sensors

Article Design of a Piezoelectric with High Sensitivity and Low Transverse Effect

Bian Tian, Hanyue Liu *, Ning Yang, Yulong Zhao and Zhuangde Jiang

State Key Laboratory for Mechanical Manufacturing Systems Egineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (B.T.); [email protected] (N.Y.); [email protected] (Y.Z.); [email protected] (Z.J.) * Correspondence: [email protected]; Tel.: +86-29-8339-5171

Academic Editor: Jörg F. Wagner Received: 19 July 2016; Accepted: 21 September 2016; Published: 26 September 2016

Abstract: In order to meet the requirements of cable fault detection, a new structure of piezoelectric accelerometer was designed and analyzed in detail. The structure was composed of a seismic mass, two sensitive beams, and two added beams. Then, simulations including the maximum stress, natural frequency, and output voltage were carried out. Moreover, comparisons with traditional structures of piezoelectric accelerometer were made. To verify which vibration mode is the dominant one on the and the space between the mass and glass, mode analysis and deflection analysis were carried out. Fabricated on an n-type single crystal silicon wafer, the sensor chips were wire-bonged to printed circuit boards (PCBs) and simply packaged for experiments. Finally, a vibration test was conducted. The results show that the proposed piezoelectric accelerometer has high sensitivity, low resonance frequency, and low transverse effect.

Keywords: piezoelectric accelerometer; high sensitivity; low resonance frequency; low transverse effect

1. Introduction With the development of urbanization, cables have gradually been replacing overhead lines due to their merits of being a reliable power supply, having a simple operation, and being a less occupying area. However, cable faults occur frequently because of the manufacturing processes, the operation environment, and insulation aging problems [1]. Moreover, cables are usually buried in the ground. Once the faults occur, they cannot be directly found by observation. Thus, methods to find faults quickly and accurately has become significantly important. When cable faults occur, the electrical insulating layer breaks down, which causes an electric spark and generates a weak vibration [2]. The weak vibration caused by an electric spark can be measured as a characteristic signal, which can be extracted by vibration sensors or [3]. Normally, the weak vibration signal is minute, directional, and low frequency (300~500 Hz) [4]. Therefore, the sensors should have the characteristics of high sensitivity, low transverse effect, and low resonance frequency response. In order to meet the requirements of the working environment, a new acceleration sensor was designed to detect a weak vibration signal. Piezoelectric thin films have caused great interest in the design of accelerometers due to their potentially high sensitivity [5]. Several groups have previously reported on the use of piezoelectric thin films accelerometers. Eichner et al. [6] designed and fabricated bulk-micro machined accelerometers. A seismic mass and two silicon beams were used as the sensing structure; an average sensitivity of 0.1 mV/g was measured. Yu et al. [7] presented and fabricated a PZT (piezoelectric lead zirconate titanate) microelectromechanical accelerometer using interdigitated electrodes, which resulted in high acceleration sensitivity. The voltage sensitivities in the range of 1.3–7.86 mV/g with corresponding

Sensors 2016, 16, 1587; doi:10.3390/s16101587 www.mdpi.com/journal/sensors Sensors 2016, 16, 1587 2 of 14 resonance frequencies in the range of 23–12 kHz were obtained. A bimorph tri-axis piezoelectric accelerometer with high sensitivity, low cross-axial sensitivity, and compact size was fabricated by Zou et al. [8]. The un-amplified sensitivities of the X-, Y-, and Z-axes electrodes in response to in X-, Y-, and Z-axes were 0.93 mV/g, 1.13 mV/g and 0.88 mV/g, respectively. Moreover, the cross-axis sensitivity was less than 15%. In this study, a novel structure for piezoelectric accelerometer on the basis of piezoelectric effect of piezoelectric material was established to further improve the sensitivity and reduce the transverse effect, including the principle of the piezoelectric accelerometer. Then, according to the FEM (finite element method), stress status, resonance frequency, and output voltage of different piezoelectric accelerometers were simulated and analyzed in detail. In accordance with the analysis results, the optimal structure of the piezoelectric accelerometer was obtained.

2. Design The piezoelectric accelerometer is generally composed of a seismic mass, one or several beams, and piezoelectric thin films. When an acceleration signal is applied on the seismic mass, the generated by the mass causes the beams to bend. Then, the piezoelectric thin films are under strain. Through the piezoelectric effect, the strain in the piezoelectric thin films is converted to an electrical charge [9]. By detecting the output voltage, the acceleration is obtained. Commonly, an ideal acceleration sensor is only sensitive to the vibration signal that is perpendicular to the plane of the device, and it is not sensitive to the vibration signal in other directions. In fact, most acceleration sensors developed in the past could not avoid the transverse effect. The reason for accelerometer with transverse effect is mainly due to the fact that the center of the seismic mass is not in the same plane as that of the beams. When transverse acceleration is applied to the accelerometer, the seismic mass rotates around one axis, producing transverse interference. The transverse effect is described by the transverse effect coefficient [10,11]. The transverse effect coefficient is defined as

ε0 R = . (1) ST ε

0 Here, RST is the transverse effect coefficient, ε is the transverse strain, and ε is the longitudinal strain. In this paper, the longitudinal strain direction is the Z direction. The X direction and the Y direction are the two transverse strain directions. The transverse effect coefficient describes the influence of the transverse acceleration on the output of the sensors. The smaller the value is, the better the accelerometer performs. A traditional accelerometer consists of a single sensitive beam, double sensitive beams, or four sensitive beams. The piezoelectric accelerometer with a single sensitive beam has a narrow bandwidth. Thus, the measuring range is narrow. The sensitivity of the piezoelectric accelerometer with double sensitive beams is not so high, but the transverse effect coefficient is quite large, which is susceptible to the lateral acceleration. The transverse coefficient of the piezoelectric accelerometer with four sensitive beams is small, but the sensitivity is low, as compared to a single sensitive beam and double sensitive beams [4]. To overcome defects of these structures, a structure with two sensitive beams and two added beams were designed, as shown in Figure1. The seismic mass is suspended by the two sensitive beams and the two added beams. The length of the sensitive beams is shorter than that of the added beams while the width is larger. In addition, the thickness of both beams is the same. Sensors 2016, 16, 1587 3 of 14 Sensors 2016, 16, 1587 3 of 13

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FigureFigure 1. Diagram 1. Diagram of the of the design design structure. structure.

