Piezoelectric Device with Volume Elastic Wave for Ultrasonic Pulses Generating

Transcription

1 Piezoelectric Device with Volume Elatic Wave for Ultraonic Pule Generating MARIAN PEARSICĂ Henri Coanda Air Force Academy Mihai Viteazu St. 160, 00, Braov, ROMANIA CIPRIAN RĂCUCIU, NICOLAE JULA, DAN RĂDUCANU Military Technical Academy George Coșbuc Blvd , Sector 5, Bucharet, ROMANIA Abtract: - The device contitute a pecific application of piezoelectric tranducer, which allow the generation of impule erie with contant or peudo-aleatory frequency, and contant or variable paue. The device work i baed on the proce of achieving invere piezoelectric effect, meaning the deformation of the crytalline network of the piezoelectric device caued by an external electrical field. The equipment wa phyically realized and it contituted the object of a reearch contract of the Optical-Electronic Intitute of Bucharet. In thi paper i alo preented a PSpice imulation of the projected equipment. Key-Word: - piezoelectric tranducer, generator, acoutic power, electrical field, ultraonic ignal, peudoaleatory frequency 1 Introduction In the indutrial application, beide defect detection, ultraound can alo be ued to determine ignificant material characteritic uch a denity, thickne, mechanical propertie, level ening, and more. By propagating a wave in a given medium, ueful information about the medium can be generated by analyzing the tranmitted or reflected ignal. The piezoelectric tranducer preent the advantage: high poitioning accuracy with a reolution of 0,005μm/V, high tiffne, fat repone, high efficiency. Ultraound i applicable to all tate of matter, with the exception of plama and vacuum. Propagation of ultraound in a material i not affected by it tranparency or opacity. Poible application: optical ytem and meaurement technology, laer tuning, fiber poitioning, microelectronic, micro-lithography, acoutic propagation ytem, preciion mechanic and mechanical engineering. Ultraonic ignal generating device, baed on piezoelectric crytal, i ued to obtain ultraonic ignal with fixing frequency between 0 10kHz. The device could alo generate packet of ultraonic ignal with contant or peudo-aleatory frequency, and contant and variable paue [6]. It i a pecific application of the piezoelectric crytal, which allow the obtaining ultraonic ignal, baed on unconventional method. It work i baed on the invere piezoelectric effect, obtained by the deformation of the crytalline network of the piezoelectric device under the action of an external electric field. The generator i a piezoelectric tranducer, which belongk to the erie of piezoelectric device with elatic wave of volume. The configuration of the electro-elatic piezoelectric tranducer i preented in Figure 1. Fig.1 Configuration of the electro-elatic piezoelectric tranducer For certain value of frequency, the equivalent electro-elatic diagram could be repreented a a circuit with dicreet element (Fig.). Fig. Equivalent diagram of piezoelectric tranducer ISSN: ISBN:

2 The element of equivalent cheme have the following expreion and ignification []: 1 1 C = Im o πf Z (1) oe Qe R e = () πf C om vf o Z λ L = = Zom (3) 8n 16πf n 4n C = (4) π f Zom π Zom R = (5) 8Q m n where: F i the repreentative in implified complex of elatic force at mechanical gate; V the repreentative in implified complex of vibration peed at mechanical gate; U, I the repreentative in implified complex of voltage and electrical current at electrical gate; n the tranformation ratio of ideal electro-elatic tranformer; f the ocillation frequency of electrical applied field; λ the wave length of the gradual elatic wave; Z om the characteritic elatic impedance of the tranducer; Q m, Q e the qualitative mechanical and electrical factor of the tranducer; f the value of the frequency for which the contant phae of the tranducer ha the particular characteritic value at the mechanical reonance; v f the propagation peed of the elatic wave in the piezoelectric tranducer; U e, Z i the complex electromotive voltage and the internal complex impedance of the upply ource for the piezoelectric tranducer; Z S the complex load impedance. Concrete value of element in equivalent diagram are determined by material parameter correponding to tranducer configuration and contructive ize. In both the direct and invere piezoelectric effect, the train and tre are related to the electrical parameter by the piezoelectric contant, d ij, g ij, h ij and e ij, which are different value for different direction in the material. The mot commonly meaured of thee contant i the piezoelectric train contant d ij. In the longitudinal mode of X-cut crytal, the applicable value i d 11. For an applied voltage, U in, d 11 will determine the reultant thickne change, Δl out, repectively: Δ lout = d11 U in (6) To determine the reultant voltage for the direct piezoelectric effect two different contant are ued. The piezoelectric deformation contant h ij i ued to relate the reultant voltage to a given deformation. In thi cae the thickne change, Δl in, produce an output voltage, U out, according to: U out = h ij Δlin (7) A econd contant, the piezoelectric preure contant, g ij i ued to relate the reultant voltage to a given applied preure, P. The reultant voltage, U out, i given by: U out = g ij P (8) For many application the material contant of interet i the electro-mechanical coupling factor, k ij, which i a meaure of the piezoelectric material ratio of output energy to input energy or efficiency: U out Δlout k ij = = h ij d ij (9) Δlin U in The coupling factor i electrically determined uing the reonance frequency data [1]. The wavelength of the ultraound ued ha a ignificant effect on the probability of detecting a dicontinuity. In ultraonic teting, the horter wavelength reulting from an increae in frequency will uually provide for the detection of maller dicontinuitie. Changing the frequency when the ound velocity i fixed will reult in a change in the wavelength of the ound. The general relationhip between the peed of ound in a olid and it denity and elatic contant i given by the following equation: e ij V = (10) ρ Where V i the peed of ound, e ij i the elatic contant, and ρ i the material denity. Thi equation may take a number of different form depending on the type of wave (longitudinal or hear) and which of the elatic contant that are ued. Equipment block diagram. Principle of operation. The emitter ection of the ultraonic emitterreceiver generate hort, large amplitude electric pule of controlled energy, which are converted into ultraonic pule when applied to ultraonic tranducer. Mot emitter ection have very low impedance output to better drive tranducer. Control function aociated with the emitter circuit include: - Pule length or damping (The amount of time the pule i applied to the tranducer.) ISSN: ISBN:

