and sensing coils, Kc is the coupling coefficient, and Usc is the coupling voltage in the sensing coil coupled from the driving signal.

Appl. Sci. 2018, 8, 2412

5 of 11

The coupling voltage Usc in the sensing coil coupled from the driving signal can be expressed as .

Usc = M I d .

(10)

By substituting Equation (2) to Equation (10), the coupling voltage Usc in the sensing coil can be given as a function of the driving voltage: Usc = M q

Ud0 ω Rd 2 + (ωLd )2

 π sin ωt + . 2

(11)

According to Equation (11), we can obtain that the amplitude of the coupling voltage in the sensing coil is proportional to that of the driving voltage. The coupling voltage in the sensing coil has a phase difference of π/2 with the driving voltage. Therefore, while considering the coupling voltage in the sensing coil coupled from the driving signal, the output voltage Us of the integrated angle sensor is Us = ε s + Usc .

(12)

According to Equations (6), (9), (11), and (12), the output voltage of the integrated angle sensor can be calculated as s  2 MK 2 sin(ωt + ϕ), (13) Us = θ0 ω ( BMs ) + BMd Q where ϕ = arcsin r

1  B 2 Md Ms Q 2 MK

,

(14)

+1

where ϕ is the phase difference between the output voltage Us and the moving-induced electromotive force εs . Equations (13) and (14) give the relationship of the output voltage of the sensor and the deflection angle of the micromirror considering the coupling effect in theory. 3. Experimental Results and Discussion Figure 3a shows the schematic drawing of the measurement setup of the angle sensor. The micromirror works at the resonance point (361.8 Hz). The scan angle is controlled by adjusting the driving voltage amplitude, and can be calculated according to the length of laser scan line and the distance between the optical screen and the micromirror. The length of laser scan line can be measured accurately through a position sensitive detector (PSD). Meanwhile, the corresponding output signal of the angle sensor is amplified by a simple voltage amplifier with a gain of 400, and then measured using an oscilloscope. After this amplifier, the signal amplitude can reach a level of several hundred millivolts, which is large enough to be detected by the oscilloscope. The photograph of the real experimental setup is shown in Figure 3b. In order to measure the coupling voltage coupled from the driving signal, and keep the device virtually undamaged at the same time, we take away the magnets of the assembled micromirror, so that the mirror plate cannot move due to the absence of a magnetic field, and then excite the driving coil to see the output of the sensing coil. Figure 4 illustrates the comparison of the driving signal, sensor output, and coupling signal in the sensing coil coupled from the driving signal in time domain. We can observe that the sensor output signal is nearly in phase with the driving signal. The small phase difference of 5.1◦ is caused by the superposed coupling signal in the sensing coil, and it is close to the theoretical value of 4.4◦ calculated by Equation (14). Meanwhile, the coupling signal in the sensing coil has a phase difference of 89.6◦ with the driving signal, which is also very close to the theoretical value of 90◦ calculated

Appl. Sci. 2018, 8, x FOR PEER REVIEW

6 of 12

Appl. Sci. 2018, 8, 2412

6 of 11

from Equation (11). Both the minor errors between the experimental and theoretical values could be Appl. Sci. 2018, x FOR PEERnoise REVIEW 6 of 12 attributed to8,electronic and a little measurement error.

Figure 3. (a) Schematic drawing and (b) photograph of the measurement setup.

Figure 4 illustrates the comparison of the driving signal, sensor output, and coupling signal in the sensing coil coupled from the driving signal in time domain. We can observe that the sensor output signal is nearly in phase with the driving signal. The small phase difference of 5.1° is caused by the superposed coupling signal in the sensing coil, and it is close to the theoretical value of 4.4° calculated by Equation (14). Meanwhile, the coupling signal in the sensing coil has a phase difference of 89.6° with the driving signal, which is also very close to the theoretical value of 90° calculated from Equation (11). Both the minor between experimental and theoretical Figure 3. (a) and (b) (b)errors photograph of the thethe measurement setup. Figure 3. (a) Schematic Schematic drawing drawing and photograph of measurement setup. values could be attributed to electronic noise and a little measurement error. Figure 4 illustrates the comparison of the driving signal, sensor output, and coupling signal in the sensing coil coupled from the driving signal in time domain. We can observe that the sensor output signal is nearly in phase with the driving signal. The small phase difference of 5.1° is caused by the superposed coupling signal in the sensing coil, and it is close to the theoretical value of 4.4° calculated by Equation (14). Meanwhile, the coupling signal in the sensing coil has a phase difference of 89.6° with the driving signal, which is also very close to the theoretical value of 90° calculated from Equation (11). Both the minor errors between the experimental and theoretical values could be attributed to electronic noise and a little measurement error.

