Dissipationless physical systems, such as simple planetary motion or a swinging pendulum, exhibit time-reversal invariance. This means that if time were reversed the way a film projector can be, then the resulting motion would be allowed by the equations governing the system. It follows that if all velocities in a dissipationless system are suddenly reversed, the system will follow a time-reversed trajectory.
Dissipationless or adiabatic wave propagation is time-reversal invariant. Phase Conjugate Optics involves the deliberate reversal of complex light paths. A customer is doing Phase Conjugate Sonar using sound instead of light.
The customer has a generator hydrophone immersed in water that generates a high power pulse of sound with a central frequency of about 30 kHz. The generator is at the center of a circular arc on which eight receiver hydrophones are situated. The direct generated sound pulse travels at approximately 1.5 km/sec through the water and is simultaneously received by the eight hydrophones. Following this main pulse in time, smaller trailing echoes are also received by the hydrophones. These echoes are sound that is scattered by objects in the water.
In order to phase-conjugate the scattered sound, the customer will modify the signals captured from each hydrophone. The biggest pulse from the direct beam will be cut out of the data record. The remaining record, consisting only of the scattered sound signal, will be time-reversed. The first point in the new data record is the last point in the captured data record and vice versa. This time-reversed record will then be used to excite the eight hydrophones. The result is that the phenomenon is roughly time-reversed. Sound traveling towards the objects is scattered by the object. The scattering reconstructs the sound into a time-reversed version of the original generated pulse that scattered from the object. Only sound incident on the object was originally scattered. Consequently, the phase conjugation results in a highly directional sound pulse that emanates from the object. This sound can be received by a directional receiver in order to locate the object. The reconstruction is not perfect, since ideal phase conjugation would require an infinite array of hydrophones. Apparently, however, eight hydrophones work reasonably well. The customer requires an integrated, flexible A/D - D/A system to control the eight hydrophones.
The GaGe solution consists of four CompuScope 512 A/D cards and eight CompuGen 1100 D/A cards in an Industrial GaGePC 586. The 20 ISA slot backplane in the GaGePC can easily accommodate the 12 ISA cards and provides a compact, elegant and integrated solution. The setup is shown schematically in the figures below:
Upon signal detection and generation, the hydrophones are switched between the CS512 cards and CG1100 cards that are connected through appropriate amplifiers. Both the CS512 cards and the CG1100 cards provide 512 kS of memory per channel. The Master/Slave configuration of the CS512 cards and CG1100 cards ensures simultaneous signal capture and generation on all eight channels - an absolute requirement of the application.
The 12-bit resolution of the CS512 allows precise detection of much smaller echoes compared to 8-bit A/D. Six different software-selectable input ranges allow detection of sound pulses with widely varying volumes. Data acquisition at 5 MS/s on all eight CS512 channels is triggered internally on one of the hydrophone channels by the direct sound pulse. Alternatively, data acquisition could be triggered by an external trigger signal. Availability of pre-trigger data ensures capture of the direct pulse on channels where it may arrive before the trigger due to hydrophone misalignment.
With 512 kS of memory per channel, data capture times of up to 512 kS / (5 MS/s) = 100 ms are possible. This allows detection of echoes from objects whose path to the eight hydrophones can vary by up to 100 ms / (1.5 km/s) = 67 m, which exceeds the customer's requirement. If the experiment is scaled up in future, up to 8 MS per channel of on-board memory can be added to both the CS512 cards and the CG1100 cards in order to increase the range of the system.
Data records captured by the CS512 cards are reversed and uploaded to the CG1100 cards. Because the CG1100 has 12-bit resolution and can generate at 5 MS/s like the CS512, no data interpolation of any kind is necessary in moving data from the CS512 cards to the CG1100 cards. Like the CS512 cards, the CG1100 cards have six output ranges, allowing for a wide range of output volumes. The CG1100 cards can be configured to generate repetitively in loop mode or in one-shot mode when an external trigger signal or software command occurs.
While the customer uses MATLAB, the hardware can be completely controlled using GaGe standalone software. From GaGeScope for Windows, data records captured by the CS512 cards can be saved in GaGe's binary SIG file format. SIG files can be converted to text files with GaGe's SIG file conversion utilities and can then be time-reversed by a simple program. The resulting files can then be converted back to SIG format and uploaded directly to all eight CG1100 cards using CGWin, GaGe's Windows-based software for controlling CG1100 cards. Powerful GaGe software allows the customer to quickly set up and test the application without writing a line of hardware-controlling computer code.
By modifying the sample programs provided in the MATLAB Software Development Kit, the customer was easily able to develop a MATLAB utility to control the application. At the 1 MS/s data transfer rate of the ISA bus, the 4 MS of total memory on the CS512 and CG1100 cards can be downloaded and uploaded in about 4 seconds. This means that the customer is able to update the phase conjugate signals by recapturing the hydrophone signals and refreshing the CG1100 cards in under 10 seconds. This application demonstrates how GaGe can provide an integrated, flexible and complete solution to a demanding multi-channel requirement.
We encourage you to contact us and discuss your research & development application in more detail with our engineering team. GaGe can provide tailored custom data acquisition hardware and software solutions to meet specific application requirements.