3 - Pule energy (The voltage applied to the tranducer.) In the receiver ection the voltage ignal produced by the tranducer, which repreent the received ultraonic pule, are amplified. The amplified ignal i available a an output for diplay or capture for ignal proceing. The equipment conit in a piezoelectric generator and a command and upply ytem. Under the action of the ocillating electrical field produced by the excitation ource, the piezoelectric generator will pule with the frequency of the command impule (will expand and repectively contract). If the frequency of the AC field correpond with the frequency where the thickne of the crytal repreent half a wavelength, the amplitude of the crytal vibration will be much greater. Thi i called the crytal fundamental reonance frequency. The power module i achieved uing Power MOSFET tranitor, which are connected in derivation. The high-voltage tranformer enure the neceary voltage for the upply of the piezoelectric device and alo the adaptation to the load. A pecific computer program i ued to generate the erie of datum impule. The block diagram of ultraonic ignal generating et, which i preented in Figure 3, ha two component: a urface equipment (A) and an equipment inide a tainle teel cylinder (b), which function at 300m depth. Fig.3 The block diagram of ultraonic ignal generating et Technical condition for input and output were the following: the equipment upply wa realized from the upply network 0±15%V AC, f = 50Hz; operating regime a) generation of pule train with peudo-aleatory frequency (0 10kHz), and contant and variable paue, b) generation of pule train with contant frequency (etablihed by the uer between 0 and 10kHz), and contant and variable paue; maximal voltage on piezoelectric device, U amax = 1kV; efficiency of excitation electronic ource, η 80%; the upply and command of the piezoelectric device were realized at maximum depth of 300m; electrical reitance of upply cable, R C = 80Ω/km; impoed acoutic power, P a = 50W; the programmable time of pule train between 0 35m, and paue between pule train, 0 75m; variation of excitation voltage value will determine the variation of acoutic output power value; protection againt overload, upravoltage, electrocution, radio-electronic interference in/from network; the electronic ource and piezoelectric device were introduced into a tainle teel cylinder with inide diameter 105mm, outide diameter 110mm and maximal height 0.5m. Conidering the input and impoed output condition were etablihed functional unit, which compoe the generating equipment. The functional unit of the urface equipment (A) are the following: protective unit againt radioof ±9 V for lifting motor electronic interference in/from network (A 1 ); rectify and filtering unit for upply voltage (A ); auxiliary power upply for lifting motor (A 3 ); auxiliary power upply of ±18 V (A 4 ); computer with keyboard (A 5 ); diplay (A 6 ); interface and command unit (A 7 ). The equipment (B) ha the following functional unit: teady voltage upply (B 1 ); teady voltage of ±15 V (B ); low-pa filter (B 3 ); circuit for amplitude dicrimination of command pule (B 4 ); circuit for width dicrimination of command pule (B 5 ); command circuit of power module (B 6 ); filter for radioelectronic interference (B 7 ); condener battery (B 8 ); high-voltage tranformer (B 9 ); power module (B 10 ); protection overload unit (B 11 ); reaction circuit (B 1 ); upra-voltage protection unit (B 13 ); piezoelectric generator (B 14 ). The witching element of power module i realized by Power MOSFET tranitor, connected in derivation, that allow obtaining an electrical power of 600W [3, 4]. Input and output parameter are preented on diplay, and working condition are etablihed with keyboard computer. ISSN: ISBN:

