The Navy had a long established policy that transducers should be designed so that they can be repaired easily at the Navy yard transducer maintenance facilities. In order to accomplish this goal, the general practice requires the housing assembly to be held together by bolts, and that removable gaskets should be available to seal the separable sections so that water does not enter into the transducer.
For the earlier types of sonar transducers, this kind of construction was a practical requirement. The entire assembly in these sonar transducers was confined to a single housing. However, this policy became meaningless with transducers increasingly becoming larger and requiring arrays of hundreds of individual transducer elements
Expendable Modular Design
Principally, if the transducer module is designed to allow disassembly, the seals and the related hardware needed to accommodate the separable parts become a major source of mechanical failure. This reduces the element’s reliability.
The extra parts and additional machining required increases the structure’s manufacturing cost. The need to provide spare parts for all of the individual structural components, which must be expensively and specially packed to prevent deterioration in storage, adds further to the manufacturing costs.
Finally, the maintenance depots require highly skilled labor to disassemble failed transducer elements and reconstruct them with spare parts.
Together, these factors considerably add to the actual cost of the transducer and at the same time defeats the fundamental design objectives described in MIL-E-16400F (Navy), which states: “The basic design objectives are that the equipment will meet the needs of the Naval service and that the final product will reflect the utmost in simplicity, have maximum reliability and minimum weight consistent with the state of the art, and be easy to install and maintain."
In order to address the objections related to the repairable modular construction and simultaneously meet the Navy reliability objectives, Massa developed a replacement transducer for the SQS-23 sonar as a low-cost, hermetically sealed assembly.
If a module fails, it is simply replaced by an identical spare. Such a design can eliminate the additional costs associated with a repairable structure. When the spare mechanical seals and parts are eliminated from the expendable design, a key source of corrosion and mechanical failure can be easily eliminated. In the sealed modular design, the assembly can be simplified and the number of parts can be reduced.
The expendable modular design enables the use of a non-corrosive, sealed, low-cost protective covering, which removes the requirement of expensive stainless steel fittings and housings that are often required for traditional designs.
Only replacement modules are required for spare parts and field repairs are fully eliminated with the expendable module. As the module is a waterproof operating unit, there is no need for expensive packing. Since precision production tools were used to build the spare module, it will have similar operating characteristics to the module that it replaces.
Ultra Long-Range Sonar Needs
Over the past two or three decades, significant effort has been made towards the development of high-power transducers for applications in the low audible frequency region to satisfy the needs of future ultra-long-range sonar systems, which are yet to be fully defined. A few examples are discussed in the following sections.
Magnetostrictive Ring Transducer
When an AC current is superimposed on a DC biasing current supplied to the winding, a large-diameter ring containing a cemented multiple layer scroll of nickel strip will begin to vibrate in the radial resonance mode. This nickel strip is wound with a toroidal coil of insulated wire.
However, providing a uniform film of high strength cement across the whole surface of the continuous nickel strip is very difficult while the nickel strip is being wound. Therefore, the nickel scroll will delaminate if there is a cement failure during high-power operation.
Ceramic Ring Transducer
High-power low-frequency sound generators were also realized by fabricating large-diameter ceramic rings from wedge-shaped ceramic sections, which were joined together to form the ring.
However, when ceramic rings are operated at high-power levels, they carry a high risk of structural failure. Also, the ring design is such that it cannot be employed in planar arrays to form high-power directional beams.
Megawatt Low-Frequency Electromagnetic Transducer Array
Among all transducers, electro-magnetic transducers that work by electromagnetic forces have shown to be most effective. The forces are produced in an air gap that exists between the magnetic laminations joined to the massive inertial non-radiating structure and the magnetic laminations attached to the vibratile structure.
Massa manufactured the world’s largest transducer array - a 300,000 pound array, 1500 square feet in area - for the Office of Naval Research as a replacement for a magnetostriction scroll array, which failed while working at high-power levels.
During its use in a five-year research program, the electromagnetic structure operated effectively to make very-long-range sonar propagation studies to determine the potential of designing a distant early warning submarine detection sonar. Operating at a quantified 60% efficiency, the huge array could be powered with up to 1 MW of audio power in the 400 to 500 Hz frequency band.
Figure 1. Lightweight (10 pound) transducer module delivers 3 kilowatts in the 3 kHz frequency region.
High-power Electromagnetic Transducer for Very Great Depths
A low-frequency, spherical transducer element assembly measuring approximately 2 ft in diameter is electro-magnetically driven, push- pull, from opposite sides of a circular disk placed on the sphere’s equatorial plane. The spherical shell oscillates as a dipole, and when it is rho-c loaded it provides 2 to 4 KW of sound with a Q between 3 and 4 based on the thickness of the spherical housing and the frequency range of operation.
The spherical housing in turn relies on whether the operation depth is 20,000 or 2,000 ft. An underwater horn was developed to increase the radiation resistance on one face of the spherical dipole by more than an order of magnitude over the loading on the sphere’s rear face. This removes the need for a pressure release baffle.
Electromagnetic Transducer for High-power Low-frequency Arrays.
An inertial mass-loaded electromagnetically driven vibrating piston, which was developed recently, is a stiff 17-inch-diameter plate spring placed on the inertial mass part of the vibrating structure.
E-laminations that comprise part of the magnetic circuit and heavy copper coils are bonded to the large steel plate and these contribute to the overall inertial mass. The lighter I-laminations that complete the magnetic circuit, are fixed to the internal surface of the vibratile plate.
The vibrating plate is separated from the inertial mass via a circumferential row of springs. The resonance frequency of the transducer is determined by the spring stiffness.
The transducer design acquires a low Q of less than unity for a rho-c loaded array. All transducer elements are able to provide 2 to 3 KW of sound at approximately 50% efficiency across an octave bandwidth within the low-frequency region of less than 1 kHz.
Lightweight Sonar Systems
In order to achieve a greater range, there has been a continued reduction of sonar frequencies, and this has led to massive sonar transducer arrays that weigh several tons. A standard scanning sonar transducer that operates between the 3 - 4kHz region weighs roughly 20,000 lb, but a new modular high-power transducer weighs just 10 lb and creates 3 kW of sound in the 3 kHz region when employed in an array for a radically new sonar system.
At opposite ends of a lead zirconate titanate ceramic stack, two conically tapered pistons are driven. While these pistons are moving in phase, a stationary node is established at the middle of the stack, equivalent to an inertial loading for both pistons of infinate mass, but zero weight.
When a number of modules are aligned along the vertical axis of a tubular housing, the tapered pistons creates a radial underwater horn at the intersection of each pair of modules. The circular horn structure causes increased resistance loading that leads to a Q of 2, which achieves an octave broadband response of 1⁄2. A line array measuring 9” in diameter by 50” in length weighs 125 lb and creates 20 kw of sound in an omnidirectional horizontal beam with 25° vertical beam angle.
The new lightweight transducer has been included in an engineering model of a portable long-range sonar system and is placed axially inside a 9-inch-diameter housing. The unit also includes a power supply as well as a high precision nine-element line hydrophone receiving array, which accomplishes a target bearing accuracy of 2°.
The entire underwater part of the Model M-1002 long-range 3 kHz portable sonar system weighs 250 lb. The latest portable sonar system has a bearing accuracy of 2° and a range of 20,000 m. Initial tests were performed in Massachusetts Bay which detected targets at ranges of 5 to 10 miles.
This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.
For more information on this source, please visit Massa Products Corp.