Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Reexamination Certificate
2002-08-28
2004-01-13
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
C367S102000
Reexamination Certificate
active
06678210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to underwater sound technology and, in particular, concerns sonar systems with multiple acoustic beams being formed and steered by frequency division.
2. Description of the Related Art
Nearly all under-water vehicles, whether manned or unmanned, are equipped with an ahead-look sonar (ALS). Common applications include obstacle avoidance, mine detection, and rendezvous and docking capabilities. In general, these sonars use electronically beamformed arrays, although some use simpler systems in which multiple, separate directional hydrophones are used to provide multiple preformed beams. The electronically beamformed systems offer improved performance but are substantially more costly because of the complexities of the beamforming circuitry. A typical multi-beam sonar forms about 100 beams over an angular sector of about 150 degrees. The beamforming circuitry for this sonar requires about 100 receiver channel amplifiers to raise the received signal to a level sufficient for digital beamforming of the 100 beams. Because of this receiver amplifier complexity which is proportional to the beam resolution (beamwidth) and the number of beams formed, the multiple physical beam approach is typically limited to a relatively small number of low resolution beams and may be relatively heavy because of the excess of ceramic material in the multiple hydrophones.
Another method of forming multiple beams without an electronic beamformer is the use of an acoustic lens with a multi-element retina of small hydrophones. Although appealing in principle, lens arrays have proven difficult in practice due to issues such as temperature instability and toxicity of fluid materials and shear wave effects in solid lenses. Lens sonars also have problems in terms of the size and weight of the physical beamformer.
Thus, present beamforming techniques have practical cost, size and weight deficiencies. These are particularly important in applications such as small, low cost unmanned under-water vehicles (UUVs) where size and weight are at a premium and the cost of individual subsystems such as sonars preferably needs be kept low.
A frequency scanning technique has been used in radar for many years. Instead of fixed phase shifts between elements, however, the radar implementations use long delay lines between antenna elements or radiating slots in a dispersive delay line. The typical application is to provide vertical scanning of an array where azimuthal scanning is provided by either mechanical rotation or another electronic phase shifting technique.
Hence, there is a need for a sonar system that permits sequential scanning through multiple beams or forming multiple simultaneous acoustic beams. There is need for such sonar system to be implemented in a simple cost and size/weight effective manner.
SUMMARY OF THE INVENTION
In one aspect, the aforementioned needs are satisfied by a sonar system for forming a steerable underwater acoustic beams. The system comprises an array of acoustic transducers and a beamforming system that associates a signal to each of the transducers to form an acoustic beam with a direction. The signal is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer and the direction of the acoustic beam is determined by the frequency of the signals. The beamforming system is adapted to vary the frequency of the signals so as to permit steering of the acoustic beam.
In one embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. In another embodiment, the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. In another embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam.
In one embodiment, a formula cos &thgr;=(&Dgr;&phgr;/2&pgr;)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency, where &thgr; represents a direction angle relative to a plane defined by the transducers, &Dgr;&phgr; represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase shift &Dgr;&phgr; is selected to be a substantially constant value andy the direction angle &thgr; is varied by varying the frequency about a center frequency f
0
. The phase shift &Dgr;&phgr; is selected such that a signal associated with a given acoustic transducer is a simple linear combination of signals proportional to cos &ohgr;t and sin &ohgr;t, where &ohgr;=2&pgr;f and t represents time. In one implementation, the phase shift &Dgr;&phgr; between the adjacent acoustic transducers is selected to be approximately &pgr;/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos &ohgr;t, sin &ohgr;t, −cos &ohgr;t, and −sin &ohgr;t. In another implementation, the phase shift &Dgr;&phgr; between the adjacent acoustic transducers is selected to be approximately 3&pgr;/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos &ohgr;t, −1/{square root over (2)} cos &ohgr;t+1/{square root over (2)} sin &ohgr;t, −sin &ohgr;t, 1/{square root over (2)} cos &ohgr;t +1/{square root over (2)} sin &ohgr;t, −cos &ohgr;t , 1/{square root over (2)} cos &ohgr;t−1/{square root over (2)} sin &ohgr;t, sin &ohgr;t, and −1/{square root over (2)} cos &ohgr;t−1/{square root over (2)} sin &ohgr;t. In one implementation, the frequency f of the signals is varied in a range of approximately 0.75f
0
to approximately 1.25f
0
.
In another aspect, the aforementioned needs are satisfied by an underwater sonar system comprising an array of acoustic transducers and a beamforming system that simultaneously associates signals with a range of frequencies to the transducers. A signal to a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer. The phase shifted signals with the range of frequencies form an acoustic signal with a range of directions. A given direction of propagation within the range of directions corresponds to a specific frequency of the signals within the range of frequencies.
In one embodiment, the beamforming system comprises a broadband transmitter that simultaneously supplies signals with a range of frequencies to the array so as to form transmitted acoustic signals with a range of directions. In another embodiment, the beamforming system comprises a receiver having a spectrum analyzer that simultaneously processes signals from the array that result from received acoustic signals from a range of directions. In another embodiment, the beamforming system comprises a broadband transmitter and a receiver having a spectrum analyzer. The broadband transmitter simultaneously supplies signals with a range of frequencies to the array so as to form transmitted acoustic signals with a range of directions, and the spectrum analyzer simultaneously processes signals from the array that result from received acoustic signals from a range of directions.
A formula cos &thgr;=(&Dgr;&phgr;/2&pgr;)(c/fd) represents a relationship between the direction of the acoustic signal and the frequency, where &thgr; represents a direction angle relative to a plane defined by the transducers, &Dgr;&phgr; represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers. The phase shift &Dgr;&phgr; is selected to be a substantially constant value and t
Knobbe Martens Olson & Bear LLP
Pihulic Daniel T.
Rowe-Deines Instruments, Inc.
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