When the acceleration along Z-axis (the normal direction to the sensor chip) is applied on the When the acceleration along Z-axis (the normal direction to the sensor chip) is applied on the chip, the beams is involved in a bending movement along with the vertical of the seismic mass. Since chip, the beams is involved in aFigure bending 1. Diagram movement of the along design with structure. the vertical of the seismic mass. Since there is no rotation in the movement of the seismic mass, displacements of the beams are the same. there is no rotation in the movement of the seismic mass, displacements of the beams are the same. Dimensions (length × width × thickness) of the sensitive beams and the added beams are DimensionsWhen the (length acceleration× width along× thickness) Z-axis (the of the normal sensitive direction beams to and the the sensor added chip) beams is applied are a1 × onb1 ×theh a1 × b1 × h and a2 × b2 × h, respectively. Let F1 and F2 be the total applied to the sensitive chip,and athe× beamsb × h, is respectively.involved in a Let bending F and movement F be the total along forces with applied the vertical to the of sensitivethe seismic beams mass. and Since the beams and2 the2 added beams, as shown1 in Figure2 2. In the analytical model, the mass of the beams thereadded is beams, no rotation as shown in the in movement Figure2. In of the the analytical seismic model,mass, displacements the mass of the of beams the beams and bending are the ofsame. the and bending of the seismic mass are neglected. According to the basic principle of mechanics and Dimensionsseismic mass (length are neglected. × width According × thickness) to the basicof the principle sensitive of mechanicsbeams and and the under added the assumptionbeams are under the assumption of small deflection [12], the following equations can be derived: aof1 × small b1 × deflection h and a2 [×12 b],2 the× h, following respectively. equations Let F1 can and be F derived:2 be the total forces applied to the sensitive beams and the added beams,EI asω shown′′ (x) = in FFigurex−M 2. In(0≤x≤a the analytical), model, the mass of the beams (2) and bending of the seismic mass are00 neglected.1 According to the basic principle of mechanics and EI1ω (x) = F1x − Mo1 (0 ≤ x ≤ a1), (2) EI ω′′ 1(y) = F2y −M (0≤y ≤a ), under the assumption of small deflection [12], the following equations can be derived: (3)

00 ′′ 1 EIEIωω(y(Fx))+F== =F=maFyx−M− M(,0≤x≤a (and0 ≤ y ≤ a), ), (4)(2) (3) 1 2 2 1 o2 2 EI ω′′ (y)ω=(aF) =y −Mω(a)(,0≤ y ≤a ), (5) F1 + F2 = F = ma, and (3) (4) where w1(x) and w2(y) are the displacement of the sensitive beams and the added beams, E is Fω+F1 (a1=F=ma) = ω2 (a2, )and, (4) (5) Young’s modulus of Si, M01 and M02 are restrictive moments to be determined, Ii = bih3/12(i = 1,2). Thewhere boundary w1(x) conditions and w2(y) arefor thethese displacement equationsω are of(a the) = sensitiveω(a), beams and the added beams, E is Young’s(5) 3 modulus of Si, M01 and M02 are restrictive moments to be determined, Ii = bih /12(i = 1,2). The where w1(x) and w2(y) areω (the0) = displacementω′ (0) = ω of(0 )the= sensitiveω′ (0) =0 beams, and and the added beams, (6) E is boundary conditions for these equations are 01 02 i i 3 Young’s modulus of Si, M and M are′ restrictive′ moments to be determined, I = b h /12(i = 1,2). ω(a) = ω(a) = 0. (7) The boundary conditions for these equations0 are 0 ω1 (0) = ω1 (0) = ω2 (0) = ω2 (0) = 0, and (6) ′ ′ ω(0) = ω(0) = ω(0) = ω(0) =0, and (6) ω0 (a ) = ω0 (a ) = 0. (7) 1 ′ 1 2′ 2 ω(a) = ω(a) = 0. (7)

Figure 2. Mechanical analysis of the structure.

From Equations (2)–(7), we have: FigureFigure 2. 2. MechanicalMechanical ∙analysis analysis of of the the structure. structure. F = ∙ma, (8) ∙∙

∙ From Equations (2)–(7), we have: F = ∙ma, ∙ ∙ (9) ∙ F = ∙ma, (8) ∙∙ ∙ F = ∙ma, (9) ∙∙

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From Equations (2)–(7), we have:

a3·b = 2 1 · F1 3 3 ma, (8) a2·b1 + a1·b2

a3·b = 1 2 · F2 3 3 ma, (9) a2·b1 + a1·b2

00 F1 ω1 (x) = (a1 − 2x) , and (10) 4EI1

00 F2 ω2 (y) = (a2 − 2y). (11) 4EI2 The generated stress along the longtitudinal direction of the sensitive beams and the added beams can be obtained as 3a3·ma· (a − 2x) σ (x) = 2 1 , and (12) 1 3 3  2 2 a2·b1 + a1·b2 ·h 3 3a ·ma· (a2 − 2y) σ (y) = 1 . (13) 2 3 3  2 2 a2·b1 + a1·b2 ·h Then, the maximum stress of the sensitive beams and the added beams are

3a3·ma·a σ (max) = σ (0) = 2 1 , and (14) 1 1 3 3  2 2 a2·b1 + a1·b2 ·h

3 3a ·ma·a2 σ (max) = σ (0) = 1 . (15) 2 2 3 3  2 2 a2·b1 + a1·b2 ·h The ratio of the sensitive beams’ maximum stress and the added beams’ maximum stress is

σ (max) a3·a a2 a 2 1 = 2 1 = 2 = ( 2 ) . (16) σ ( ) 3 2 2 max a1·a2 a1 a1

Here, the length of the sensitive beams is longer than that of the added beams, so

σ1(max) > σ2 (max). (17)

That is to say, under the acceleration along the Z-axis, the sensitive beams will obtain a larger stress compared with the added beams. Therefore, the stress of the sensitive beams can be investigated as a key parameter that directly determines the accelerometer sensitivity. The maximum strain of the sensitive beams is 3a3·ma·a ε = 2 1 . (18) max 3 3  2 2E a2·b1 + a1·b2 ·h The elastic constant of the structure is

b h3 b h3 = ( 1 + 2 ) K 2E 3 3 . (19) a1 a2

Therefore, the resonance frequency of the accelerometer can be expressed as

r s 1 K 1 2E b h3 b h3 f = = ( 1 + 2 ). (20) π π 3 3 2 m 2 m a1 a2 Sensors 2016, 16, 1587 5 of 14

Sensors 2016, 16, 1587 5 of 13 From Equations (18) and (20), it is observed that, if dimensions of the seismic mass and the added beamsThe designed remain structure unchanged, is related the maximum to multiple strain variables. of the Thus, structure we used increases methods with of the controlling increase in thevariables length of to sensitive divide the beams, multi-factor while theproblem frequency into single decreases factors. when The thedimensions length of of the the sensitive seismic mass beams increases.and the Additionally,length and width as regards of the theadded width beams or thickness should be of theconstant. sensitive Furthe beams,rmore, the considering maximum strainthe andsmall frequency size of showsensors, an oppositethe dimensions result. should be as small as possible. In this project, the size of the seismicTo verify mass the was developed set to be model 2400 andμm determine× 2400 μm the × 400 optimal μm. dimensionsThe length ofof thethe structure,added beams simulations was based1300 on μm, COMSOL and the width software was were100 μ executed.m, considering Then, the the manufacturing location of the process. piezoelectric Then, the thin impacts films of was obtainedthe length, after width, the optimal and thickness dimensions of the and sensitive the placement beams on of the the sensitivity maximum and strain frequency were determined.are studied as shown in Figures 3–5. 3. Simulation and Analysis

