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Techniques for Tailoring Sonar Transducer Responses

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Techniques for Tailoring Sonar Transducer Responses TECHNIQUES FOR TAILORING SONAR TRANSDUCER RESPONSES by Geoffrey A. Steel B.Sc Submitted for the Degree of Doctor of Philosophy Department of Electronic and Electrical Engineering, University of Birmingham. October 1986. *" University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. SYNOPSIS This work is concerned with the design of sonar transducers operating in the frequency range 100 kHz to 1 MHz. The transducer frequency responses are predicted using a one-dimensional transmission line analysis. Differences between predicted and measured results are shown to be caused by intermodal coupling between planar and thickness modes of vibration. Conventional transducer designs achieve wide bandwidth using quarter-wave matching layers. In this work the piezoelectric-tunable transducer is investigated as a possible alternative. This structure consists of a pair of ceramics, one of which is driven by a voltage source and the other has a passive electrical load. It is shown that the resonant frequency is variable over more than one octave but the instantaneous bandwidth is only around 10% of the centre frequency. The same transducer can be controlled actively by applying voltages to both ceramics. In this case transducer characteristics are determined by the relative amplitude and phase of the two voltage sources, which can be chosen to give the same results as with passive control. Data is often required for the velocity and attenuation of sound in the materials being used. For this purpose several measuring techniques are described, all of which use a solid buffer rod in place of the more common water tank measurements. Acknowled gement s This research was funded by an industrial studentship arranged between S.E.R.C. and Marconi Underwater Systems Limited. I am employed by M.U.S.L at Croxley Green, where the transducer design team is run by Mr C.Pearcy. His valuable advice and encouragement have maintained my interest in acoustics both before and during ray postgraduate studies. I am grateful to my supervisors, Dr B.V.Smith and Dr B.K.Gazey, for their guidance, without which the work would not be possible. I also wish to thank Mr J.Nibblett for valuable help with mechanical problems and Mr J.Dunn whose practical experience is always at hand. CONTENTS Chapter Page 1. Introduction. 1 2. A Review of Published Information. 2.1 Equivalent circuits 5 2.2 Mechanical transmisson line representation 10 2.3 Common designs for high frequency sonar transducers 11 3. Computer Simulation of Transducer Performance and Comparison with Practical Results. 3.1 Transmission line analysis 15 3.2 Description of the computer program 17 3.3 Comparison of predicted and measured results 18 3.3.1 Results for TB1 21 3.3.2 Results for TB2 22 3.3.3 Standing waves 24 3.3.4 Radiation balance measurements 27 3.4 Discussion of program limitations 30 4. Measurement of Material Properties. 4.1 Relevant data for transducer design 31 4.2 Useful measurement techniques 32 4.3 Principle of buffer rod measurements 34 4.4 Effect of coupling layers 39 4.5 Accurate velocity and absorption measurement 41 4.6 Some experimental results 44 4.7 Interference techniques 49 4.8 Improvements to measuring techniques 52 5. The Piezoelectric-Tunable Transducer. 5.1 Harmonics of the thickness resonance in ceramic discs 53 5.2 Control of resonances using passive electrical components 55 5.3 Effects of parameter variation 60 5.3.1 Electrical impedance and backing impedance variation 60 5.3.2 Position of the active ceramic 65 5.3.3 Ceramic thickness ratio 66 5.3.4 Effect of bond thickness 68 5.4 Design and construction of TC4 71 5.5 Performance of TC4 with passive control 75 5.5.1 Conductance measurements 75 5.5.2 Output voltage 78 5.5.3 Power output and efficiency 81 5.6 Stress analysis for TC4 82 5.7 Improvements to transducer design 86 5.8 Construction of TC5 90 5.9 Performance of TC5 with passive control 92 5.9.1 Conductance measurements 92 5.9.2 The effects of series resistance 95 5.9.3 Output voltage 97 5.9.4 Pressure and efficiency measurements 101 5.10 Stress analysis for TC5 103 5.11 Discussion of passive control 106 6. Active Control. 6.1 Low frequency transducer tests 108 6.2 Active backing impedance at high frequency 110 6.3 Calculation of active loads by computer 114 6.4 Basic design requirements 115 6.5 Effects of parameter variation 116 6.5.1 Ceramic thickness ratio 116 6.5.2 Backing impedance and coupling layer thickness 117 6.6 Synthesis of real backing impedances for TC4 120 6.7 Synthesis of reactive backing impedances 125 6.8 Active control of resonant frequency for TC5 127 6.9 Performance of TC5 with active control 128 6.9.1 Drive ceramic admittance 132 6.9.2 Control ceramic admittance 138 6.9.3 Pressure and efficiency measurements 140 6.10 Stress analysis with active control 146 6.11 Discussion of active control 146 7. Conclusion. 