50kHz split beam 음향변환기의 설계 및 성능특성

Abstract

Abstract

In a split beam echo sounder, the transducer design needs to have minimal side lobes because the angular position and level of the side lobes establishes the usable signal level and phase angle limits for determining target strength. In order to suppress effectively the generation of unwanted side lobes in the directivity pattern of split beam transducer, the spacing and size of the transducer elements need to be controlled less than half of a wavelength. With this purpose, a 50kHz tonpilz type transducer with a half-wavelength diameter in relation to the development of a split beam transducer was designed using the equivalent circuit model, and the underwater performance characteristics were measured and analyzed. From the in-air and in-water impedance responses, the measured value of the electro-acoustic conversion efficiency for the designed transducer was 51.6%. A maximum transmitting voltage response(TVR) value of 152.38dB re 1μPa/V at 1m was achieved at 52.92kHz with a specially designed matching network and the quality factor was 10.3 with the transmitting bandwidth of 5.14kHz. A maximum receiving sensitivity(SRT) of -183.57dB re 1V/μPa was measured at 51.45kHz and the receiving bandwidth at -3dB was 1.71kHz. These results suggest that the designed tonpilz type transducer can be effectively used in the development of a split beam transducer for a 50kHz fish sizing echo sounder. An improved split-beam transducer for a 50kHz fish-sizing echo sounder was developed. The main attempts in this design were to minimize the side lobe level in the beam pattern and the distance between acoustic centers for adjacent transducer quadrants in the geometrical arrangement of array elements while maintaining a given number of transducer elements and beam width required for fish sizing in the fishing field. To achieve these goals, a 32 elements planar array transducer(6×6 array with one elements in each corner missing) was designed by using Dolph-Chebyschev shading function to suppress side lobes in the array beam pattern and fabricated by arranging with the inter-element spacing being substantially equal to half the wavelength using the transducer element of 0.4 times the wavelength in diameter. The performance characteristics of this split-beam transducer were evaluated in the experimental water tank of L5 x H5 x W6m. In this study, the design goal of the beam width and side lobe level for transmitting beam pattern was initially set at 21° and -30dB, respectively. However, the measured beam width at 3dB was 21° all in both directions with the side lobe levels of -24.7dB in the horizontal plane and -25.6dB in the vertical plane. The averaged beam width at -3dB of the receiving beam patterns for four receiving quadrants was 31.4°. The transmitting voltage response was 161.5dB(re 1μPa/V at 1m) at 50.23kHz with a band width of 2.16kHz and the averaged receiving sensitivity for four receiving quadrants was -178.13dB(re 1V/μPa) at 49.8kHz with a bandwidth of 2.64kHz. Furthermore, the split beam transducer for fish sizing needs to maintain a wide bearing angle region for fish tracking without interference from side lobes and lower sensitivity to fish echoes outside of the main lobe region to measure correctly the angular location of free-swimming fishes in the sound beam. To evaluate these performance, the angular location of a target was derived from the electrical phase difference between the resultant signals for the pair of transducer quadrants in the horizontal and vertical planes consisting of 32 transducer elements. In particular, the side lobe level of -25.2dB and the inter-center spacing of 0.96λ(λ : wavelength) obtained in this study suggest that the angular location of fish target distributing within a range of approximately ±28° without interference from side lobes can be measured.

- 요약 수산자원의 효율적인 이용 및 관리를 위해서는 무엇보다도 생물학적 성숙연령에 미달하는 어린 개체들의 보호가 필수적이다. 이를 위해 본 연구에서는 과거 조업 경험을 바탕으로 많은 정보가 축적되어 온 50kHz용 체장어군탐지시스템을 개발하기 위한 연구의 일환으로 50kHz split beam 음향변환기를 설계, 제작하고, 이것에 대한 지향성 보정, 위치각의 추정 및 검증 실험 등을 수행하여 어로 현장에서 실용적으로 사용할 수 있는 50kHz용 split beam 음향변환기를 구현하였다. 본 연구의 성과를 요약하면 다음과 같다.

첫째, 우리나라 주변해역의 어로현장에서 실용적으로 사용할 수 있는 50kHz split beam 음향변환기를 시험적으로 설계, 개발하였다. 체장어군탐지기용 음향변환기를 개발하는데 있어 가장 기본적인 조건의 하나는 음향변환기의 side lobe level의 적극적인 억제와 음향중심 간격의 최소화인데, 본 연구에서는 이러한 문제점을 해결하기 위해 4분할 빔 음향변환기를 설계할 때, 6×6 배열에서 네모퉁이의 진동소자를 각각 제거하여 총 32개로 구성된 원형의 array 패턴을 도출하고, 각 진동소자의 직경을 반파장 이하로 최소화시켜 가중회로를 통해 이들 각 진동자 그룹을 분할함으로써 side lobe level을 -25dB이하로 실현하였다.

둘째, 우리나라 연근해 어장의 수심범위를 고려하여 Dolp-Chebyshev shading 기법을 이용하여 빔 폭과 side lobe level을 최적화하여 어류의 위치추적 dynamic range를 증대시키고, 4분할된 수신진동자 그룹의 음향중심(위상 중심) 간격을 한 파장으로 최소화함으로써 main lobe의 약 56°의 지향성 범위내에서 유영하는 모든 어류의 탐지가 가능한 50kHz split beam 음향변환기를 구현하고 성능특성을 분석, 평가하였다.

셋째, 최종적으로 본 연구에서 개발한 50kHz split beam 음향변환기에 대한 신뢰도를 평가하기 위해 수신 지향성 빔에 대한 음향 중심 간격, 어류의 위치각 추정, 또한, 위치각 범위와 side lobe level의 관계 등을 종합적으로 평가, 분석하고, ES70 split beam echo sounder(Simrad, Norway) 등에 의한 위치각의 측정결과와 비교, 분석하여 신뢰성 검증을 수행하였다.

본 연구의 성과는 향후 우리나라 연근해 해역에서 TAC 제도의 체계적인 시행과 관련하여 도입되어야 할 체장어군탐지기의 국산화에 필요불가결한 각종의 핵심적인 요소 및 기반 기술을 제공할 수 있을 것으로 사료된다.