3.1. Static Analysis The dimensions of the piezoelectric accelerometer were determined by measuring an environment that is minute, directional, and low frequency (300~500 Hz). Therefore, the acceleration sensor should satisfy the requirements of high sensitivity (500~600 µε), low transverse effect, i.e., a low range Sensors 2016, 16, 1587 5 of 13 (50~100 g), and low resonance frequency (1000 Hz). TheThe designed designed structure structure is is related related toto multiplemultiple variables. Thus, Thus, we we used used methods methods of ofcontrolling controlling variablesvariables to to divide divide the the multi-factor multi-factor problem problem intointo singlesingle factors. The The dimensions dimensions of of the the seismic seismic mass mass andand the the length length and and width width of theof the added added beams beams should shou beld constant.be constant. Furthermore, Furthermore, considering considering the the small sizesmall of sensors, size of sensors, the dimensions the dimensions should beshould as small be as as small possible. as possible. In this In project, this project, the size the of size the of seismic the µ × µ × µ µ massseismic wasFigure mass set 3. to Maximumwas be 2400set to strain mbe 2400 and2400 μfrequencym m× 2400400 with μm m.differen × 400 The tμ lengthlengthsm. The of length sensitive the added of beamsthe beamsadded (thickness wasbeams 1300is was m, and1300 the 20μ widthm, μm; and width was the is 100width 210µ μm,m). was considering 100 μm, considering the manufacturing the manufacturing process. Then, process. the Then, impacts the ofimpacts the length, of width,the length, and thickness width, and of thethickness sensitive of the beams sensitive on the beams sensitivity on the andsensitivity frequency and arefrequency studied are as studied shown in Figuresas shownFigure3–5. in 3 Figures shows the3–5. maximum strain and frequency of different lengths of sensitive beams when the thickness of the sensitive beams is 20 μm and the width is 210 μm. Figure 4 shows the maximum strain and frequency with different widths of sensitive beams when the length of the sensitive beams is 1200 μm and the thickness is 20 μm. Figure 5 shows the maximum strain and frequency with different thicknesses of sensitive beams when the length of the sensitive beams is 1200 μm and the width is 210 μm. It can be observed that the maximum strain increases as the length of sensitive beams increases. The maximum strain decreases with the increase of the width or the increase of the thickness of the sensitive beams. The results of frequency are opposite compared with the maximum strain, which shows the consistency of the previous theoretical analysis. To obtain high sensitivity and ensure the linearity and precision of the accelerometer, the maximum strain should not exceed 1/5~1/6 of the strain limit of silicon [13]. That is to say, the strain should be less than 500~600 με. As we can see from the figures, when the size of the sensitive beams is 1200 μm × 210 μm × 20 μm (length × width × thickness), the value of maximum strain is up to 597Figure Figureμε, 3.which Maximum3. Maximum is close strain strain to and and600 frequency frequencyμε. Therefore, with with different differen the lengths tdimensions lengths of sensitive of sensitive of beamsthe beamssensitive (thickness (thickness beams is 20 isµ m; are 1200width 20μm μ ism;× 210210 width µμm).m is × 210 20 μμm).m.

Figure 3 shows the maximum strain and frequency of different lengths of sensitive beams when the thickness of the sensitive beams is 20 μm and the width is 210 μm. Figure 4 shows the maximum strain and frequency with different widths of sensitive beams when the length of the sensitive beams is 1200 μm and the thickness is 20 μm. Figure 5 shows the maximum strain and frequency with different thicknesses of sensitive beams when the length of the sensitive beams is 1200 μm and the width is 210 μm. It can be observed that the maximum strain increases as the length of sensitive beams increases. The maximum strain decreases with the increase of the width or the increase of the thickness of the sensitive beams. The results of frequency are opposite compared with the maximum strain, which shows the consistency of the previous theoretical analysis. To obtain high sensitivity and ensure the linearity and precision of the accelerometer, the

maximum strain should not exceed 1/5~1/6 of the strain limit of silicon [13]. That is to say, the strain shouldFigureFigure be 4. less 4.Maximum Maximum than 500~600 strain strain andμε and. frequencyAs frequency we can with withsee fromdifferentdifferent the widthsfigures, of of whsensitive sensitiveen the beams beamssize of (length (lengththe sensitive is is1200 1200 μ m;beamsµm; is thickness1200thickness μm is× is 20210 20µ m).μμm).m × 20 μm (length × width × thickness), the value of maximum strain is up to 597 με, which is close to 600 με. Therefore, the dimensions of the sensitive beams are 1200 μm × 210 μm × 20 μm.

Figure 4. Maximum strain and frequency with different widths of sensitive beams (length is 1200 μm; thickness is 20 μm).

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FigureFigure 5. Maximum 5. Maximum strain strain and and frequency frequency with with differentthicknesses thicknesses of of sensitive sensitive beams beams (length (length is is 12001200 μm;µ widthm; width is 210 is 210 μm).µm).