Figure 4. 4. Comparison of the driving signal, sensor output and coupling signal in the sensing sensing coil coil coupled from from the the driving driving signal signal in in time time domain. domain.

Figure 5a between the the coupling voltage in theinsensing coil andcoil theand driving Figure 5a plots plotsthe therelationship relationship between coupling voltage the sensing the voltage. We can see that the amplitude of the coupling voltage is proportional to that of the driving driving voltage. We can see that the amplitude of the coupling voltage is proportional to that of the voltage. The testThe datatest is data in good with the theoretical result result obtained by Equation (11), driving voltage. is inagreement good agreement with the theoretical obtained by Equation although there there is a little between them. The deviation is caused the minor error between (11), although is adeviation little deviation between them. The deviation is by caused by the minor error the real and simulated mutual inductances. Owing to the difference between the real and simulated between the real and simulated mutual inductances. Owing to the difference between the real and material parameters,parameters, the simulated inductance inevitably deviates a little from the real value. simulated themutual simulated mutual inductance inevitably deviates a little Figure material 4. Comparison of the driving signal, sensor output and coupling signal in the sensing coilfrom According to Figure 5b, we can see a clear linear relationship between the sensor output voltage the real value. coupled from the driving signal in time domain. and the optical scan angle of the micromirror. The correlation coefficient r can reach 0.9995. This is well consistent with the theoretical relation determined by Equation (13). The sensitivity of the angle sensor Figure 5a plots the relationship between the coupling voltage in the sensing coil and the can be determined as 55.0 mVp /◦ , which is also very close to the theoretical value of 55.2 mVp /◦ , driving voltage. We can see that the amplitude of the coupling voltage is proportional to that of the calculated by Equation (13). The difference between the experimental and theoretical values can be driving voltage. The test data is in good agreement with the theoretical result obtained by Equation attributed to the process error. In addition, a little nonlinear behavior of the test result is observed in (11), although there is a little deviation between them. The deviation is caused by the minor error Figure 5b. It is caused by the coupling effect between the driving and sensing coils. By comparing between the real and simulated mutual inductances. Owing to the difference between the real and Equation (9) with (13), we can find that the parameter of torsional stiffness K is introduced in simulated material parameters, the simulated mutual inductance inevitably deviates a little from Equation (13) when considering the coupling voltage in the sensing coil coupled from the driving signal. the real value. Owing to the nonlinear spring effect of the torsion bars [15,24], K is not a constant, but varies with the change of the scan angle. Thus, a little nonlinear behavior of the test result is observed. The theoretical result is perfectly linear because K is assumed to be a constant in the theoretical calculation.

Appl. Sci. 2018, 8, 2412

7 of 11

Appl. Sci. 2018, 8, x FOR PEER REVIEW

7 of 12

Figure 5. (a) Relation between the coupling voltage in the sensing coil and the driving voltage. (b) Relation between the sensor output and the optical scan angle. Note: all the voltage values are measured as peak value.

According to Figure 5b, we can see a clear linear relationship between the sensor output voltage and the optical scan angle of the micromirror. The correlation coefficient r can reach 0.9995. This is well consistent with the theoretical relation determined by Equation (13). The sensitivity of the angle sensor can be determined as 55.0 mVp/°, which is also very close to the theoretical value of 55.2 mVp/°, calculated by Equation (13). The difference between the experimental and theoretical values can be attributed to the process error. In addition, a little nonlinear behavior of the test result is observed in Figure 5b. It is caused by the coupling effect between the driving and sensing coils. By comparing Equation (9) with (13), we can find that the parameter of torsional stiffness K is introduced in Equation (13) when considering the coupling voltage in the sensing coil coupled from the driving Owing to the nonlinear spring of thecoil torsion bars [15,24], K is(b)not a Figure 5. 5.signal. (a) between the coupling ineffect thethe sensing andand the driving voltage. Figure (a) Relation Relation between the couplingvoltage voltage in sensing coil the driving voltage. constant, but between varies with the change ofand the scan angle. Thus, a little nonlinear behavior of are the test Relation thethe sensor output scan angle. Note: all are (b) Relation between sensor output andthe theoptical optical scan angle. Note: all the the voltage values result is observed. The theoretical result is perfectly linear because K is assumed to be a constant in measured as value. measured as peak peak value. the theoretical calculation. The frequency frequency spectrum from to (surrounding the was to According to Figure 5b,from we 00can see Hz a clear linear relationship between the sensor output The spectrum to 800 800 Hz (surrounding the 361.8 361.8 Hz) Hz) was measured measured to identify identify the signal-to-noise ratio (SNR) of the angle sensor. In this test, the sensor signal is recorded in voltage and the optical angle of the micromirror. coefficient reach 0.9995. the signal-to-noise ratioscan (SNR) of the angle sensor. In The this correlation test, the sensor signal riscan recorded in the the oscilloscope after going through the aforementioned amplifier and a fourth-order bandpass filter. This is well consistent with the theoretical relation determined by Equation (13). The sensitivity of oscilloscope after going through the aforementioned amplifier and a fourth-order bandpass filter. Then, the frequency spectrum can be obtained through a fast Fourier transform of the time-domain the angle can spectrum be determined asobtained 55.0 mVpthrough /°, whichaisfast also very close to the of theoretical value of Then, the sensor frequency can be Fourier transform the time-domain ◦ , and the signal. Figure 6a,b measured frequency spectrum at scan 55.2 mV p/°, calculated Equation (13). The difference between the experimental and theoretical signal. Figure 6a,b show showbythe the measured frequency spectrum at the the optical optical scan angle angle of of 4.6 4.6°, and the corresponding angle sensor output in time domain, respectively. This signal shows a great values can be attributed to the process error. In addition, a little nonlinear behavior of the test result corresponding angle sensor output in time domain, respectively. This signal shows a great signal signal quality of 71.9 dB SNR, which is beneficial for feedback control. is observed in dB Figure It is caused by the between the driving and sensing coils. quality of 71.9 SNR,5b. which is beneficial forcoupling feedbackeffect control. By comparing Equation (9) with (13), we can find that the parameter of torsional stiffness K is introduced in Equation (13) when considering the coupling voltage in the sensing coil coupled from the driving signal. Owing to the nonlinear spring effect of the torsion bars [15,24], K is not a constant, but varies with the change of the scan angle. Thus, a little nonlinear behavior of the test result is observed. The theoretical result is perfectly linear because K is assumed to be a constant in the theoretical calculation. The frequency spectrum from 0 to 800 Hz (surrounding the 361.8 Hz) was measured to identify the signal-to-noise ratio (SNR) of the angle sensor. In this test, the sensor signal is recorded in the oscilloscope after going through the aforementioned amplifier and a fourth-order bandpass filter. Then, the frequency spectrum can be obtained through a fast Fourier transform of the time-domain signal. Figure 6a,b show the measured frequency spectrum at the optical scan angle of 4.6°, and the corresponding angle sensor output in time domain, respectively. This signal shows a great signal quality of 71.9 dB SNR, which is beneficial for feedback control.

◦ . (b) The Figure Figure 6. 6. (a) Frequency spectrum of the sensor output at the optical optical scan scan angle angle of of 4.6 4.6°. (b) The corresponding signal in time domain. corresponding signal in time domain.

