New Virtual Sonar and Wireless Sensor System Concepts J. A. Bucaro, B. H. Houston, and A. J. Romano Naval Research Laboratory, Washington, D.C. 20375 Recently, exciting new sensor array concepts have been proposed which, if realized, could revolutionize how we approach surface mounted acoustic sensor systems for underwater vehicles. Two such schemes are discussed here — so-called "virtual sonar" which is formulated around Helmholtz integral processing and "wireless" systems which transfer sensor information through radiated rf signals. The "virtual sonar" concept provides an interesting framework through which to combat the deleterious effects of the structure on surface mounted sensor systems including structure-borne vibration and variations in structure-backing impedance. The "wireless" concept would eliminate the necessity of a complex wiring or fiber-optic external network while minimizing vehicle penetrations. INTRODUCTION The growing possibility of being able to implement acoustic systems with high sensor counts (~ 10 4) has motivated consideration of how we might exploit such a capability when it does indeed become a reality. In particular, fiber optic sensor arrays utilizing in-fiber Bragg gratings and the revolution underway in MEMS/NEMS silicon-based sensor technologies suggest that such high sensor count systems might be “just around the corner.” Our considerations of how one might exploit this future technology for underwater vehicle sonars has centered on long-standing technical and engineering issues which have hampered these applications. These are, first and foremost, the deleterious effects FIGURE 1. Helmholtz integral reconstruction for an of structure-borne noise and hull impedance spatial incident plane wave, point force, and the two combined. and temporal variability. But they include as well limited apertures and the necessity for multiple hull over the surface of an underwater structure for which penetrations for sensor signal feed through. Two new there is high spatial density acoustic pressure and sensor array concepts — one called “virtual sonar”[1] normal velocity sensor data. The important result and the other involving wireless arrays based on here is that for the case of a structure excited by both cellular communication[2] — offer a new perspective an incident acoustic signal and interior noise sources on how to approach surface mounted acoustic sensor applied to the hull, when the acoustic field is systems in the attempt to mitigate these problems. evaluated inside the surface of the structure, only the The "virtual sonar" concept provides an interesting “virtual” incident field remains. This is illustrated in framework through which to combat unwanted Figure 1 taken from Reference[1] for an evacuated, effects of the structure on surface mounted sensor thin cylindrical shell at kaa = 5, where ka is the systems. The "wireless" concept would eliminate the acoustic wavenumber and a is the shell radius. To necessity of a complex wiring external network while illustrate the structural noise reducing properties of minimizing vehicle penetrations. In the following, we “virtual” sonar processing, Figure 2 displays the discuss these concepts and how they might be spatial Fourier transform of the pressure over the synergistically applied in the case of an underwater length of the cylindrical shell section for this case. AUV. The lowest curve is the transform of the virtual sonar evaluated along the central axis, as well as for the ARRAY CONCEPTS true 1 Pa incident field (these two curves overlap). The upper curves are the transforms of the total Virtual Sonar surface response along a line on the lateral surface of the cylinder when the interior force (normalized by the square of the shell thickness) is 104 Nt/m2 and 103 The “virtual sonar” concept[1] is based on simple Nt/m2, respectively. One can see the superior considerations of evaluating the Helmholtz integral performance of the virtual interior sonar regarding structure-borne noise across the entire wavenumber spectrum including the acoustic domain (-ka to + ka). 10000 104 Nt/m2 1000 Exterior Line Array 103 Nt/m2 100 Virtual Sonar 10 FIGURE 3. Wireless array on an AUV In the case of a small AUV, an individual cell 1 might access sensors located within approximately a -k k a a half meter radius. At the high rf frequencies 0.1 -20 -15 -10 -5 0 5 10 15 20 involved, the high absorption in water necessitates Wavenumber FIGURE 2. Spatial Fourier transforms of the pressure over the introduction of a suitable waveguide material to an exterior line and for the interior “virtual” line. The allow sufficient propagation even over these modest excitation is an incident plane wave and an interior point base-cell distances. The field strength of the lowest force. The transform for the incident field overlaps that for TEM order mode is approximately proportional to the the virtual sonar. following factors Results of the type shown above require sensor 2 - f r tan G ~ c/ 2 / p f r e (1) spatial sampling on the order of two per structural 0 wavelength. Azimuthally, the number of sensors, N, where c is the speed of light, is the dielectric would be 2nmax, i.e twice the highest flexural circumferential harmonic. Axially, N would be constant, f the frequency, r the propagation distance, the permeability, and the loss factor. Eq. (1) 2L/lf, i.e. twice the number of flexural wavelengths 2/5 indicates that what is desired is a material with a low (lf) along the shell of length L. Now nmax µ (k aa) 1/2 index of refraction and a low loss factor. In water, and lf µ (ka a) . Calculations for the flexural wave the rf signal decays by 10-5 in 1 cm. This compares dispersion and cut-off frequencies versus nmaxfor a plastic-like, 1.8m AUV shell structure predict the to 2m, 12m, 20m, and 1km for polyurethane, nylon, 3 Teflon, and Styrofoam, respectively. Thus, a thin following. At kaa = 1, about 10 measurement points layer of a material of this type would provide a would be required, while for kaa =10, about 8600. This requires indeed a large number of sensors; sufficiently low loss waveguide in which to however, it would enable a “noiseless” array. In propagate the sensor-to-base cell signals. addition, with both pressure and velocity Estimates indicate that on the order of 1.5mW of (acceleration) sensors employed, hull impedance electrical power would have to be supplied to each spatial and temporal variations would be of no sensor/radio pair assuming 10nJ/bit. This could be consequence. distributed in a number of ways including power broadcasting and energy harvesting of thermal gradients or fluid flow. Wireless Array ACKNOWLEDGEMENT One approach for accessing such a large sensor count system is a wireless array. As depicted in This work is supported in part by ONR. Figure 3, wireless sensor/radios would be distributed over the surface of the structure. As many as one REFERENCES thousand sensor/radio pairs could be tied in to a specific cell cite, and there could be tens of cells over the body. The sensor information from this relatively 1. A.J. Romano, J.A. Bucaro, B.H. Houston, and small number of cells could be fed onto a single line E. G. Williams, J.Acoust. Soc. Am. 108, 2823-2828 which then penetrates the hull. The two major (2000) technology issues here involve propagation of the 2. A. Mehrotra, Cellular Radio: Analog and Digital radiating gigahertz rf signals from sensors to base Systems, Artech House, Boston, MA, 1994 cell and powering of the individual sensor/radio devices. Development of Thin, Low Frequency Electroacoustic Projectors for Underwater Applications T. R. Howartha and J. F. Tresslerb aNAVSEA Division Newport, Newport, RI USA bNaval Research Laboratory, Washington, DC USA Two acoustic transducer panels have been designed, fabricated and electroacoustically evaluated. These panels featured ‘cymbal’ drivers sandwiched between a radiating cover plate and a tungsten backing plate. The acoustic output shows resonance frequencies of both transducer panels below 1 kHz. INTRODUCTION Materials Research Laboratory at Penn State in the mid 1990’s and have been investigated for use in a The two most common acoustic projector technologies number of applications [3]. A cymbal consists of an used on unmanned underwater vehicle (UUV) electroactive ceramic disk sandwiched between and platforms are tonpilz transducers and piezocomposites. mechanically bonded to two thin metal caps. Each cap Both of these technologies, however, are typically is shaped in a die press so that it will contain a shallow designed for use at frequencies above 10 kHz. A U.S. air cavity underneath its inner surface after it is bonded Navy designed ‘1-3’ piezocomposite 2.54 cm in height to the face of the ceramic disk. The caps serve as with a projector radiating face of 15.24 cm by 7.62 cm mechanical transformers for converting the small was demonstrated to exhibit broadband characteristics radial displacement and vibration velocity of the between 10 kHz and 100 kHz [1]. At its resonance electroactive disk into a much larger axial direction frequency of 100 kHz, the TVR was measured to be displacement and vibration velocity normal to the apex 174 dB// Pa/V @ 1 m. At 1 kHz, the TVR was less of the caps. Hence, the cymbal driver primarily than 110 dB// Pa/V @ 1 m. utilizes the ‘31’ contribution of the active ceramic to To generate high acoustic output at frequencies below achieve flexure in the caps. 10 kHz, free-flooded piezoelectric ceramic rings, Cymbals with two different diameters (12.7-mm and electromagnetic drivers, and flextensional transducers 15.9-mm, designated as Type 1 and Type 2, have traditionally been used. However, due to their respectively) were used in this study. Titanium was selected as the cap material because of its low density large size and weight, these technologies are not easily 3 adaptable or convenient for use in UUV platforms.