The Figurestrain3 distributionshows the maximum of the strainstructure and frequencyis shown of in different Figure lengths 6. Because of sensitive there beams are differences when betweenthe thickness sensitive of beams the sensitive and added beams isbeams 20 µm in and the the leng widthth isand 210 width,µm. Figure the strain4 shows distributions the maximum of the two strainkinds andof beams frequency are with different. different It widthscan be of observ sensitiveed beamsthat the when maximum the length strain of the is sensitive generated beams on the sensitiveis 1200 beams.µm and In theorder thickness to get is high 20 µ m.sensitivity, Figure5 shows piezoelectric the maximum thin films strain were and frequency distributed with at the maximumdifferent stress thicknesses spots. As of sensitiveshown in beams Figure when 6, the the valu lengthe of of the the strain sensitive is from beams the is center 1200 µ mof andthe mass the to µ the edgewidth of is the 210 sensitivem. It can bebeams. observed At the that edge the maximum of the se strainnsitive increases beams, as the the lengthvalue of sensitivethe strain beams is up to increases. The maximum strain decreases with the increase of the width or the increase of the thickness maximum and the strain focuses on the area from 2.3 mm to 2.4 mm. In order to get the maximum of the sensitive beams. The results of frequency are opposite compared with the maximum strain, output voltage, the piezoelectric thin films were arranged in this area. which shows the consistency of the previous theoretical analysis. To obtain high sensitivity and ensure the linearity and precision of the accelerometer, the maximum strain should not exceed 1/5~1/6 of the strain limit of silicon [13]. That is to say, the strain should be less than 500~600 µε. As we can see from the figures, when the size of the sensitive beams is 1200 µm × 210 µm × 20 µm (length × width × thickness), the value of maximum strain is up to 597 µε, which is close to 600 µε. Therefore, the dimensions of the sensitive beams are 1200 µm × 210 µm × 20 µm. The strain distribution of the structure is shown in Figure6. Because there are differences between sensitive beams and added beams in the length and width, the strain distributions of the two kinds of beams are different. It can be observed that the maximum strain is generated on the sensitive beams. In order to get high sensitivity, piezoelectric thin films were distributed at the maximum stress spots. As shown in Figure6, the value of the strain is from the center of the mass to the edge of the sensitive beams. At the edge of the sensitive beams, the value of the strain is up to maximum and the strain focuses on the area from 2.3 mm to 2.4 mm. In order to get the maximum output voltage, the piezoelectric thin films were arranged in this area. PZT has superior piezoelectric properties compared with other piezoelectric materials [14]. Therefore, this work focuses on the use of PZT films. The width of the PZT films is the same as that of the sensitive beams. The length of the PZT films is 100 µm, which is at the area from 2.3 mm to 2.4 mm.Figure Considering 6. The strain the actual distribution machining from precision, the center the of thickness mass to the of the edge PZT of filmssensitive is 3 µbeams.m. Moreover, it is supposed that the top surface of the PZT films is zero potential. The output voltage is shown in FigurePZT has7. The superior maximum piezoelectric output voltage properties is 1.67 V, whichcompared is in thewith back othe of ther piezoelectric PZT films. materials [14]. Therefore, this work focuses on the use of PZT films. The width of the PZT films is the same as that of the sensitive beams. The length of the PZT films is 100 μm, which is at the area from 2.3 mm to 2.4 mm. Considering the actual machining precision, the thickness of the PZT films is 3 μm. Moreover, it is supposed that the top surface of the PZT films is zero potential. The output voltage is shown in Figure 7. The maximum output voltage is 1.67 V, which is in the back of the PZT films.

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Figure 5. Maximum strain and frequency with different thicknesses of sensitive beams (length is 1200 μm; width is 210 μm).

The strain distribution of the structure is shown in Figure 6. Because there are differences between sensitive beams and added beams in the length and width, the strain distributions of the two kinds of beams are different. It can be observed that the maximum strain is generated on the sensitive beams. In order to get high sensitivity, piezoelectric thin films were distributed at the maximum stress spots. As shown in Figure 6, the value of the strain is from the center of the mass to the edge of the sensitive beams. At the edge of the sensitive beams, the value of the strain is up to maximum and the strain focuses on the area from 2.3 mm to 2.4 mm. In order to get the maximum Sensors 2016, 16, 1587 7 of 14 output voltage, the piezoelectric thin films were arranged in this area.

Sensors 2016, 16, 1587Figure 6. The strain distribution from the center ofof massmass toto thethe edgeedge ofof sensitivesensitive beams.beams. 7 of 13

PZT has superior piezoelectric properties compared with other piezoelectric materials [14]. Therefore, this work focuses on the use of PZT films. The width of the PZT films is the same as that of the sensitive beams. The length of the PZT films is 100 μm, which is at the area from 2.3 mm to 2.4 mm. Considering the actual machining precision, the thickness of the PZT films is 3 μm. Moreover, it is supposed that the top surface of the PZT films is zero potential. The output voltage is shown in Figure 7. The maximum output voltage is 1.67 V, which is in the back of the PZT films.

FigureFigure 7. 7.TheThe result result of outputoutput voltage. voltage.

3.2. Mode3.2. Analysis Mode Analysis and Frequency and Frequency Domain Domain Normally, the response of the accelerometer is linear on a wide frequency range [15]. In order Normally,to obtain reliablethe response and accurate of the results,accelerometer the working is line frequencyar on a wide of the frequency sensor should range be lower[15]. In than order to obtain thereliable resonance and frequencyaccurate ofresults, the accelerometer. the working To frequency obtain the resonance of the sensor frequency should and be verify lower which than the resonancevibration frequency is the dominant of the oneaccelerometer. on the accelerometer, To ob modetain analysisthe resonance was carried frequency out, as shown and in verify Table1 which vibrationand is Figure the 8dominant. Table1 shows one that on thethe first accelerometer, natural frequency mode of the analysis structure was was carried 1279.1 Hz, out, which as shown is in Table 1also and the Figure working 8. modalTable of1 theshows structure. that the The first first natural frequency frequency is higherof the thanstructure the frequency was 1279.1 of Hz, which ordinaryis also the external working excitation modal and of distant the fromstructure. other naturalThe first frequencies, natural sofrequency the accelerometer is higher could than the resist the interference of the external signal and obtain the vibration signal accurately. frequency of ordinary external excitation and distant from other natural frequencies, so the accelerometer could resist the interferenceTable 1. The resultof the of theexternal modal analysis. signal and obtain the vibration signal accurately. Vibration Mode Frequency (Hz) TableThe first1. The mode result of the modal 1279.1 analysis. The second mode 1841.5 VibrationThe third Mode mode Frequency 2772.5 (Hz) The fourth mode 20,421 The Thefirst fifth mode mode 45,272 1279.1 The secondThe sixth mode mode 65,700 1841.5 The third mode 2772.5 The fourth mode 20,421 The fifth mode 45,272 The sixth mode 65,700

(a)(b)(c)

(d)(e)(f)

Figure 8. The first to sixth vibration mode of the structure. (a) The first mode; (b) The second mode; (c) The third mode; (d) The fourth mode; (e) The fifth mode; (f) The sixth mode.

Frequency domain describes the response of the accelerometer at different frequencies. Figure 9 is the frequency domain of the structure. It illustrates that, when the frequency is lower than the

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Figure 7. The result of output voltage.

3.2. Mode Analysis and Frequency Domain Normally, the response of the accelerometer is linear on a wide frequency range [15]. In order to obtain reliable and accurate results, the working frequency of the sensor should be lower than the resonance frequency of the accelerometer. To obtain the resonance frequency and verify which vibration is the dominant one on the accelerometer, mode analysis was carried out, as shown in Table 1 and Figure 8. Table 1 shows that the first natural frequency of the structure was 1279.1 Hz, which is also the working modal of the structure. The first natural frequency is higher than the frequency of ordinary external excitation and distant from other natural frequencies, so the accelerometer could resist the interference of the external signal and obtain the vibration signal accurately.

Table 1. The result of the modal analysis.

