Design and System Modeling of a Tri-Axial Microaccelerometer Using Piezoelectric Thin Films

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1 Ferroeectrics, 85:, 69 74, 009 Reprints avaiae directy fro the puisher DOI:.80/ URL: 69 Jyh-Cheng Yu et a. 009 Tayor & Francis ISSN: print / 56-5 onine Design and Syste Modeing of a Tri-Axia Microacceeroeter Using Piezoeectric Thin Fis JYH-CHENG YU, * CHUNGDA LEE, and CHIAOKAI CHANG Departent of Mechanica and Autoation Engineering Nationa Kaohsiung First University of Science and Technoogy, Juoyue Rd., Nantz, Kaohsiung 8, Taiwan, R.O.C. Departent of Mechanica and Autoation Engineering I-Shou University,, Sec., Syuecheng Rd., Dashu, Kaohsiung County 840, Taiwan, R.O.C. This study presents the syste odeing of a tri-axia icroacceeroeter that consists of a quadri-ea suspension, a seisic ass, and dispaceent transducers using piezoeectric thin fis. Both ends of the seisic ass are supported y two suspension eas. Therefore, the out-of-pane acceeration wi resut in a end, and in-pane acceerations wi produce a end and torsion. The eectrodes are configured and interconnected to easure one out-of-pane acceeration and two in-pane acceerations seectivey. Resonant frequency and sensor sensitivities to triaxia acceeration are investigated using FEM anaysis. Sensitivity study of ajor diensiona paraeters are presented for sensor design. Keywords: Microsensor, Piezoeectric Acceeroeter, Tri-axia, Thin fi Introduction Microacceeroeters have een successfuy appied in autootive industry such as contro of airags and suspension systes and interactive entertainent eectronics. Potentia arkets aso exist in the areas of ioechanics and aerospace technoogy. Piezoeectric acceeroeters have the advantages of easy integration with existing easuring systes. Due to the exceent dynaic perforance and inearity, they have een widey used in condition onitoring systes to easure achinery viration. Two types of piezoeectric icro-acceeroeters are reported ased on farication approaches. Surface icroachining ased piezoeectric acceeroeters are ore cost effective due to its reativey sipe farication []. On the other hand, uk icroachining ased piezoeectric acceeroeters have a ower detection eve that is suitae for precision easureent []. Van Kapen and Woffenutte anayzed utipe-supported ea structures and odeed the echanica ehavior of uk-icroachined siicon acceeroeters using a static echanics anaytica technique []. Yu and Lan used a sipified spring-ass syste to represent a z-axia icro-acceeroeter with piezoeectric thin fi sensing [4]. Soe takes into account oth the sustrate and PZT thin fi in eastic properties of suspension eas y use of ainated pate theory [5][7]. Severa design of tri-axia Received August, 008; in fina for Deceer, 008. * Corresponding author. E-ai: jcyu@ccs.nkfust.edu.tw 69

2 70 Jyh-Cheng Yu at a. piezoeectric icroacceeroeter have een reported [6][7]. Theoretica odes agree we with FEM anaysis. However, a the iteratures negect ass and eastic effects of patterned eectrodes ecause of copexity. FEM anaysis sees an effective too for sensor design. Design of Microacceeroeter The proposed icroacceeroeter consists of a quadri-ea suspension, a seisic ass, and eight piezoeectric dispaceent transducers arranged as shown in Figure. The device is faricated fro wet etching of <0> SOI siicon and dry etching of suspension eas. Both ends of the seisic ass are supported y two suspension eas. Two transducers are designed on each suspension as arranged in Figure such that the discontinuous regions at oth ends and ow stress region at the idde of the ea are negected. The out-of-pane (z axia) acceeration wi resut in a viration, and in-pane (x and y axia) acceerations wi produce a and torsiona virations. The inertia force wi introduce ending and torsiona stress on the eas that wi produce eectrica charge y the piezoeectric transducers. The eectrodes are configured and interconnected as in Tae so the out-of-pane acceeration and in-pane acceerations can e seectivey easured. For instance, the out-of-pane acceeration wi end the eas as shown in Figure (a) where the inward transducers (cose to seisic ass) and the outward transducers are suject to stresses of different directions. The charge coected fro transducers, 4, 5 and 8 inus the charge coected fro transducers,, 6 and 7 wi e in proportiona to the out-of-pane acceeration whie the effects due to in-pan acceerations wi e canceed out. This design not ony increases transducer sensitivities, ut aso reduces noise effects. 4 w w w h Figure. Scheatic view of the piezoeectric icro-acceeroeter t (a) Syetric end (z-axia) () A end (x-axia) (c) Torsiona (y-axia) Figure. Three asic viration odes of the acceeroeter due to tri-axia acceeration 70