Figure 7 shows the frequency characteristics of the angle sensor compared to the optical scan angle obtained from PSD measurement. In the measurement, the micromirror is actuated by the sinusoidal sweep signal from 348 Hz to 378 Hz. The frequency response functions of the micromirror are measured by the angle sensor and PSD, respectively. We can see that two frequency response functions have almost the same overall profile, which means that the angle sensor can still work, even though the micromirror is off the resonance. In addition, the phase curve in Figure 7 shows that the phase of the sensor output voltage will change when the micromirror oscillates off the resonance. Therefore, the resonance can be tracked by monitoring the phase difference between the sensor output and driving signals, i.e., a phase-lock loop [21]. Figure 6. (a) Frequency spectrum of the sensor output at the optical scan angle of 4.6°. (b) The corresponding signal in time domain.

micromirror are measured by the angle sensor and PSD, respectively. We can see that two frequency response functions have almost the same overall profile, which means that the angle sensor can still work, even though the micromirror is off the resonance. In addition, the phase curve in Figure 7 shows that the phase of the sensor output voltage will change when the micromirror Appl. Sci. 2018, 8, 2412 8 of 11 oscillates off the resonance. Therefore, the resonance can be tracked by monitoring the phase difference between the sensor output and driving signals, i.e., a phase-lock loop [21].

Figure 7. 7. Frequency Frequency response response function function of of the the micromirror micromirror measured measured by by the the angle angle sensor sensor and and position position Figure sensitive andand phase curve of theofangle The phase the phase the difference sensitivedetector detector(PSD) (PSD) phase curve the sensor. angle sensor. Therepresents phase represents phase between thebetween sensor output andoutput drivingand voltage. difference the sensor driving voltage.

Based on angle sensor signalsignal quality, a simple control circuit wascircuit developed Based onthe thegreat great angle sensor quality, a closed-loop simple closed-loop control was to further verify the sensor’s good feasibility for feedback control. Figure 8a demonstrates developed to further verify the sensor’s good feasibility for feedback control. Figure the 8a closed-loop control scheme. The micromirror is actuated to oscillate around the torsion bars the by demonstrates the closed-loop control scheme. The micromirror is actuated to oscillate around the sinusoidal driving signal. Meanwhile, the generated sinusoidal angle sensor signal,angle depending torsion bars by the sinusoidal driving signal. Meanwhile, the generated sinusoidal sensor on the deflection angle of the micromirror, is detected by the angle sensor signal acquisition module. signal, depending on the deflection angle of the micromirror, is detected by the angle sensor signal After goingmodule. throughAfter the rectifier-filter the signalmodule, is converted to aisDC voltagetosignal. acquisition going throughmodule, the rectifier-filter the signal converted a DC To stabilize theTo deflection the amplitude gain control module wasmodule introduced. The signal voltage signal. stabilize angle, the deflection angle, the amplitude gain control was introduced. of thesignal voltage the between DC voltage, the deflectionthe angle and theangle reference The of difference the voltagebetween difference the representing DC voltage, representing deflection and voltage signal,voltage is inputsignal, to the module. amplitude is mainly accomplished by the reference is input Then, to the the module. Then,gain thecontrol amplitude gain control is mainly multiplying the voltage difference with the sinusoidal signal generated by signal generation module. accomplished by multiplying the voltage difference with the sinusoidal signal generated by signal Therefore, amplitude of the driving signal can real time according to the generationthe module. Therefore, the amplitude ofbe theregulated driving in signal can be regulated in feedback real time signal of the angle sensor. according tointegrated the feedback signal of the integrated angle sensor. Long-term stability tests on the micromirror were operated continuously for 12 h, both with and without closed-loop control. Figure 8b shows the test result. The maximum variations of the optical scan angle are 3.04% and 7.83%, respectively. Hence, the micromirror operated with closed-loop control possesses better long-term stability. This result indicates the angle sensor’s good feasibility for feedback control. Finally, temperature stabilities of the micromirror, operated both with and without closed-loop control, were also tested. In this test, the micromirror is placed in a high and low temperature test chamber. The temperature is increased from 20 to 50 ◦ C in increments of 5 ◦ C under atmospheric pressure. The test result is shown in Figure 9. With the change of temperature, the resonant frequency of the device varies, and the micromirror will work off the resonance [21,25]. Hence, we can see that the optical scan angle decreases with the increase of temperature. Without closed-loop control, the optical scan angle decreases from 4.6◦ to 1◦ as the temperature rises from 20 to 50 ◦ C. On the other hand, with closed-loop control, the optical scan angle decreases from 4.6◦ to 2.5◦ . By comparison, the micromirror operated with closed-loop control possesses relatively better temperature stability. It indicates that the feedback control, based on the integrated angle sensor, is workable and is beneficial to improve the micromirror’s temperature stability. However, due to the limited adjustable voltage range of this analog closed-loop control circuit, and the lack of tracking-resonance function, the maximum variation of scan angle still reaches 45.65%. Therefore, it is highly desirable to develop a more precise digital closed-loop control circuit that can regulate the amplitude and frequency of the driving signal in follow-up work.