Vibration Mode Frequency (Hz) The first mode 1279.1 The second mode 1841.5 The third mode 2772.5 The fourth mode 20,421

Sensors 2016, 16, 1587 The fifth mode 45,272 8 of 14 The sixth mode 65,700

(a)(b)(c)

(d)(e)(f)

Figure 8. The firstfirst toto sixthsixthvibration vibration mode mode of of the the structure. structure. (a ()a The) The first first mode; mode; (b )(b The) The second second mode; mode; (c) The(c) The third third mode; mode; (d) ( Thed) The fourth fourth mode; mode; (e) The(e) The fifth fifth mode; mode; (f) The(f) The sixth sixth mode. mode.

Frequency domain describes the response of the accelerometer at different frequencies. Figure 9 is Frequency domain describes the response of the accelerometer at different frequencies. Figure9 is Sensorsthe frequency 2016, 16, 1587 domain of the structure. It illustrates that, when the frequency is lower than8 of the 13 the frequency domain of the structure. It illustrates that, when the frequency is lower than the natural naturalfrequency, frequency, with the with increase the increase in the frequency, in the frequency, displacement displacement increases. increases. When the When frequency the frequency equals the equalsnatural the frequency, natural frequency, the maximum the responsemaximum appears. response appears.

(a) (b)

FigureFigure 9. FrequencyFrequency domain domain and and displacement displacement of of the the structure structure in in natural natural frequency. frequency. ( (aa)) Frequency Frequency domain;domain; ( (bb)) Displacement Displacement of of the the structure structure in natural frequency.

3.3. Deflection Analysis 3.3. Deflection Analysis Generally, the accelerometer needs to be bonded with a glass to prevent overload. There is a Generally, the accelerometer needs to be bonded with a glass to prevent overload. There is a certain space between the seismic mass and the glass to guarantee that the seismic mass can vibrate certain space between the seismic mass and the glass to guarantee that the seismic mass can vibrate under normal working conditions. Moreover, it can protect the structure from being destroyed when under normal working conditions. Moreover, it can protect the structure from being destroyed when there is a great impact. Therefore, in order to determine the size of space and the measuring range, there is a great impact. Therefore, in order to determine the size of space and the measuring range, deflection analysis was carried out without exceeding the silicon’s stress limit of 450~500 MPa. deflection analysis was carried out without exceeding the silicon’s stress limit of 450~500 MPa. The results are shown in Figure 10. It can be observed that the maximum stress and deflection The results are shown in Figure 10. It can be observed that the maximum stress and deflection increase with the increase in applied acceleration. When the applied acceleration is 80 g, the value of increase with the increase in applied acceleration. When the applied acceleration is 80 g, the value of maximum stress is 71.9 MPa, and the deflection is 12.4 μm. As the acceleration is 500 g, the value of maximum stress is 71.9 MPa, and the deflection is 12.4 µm. As the acceleration is 500 g, the value of maximum stress is up to 449 MPa, and the deflection is 69.5 μm. In order to ensure that the maximum stress is up to 449 MPa, and the deflection is 69.5 µm. In order to ensure that the maximum maximum stress does not exceed the limit stress of silicon, the space between the glass and the stress does not exceed the limit stress of silicon, the space between the glass and the seismic mass must seismic mass must be between 12.4 μm and 69.5 μm. The measuring range can be up to 500 g. be between 12.4 µm and 69.5 µm. The measuring range can be up to 500 g.

Figure 10. The results of deflection analysis.

3.4. Comparisions We then compared the designed structure with the traditional structures, and the comparisons of the results are summarized in Table 2. The four structures have the same dimensions of the seismic mass and the sensitive beams. The seismic mass’s dimensions are 2400 μm × 2400 μm × 400

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natural frequency, with the increase in the frequency, displacement increases. When the frequency equals the natural frequency, the maximum response appears.

(a) (b)

Figure 9. Frequency domain and displacement of the structure in natural frequency. (a) Frequency domain; (b) Displacement of the structure in natural frequency.

3.3. Deflection Analysis Generally, the accelerometer needs to be bonded with a glass to prevent overload. There is a certain space between the seismic mass and the glass to guarantee that the seismic mass can vibrate under normal working conditions. Moreover, it can protect the structure from being destroyed when there is a great impact. Therefore, in order to determine the size of space and the measuring range, deflection analysis was carried out without exceeding the silicon’s stress limit of 450~500 MPa. The results are shown in Figure 10. It can be observed that the maximum stress and deflection increase with the increase in applied acceleration. When the applied acceleration is 80 g, the value of maximum stress is 71.9 MPa, and the deflection is 12.4 μm. As the acceleration is 500 g, the value of maximum stress is up to 449 MPa, and the deflection is 69.5 μm. In order to ensure that the maximum stress does not exceed the limit stress of silicon, the space between the glass and the Sensors 2016, 16, 1587 9 of 14 seismic mass must be between 12.4 μm and 69.5 μm. The measuring range can be up to 500 g.

Figure 10. The results of deflection analysis. Figure 10. The results of deflection analysis.

3.4. Comparisions 3.4. Comparisions We then compared the designed structure with the traditional structures, and the comparisons We then compared the designed structure with the traditional structures, and the comparisons of of the results are summarized in Table 2. The four structures have the same dimensions of the the results are summarized in Table2. The four structures have the same dimensions of the seismic seismic mass and the sensitive beams. The seismic mass’s dimensions are 2400 μm × 2400 μm × 400 mass and the sensitive beams. The seismic mass’s dimensions are 2400 µm × 2400 µm × 400 µm, and the sensitive beams’ dimensions are 1200 µm × 210 µm × 20 µm. The applied acceleration is 80 g.

Table 2. Comparison of the results.

Transverse Effect Coefficient Maximum Output Frequency Structures Strain (µε) Voltage (V) (Hz) X Direction Y Direction Single sensitive beam 0.4932 0.0847 5920 15.00 136.47 Double sensitive beams 0.7474 0.1179 817 2.25 1093.4 Four sensitive beams 0.2053 0.2097 412 1.10 1545.0 Designed structure 0.0329 0.0992 597 1.67 1279.1

It can be observed that, under the same size, the transverse effect coefficients of the structure with a single beam are 0.4932 in the X direction and 0.0847 in the Y direction. The output voltage is 15 V, which is the highest. However, the maximum strain is 5920 µε. It exceeded the strain limit of silicon (3000 µε). Thus, under the acceleration of 80 g, the structure was destroyed. In addition, the frequency was too low, which easily generated resonance. Therefore, the bandwidth was narrow and was not suitable for dynamic measurement. The structure with double sensitive beams had low transverse effect coefficient in the Y direction (0.1179). However, the transverse effect coefficient in the X direction (0.7474) was large. It was easily affected by the acceleration of the X direction. In addition, the maximum strain (817 µε) exceeded 1/5~1/6 of the strain limit of silicon, which means that the linearity and precision of the accelerometer could not be ensured. The structure with four sensitive beams had low transverse effect coefficient in both X and Y directions. However, the maximum strain and output voltage were low, and the frequency was relatively higher than other structures. Compared to other structures, the transverse effect coefficients of the designed structure were 0.0329 in the X direction and 0.0992 in the Y direction, which were the lowest. This indicates that the designed structure was less affected by transverse acceleration. The value of the maximum strain is 597 µε, which is below the strain limit of silicon. The output voltage was 1.67 V, which is relatively high. Moreover, the frequency was also close to 1000 Hz. From comparison of the results, it was found that the structure with two sensitive beams and two added beams not only improves the maximum strain and output voltage but also reduces the transverse effect coefficient. Sensors 2016, 16, 1587 10 of 14

