Saturday, June 27, 2015

Sonar Development: Towards Something That Works In Water


Bathymetric survey data from a NOAA ship.

After a few months of distractions to prepare for a new kid, build a quadcopter, and work a bunch, I'm trying to get my sonar project moving forward. As documented in previous articles, I've got a simple digital sonar working in air. It was a simple way to test the echo detection algorithms. I'm convinced if I can figure out a way to use piezo transducers to transmit sound in the water, I can make it work.

Previous installments:

Audible Frequency Chirp Sonar on the Stellaris Launchpad

Initial Experiments - Sonar in air with a conferencing speaker mic and Python

So the next challenge is how to mount the piezo element and efficiently couple the sound to the surrounding water. One way to do this appears to be to pot the transducer in a potting compound that closely matches the density of water.

This article from NOAA on building hydrophones for listening to whales details one way to pot a transducer, and also includes a high gain amplifier circuit. My current plan is to build one, and figure out how to get the ADC on the Launchpad reading the audio. From there, I can make a transmit circuit. The challenges will likely be in acoustic coupling, transducer selection, and getting enough power into the water to travel a reasonable distance.

I considered using piezo disks, but I found that getting any sort of output from them at all requires them being mounted at either their edges or nodal points in a resonant cavity known as a Helmholtz chamber. I don't think I can manufacture one to the precision needed for the small size. I'm going to work first with cylindrical piezo units as used in the hydrophone above.

I intend to try one with a resonant frequency in the audible range - that's not going to result in very good resolution, but should be easier to debug since I can hear it and use PC audio equipment to measure it. Once that works, I'll switch to higher frequencies.

The next step is to build a functioning hydrophone with a piezo element, get it working with the op-amp, and get that feeding into the ADC of the Launchpad. That will complete the receiver side, and test the methods of acoustically coupling the transducer to the water.

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