Function Matching Design of Wide-Band Piezoelectric Ultrasonic Transducer

Transcription

1 Function Matching Design of Wide-Band Piezoeectric Utrasonic Transducer Yingyuan Fan a, Hongqing An b Weifang Medica University, Weifang, , China a yyfan@126.com, b hongqingan01@126.com Abstract The matching design in wide-band piezoeectric utrasonic transducer is studied in theory in the foowing aspect: the choice of piezoeectric wafer; the acoustic impedance match between the back ayer and the piezoeectric wafer; the matching circuits of the transducer; the matrix matching ayer. According to theoretica anaysis, an utrasonic transducer is designed. Test resut shows that it has a good bandwidth and high sensitivity. Keywords Matching Design, Piezoeectric Wafer, Back Layer, Matching Circuits, Matrix Matching Layer. 1. Introduction Utrasonic transducer is a key component during the nano-partices utrasonic testing. An ideaized utrasonic transducer depends on its high sensitivity and wide bandwidth, but you can't have it a at the same time, so you shoud ony seect one aspect or ower your sights. In this artice, an utrasonic transducer has been designed and manufactured with the 20MHz mid-frequency LiNbO3 and the mixture of tungsten partices and epoxy resin. The foowing aspects are isted to increase bandwidth of it. 2. The construction and principe of the transducer The transducer is composed of piezoeectric wafer, back ayer, acoustic impedance matching ayer, matching circuits, matrix matching ayer, externa connector, inner she and outer she. (Fig.1) Fig.1 The construction of the transducer Piezoeectric wafer is a major component of transducer, which coud compete the conversion between mechanica energy and eectric energy. An acting force on the proper direction of piezoeectric wafer may be about to change its eectric poarization state with positive and negative eectric charges proportiona to the force appearing on opposite surfaces of the piezoeectric wafer. This phenomenon is caed piezoeectric effect which is the principe of the utrasonic transducer to receive utrasonic signas by converting mechanica signas into eectrica signas. Conversey, with the change of eectric 12

2 poarization state in piezoeectric wafer by an effect of eectric fied on the surfaces, the strain generated in proportion to the eectric fied intensity. This phenomenon is caed inverse piezoeectric effect which is the principe of the utrasonic transducer to transmit utrasonic signas by converting eectrica signas into mechanica signas. 3. Function matching design 3.1 The piezoeectric wafer Piezoeectric wafer is a kind of anisotropic materias, with the fundamenta performance parameter such as eectro-mechanica couping coefficient ( K ), dieectric constant ( ) and mechanica quaity factor ( Q m ). Eectro-mechanica couping coefficient ( K ) is the ratio between mechanica energy and eectric energy in piezoeectric wafer, expressed as fowing: the output mechanica energy Kt 100% (Emitting state) (1) the input power K s the output power 100% (Accepting state) (2) the input mechanica energy There are two different vibration modes in piezoeectric wafer, one is ongitudina eectrostriction and the other is atera eectrostriction. is the ongitudina eectrostrictive coefficient and is the atera eectrostrictive coefficient. The ongitudina eectrostriction is usuay used in piezoeectric wafer, and a higher of piezoeectric materia is often demanded in the transducer Dieectric constant ( ) is a physica quantity to describe the dieectric properties of piezoeectric wafer which is reated to eectrica impedance. In order to reaize appropriate impedance matching, of the piezoeectric wafer is usuay not be too big. Mechanica quaity factor ( Q ) is defined as foowing: K s Q m m K t the mechanica energy stored inpiezoeectric wafer (3) the oss of mechanica energyin onevibration period From the formua 3 we coud see that the oss of mechanica energy is smaer with a bigger ( Qm 1) and the pused-signa width woud widen. So the back ayer is aways used behind the piezoeectric wafer to reduce the vaue of and to add bandwidth of the transducer. Q m In this study, we took the choice of LiNbO3 as piezoeectric wafer which is a kind of synthetic singe-crysta materia with strong piezoeectric properties and stabe physica and chemica properties. LiNbO3 doesn't dissove in water, meanwhie, its curie point and meting point is 1210 and The ongitudina eectrostrictive coefficient ( K t ) of LiNbO3 cut aong the direction of 36 y is neary to 55%, so the energy oss of utrasonic propagating in LiNbO3 is ess than 0.05dB/cm with 500MHz high frequency utrasound. At the same time, LiNbO3 is very perfect for high-frequency transducer because of the frequency constant of it is up to 3.62MHz mm. 3.2 The acoustic impedance match between the back ayer and the piezoeectric wafer The basic requirement to the back ayer is that the acoustic impedance of back ayer as same as possibe to piezoeectric wafer. And the attenuation coefficient of back ayer is must bigger enough so the backward utrasonic generated by piezoeectric wafer coud be absorbed and an idea test waveform may be gained. In our study,the mixture of tungsten partices and epoxy resin is used for back ayer of transducer. In order to make impedance matching we, the theoretica cacuation and experiment vaidate about K s Q m 13

3 acoustic impedance of composite materias transducer back ayer is required to identify the reationships between voume concentration of partice and its impedance. Other studies have shown that the acoustic impedance and the attenuation coefficient coud be changed by controing the partice size and quantity in back ayer. 3.3 The matching circuits of the transducer The matching circuits of the transducer is an important method to add bandwidth. The impedance characteristic has been demonstrated by equivaent circuit in Fig.2. A C 0 R 0 L 1 L 2 L 3 Ln R 1 R 2 R 3 Rn B C 1 C 2 C 3 Cn Fig.2 Equivaent circuit In Fig.2, L1, R2 and C3 are equivaent inductance, equivaent resistance and equivaent capacitance, C0 is static capacitance, R0 is dieectric oss. 1 R 0 C0 tan (10) Where is anguar frequency, tan is tangent of dieectric ose ange. Different circuit branch with different L, R and C correspond to different resonance frequency. If the thickness of the piezoeectric wafer is certain, a fixed frequency f 0 existed as formua 11. V f0 (11) 2d Where V is speed of sound in piezoeectric wafer, d is thickness of piezoeectric ayers. As can be seen from Fig.2: ony the dynamic impedance of L1-R1-C1 branch is caused by piezoeectric effect. When utrasonic frequency is just equa to f 0 in piezoeectric wafer, the eectric resonance and mechanica resonance is compatibe which ead L1-R1-C1 circuit branch to pure resistance state. At this moment, the equivaent circuit of Fig.2 with ignoring the infuence of R0 can be simpified as shown in Fig.3 I A C I 0 I 1 R 0 1 B Fig.3 The equivaent circuit when frequency is f 0 As can be seen from Fig.3: When frequency is f 0, the source current (I) coud not fow hundred -percent into R1 branch because of the static capacitance (C0). That is to say the branch current (I0) reduces the power factor of the transducer. For genera piezoeectric wafer, C0 is defined as: S C0 (12) d 14

4 Where, S and d are dieectric constant, area and thickness of the piezoeectric wafer. The transmit power of the transducer coud be greaty improved by a parae inductance ( ) to cance out the capacitive reactance of C 0. L The matrix matching ayer is cacuated by: L 1 (13) 0 2 0C0 The matrix matching ayer outside of the transducer is another method to add bandwidth. This matching ayer coud be regarded as dieectric between the transducer and the measured materia. Considering from the fied of physics, it is a series circuit with the reationship of impedance conversion as foowing: Zmcos jzsin Zin Z (14) Z cos jzmsin Z in Z Where is the input impedance of transducer, is the impedance of the matrix matching ayer, is the impedance of the measured materia, is the thickness Of the matrix matching ayer and is the waveength in the matrix matching ayer. The transmission characteristic of the regenerator is discussed in three particuar cases: ⑴ If, the effect of the matrix matching ayer is ony to protect the transducer, without any impedance matching function; ⑵ If 2, it has no effect to change the reative phase reation because the matrix matching ayer coud ony offer a resonance ayer. On the contrary, it increases the refection oss; ⑶ If 4, from formua 14 we can see, Z Z Z 2 in m L 0 Z m. And then, the best impedance matching optimization is reaized as soon as Z Z in Z m. You coud get the same effect. When is odd time of 4 to thicker-ayer or muti-ayer. 4. Experimenta tests and concusion According to the above theoretica anaysis,a transducer has been designed with 20MHz center frequency LiNbO 3, tungsten partices and epoxy resin back ayer, appropriate thickness matching ayer, etc. In order to test performance of the transducer, the anaysis of the spectrum with a high frequency utrasonic signa (Fig.4) is obtained by spectrum anayzer, which is shown in Fig.5 Fig.4 Utrasonic signa 15

5 Fig.5 Utrasonic spectrum curve As can be seen from Fig.5, the transducer in this study has 18.5MHz center frequency and 5.5MHz band width the power received by 6 db. So, the reative bandwidth is 30%, and a these are enough evidence to the good bandwidth and high sensitivity of our transducer. Acknowedgements This paper was financiay supported by Weifang Medica Coege Educationa Reform Project Fund (2015Y018) and Natura Science Foundation of Shandong Provincia under Grant No.ZR2016HL45. References [1] H. Chen, Y.M. Zhang, Z.S. Li, Design of wide-band ongitudina mode piezoeectric transducers with impedance matching ayers, Journa of Appied Acoustics. Vo.20(2001) No.2, P.31-34,22. (In Chinese) [2] X. Li, Y.P. Song, Z.B. Wang, etc. The anaysis of a piezoeectric transducer equivaentimpedance mode and the design of an impedance matching circuit, Journa of Appied Acoustics, Vo.35 (2016) No.1, p (In Chinese) [3] L.X. Han, B.H. Yu, X.P. Hu, Parametric design of impedance matching circuit for power utrasonic piezoeectric transducer, Piezoeectrics & Acoustooptics,Vo.37(2015)No.4, p , 720. (In Chinese) [4] KIM Kyeong-seop, SHIN Seung-won, YOON Tae-ho, LEE Sang-min, LEE Insung, RYU Keun ho, Sound transducer caibration of ambuatory audiometric system utiizing deta earning rue. Journa of Centra South University of Technoogy, Vo.18 (2011), p [5] W.S. Chen, Y.X. Liu, J.K. Liu, S.J. Shi, Working principe and design of a doube cyinders type traveing wave utrasonic motor using composite transducer. Journa of Harbin Institute of Technoogy, Vo.18(2011) No.2, p (In Chinese) 16