The structure of the four sensitive beams was completely symmetrical; thus, force applied to the structure split into the four beams. Stress in each beam was small, so the sensitivity was low. However, the sensitive beams of the designed structure obtained a larger stress compared with the added beams. The sensitivity was improved with respect to the structure of the four sensitive beams. Because of the application of the added beams, the stiffness of the structure increased. The transverse effect was reduced compared with the structure of the double sensitive beams. Therefore, the piezoelectric accelerometer has the characteristics of high sensitivity, low transverse effect, and low frequency and can ensure the linearity and precision of the accelerometer.

4. Fabrication The piezoelectric accelerometer was fabricated by bulk-micromachining technology, and the fabrication process is shown in Figure 11. First, the (100) oriented n-type Si wafer whose thickness is 400 µm was an oxidated-silicon (SiO2) thin film via a thermal oxidization process; Second, the lithography was utilized to expose the glue, and the sputtering method was employed to sputter a layer of chromium/aurum (Cr/Au) as the lower electrode. After that, lift-off was used to form the shape of the lower electrode. Here, Cr was an adhesive layer, which was used to promote the adhesion of Au and Si/SiO2; Third, PZT thin films were prepared on the lower electrodes using the lithography, the sputtering method, and lift-off. In the fabrication process, the PZT target was a 3-in-diameter

Pb(ZrSensors 0.522016Ti, 160.48, 1587)O3 ceramic target, and 10% excess of Pb was added to compensate for the losses10 of of Pb 13 during the sputtering; Fourth, the rapid thermal annealing (RTA) was applied to make the PZT thin ◦ ◦ filmsthe PZT from thin amorphous films from into amorphous crystalline. into When crystalline. the When the was temperature up to 650 wasC at up the to rate 650 of °C 50 atC/s, the therate PZT of 50 thin °C/s, films the neededPZT thin to films incubate needed for to 5 min incuba andte cool for 5 to min room and temperature; cool to room Fifth, temperature; a layer of Fifth, Al2O a3 waslayer deposited of Al2O3 atwas the deposited edge of the at PZTthe edge thin films,of the which PZT th wasin films, to prevent which the was upper to prevent electrodes the and upper the lowerelectrodes electrodes and the from lower contacting electrodes and causingfrom contacti a shortng circuit; and causing Sixth, the a uppershort electrodescircuit; Sixth, were the made upper on theelectrodes PZT thin were films made using on the the same PZT methods thin films as the us lowering the electrodes. same methods Seventh, as the the inductively lower electrodes. coupled plasmaSeventh, (ICP) the inductively was utilized coupled to etch theplasma front (ICP) side ofwas the util wafer;ized to then, etch the the sensitive front side beams, of the added wafer; beams, then, andthe sensitive seismic mass beams, were added formed. beams, Finally, and theseismic sensitive mass beams,were formed. the added Finally, beams, the and sensitive the seismic beams, mass the wereadded released beams, byand a back-sidethe seismic ICP mass process. were released by a back-side ICP process.

(a)(b)(c)

(d)(e)(f)

Figure 11. The process of fabrication. ( a) Oxidated; ( b) The lower electrodes; ( c) PZT thin filmsfilms and RTA; (d) Al2O3; (e)The upper electrodes; (f) ICP etching. RTA; (d) Al2O3;(e)The upper electrodes; (f) ICP etching.

The fabrication was related to six masks. During the process, many chips were fragile. Most of The fabrication was related to six masks. During the process, many chips were fragile. Most the failures occurred during the release of the sensitive beams, the added beams, and the seismic of the failures occurred during the release of the sensitive beams, the added beams, and the seismic masses in the final ICP process. The fabrication was expected to be enhanced when thicker masses in the final ICP process. The fabrication was expected to be enhanced when thicker suspension suspension beams were used. In order to meet the demands of the fabrication, the thickness of the beams were used. In order to meet the demands of the fabrication, the thickness of the beams needed beams needed to increase from 20 μm to 50 μm, while the maximum strain and the output voltage to increase from 20 µm to 50 µm, while the maximum strain and the output voltage were down were down to 100 με and 103 mV, respectively. When the thickness of the beams increased, the to 100 µε and 103 mV, respectively. When the thickness of the beams increased, the sensitivity of sensitivity of the accelerometer decreased as stated above. To improve the sensitivity of the accelerometer, the optimization of the designed structure was conducted. On the basis of the structures of the two sensitive beams and the two added beams, four additional small masses were added to the seismic masses. By increasing the quality of the structure, the sensitivity improved. The distribution of the maximum strain and the output voltage are shown in Figure 12. Maximum strain increased to 383 με, and the maximum output voltage increased to 408 mV. Moreover, the natural frequency was 3313.4 Hz. The fabricated piezoelectric accelerometer chip is shown in Figure 13, and the dimensions of the final structure are described in Table 3.

(a) (b)

Figure 12. The distribution of maximum strain and the output voltage of the optimal structure. (a) The distribution of maximum strain; (b) The output voltage of the optimal structure.

Sensors 2016, 16, 1587 10 of 13 the PZT thin films from amorphous into crystalline. When the temperature was up to 650 °C at the rate of 50 °C/s, the PZT thin films needed to incubate for 5 min and cool to room temperature; Fifth, a layer of Al2O3 was deposited at the edge of the PZT thin films, which was to prevent the upper electrodes and the lower electrodes from contacting and causing a short circuit; Sixth, the upper electrodes were made on the PZT thin films using the same methods as the lower electrodes. Seventh, the inductively coupled plasma (ICP) was utilized to etch the front side of the wafer; then, the sensitive beams, added beams, and seismic mass were formed. Finally, the sensitive beams, the added beams, and the seismic mass were released by a back-side ICP process.

(a)(b)(c)

(d)(e)(f)

Figure 11. The process of fabrication. (a) Oxidated; (b) The lower electrodes; (c) PZT thin films and

RTA; (d) Al2O3; (e)The upper electrodes; (f) ICP etching.