4 The interface and command unit realize the interface between PC and functional unit (B), procee input and output data, and etablihe working condition. The command circuit (B 6 ) realize charge and dicharge the gate-drain capacitance of Power MOSFET tranitor, determining aturation or blocking them [4]. Becaue the command pule are conducted through a 300m long cable, it wa neceary to utilize ome receiving circuit for command pule, like: low-pa filter, pule height dicriminator realized with a Smith trigger, circuit for time dicrimination, circuit for reject the pike and unlike interference. High-voltage tranformer deliver neceary voltage to upply piezoelectric generator; alo, it realized the load matching. The tranfer of energy i made in-phae. Piezoelectric device conit in one piezoelectric radial polarized tranducer, which pule with frequency of the command pule. The acoutic power of piezoelectric tranducer i approximately 50W. 3 Deigning element. Obtained Reult. Many of today application of piezoelectricity ue polycrytalline ceramic intead of natural crytal. Piezoelectric ceramic material can be manufactured in almot any hape or ize, and the mechanical and electrical axe of the material can be oriented in relation to the hape of the material. The orientation of the DC poling field determine the orientation of the mechanical and electrical axe. The active element i the heart of the tranducer a it convert the electrical energy to acoutic energy and vice vera. When an electric field i applied acro the material, the polarized molecule will align themelve with the electric field, reulting in induced dipole within the molecular tructure of the material. Thi alignment of molecule will caue the material to change dimenion. The piezoelectric tranducer are made of ceramic material a PZT (titanium, zirconium, lead) and the reearch wa realized uing a material a Pb(Zr 0,53 Ti 0,47 )O 3 doped with Nb O 5, BiO 3 and MnO [5]. The dimenion and the compoition of the piezoelectric tranducer were determined conidering the frequency band (0 10 khz) of the ultraonic ignal and the impoed acoutic power (50W). After the performed experiment at Optical Electronic Intitute of Bucharet, it reulted that the piezoelectric generator repreent a preponderant capacitive load with a capacitance around 10nF. For an electrical power of 600W and a frequency of 10kHz, the electrical tored energy in piezoelectric crytal capacitance i around 5mJ. It reult the relation for excitation voltage of piezoelectric crytal: E U a = U a 1kV (11) C For a conduction time of 4μ and a required power of 600W, the electrical voltage on piezoelectric crytal increae to maximum 1000V. An important parameter that we mut conider for piezoelectric crytal deigning i maximum puling frequency. Conidering relaxation time of crytal around 4.μ, it reult the maximum working frequency around 10kHz. Another requirement for piezoelectric crytal i it output impedance to be better matching to radiation impedance of environment. So, the ocillatory circuit repreented by piezoelectric crytal mut have it own damping, and it impedance mut varie linear in frequency band (doen t have pole). The piezoelectric generator i formed by a cylindrical piezoelectric piece, radial polarized, with dimenion: φ ext = 50mm, φ int = 38mm, h = 6mm. Piezoelectric generator aembly i how in Figure 4, and it i compoed by the following element: cylindrical tainle teel tube (1), terminal for highvoltage upply (), metallic reflector (3), nonconductive metallic axle (4), inulated piece (5), piezoelectric crytal (6), ilicone oil (7), mixture of rein and wolfram powder (8), ealing piece (9). Fig.4 Piezoelectric generator aembly On both extremitie of piezoelectric crytal are two m etallic reflector, fixed by a metallic axle with thread. The piece i filled inide with a compoition made by rein and wolfram powder, which involve a a reflector and utain metallic axle. The ISSN: ISBN:

5 piezoelectric generator work into a ilicone oil medium, which allow acoutic propagation. The ocillatory circuit realized by the econdary inductance of high-voltage tranformer and by the piezoelectric generator capacitance i trongly damped becaue the medium radiation impedance, where the generator i ituated. By PSpice imulation wa teted in time and frequency domain the excitation ource of piezoelectric device. In order to implify the analyi and to reduce imulation time, from projected ource wa conidered only the power module. The excitation ource i in eence a cloed loop regulating ytem (Fig.5), and it mut be analyzed it tability. S( ) TR ( ) TM ( ) TC ( ) gain; ( ) S( ) By reolving 1 T () S() = 0 = repreent the open loop T D i the open loop tranfer function. + D equation, are determined the cloed loop tranfer function pole. The wavefor m for: grid current, drain current, drain-ource voltage, and excitation voltage of piezoelectric device are preented in Figure 6. Fig.5 Excitation ource a a cloed loop regulating ytem The converter output-control tranfer function i determinate by mediation method [7], doing Forward converter Buck converter equivalence. It i given by the following equation: 1 + U O () 1 TC () = = K (1) U C() ω ωo Q o n D 1 Where: K = Uin, ω o =, Lo Co ω S o Q = R 1, 1 =, r C - erie equivalent Lo rc Co reitance of tranducer capacitance, C o. The tranfer function of the cloed loop regulating ytem may be expreed by the equation [4]: U o () TR () TM () TC () F() = = U ref () 1 + TD () TR () TM () TC () S () () F = (13) 1+ TD () S() Where: T M () repreent the PWM chopper tranfer function, T R () i the PI controller tranfer function; T D () i the diviion circuit tranfer function; Fig.6 Electrical quantitie waveform for analyzed circuit The frequency attenuation characteritic and phae repone, for PI chopper, and converter are preented in Figure 7, and 8. Fig.7 Chopper waveform in frequency domain ISSN: ISBN:

6 Fig.8 Converter waveform in frequency domain The frequency attenuation characteritic and envelope-delay characteritic (phae repone), for excitation ource, are preented in Figure 9. The piezoelectric generator i compoed of a piezoelectric crytal upplied by impule and it pule with the command impule frequency. Many factor, including material, mechanical and electrical contruction, and external mechanical and electrical load condition, influence the behavior of a tranducer. One of the eential feature of ultraonic meaurement i mechanical coupling between the tranducer and the olid whoe tructure or propertie are to be tudied. The geometric conformation and the tructure of the piezoelectric piece were etablihed conidering firt maximal work frequency and the impoed acoutic power. The excitation ource work in witching and allow obtaining an electrical power of 600W. The piezoelectric generator work alo a a torage condener and the tranfer of energy i made inphae. A a conequence of the performed PSpice analyi, it reult that the evolution in time of the electrical quantitie and alo the obtained ignal level have a good concordance with calculated value. Fig.9 Waveform in frequency domain The open loop tranfer function of the ytem, T D () S(), ha a pole in fixed point, which i introduced by the controller, a double pole introduced by the converter, and two zero, one introduced by the controller and the other one introduced by the converter. The preence of pole in fixed point enure a high gain at low frequencie. The zero introduced by the controller i placed near the double pole introduced by the converter, o that the paing through f cro i realized with 0dB/dec. It reult f cro = 4,kHz. The phae edge i poitive and ha the value equal to 49, o. Coni dering the reult of performed analyze it i allowed to affirm that the open loop tranfer function of the ytem enure it tability. 4 Concluion The preented equipment i a complex ytem, purpoed to generate ultraonic ignal by an unconventional method, uing piezoelectric tranducer. Reference: [1] Bhardwaj, C.M., High Efficiency Non-Contact Tranducer and a Very High Coupling Piezoelectric Compozite, WCNDT-004, Montreal, Canada, September 004. [] Lovau, V., Feuillard, G., Tran Huu Hue, L.P., Thermal Effect in Piezoelectric Tranducer. Theoretical and Experimental Reult, WCU 003, Pari, September 003, pp [3] Kauczor, C., Schulte, T., Frohleke, N., Reonant Power Converter for Ultraonic Piezoelectric Converter, Proc. of Actuator 00, pp [4] Mohan, N., Undeland, T.M., Robin, W.P., Power Electronic Converter, Application and Deign, John Wiley & Son, New York, [5] Racuciu, C., Pearica, M., Jula, N., Ultraonic Signal Generator Baed on Piezoelectric Tranducer, Int. Conference Communication, Bucharet, June 004, pp [6] Racuciu, C., Pearica, M., Jula, N., Acoutic Emitter for Generating Ultraonic Pule Serie with Peodo-aleatory Frequency, European Conference on Propagation and Sytem ECPS 05, Bret, France, March 005. [7] Pearica, M., An Application of the piezoelectric Effect for Mechanical Shock Generating, ELMA 005 XI International Conference on Electrical Machine, Drive and Power Sytem, Sofia, Bulgaria, September 005, pp ISSN: ISBN: