Controllable Sonar Lenses and Prisms Based on ERFs
- Wednesday, 03 January 2007
Compact devices without moving parts would focus and steer acoustic beams.
Sonar-beam-steering devices of the proposed type would contain no moving parts and would be considerably smaller and less power-hungry, relative to conventional multiple-beam sonar arrays. The proposed devices are under consideration for installation on future small autonomous underwater vehicles because the sizes and power demands of conventional multiple-beam arrays are excessive, and motors used in single-beam mechanically scanned systems are also not reliable.
The proposed devices would include a variety of electrically controllable acoustic prisms, lenses, and prism/lens combinations – both simple and compound. These devices would contain electrorheological fluids (ERFs) between electrodes. An ERF typically consists of dielectric particles floating in a dielectric fluid. When an electric field is applied to the fluid, the particles become grouped into fibrils aligned in rows, with a consequent increase in the viscosity of the fluid and a corresponding increase in the speed of sound in the fluid. The change in the speed of sound increases with an increase in the applied electric field. By thus varying the speed of sound, one varies the acoustic index of refraction, analogously to varying the index of refraction of an optical lens or prism. In the proposed acoustic devices, this effect would be exploited to control the angles of refraction of acoustic beams, thereby steering the beams and, in the case of lenses, controlling focal lengths.
Figure 1 schematically illustrates a sonar assembly according to the proposal. A planar array of acoustic transmitting/receiving transducers would both send out acoustic signals to irradiate targets and, in the acoustic analog of a retina, sense the spatial pattern of return acoustic signals. The transmitted and return signals would be collimated and focused, respectively, by use of two acoustic lenses. The front acoustic lens would be designed to contain an ERF in multiple compartments separated by electrodes, rather than one compartment between a single pair of outer electrodes, in order to reduce the magnitudes of the potentials needed to be applied to the electrodes to vary the focal length of the lens through the required range. A prism in front of the front lens would also be constructed in layers, for the same reason. The index of refraction of the prism and, hence, the angle of refraction of the beam, would be varied by modulating the potentials applied to the prism electrodes in order to steer the outgoing and incoming acoustic beams.
The concept of ERF-filled compartments separated by electrodes could be generalized and modified to that of a multicellular device comprising a rectangular array of ERF-filled cells (see Figure 2). Electrodes would be affixed to both the row and the column walls between cells, so that an electric field of controlled magnitude and direction could be applied to the ERF within each cell. Lens shape (convex, concave, etc.) can be varied by selectively activating individual cells.
Figure 1. Electric Fields Would Be Applied to electrorheological fluids between electrodes to vary the indices of refraction of the acoustic prism and lens, thereby varying the beam direction and focal length, respectively.
Figure 2. Cells in a Rectangular Array would be filled with an electrorheological fluid. Electrodes on the walls between the cells would make it possible to apply electric fields to individual cells along the row and column directions.
This work was done by Yoseph Bar-Cohen, Stewart Sherrit, Zensheu Chang, and Xiaoqi Bao of NASA's Jet Propulsion Laboratory; Iris Paustian and Joseph Lopes of NSWC Coastal Systems Station; and Donald Folds of Ultra-Acoustics, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
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