The fabrication was related to six masks. During the process, many chips were fragile. Most of the failures occurred during the release of the sensitive beams, the added beams, and the seismic masses in the final ICP process. The fabrication was expected to be enhanced when thicker suspension beams were used. In order to meet the demands of the fabrication, the thickness of the Sensors 2016, 16, 1587 11 of 14 beams needed to increase from 20 μm to 50 μm, while the maximum strain and the output voltage were down to 100 με and 103 mV, respectively. When the thickness of the beams increased, the sensitivitythe accelerometer of the accelerometer decreased as stateddecreased above. as Tostated improve above. the To sensitivity improve of the the sensitivity accelerometer, of the the accelerometer,optimization ofthe the optimization designed structure of the designed was conducted. structure On was the basisconducted. of the On structures the basis of theof the two structuressensitive beamsof the two and sensitive the two added beams beams, and the four two additional added beams, small four masses additional were added small tomasses the seismic were addedmasses. to Bythe increasingseismic masses. the quality By increasing of the structure, the quality the of sensitivity the structure, improved. the sensitivity The distribution improved. of The the distributionmaximum strain of the and maximum the output strain voltage and the are output shown voltage in Figure are 12 shown. Maximum in Figure strain 12. increased Maximum to 383strainµε , increasedand the maximum to 383 με output, and the voltage maximum increased output to 408 voltage mV. Moreover, increasedthe to 408 natural mV. frequency Moreover, was the 3313.4 natural Hz. frequencyThe fabricated was 3313.4 piezoelectric Hz. The accelerometer fabricated piezoelectri chip is shownc accelerometer in Figure 13 chip, and is the shown dimensions in Figure of the13, and final thestructure dimensions are described of the final in Tablestructure3. are described in Table 3.

(a) (b)

FigureFigure 12. 12. The distributiondistribution ofof maximum maximum strain strain and and the the output output voltage voltage of the of optimal the optimal structure. structure. (a) The (a) The distribution of maximum strain; (b) The output voltage of the optimal structure. Sensorsdistribution 2016, 16, of1587 maximum strain; (b) The output voltage of the optimal structure. 11 of 13

(a) (b)

FigureFigure 13. 13.Fabricated Fabricated piezoelectric piezoelectric accelerometer accelerometer chip chip and and the the enlarged enlarged view. view. (a )(a Fabricated) Fabricated piezoelectricpiezoelectric accelerometer accelerometer chip; chip; (b) ( Theb) The enlarged enlarged view. view.

TableTable 3. Dimensions 3. Dimensions of theof th finale final structure. structure. Items Length ×× Items Length × Width × Thickness The sensitive beams 800 μm × 110 μm × 50 μm The sensitiveadded beams beams 1000 800 µ μmm × ×110 100µm μm× ×50 50µm μm µ × µ × µ TheSeismic added mass beams 3000 1000 μmm × 3200100 m μm ×50 400 μmm Seismic mass 3000 µm × 3200 µm × 400 µm FourFour additional additional small massesmasses 2000 2000 µ μmm × ×1800 1800µm μm× ×400 400 μmµm PZTPZT thin thin films films 100 100 µ μmm × ×110 110µm μm× ×1 µ 1m μm

5.5. Experiment Experiment and and Results Results AfterAfter the the accelerometer accelerometer chip chip was was fabricated, fabricated, the the key key problem problem was was how how to realizeto realize the the package package of of thethe sensor. sensor. The The schematic schematic of of the the packaging packaging and and the the packaged packaged accelerometer accelerometer are are shown shown in in Figure Figure 14 14.. First, the chip needed to be bonded with Pyrex glass to prevent overload and then cohered with the PCB. The electrical connection between the pads in the sensor chips and PCB was achieved by gold wire. Finally, the accelerometer chip was completely enclosed in the shell.

(a) (b)

Figure 14. The schematic of the packaging and the photo of the packaged sensor. (a) The schematic of the packaging; (b) Photo of the packaged sensor.

To test the performance of the piezoelectric output of the designed structure, a vibration test was carried out, as shown in Figure 15. The structure was on the vibration table, and different accelerations from 0 m/s2 to 50 m/s2 at 20 Hz were applied. In order to increase the output voltage, the two piezoelectric thin films were in series. The results are shown in Figure 16a. Obviously, the lines of the positive travel of measurement and the reverse travel of measurement match well; the structure kept the linearity satisfied. Based on Matlab, via calculation, the sensitivity of the piezoelectric acceleration was 0.00091 V/(m/s2), the linearity was 0.0205, and the hysteresis error was 0.0033. To measure the transverse motion of the accelerometer, we only needed to change the installation direction of the sensor. The results are shown in Figure 16b. The sensitivity of the X direction was 3.91343 × 10 V/(m/s2), and the sensitivity of the Y direction was

Sensors 2016, 16, 1587 11 of 13

(a) (b)

Figure 13. Fabricated piezoelectric accelerometer chip and the enlarged view. (a) Fabricated piezoelectric accelerometer chip; (b) The enlarged view.

Table 3. Dimensions of the final structure.

Items Length ×× The sensitive beams 800 μm × 110 μm × 50 μm The added beams 1000 μm × 100 μm × 50 μm Seismic mass 3000 μm × 3200 μm × 400 μm Four additional small masses 2000 μm × 1800 μm × 400 μm PZT thin films 100 μm × 110 μm × 1 μm

5. Experiment and Results

AfterSensors 2016the ,accelerometer16, 1587 chip was fabricated, the key problem was how to realize the package12 of 14 of the sensor. The schematic of the packaging and the packaged accelerometer are shown in Figure 14. First, the chip needed to be bonded with Pyrex glass to prevent overload and then cohered with the First, the chip needed to be bonded with Pyrex glass to prevent overload and then cohered with the PCB. The electrical connection between the pads in the sensor chips and PCB was achieved by gold PCB. The electrical connection between the pads in the sensor chips and PCB was achieved by gold wire.wire. Finally, Finally, the theaccelerometer accelerometer chip chip was was completely completely enclosedenclosed in in the the shell. shell.

(a) (b)

FigureFigure 14. The 14. Theschematic schematic of ofthe the packaging packaging and thethephoto photo of of the the packaged packaged sensor. sensor. (a) The (a) schematic The schematic of of thethe packaging; packaging; (b (b) )Photo Photo of of the packagedpackaged sensor. sensor.

To testTo testthe theperformance performance of of the the piezoelectric output output of theof the designed designed structure, structure, a vibration a vibration test was test was carried out,out, as as shown shown in Figure in Figure 15. The 15. structure The stru wascture on the was vibration on the table, vibration and different table, accelerations and different accelerationsfrom 0 m/s from2 to 0 50 m/s m/s2 to2 at5020 m/s Hz2 at were 20 Hz applied. were Inapplied. order to In increase order to the increase output voltage,the output the voltage, two the twopiezoelectric piezoelectric thin filmsthin werefilms in were series. in The series. results The are results shown are in Figure shown 16 a.in Obviously,Figure 16a. the Obviously, lines of the the lines positiveof the positive travel of travel measurement of measurement and the reverse and travelthe reverse of measurement travel of match measurement well; the structure match well; kept the structurethe linearity kept the satisfied. linearity Based satisfied. on Matlab, Based via calculation, on Matlab, the sensitivity via calculation, of the piezoelectric the sensitivity acceleration of the was 0.00091 V/(m/s2), the linearity was 0.0205, and the hysteresis error was 0.0033. To measure the Sensorspiezoelectric 2016, 16, 1587 accelerati on was 0.00091 V/(m/s2), the linearity was 0.0205, and the hysteresis error12 of 13 transverse motion of the accelerometer, we only needed to change the installation direction of the sensor. was 0.0033. To measure the transverse motion of the accelerometer, we only needed to change the The results are shown in Figure 16b. The sensitivity of the X direction was 3.91343 × 10−5 V/(m/s2), × 10 2 9.78357installationand the direction sensitivityV/(m/s ).of of The the Yresultssensor.direction illustrateThe was results 9.78357 that are× the 10shown− accele5 V/(m/s inrometer Figure2). The 16b.is results less The illustrateaffected sensitivity that by thetransverseof the 2 acceleration.X directionaccelerometer was is less3.91343 affected × 10 by transverse V/(m/s acceleration.), and the sensitivity of the Y direction was

FigureFigure 15. 15. VibrationVibration test. test.

(a) (b)

Figure 16. Testing results. (a) Positive and reverse travel of measurement; (b) Output voltage of the Z-, X-, and Y-axes.

6. Conclusions In this paper, a piezoelectric accelerometer was designed, simulated, and analyzed in terms of its maximum stress, natural frequency, and output voltage under an acceleration through the FEM. Moreover, the optimal dimensions were determined. Through the above analysis, it was found that a piezoelectric accelerometer with two sensitive beams and two added beams has the characteristics of high sensitivity, low transverse effect, and low frequency, which meets the requirements of cable fault detection. From mode analysis, the fundamental mode is known and the natural frequency is 1279.1 Hz. In order to obtain reliable and accurate detecting results, the accelerometer must work under the condition that the frequency range is lower than the natural frequency. Through deflection analysis, it was also found that the space between the seismic mass and the glass must be between 12.4 μm and 69.5 μm. Then, the sensor chip was fabricated using lithography, sputtering, ICP technology, and so on. In the fabrication process, we found that most of the chips were fragile during the release of the sensitive beams, the added beams, and the seismic masses in the final ICP process. To improve the yield of fabrication, the thickness of the beams needed to increase. However, the sensitivity decreased. To increase the sensitivity, four additional small masses were added to the seismic masses. Then, the sensor chips were wire-bonged to printed circuit boards (PCBs) and simply packaged for experiments. Finally, the vibration test was carried out, which verifies that the designed structure has good piezoelectric output characteristics. In the future, the designed structure can be optimized to meet different application demands, including the activation of automotive safety systems, machine and vibration monitoring, and biomedical applications for activity monitoring.

Sensors 2016, 16, 1587 12 of 13

9.78357 × 10 V/(m/s2). The results illustrate that the accelerometer is less affected by transverse acceleration.

Sensors 2016, 16, 1587 13 of 14 Figure 15. Vibration test.

(a) (b)

Figure 16.Figure Testing 16. Testing results. results. (a) Positive (a) Positive and and reverse travel travel of measurement;of measurement; (b) Output (b) voltageOutput of voltage the Z-, of the Z-, X-, andX-, andY-axes.Y-axes.

6. Conclusions6. Conclusions In this paper, a piezoelectric accelerometer was designed, simulated, and analyzed in terms of In thisits maximum paper, a stress, piezoelectric natural frequency, accelerometer and output was voltage designed, under simulated, an acceleration and through analyzed the FEM. in terms of its maximumMoreover, stress, the optimalnatural dimensions frequency, were and determined. output voltage Through under the above an analysis, acceleration it was foundthrough that athe FEM. Moreover,piezoelectric the optimal accelerometer dimensions with were two sensitive determined beams. andThrough two added the above beams hasanalysis, the characteristics it was found that a piezoelectricof high sensitivity,accelerometer low transverse with two effect, sensitive and low beams frequency, and whichtwo added meets the beams requirements has the of characteristics cable of high sensitivity,fault detection. low From transverse mode analysis, effect, the and fundamental low frequency, mode is which known andmeets the the natural requirements frequency is of cable 1279.1 Hz. In order to obtain reliable and accurate detecting results, the accelerometer must work under fault detection.the condition From that mode the frequency analysis, range the is lowerfundamen than thetal natural mode frequency. is known Through and the deflection natural analysis, frequency is 1279.1 Hz.it was In alsoorder found to obtain that the reliable space between and accura the seismicte detecting mass and theresults, glass mustthe accelerometer be between 12.4 µmustm work under theand condition 69.5 µm. Then, that the the sensor frequency chip was fabricatedrange is usinglower lithography, than the sputtering, natural ICPfrequency. technology, Through deflectionand analysis, so on. In theit was fabrication also found process, that we foundthe sp thatace most between of the chips the wereseismic fragile mass during and the the release glass of must be betweenthe 12.4 sensitive μm and beams, 69.5 the μm. added Then, beams, the andsensor the seismic chip was masses fabric in theated final using ICP process. lithography, To improve sputtering, the yield of fabrication, the thickness of the beams needed to increase. However, the sensitivity ICP technology,decreased. and To increase so on. the In sensitivity,the fabrication four additional process, small we masses found were that added most to of the the seismic chips masses. were fragile during theThen, release the sensor of the chips sensitive were wire-bonged beams, the to printedadded circuit beams, boards and (PCBs) the seismic and simply masses packaged in the for final ICP process. experiments.To improve Finally, the the yield vibration of fabrication, test was carried the out, thickness which verifies ofthat the the beams designed needed structure to has increase. However,good the piezoelectric sensitivity output decreased. characteristics. To increase In the future,the sensitivity, the designed four structure additional can be optimizedsmall masses to were added tomeet the different seismic application masses. demands,Then, the including sensor the chips activation were of wire-bonged automotive safety to systems,printed machine circuit boards and vibration monitoring, and biomedical applications for activity monitoring. (PCBs) and simply packaged for experiments. Finally, the vibration test was carried out, which verifies Acknowledgments:that the designedThis structur work is supportede has good by Natural piezoelectric Science Basic Researchoutput Plancharacteristics. in Shaanxi Province In ofthe China future, the (Program No. 2016JQ5088) and Collaborative Innovation Center of Suzhou Nano Science and Technology. designed structure can be optimized to meet different application demands, including the activation Author Contributions: Bian Tian directed the research study; all authors contributed to the design of the of automotiveaccelerometer; safety Hanyue systems, Liu planned machine and and analyzed vibration the structure, monitoring, performed and the experiments, biomedical analyzed applications the for activity monitoring.data, and wrote the paper; Ning Yang helped to perform the experiments; Yulong Zhao and Zhuangde Jiang supervised the overall study and experiments and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.

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