Submarine Sonar Wet End Development in ASELSAN Muhammed N. enlik, - PDF document

UDT 2020 UDT Extended Abstract Template Presentation/Panel Submarine Sonar Wet End Development in ASELSAN Muhammed N. Ş enlik, Erdem Ça ğ atay , Ömer Özdemir, Osman Yalçınkaya, Kerim Çepni, Aykut Şahin ASELSAN, Ankara, Turkey Abstract — PREVEZE Class Submarines are integral part of Turkish Navy Submarine Fleet. They are about to enter half life-cycle full modernization phase. In the context of this phase, the whole wet end made of the analog output sensors shall be replaced with the modern digital output sensors. ASELSAN is the prime contractor for the modernization of wet end. Six different sensors, most of which operate at the low frequency regime, shall be designed, produced and verified. In this paper, the concentration shall be on low frequency transducers, specifically on their characterization. The low frequency measurements typically require deep water reservoirs due to increased wavelength, or equivalently to prevent reflections from surface and bottom of the reservoir. There are several approaches to overwhelm this problem. Bobber has discussed many of the methods in his classical book ―Underwater Electroacoustic Measurements ‖, especially concentrating on the near -field methods. Here, we will walk through the most classical approach, ―pulse - echo measurement‖, and use the sin gle cycle measurement technique with the aid of equivalent circuits . We will conclude with the presentation of the measurements taken at ODTU Yalıncak Facility, which is in the close proximity of ASELSAN, Ankara. The facility has a reservoir of 9 m depth. 1 Introduction of characterization of a directional hydrophone, it may be required to use travelling wave methods based on pulse- Submarines are integral part of the navies due to their echo measurement. capability of invisibility. The price comes with the need of sole trust to the sonars, a combination of both wet end In this work, measurement of the free-field sensitivity and algorithms. The use of active sonar is not preferred (RVS) of a directional hydrophone is presented and since it removes the invisibility cloak on the submarine. compared with the results obtained from equivalent Passive sonars with the lowest achievable frequency are circuit. First, the measurement setup is described and the preferred. The upper end of the frequency band is reference measurements are presented. The measurement determined by the need and/or sonar dome; whereas the data is verified using equivalent circuit approach. Then, low frequency end is determined by the finite input the measurement method for a directional hydrophone is impedance of the receiver circuitry and the self-noise of given. Both the experimental and simulation results are the submarine. The typical lowest frequency is in the presented. It is shown that they are in good agreement order of few 10’s of Hz for a flank array [1]-[3], whereas and can be used for calibration and/or verification of a few 100’s of Hz for a cylindrical array [1]-[3]. In this output of mass production. work, an element so called ―stave‖ of a cylindrical array (cylindrical hydrophone array), which is an integral part 2 Reference Measurement of PREVEZE Class Submarines of Turkish Navy Submarine Fleet, shall be characterized in the low For reference measurements, a pulse-echo measurement frequency regime, namely below 1 kHz. setup shown in Fig. 1 is being constructed at ODTU Yalıncak Facility, in the close proximity of ASELSAN, A general characterization method, pulse-echo Ankara. At the measurement point, the reservoir has a measurement, is given in [4]. It is critical that a sufficient depth of 9 m. The reference projector and hydrophone is depth in the measurement area is present, in order to placed around 4.5 m depth to maximize the maximum successfully discriminate the input and reflected pulses. allowable pulse width which is close to 5 ms, rather than Also, in order to optimize the use of depth, all equipment advised one which is 2/3 of actual depth [5]. The should have high bandwidth, to prevent transients and reference projector is Neptune D11 [6] and the reference increase the possibility of single cycle measurements. hydrophone is Neptune D140H [6]. The measurement When these criteria are not met, the use of near field location suffers from electromagnetic interference. A measurement techniques can be used. These techniques Butterworth filter with a low frequency cut-off at 100 Hz are investigated by Bobber in his classical book is used to clean the output of the hydrophone from the ―Underwater Electroacoustic Measurement s ‖ [5]. Both of environmental noise. these methods, DRL and Trott, require tight alignment of the projector and hydrophone sometimes in the order of mm’s . There are also various methods for characterization of the equipment at low frequency regime. However, these methods are mostly useful for the omnidirectional hydrophones and projectors. In the case

UDT 2020 Presentation/Panel UDT Extended Abstract Template 3 Actual Measurement The actual hydrophone to be characterized is a directional hydrophone, which is approximately 65 cm in length. A baffle composed of a hard and soft impedance material is implemented inside the hydrophone [8] [9]. This gives the directional characteristics of hydrophone, which loses its characteristics below 1 kHz. The preliminary measurements indicate that the hydrophone has a very narrow bandwidth, which is predicted by [8] [9]. An equivalent circuit based on Fig. 1. Test setup Error! Reference source not found. including the transfer function of the hydrophone is implemented. To verify the measurements, an equivalent circuit shown in Fig. 2 is constructed using Pathwave Advanced Design System (ADS) of Keysight Technologies [7]. ADS is preferred due to its ability to use frequency dependent model in the transient solutions. Fig . 4 shows the measurement data and equivalent circuit output at 500 Hz. The results show that both measurement data and equivalent circuit data are in good agreement. The initial rise at the first cycle of the output is due finite response of the filter used in the measurements. The variations at the higher cycles are due Fig. 2. Equivalent circuit of reference measurement setup. to the narrow bandwidth of the hydrophone. The equivalent circuits parameters are calculated using [8], assuming that the reference transducers have a spherical shape. The material is assumed to PZT-4 for D11, whereas PZT-5H for D140H. The main motivation in this work is the characterization of a directional hydrophone below 1 kHz. Fig. 3 shows a comparison of measurement data and output obtained from equivalent circuit. As can be seen, the results are in good agreement. The transient response at 11 kHz due to the finite bandwidth of the projector is well resolved with the equivalent circuit. Fig. 4. Measurement data and equivalent circuit output. Measurement and simulation are performed at 500 Hz. Fig. 5 also shows the measured RVS data below 2 kHz. The dip around 1 kHz corresponds to the resonance of the soft and hard impedance materials. Fig. 3. Measurement data and equivalent circuit output. Measurement and simulation are performed at 500 Hz.