DEVELOPMENT OF DOUBLE MATCHIN LAYE FO ULTASONIC POWE TANSDUCE unn Hang, WooSub Youm and Sung Q Lee ETI, Nano Convergence Sensor esearch Section, 8 ajeong-ro, Yuseong-gu, Daejeon, 349, South Korea email: hangun@etri.re.kr The ultrasonic transmitter is designed for implantable devices in human body to supply electric poer. For implantable devices, it is necessary to design a ireless poer unit ith rechargeable batteries. The ultrasonic poer transmission system consists of to piezoelectric transducers, facing each other beteen skin tissues and converting electrical energy to mechanical vibration energy or vice versa. The ultrasonic poer transmission system is free from the electromagnetic coupling effect and medical frequency band limitations hich making it a promising candidate for implantable devices. This paper is focused on the development of the double matching layer of ultrasonic transmitter to maximize energy conversion efficiency. The double matching layer of the transmitter is calculated based on the acoustic impedance theory, designed through numerical analysis, and tested to get the optimal matching layers thickness of the ultrasonic transmitter ith the -3 composite piezoelectric ceramic structure of 3 mm diameter, 3 mm thickness. The tested -3 composite piezoelectric ceramics is PMN-3PT. The polyoxymethylene (POM), also knon as acetal and polyacetal, and a titanium(ti) layer for biocompatibility packaging are chosen as base materials of the matching layer. The maximum energy conversion efficiencies of the optimal matching layer of the transmitter are 79.4 % at. mm thickness of POM matching layer and 67.5 % at. mm thickness of POM matching layer and. mm thickness of Ti layer for PMN-3PT, experimentally.. Introduction The ultrasonic transmitter is designed for implantable devices in human body to supply electric poer. For implantable devices, it is started to be necessary to have a ireless poer unit ith rechargeable batteries to reduce the number of the operations to replace the primary cell batteries. The ultrasonic poer transmission system consists of to piezoelectric transducers, facing each other beteen skin tissues and converting electrical energy to mechanical vibration energy or vice versa. The ultrasonic poer transmission system is free from the electromagnetic coupling effect and medical frequency band limitations hich making it a promising candidate for implant devices []. The biocompatibility of the devices is the first requirement as medical implant devices. Titanium has been idely used as a base material to meet the biocompatibility requirement in medical industry. It is very important for an ultrasonic transducer to have the high energy transfer efficiency as an energy converter. This paper is focused on the development of the experimental study of the double matching layer of ultrasonic transmitter for ireless poer transmission to have the maximum energy transfer efficiency. The polyoxymethylene (POM), also knon as acetal and polyacetal, and titanium (Ti) layer for biocompatibility packaging are chosen as materials of matching layers of the ultrasonic transducer.
. Theory For ultrasonic transducers, the efficiency is defined the ratio of the output acoustic poer to the input electric poer. The input electric poer is calculated from input voltage and current. The output acoustic poer is obtained by the acoustic intensity integration of the total radiated area of the ultrasonic acoustic poer. Hoever, it is very difficult and energy consumption for the output acoustic poer calculation to get the acoustic intensity to the total radiated area. Instead of this acoustic intensity measurement, this study estimates the efficiency of an ultrasonic transducer ith a relative simple method using a mathematical model of the ultrasonic transducer and an impedance analyser [].. overning Equations The mathematical model of the ultrasonic transducer that converts electric energy to acoustic energy is shon as Fig.. ZEB is electrical impedance hen the displacement of the transducer is blocked to zero and a capacitance of the transducer for a piezo electric transducer. Zms is mechanical impedance hen the signal input terminal is shorted. Zr is radiation impedance, and ø transformation factor, respectively. The electrical impedance and the mechanical impedance that could be converted to an electrical impedance ith a transformation factor, ø, is related ith a parallel structure as shon in Fig. []. Figure : Mathematical model of the ultrasonic transducer []. When the ultrasonic transducer is at resonant mode, the efficiency of the transducer is obtained as follos: r / EM MA. () ( ) here =, is an electric-to-mechanical energy conversion efficiency, is a mechanical-to-acoustic energy conversion efficiency, m mechanical resistance and r radiation resistance. The conductance,, is defined as: ICSV3, Athens (reece), -4 July 6
E = + X. (). (3) hen the ultrasonic transducer is resonant condition ( = ), = and as shon in Fig. : Figure : Conductance response curve of ultrasonic transducer [, 3]. In Fig., the conductance,, measured ith vacuum condition, is obtained as follos: Evacuum. (4) vacuum here = and = at. The conductance,, measured in ater, is as follos: Eater here = and = at. From Eqs. (4) and (5),, and are obtained as follos: vacuum. (5). (6) (/ ) Evacuum. (7) (/ ) r Eater vacuum. (8) From Eqs. () and (8), the efficiency of the ultrasonic transducer is obtained as follos: vacuum EM MA ( ). (9) here is a resistance of DC or lo frequency condition and obtained as follos: ICSV3, Athens (reece), -4 July 6 3
vacuum E E ater (@ ). () here and are initial conductances in vacuum and ater condition, respectively.,, and are obtained that the maximum conductance in vacuum and ater condition, respectively, are substituted as and and applied into Eqs. (6), (7), and (). Finally, the efficiency of the ultrasonic transducer, η, is obtained from Eq. (9) ith,, and. The characteristic acoustic impedance, Z, of medium such as air or ater is = here ρ is the density and c is the sound of speed of the medium. The conductance,, measured in air of a transducer used for ater is supposed to be equal to, because the characteristic acoustic impedance of ater is.478 Mrayls and larger than that of air,.45 Mrayls.. Measurements ith Impedance Analyzer The measurement mode of the Agilent technologies 494A impedance analyzer, is selected ith parallel mode because the mathematical model of the ultrasonic transducer is parallel type as shon in Fig.. Based on the measurement parameter data, p (parallel conductance) and Bp (parallel susceptance) are selected as measurement parameters, conductance and susceptance, of a ultrasonic transducer and measured those value in air and ater condition, respectively [4]. Figure. 3 shos the schematic diagram for experimental setup of -3 piezocomposite transducer. The diameter and thickness of the -3 composite transducer are 3 mm and 3 mm. The polyoxymethylene (POM), also knon as acetal and polyacetal, and titanium (Ti) are chosen as base materials of the matching layer of the transducer. The. mm,. mm, and.3 mm thickness of the POM layer and the. mm thickness of the Ti layer are tested. The measurement is conducted in air and ater, respectively. The base material of the piezoelectric ceramic is PMN-3PT made by Ceracomp Co., Korea [5]. The tested volume% of the -3 piezocomposite is 44/6/74. Figure 3: Schematic diagram for experiment of -3 piezocomposite transducer. 3. esults The maximum efficiencies of the -3 composite transducer are 8.4% at. mm thickness of POM layer (M) and 67.5% at. mm thickness of POM layer and. mm thickness of Ti layer 4 ICSV3, Athens (reece), -4 July 6
(M+Ti), respectively as shon in Fig. 4. The mean efficiencies of the experimental results are 78.3% for POM layer and 63.% for POM+Ti layer. The differences of efficiency beteen POM layer and POM+Ti layer are from.9% to 8.6% as shon in Table. 9 Efficiency(%) 8 7 6 5 4 M M+Ti...3.4 POM layer thickness (mm) Figure 4: Experimental results of the efficiency of -3 piezocomposite transducer ith POM layer vs ith POM+Ti layer. Table : Experimental results of the efficiency comparison ith POM layer vs POM+Ti layer. POM thickness POM layer(a) POM+Ti layer(b) difference(a-b). 79.4 67.5.9. 8.4 6.7 7.7.3 76.5 63.3 3..3 77. 58.4 8.6 mean 78.3 63. 5.4 Figure 5 shos the experimental results of conductance () ith. mm thickness of POM layer (M.) and ith. mm thickness of POM layer and. mm thickness of Ti layer (Ti.M.), respectively. There are to peaks of the conductance graph measured in air in Fig. 5. These peaks is merged to one peaks and the values are loered hen it is measured in ater. Figure 5: Experimental results of the conductance of -3 piezocomposite transducer ith POM layer vs POM+Ti layer. ICSV3, Athens (reece), -4 July 6 5
4. Discussion and conclusion This paper is focused on the development of the double matching layer of ultrasonic -3 composite transducer for energy conversion efficiency used for ireless poer transmission in human body. The efficiency of the transmitter ith double matching layer is calculated based on the acoustic impedance theory, designed through numerical analysis, and tested to get the optimal matching layer of the ultrasonic transmitter ith the -3 composite piezoelectric ceramic structure of 3 mm diameter, 3 mm thickness. The tested -3 composite piezoelectric ceramics is PMN- 3PT. The polyoxymethylene (POM), also knon as acetal and polyacetal, is chosen as a material of the inner layer for impedance matching. The titanium plate ith. mm thickness is used as a base material of the outer layer for protection of the system and biocompatibility against human body. The maximum energy conversion efficiency of the transmitter are 67.5 % at. mm thickness of matching layer and. mm thickness of Ti layer. The next step is to find the optimum matching layer thickness based on this experimental results and simulation results that ill be studied. 5. Acknoledgements This ork as supported by the &D Program of MOTIE/KIAT, South Korea [N897, Development of sustainable poer module for implantable medical devices based on ultrasonic ireless poer transfer], and e ould like to thank them for their financial assistance. EFEENCES Hang,., Youm, W. and Lee, S., Development of matching layer for ultrasonic ireless poer transmitter, Proceedings of the 44th International Congress and Exposition on Noise Control Engineering, California, U.S.A., 9 - August, (5). Kinsler, L. Ed., Fundamentals of Acoustics, John Wiley & Sons, Hoboken, NJ (). 3 Sherman, C. and Butler, J., Transducers and Arrays for Underater Sound, Springer, Cohasset, MA (7). 4 Yanaga K., Agilent 494A Precision Impedance Analyzer Operating Manual, Agilent Technologies, Hyogo, Japan, (). 5 Ceracomp, Co., Ltd., http://.ceracomp.com/ 6 ICSV3, Athens (reece), -4 July 6