148 8. Suggestions for future work. 152 References. 154 Appendices: 1. Calculation of terminating impedance 162 2. Calculation of bond thickness 164 3. Effect of grease bond on reflected signals 166 4. Calculation of absorption without interference 169 5. Calculation of absorption with interference 170 6. The radiation balance 172 7. Detailed conductance measurements for TC5 175 8. Source simplification 181 9. Derivation of active control parameters 185 10. Active control electronics 188 11. Stress distribution in piezoelectric material 191 12. "A technique for measuring acoustic properties of materials using a buffer rod" I.O.A. Proceedings, Underwater acoustic calibration and measurement, 1984. 13. "Tunable sonar transducer" Electronics letters, Vol.22, July 1986. LIST OF SYMBOLS Symbols Common to all Chapters ^ = density H = efficiency c = speed of sound # = propagation constant f = frequency a = absorption k = wavenumber (=°/c) hsi= piezoelectric constant 1 = thickness £ = permittivity \ = wavelength <*> = angular frequency Z F = impedance on front of ceramic (=Rp+jXp) Z 6 = " " back " " (=R B+JX B ) Z T = terminating impedance (=RT+jXr ) Z c = ceramic impedance f 0 = resonant frequency g0 = conductance at resonance Q = magnification factor (=f0 /bandwidth) Symbols for Chapter 2 %c = density of ceramic c c = speed of sound in ceramic Z n = matching layer impedance Symbols for Chapter 3 Z 0 = characteristic impedance of transmission line x = length of transmission line Symbols for Chapter 4 Impedance Thickness Wavenumber Velocity Absorption Perspex Z p l p kp C P a P Sample Z s 1$ k s c s a s Bond Z b l b k b |R| = modulus of reflection coefficient <f> = phase & = phase shift on transmission from rod to sample ^ = electrical phase shift Pr = received pressure Pt = transmitted pressure .A. = maximum/minimum ratio in interference pattern Zb +ZpZg F = impedance ratio: z b(z p+zs ) Symbols for Chapter 5 Z fL = impedance of passive electrical load ( =RPl+jXPL) 10 = drive ceramic thickness 1, = control ceramic thickness Symbols for Chapter 6 Drive ceramic Control ceramic Coupling layer Voltage E, Capacitance Co C, Impedance Z c Z c ZfV\ Thickness 1, In Wavenumber k m Zs = impedance to be synthesised Vm = coupling layer propagation constant (=< 0 = phase of control voltage relative to drive voltage F0 = Thevenin equivalent source for drive ceramic Z 0 = " " impedance " F, = " " source for control ceramic Z, = " " impedance " II II v+j<r = impedance ratio (=F|/F0 ) Z po ,Z Qo ,Z P, ,ZQ( : mechanical impedances, see section 6.2 CHAPTER 1 Introduction Useful sonar frequencies extend from infrasonic signals of a few Hertz up to ultrasonic signals of several MHz. The choice of operating frequency for a particular sonar depends largely on range and resolution requirements. At high frequencies the wavelength is small so high resolution can be achieved, but sound absorption and thermal background noise both increase with frequency and limit the operating range. Therefore long range sonars always use low frequencies which give comparatively poor resolution. Infrasonic frequencies are often produced by high power non-reversible sources such as explosives. The rest of the frequency range can be covered by transducers with linear and reversible properties. Early transducer designs were based on piezoelectric crystals, such as quartz and Rochelle salt, which have simple geometrical shapes and are only suitable for a limited number of applications. It is now more common to use electrostrictive ceramics, such as barium titanate and lead zirconate titanate (PZT), which can be manufactured in any required shape. Transducers can also be constructed using magnetostrictive materials although these are only suitable for operation up to 200 kHz. Most sonar systems require transducers to be easily manufactured and have reliable properties which are stable over the expected operating life. An important mechanical design consideration is the ability to withstand the high hydrostatic pressure experienced in deep water operation. High power transducers must also withstand - 2 - large temperature variations and prolonged temperature cycling. Sophisticated signal processing techniques are often used to improve range resolution and signal-to-noise ratio. These techniques depend on the use of short acoustic pulses or swept frequency signals, both of which require very wide bandwidth transducers and therefore present considerable design problems.
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