3 Tri-Axia Microacceeroeter/Piezoeectric Thin Fis 7 Tae. Arrangeent of eectrodes for tri-axia acceeration sensing Syetric PZT(+4+5+8) - PZT(++6+7) A PZT(+4+6+8) - PZT (++5+7) Torsiona PZT(+6+4+7) - PZT (+5++8) Structure Modeing The copete syste of the sensor can e divided into a echanica susyste and an eectric susyste. The perforance of the echanica susyste is deterined y the equivaent stiffness of the suspension and the seisic ass, whie the eectric susyste is deterined y the piezoeectric transducers [4]. The andwidth of the acceeroeter is constrained y the resonance frequency. Resonance frequency ω = K n M () M Mechanica sensitivity S () K where K is the structure stiffness and M is the seisic ass. This study appies energy ethod - Castigiano s second theore to derive structure stiffness. Differentia of structure strain energy to a given oad provides the dispaceent at the direction of the given oad. For the instance of otion, the derived stiffness is 48EI z K = () wt where I z is the oent of inertia of suspension ea I Z = Negecting the iperfection of convex corner copensation, a sipe integration of the structure wi give the ass: 4 M = ρ wh h ( + w )cotθ + h cot θ + ρ wt (4) where ρ is the density of Siicon and θ = 54.7 for <0> siicon. The echanica sensitivity is as foows: M M S = = (5) K EI z For a sape diensiona design as in Tae, the derived resonance frequency of otion is 95 Hz that is cose to a FEM resut of 0 Hz. Tae. Initia diensions of the icro-acceeroeter (unit: μ) w t w h Finite Eeent Siuation The axiu oading of the sensor without structure faiure is 5g. To verify the configuration of the eectrodes, four different oads are appied incuding unit

4 7 Jyh-Cheng Yu at a. acceeration at three principa directions and a tri-axia acceeration. The FEM resuts are shown as Tae. The ateria properties can e found in previous iterature [4]. The resut shows that even a triaxia acceeration is appied to the sensor; the sensor can detect and provide the sae resuts as uniaxia acceeration using the interconnection ayout of Tae. Tae. Sensitivity coparisons of uni-axia acceeration and tri-axia acceeration (unit: V) & x v & y v z v v r r & & x + & y + & z Syetric Torsiona A The frequency response of the device is shown in Figure. The upper iit of andwidth is aout ω n /5. Since the resonance frequency of the otion is the owest aong three otions, the andwidth of the icroacceeroeter wi e constrained y the andwidth of otion. Aso, Figure shows that the sensitivity ratio etween and torsiona otions is aout :. Paraeter designs shoud e introduced to cose up the gap a torsiona Hz=00 V 0 Hz=546.7 Hz= Hz Figure. Frequency response of the acceeroeter in triaxia acceeration The study derives the sensitivities of ω x /ω z and K x /K z versus the ength of seisic ass ( ) and the ength of suspension eas ( ) as shown in Figure 4 using anaytica ode. The sensitivity of otion is higher than a otion. A shorter ea and a arger ass wi reduce the echanica sensitivity difference. However, a shorter ea wi reduce the size of piezoeectric transducer and thus the eectric sensitivity. The effects of the ength of suspension ea and ass on the overa sensor sensitivities are siuated using ANSYS. As shown in Figure 5, the sensor sensitivity ratio etween torsiona and otions are insensitive to the change of the ength of seisic ass and suspension eas, whie the reative torsiona sensitivity increases with the increasing of the ength of seisic ass. Given a andwidth requireent, diension optiization can e perfored to increase sensitivities and reduce the sensitivity difference aong three axes. 7

5 Tri-Axia Microacceeroeter/Piezoeectric Thin Fis 7 4 ωx/ωz Kx/Kz 6 5 ωx/ωz Kx/Kz (μ) (a) (μ) Figure 4. Resonance frequency ratio ω x /ω z and stiffness ratio K x /K z versus the ength of seisic ass ( ) and the ength of suspension eas ( ) ().E+0 a torsiona.e+0 a torsiona sensitivity( V//s ).E+00.E-0 sensitivity(v//s ).E+00.E-0.E (μ) (a).e (μ) Figure 5. Sensor sensitivities versus the ength of seisic ass ( ) and the ength of suspension eas ( ) () Concusion A tri-axia piezoeectric icroacceeroeter is investigated and the interconnection ayout of transducers is presented. The proposed design can seectivey easure triaxia acceeration. The perforances in three axes are anayzed using FEM anaysis. Differences of triaxia sensitivities are noted. Discussions of sensor perforance tradeoff versus sensor paraeters are presented. The resut deonstrates the feasiiity of sensor design, and future direction for design optiization is propted. References [] Chen PL, Muer RS, Joy RD, Haac GL, White RM, Andrews AP, Li TC, Motaedi ME: Integrated siicon icroea PI-FET acceeroeter, IEEE Trans. Eectron Devices, 98; 9, : 7-. [] DeVoe DL, Pisano AP: A Fuy Surface-Microachined Piezoeectric Acceeroeters, The 9th Internationa Conference on Soid State Sensors and Actuators - Transducers'97, 997; :

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