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I finally got a chance to take a look at this forum, as I had been urged to do by Jan & Patrick. I've still got an hour or two of reading to do to catch up - you guys have been busy! I thought I'd jump in with some random observations.
My project had as its objective a quick, easy, cheap and lightweight sidescan sonar system for the starving shipwreck hunter. I usually work from a small boat (20 foot - I recently moved up from a 16 footer) and I often work solo, so the fish and cable had to be small and light. I don't have a laptop or a source of 120 VAC power on the boat, so I haven't explored any computer applications to sidescan. I have sought to use off-the-shelf components wherever possible to keep cost and complexity down.
TOWFISH: The towfish is made from cheap, readily available & easily worked materials - PVC pipe and aluminum. The design objectives were simple - mount the transducer array so that it points in the right direction, and stabilize the fish so it tows smoothly. There are a lot of refinements that could be made - a more rounded nose cone, a more tapered tail cone, more streamlined tail fins - but the gains would be incremental. As it is, the fish "flies" smoothly, so I haven't changed much.
TRANSDUCERS: A single transducer will give a useable sidescan picture. An array of transducers will result in a narrower horizontal beamwidth - the more transducers, the narrower the horizontal beamwidth and the higher the resolution. Using fishfinder transducers to construct an array is a bit of an inexact science. As I understand the construction of these transducers, the puck-type transducer consists of a disk-shaped ceramic element potted into a PVC cup with epoxy. I mounted the PVC faces of the transducers in the same plane, in hopes of keeping them exactly in phase. However, since the wavelength of sound in water at 200 kHz is a mere 7.5 mm, a difference of one millimeter in the positioning of the transducer element in the PVC cup would result in a 48 degree phase difference. For my next sonar, I will probably try to remove the ceramic transducer elements from their mounts (perhaps I could use acetone or MEK to dissolve the epoxy and PVC) and epoxy the bare elements directly onto a "window" of plexiglass or some other plastic. (This is where acoustic impedance matching, which I know nothing about, comes into play.)
TRANSMITTER / RECEIVER / TRIGGER: While one could easily enough build a pulsed CW transmitter of the appropriate frequency, as well as a receiver and trigger pulse circuits, it would seem to me to be easier to use a fishfinder with the highest possible output power and merely tap into its trigger pulse circuit and into its receiver after a stage or two of amplification, just before it goes through any signal processing. You could then digitize it for input ot your custom computerized application.
Well, I should get back to reading the last two week's worth of messages you guys have been sending, and try to get up to speed with your project.
Dan
My project had as its objective a quick, easy, cheap and lightweight sidescan sonar system for the starving shipwreck hunter. I usually work from a small boat (20 foot - I recently moved up from a 16 footer) and I often work solo, so the fish and cable had to be small and light. I don't have a laptop or a source of 120 VAC power on the boat, so I haven't explored any computer applications to sidescan. I have sought to use off-the-shelf components wherever possible to keep cost and complexity down.
TOWFISH: The towfish is made from cheap, readily available & easily worked materials - PVC pipe and aluminum. The design objectives were simple - mount the transducer array so that it points in the right direction, and stabilize the fish so it tows smoothly. There are a lot of refinements that could be made - a more rounded nose cone, a more tapered tail cone, more streamlined tail fins - but the gains would be incremental. As it is, the fish "flies" smoothly, so I haven't changed much.
TRANSDUCERS: A single transducer will give a useable sidescan picture. An array of transducers will result in a narrower horizontal beamwidth - the more transducers, the narrower the horizontal beamwidth and the higher the resolution. Using fishfinder transducers to construct an array is a bit of an inexact science. As I understand the construction of these transducers, the puck-type transducer consists of a disk-shaped ceramic element potted into a PVC cup with epoxy. I mounted the PVC faces of the transducers in the same plane, in hopes of keeping them exactly in phase. However, since the wavelength of sound in water at 200 kHz is a mere 7.5 mm, a difference of one millimeter in the positioning of the transducer element in the PVC cup would result in a 48 degree phase difference. For my next sonar, I will probably try to remove the ceramic transducer elements from their mounts (perhaps I could use acetone or MEK to dissolve the epoxy and PVC) and epoxy the bare elements directly onto a "window" of plexiglass or some other plastic. (This is where acoustic impedance matching, which I know nothing about, comes into play.)
TRANSMITTER / RECEIVER / TRIGGER: While one could easily enough build a pulsed CW transmitter of the appropriate frequency, as well as a receiver and trigger pulse circuits, it would seem to me to be easier to use a fishfinder with the highest possible output power and merely tap into its trigger pulse circuit and into its receiver after a stage or two of amplification, just before it goes through any signal processing. You could then digitize it for input ot your custom computerized application.
Well, I should get back to reading the last two week's worth of messages you guys have been sending, and try to get up to speed with your project.
Dan
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