An experimental study on transducer for the DDS technology demonstrator

AN EXPERIMENTAL STUDY ON TRANSDUCER FOR THE DDS TECHNOLOGY DEMONSTRATOR ANDRZEJ ELMINOWICZ, WALDEMAR LIS*, WŁADYSŁAW MĘCIŃSKI R&D Marine Technology Centre Dickmana 62, 81-109 Gdynia, Poland, *University of Technology *Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland, , * R&D MTC has began designing and building the next generation wideband Diver Detection Sonar (DDS, with cylindrical multielement piezocomposite transducer. The paper will focus on key features of DDS technology demonstrator, with special attention on technologies of the transducer. The new transducer is designed at the cooperation with the producer of the transducer – Materials Systems Inc. (Littleton, USA), remainder hardware and software solutions are designed in R&D MTC. In this article specific theoretical and technological principles of the wideband cylindrical piezocomposite transducer will be presented. The implementation of the transducer to the wideband Diver Detection Sonar (DDS) will be described. The results of transducer measurements, transmitter modules, matching circuits modeling and examples of application will be also presented. INTRODUCTION Piezocomposite 1–3 technology using shading technique shapes the transducer response (beampattern) without resort to complex electronics – it is possible due to ceramic rod is electrically isolated from its neighbour as well as due to ceramics rods are embedded in polymer matrix which significantly reduces coupling between the different modes in the ceramic that effectively provides a uni–modal response without unwanted resonances. Composites materials are inherently wideband because of damping provided by the polymer matrix and furthermore the introduction of specially designed matching layer between the outer electrode and the sea water. The composite structures reduces the acoustic impedance closing it to impedance of the sea water – it means the higher power is transferred, so efficiency increases. The electro-acoustic performance of these piezoelectric materials embedded in polymer matrix, sophisticated data processing and software solutions have enabled improvement in operational parameters of sonars, adaptation of sonar to specified protected object, propagation and environment condition. The designed technology demonstrator of DDS is a first attempt of our firm leading to next DDS generation intended to operate in shallow water. The new array has been created at the cooperation with Materials Systems Inc. (Littleton, USA), remainder hardware and software solutions are being formed by R&D MTC. Technology demonstrator (TD) of DDS has been designed as two modules: underwater and above water [4]. Both modules connect by cable: power + data (ETHERNET). Underwater parts will be in cylinder form with possibility to be placed on the sea bottom or suspended in column water. In the further parts of the paper, we described in details only one of the most important sub–assembly: transducer. The following assumptions for TD’s transducer have been established: – tranducer will be made as curved piezocomposite (piezoelectric ceramic rods in a polymer matrix) array – cylindrical array with 180o aperture and 64 elements, – tranducer will operate in shallow water (survival depth – 100 m), – frequency range: 60 kHz ÷ 80 kHz, – source level: ≥ 205 dB, – vertical side lobe level: -21 dB*, – diameter: ≤ 350 mm, – transducer height: 120 mm, – elements spacing of transducer: < ½ λ, * – vertical side lobe level without shaping is 13.4 dB. 1. DESIGN OF TRANSDUCER Technology demonstrator (TD) of DDS utilizes piezoelectric materials in the transducer to generate and receive the acoustic signals. The transducer determines the performance limits of the sonar system. The use of piezocomposite improves transducer’s performance. Advantages of piezocomposite transducers for sonars: – increased sensitivity, – better resolution by broader bandwidth, – improved image contrast by reduced side lobes, – better efficiency (increased signal to noise ratio), by improved impedance matched to water, – low interelement cross talk, – greater element to element phase and amplitude uniformity, – low cost construction. Piezocomposite design goals: – maximization of capacitance, – electrical impedance match to system, – acoustic impedance match to water, – maximization of electromechanical coupling, – electrical loss tangent minimization, – mechanical loss (1/Qm) minimization. Any composite design is a compromise among these parameters. For environment condition of Baltic Sea: temperature 10o C and salinity = 7 o/oo sound speed is approx. 1450 m/s and element spacing of the transducer for 70 kHz is 10.36 mm and for 80 kHz is 9.06 mm. Calculated, at assumed transducer dimensions, spacing is: d = π * 175/64 ≈ 8.6 mm We assumed that developed transducer will have 64 elements with width = 7.8 mm and gap = 0.8 mm; assumed spacing (see Fig. 1) is less then theoretical spacing for 80 kHz by 5.1 %, which protects against spatial aliasing at lowest frequencies. Average 3 dB beam width is: Θh ≈ 50.7 *λ/L ≈ 9o where: λ is wave length and L = 120 mm (see Fig. 1). Beams spacing Θs ≈ 3o. Fig.1. Elements shape and spacing Transducer element is a piezocomposite block with ceramic rods embedded in polymer matrix; it should be noted, that for ceramic rods with height compared to their lateral dimensions prevail length mode of vibration resulting in improved electroacoustic efficiency, then sensor sensitivity and signal/noise ratio increased. Other benefits, related to applying piezocomposite transducer to sonars, are: better resolution due to broader bandwidth, reduced side lobes and moreover low cost construction [2]. 2. FACTORY ACCEPTANCE TESTS Materials Systems Inc. invited CTM representative to visit MSI facility and laboratory to carry out the factory acceptance test of the sonar transducer. Tests conducted for following parameters: – TVR (all elements), – RVS (all elements), – Impedance in water (all elements), – Horizontal directivity with all other elements electrically shorted (all elements). Tests were conducted in MSI tank for the transducer connected with all elements open circuited except for the element under test. It was determined that capacitive cross coupling between element’s wire pairs within 5 meters cable adversely affected element’s TVR, RVS and directivity. To eliminate the cross coupling and better simulate performance under actual operating condition (preamps and power amps located within 40 cm of the array), every element was retested for RVS and TVR with all other elements short circuited. This eliminated the performance irregularities. Test results: – One element’s TVR (ref. 1µP/V for 1 m), predicted 125 dB and measured 144 dB, – One element’s RVS (ref. 1V/1µP), predicted – 188 dB and measured –176 dB, – Transmit bandwidth (-3 dB), predicted 60 kHz to 80 kHz and measured 61 kHz to 82 kHz, – Receive bandwidth (-3 dB), predicted 60 kHz to 80 kHz and measured 65 kHz to 81 k (...truncated)