Appl. Sci. 2018, 8, 2412

9 of 11

Appl. Sci. 2018, 8, x FOR PEER REVIEW

9 of 12

Figure 8. diagram of of the the closed-loop closed-loop control control for for the the FR4 FR4 micromirror. micromirror. 8. Closed-loop control: (a) Block diagram (b) thethe FR4 micromirror operated withwith closed-loop control and (b) Comparison Comparisonofoflong-term long-termstabilities stabilitiesforfor FR4 micromirror operated closed-loop control Appl. Sci. 8, x FOR PEERcontrol. REVIEW 10 of 12 without closed-loop and2018, without closed-loop control.

Long-term stability tests on the micromirror were operated continuously for 12 h, both with and without closed-loop control. Figure 8b shows the test result. The maximum variations of the optical scan angle are 3.04% and 7.83%, respectively. Hence, the micromirror operated with closed-loop control possesses better long-term stability. This result indicates the angle sensor’s good feasibility for feedback control. Finally, temperature stabilities of the micromirror, operated both with and without closed-loop control, were also tested. In this test, the micromirror is placed in a high and low temperature test chamber. The temperature is increased from 20 to 50 °C in increments of 5 °C under atmospheric pressure. The test result is shown in Figure 9. With the change of temperature, the resonant frequency of the device varies, and the micromirror will work off the resonance [21,25]. Hence, we can see that the optical scan angle decreases with the increase of temperature. Without closed-loop control, the optical scan angle decreases from 4.6° to 1° as the temperature rises from 20 to 50 °C. On the other hand, with closed-loop control, the optical scan angle decreases from 4.6° to 2.5°. By comparison, the micromirror operated with for closed-loop control operated possesses Figure 9. of temperature stabilities the FR4 withrelatively closed-loop 9. Comparison FR4 micromirror micromirror operated closed-loopbetter temperature stability. indicates control. that the feedback control, based on the integrated angle sensor, is control and without withoutItclosed-loop closed-loop control. workable and is beneficial to improve the micromirror’s temperature stability. However, due to the 4. Conclusions 4. Conclusions limited adjustable voltage range of this analog closed-loop control circuit, and the lack of tracking-resonance function, the maximum variation of scan angleinstill reaches 45.65%. Therefore, it In this In this work, work, an an electromagnetic electromagnetic angle angle sensor sensor integrated integrated in the the FR4-based FR4-based micromirror micromirror is is is highly desirable to develop a more precise digital closed-loop control circuit that can regulate the investigated model was was developed developed to to predict predict the the sensor sensor output, investigated in in detail. detail. An An accurate accurate theoretical theoretical model output, amplitude considering and frequency ofcoupling the driving signal in follow-up work. especially especially considering the the coupling effect effect between between the the driving driving and and sensing sensing coils. coils. The The coupling coupling parameters (i.e., inductances of the driving coil and sensing coil, mutual inductance, and coupling parameters (i.e., inductances of the driving coil and sensing coil, mutual inductance, and coupling coefficient) in the model were obtained by electromagnetic field FEA simulation. Test results of the coupling voltage in the sensing coil and the sensor output are in good agreement with the theoretical results, which indicates the high accuracy of the theoretical model. Although the coupling effect of the driving coil on the sensing coil affects the linearity of the angle sensor, it can still reach 0.9995. Moreover, the integrated angle sensor has a sensitivity of 55.0 mVp/° and a high SNR of 71.9 dB.

Appl. Sci. 2018, 8, 2412

10 of 11

coefficient) in the model were obtained by electromagnetic field FEA simulation. Test results of the coupling voltage in the sensing coil and the sensor output are in good agreement with the theoretical results, which indicates the high accuracy of the theoretical model. Although the coupling effect of the driving coil on the sensing coil affects the linearity of the angle sensor, it can still reach 0.9995. Moreover, the integrated angle sensor has a sensitivity of 55.0 mVp /◦ and a high SNR of 71.9 dB. Additionally, the frequency response and phase-frequency characteristics of the angle sensor are also measured and discussed. Finally, the sensor’s good feasibility for feedback control has been verified through a simple closed-loop control circuit. Experimental results show that the micromirror operated with closed-loop control possesses better long-term stability and temperature stability than that operated without closed-loop control. In the future, the long-term stability and temperature stability of the micromirror can be further improved by developing a more precise digital closed-loop control circuit. Author Contributions: Q.W., H.L. and Z.W. conceived and designed the integrated angle sensor; H.L., F.Y. and D.L. deduced the theory; H.L., Y.S., J.H. and L.H. performed the experiments and analyzed the data; H.L. and F.Y. conceived and developed the closed-loop control circuit; Q.W. and H.L. wrote the paper; Z.W. revised it. Funding: This research was funded by the National Key R&D Program of China (Grant No. 2018YFF01011200), the Special Funds of the National Natural Science Foundation of China (Grant No. 61327002), the National Natural Science Foundation of China (Grant No. 61804016) and the Fundamental Research Funds for Central Universities (Grant No. 106112017CDJXY120006). Acknowledgments: The authors want to thank Xiaokang Yang, Chao Yang and Shan Lu for their valuable advice about the experiments. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

4. 5.

6. 7. 8. 9.

10. 11. 12.

Yalcinkaya, A.D.; Ergeneman, O.; Urey, H. Polymer magnetic scanners for bar code applications. Sens. Actuators A Phys. 2007, 135, 236–243. [CrossRef] Duan, C.; Tanguy, Q.; Pozzi, A.; Xie, H. Optical coherence tomography endoscopic probe based on a tilted mems mirror. Biomed. Opt. Express 2016, 7, 3345–3354. [CrossRef] [PubMed] Ito, K.; Niclass, C.; Aoyagi, I.; Matsubara, H.; Soga, M.; Kato, S.; Maeda, M.; Kagami, M. System design and performance characterization of a mems-based laser scanning time-of-flight sensor based on a 256 × 64-pixel single-photon imager. IEEE Photonics J. 2013, 5, 6800114. [CrossRef] Ye, L.; Zhang, G.; You, Z. Large-aperture khz operating frequency ti-alloy based optical micro scanning mirror for lidar application. Micromachines 2017, 8, 120. [CrossRef] Wang, Y.; Gokdel, Y.D.; Triesault, N.; Wang, L.; Huang, Y.Y.; Zhang, X. Magnetic-actuated stainless steel scanner for two-photon hyperspectral fluorescence microscope. J. Microelectromech. Syst. 2014, 23, 1208–1218. [CrossRef] Jeong, H.M.; Park, Y.H.; Lee, J.H. Slow scanning electromagnetic scanner for laser display. J. Microlithogr. Microfabr. Microsyst. 2008, 7, 1589–1604. Zuo, H.; He, S. Fpcb micromirror-based laser projection availability indicator. IEEE Trans. Ind. Electron. 2016, 63, 3009–3018. [CrossRef] Ataman, Ç.; Urey, H. Compact fourier transform spectrometers using fr4 platform. Sens. Actuators A Phys. 2009, 151, 9–16. [CrossRef] Zhou, Y.; Wen, Q.; Wen, Z.; Huang, J.; Chang, F. An electromagnetic scanning mirror integrated with blazed grating and angle sensor for a near infrared micro spectrometer. J. Micromech. Microeng. 2017, 27, 125009. [CrossRef] Huang, J.; Wen, Q.; Nie, Q.; Chang, F.; Zhou, Y.; Wen, Z. Miniaturized nir spectrometer based on novel moems scanning tilted grating. Micromachines 2018, 9, 478. [CrossRef] [PubMed] Namazu, T.; Isono, Y. Fatigue life prediction criterion for micro–nanoscale single-crystal silicon structures. J. Microelectromech. Syst. 2009, 18, 129–137. [CrossRef] Urey, H.; Holmstrom, S.; Yalcinkaya, A.D. Electromagnetically actuated fr4 scanners. IEEE Photonics Technol. Lett. 2007, 20, 30–32. [CrossRef]

Appl. Sci. 2018, 8, 2412

13. 14. 15. 16. 17. 18. 19. 20.

21. 22.

23. 24. 25.

11 of 11

Zuo, H.; He, S. Fpcb ring-square electrode sandwiched micromirror based laser pattern pointer. IEEE Trans. Ind. Electron. 2017, 64, 6319–6329. [CrossRef] Zuo, H.; He, S. Double stage fpcb scanning micromirror for laser line generator. Mechatronics 2018, 51, 75–84. [CrossRef] Lei, H.; Wen, Q.; Yu, F.; Zhou, Y.; Wen, Z. FR4-based electromagnetic scanning micromirror integrated with angle sensor. Micromachines 2018, 9, 214. [CrossRef] [PubMed] Han, F.; Wang, W.; Zhang, X.; Xie, H. Modeling and control of a large-stroke electrothermal mems mirror for fourier transform microspectrometers. J. Microelectromech. Syst. 2016, 25, 750–760. [CrossRef] Han, F.T. Miniature fourier transform spectrometer with a dual closed-loop controlled electrothermal micromirror. Opt. Express 2016, 24, 22651. [CrossRef] [PubMed] Sasaki, M.; Tabata, M.; Haga, T.; Hane, K. Piezoresistive rotation angle sensor integrated in micromirror. Jpn. J. Appl. Phys. 2006, 45, 3789–3793. [CrossRef] Chi, Z.; Zheng, Y.; Hu, H.; Li, G. Study on a two-dimensional scanning micro-mirror and its application in a moems target detector. Sensors 2010, 10, 6848–6860. Park, Y.H. Low power and highly precise closed-loop driving circuits for piezoelectric micromirrors with embedded capacitive position sensors. In MOEMS and Miniaturized Systems XV; SPIE Press: Bellingham, WA, USA, 2016. Naono, T.; Fujii, T.; Esashi, M.; Tanaka, S. A large-scan-angle piezoelectric mems optical scanner actuated by a nb-doped pzt thin film. J. Micromech. Microeng. 2013, 24, 5010. [CrossRef] Gu-Stoppel, S.; Giese, T.; Quenzer, H.J.; Hofmann, U.; Benecke, W. Piezoelectrically driven and sensed micromirrors with large scan angles and precise closed-loop control. In Proceedings of the MikroSystemTechnik 2017 Congress, Munich, Germany, 23–25 October 2017; p. 561. Watanabe, Y.; Kobayashi, S.; Iwamatsu, S.; Yahagi, T.; Sato, M.; Oizumi, N. Motion monitoring of mems actuator with electromagnetic induction. Electron. Commun. Jpn. 2014, 97, 52–57. [CrossRef] Kundu, S.K.; Ogawa, S.; Kumagai, S.; Fujishima, M.; Hane, K.; Sasaki, M. Nonlinear spring effect of tense thin-film torsion bar combined with electrostatic driving. Sens. Actuators A Phys. 2013, 195, 83–89. [CrossRef] Ishikawa, N.; Ikeda, K.; Sawada, R. Temperature dependence of the scanning performance of an electrostatic microscanner. J. Micromech. Microeng. 2016, 26, 035002. [CrossRef] © 2018 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/).