Ultrasonic Sensors - Underwater

John R.

Has anyone·tried any of the ultrasonic sensors underwater?

I would anticipate that the distance would need to be "adjusted" due to the difference in the speed of sound (or "ultrasound").

I'm working on an ROV, and am looking for something less expensive than hacking a "fish finder".· Idealy, I'd have an "array" of sensors for depth below, depth above (assuming I could get a return off the surface) and objects in front and/or around.




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John R.

8 + 8 = 10

steve_b

I did some icefishing in Northern Ontario and one guy has a fish finder. (To note: this doesn't give any additional advantage since you've augered your holes and are pretty well stuck looking at the same spot).

I guess the way the cheaper fishfinders work is by assuming the temperature of the water you're in....and then they determine the temperatures due to its depth. Temperature differences do play a part in their workings. The more expensive models have a temperature sensor built in for a proper starting point (check the temperature sensors range if you plan to use it in the bahama's and the great white north).

My future father-in-law says there's a heat layer in the lake that fish tend to swim in. The Magic number is something near 48degF....so I'm in teh process of building him a temp sensor line that he can hang in the water and see where this 'heat layer' is.

I think the biggest part of using a standard US sensor in water is the waterproofing.· If the transducer is exposed, it has to push water at ultrasonic speeds to create the sound (which means a big driver)....if its contained, then you have to account for the echo in its chamber and also the attenuation before the sound goes where you want it.

I think this is why waterproofed US sensors cost so much!

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

John R.

I can relate to the problems of depth when ice fishing. This can be somewhat overcome by a gasoline powered ice auger. It takes the pain out of drilling a hole only to find your too far in/out. We've also got some "high end" ice fishing fish finders. They are the old rotating bulb type, but use a different color for the bottom and "fish" returns. The nice thing is that they show the presence and depth of the fish, as well as where you're jig is. You drop or raise the jig to the leve of the fish. Sweet...

My understanding of these units is that the "logic" is in the unit, and the part you put in the water is only the sensor(s). I may need to do a little more poking around to confirm this. If there are some "brains" in the sensors, then it wouldn't be so bad to use one of these units. For what I'm after, I could even "borrow" one, especially from the one of the ice fishing units (they have a very narrow cone, are very senstive, and don't work that well in "open" water for fishing purposes).

Has anyone worked with any of these sensor units?

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John R.

8 + 8 = 10

rtowler

Wow. Sounds like a cool project.

Ignoring the waterproofing issues, the common ultrasonic sensors will not work underwater without modification since the piezoceramic transducers are designed for transmitting in air and there is a very big acoustical impedence mismatch between air and water I don't know exactly what you'll see, but I would guess that there would be a pretty big loss in energy, if anything at all. And since attenuation is so much higher in water you may not be able to drive them hard enough to get any range.

Your "fish finder" transducer may work... Typically there aren't any "brains" embedded in the transducer. It will most certainly be a single beam, 2 wire type. You'll need to play around with the deck unit to determine how to drive it (generally you don't run the deck unit w/o the transducer attached.) Check the specs for the frequency. You'll then need to work up your xmit and recv circuits. Power may be an issue depending on the size of your ROV.

On the software side, if you want to do anything more than bottom/surface detection you'll probably need to apply a time varied gain to account for attenuation from spreading and absorption before doing any sort of target detection. Once you get this far though there is a lot of info out there to go from power received to some sort of useable values. Google "sonar equation".

have fun!

Bongo

I did quite a lot with ultrasonics last year. Searching for transducers I came across a few companies that also make water versions. They are different, as the elements need to be in (indirect) contact with water. Typically they also use different frequencies than the usuall 40kH for air. This is no problem if you are using a micro to drive, you can program to suit. Also noticed they are much higher power. Have seen up to 2kV peak to peak. So the TX amp and power supply will need to be rather different. For your idea., I would be sampling the reciever amp into an ADC so as differenting between fish and a hard bottom could be programmed in. Would also be considering a variable output driver (amp) so the micro can limit output for shallower water to limit the power of multiple echo returns.

Also tried to think of a way to swing the transducer backwards and forwards across an arc so as a region can be scanned like radar. That would be a scream, but not so easy when the transducer needs to be in water, or a wet box for through hull reception.

Given that water is a much better medium for sound than air, I think you will find the possibilities here quite interesting.


bongo

Gadgetman

You may not need a 'wet box'...

Installation documents I've seen for echo-sounders and fish-finders for use in boats often state that in fibre-glass hulls, it's sufficient to glue the transmitter to the inside of the hull with epoxy if you're careful not to get any airbubbles in the epoxy.

It won't work in wood or steel hulls, obviously.
I also don't know if it'll work in plastic hulls, but if anyone is willing to donate the equipment, I'll be happy to test it in my cayak.(It's made of PolyEthyleneSomething - PET - plastic)

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Don't visit my new website...

John R.

As one of my "teenage" jobs, I worked in a boat shop, and I can verify that many of the tranducers can work "through" the hull on Fiberglass hulls. We would use fiber glass resin to mount the tranducers to the inside of the hull. This only worked well if there was a "flat spot" on the hull we could mount to. If the transducer were mounted on an area of the hull not parallel with the surface of the water, readings were "skewed" because of the angle. For some cusomers, this wasn't a problem, they just wanted to find the drop off or see the bottom structure, and preferred not to have the transducer external.

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John R.

8 + 8 = 10

steve_b

Do the transducers use the boat hull as a reverberant chamber....as an amplifier of sorts?

Do the transducers' ultrasonics pass thru the hull or do they vibrate against it? Curious why steel wouldn't work (although it would certainly prevent it from passing thru).

If fibreglass works, I'd assume plastic would.

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

John R.

I must confess, I don't know why/how it worked, but it did work. It also "usually" worked when you had a hull that was a sandwich of fiberglass, plywood, fiberglass. The "gotcha" was that if the plywood had an unfilled void, things would get flakey, and the customer would not be a happy camper. It was about a 75% success rate on that (sandwiched) construction.

I assume that the "sound" waves were just transmitted through. I am assuming that the chances of finding a hull with the right harmonics to amplify the waves would be between slim and none. I can also vouch for the fact that your fiberglass job better be good, and free of bubbles. It only took one or two times of having to grind and chip the stuff away for you to realize you had better do it right the first time

The only conern I might have with a plastic hull is that some of them are built up with laminated layers of different material. If there are any air gaps, or if the hull starts to de-laminate, I would expect to see some "flakey" behavior of the unit.

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John R.

8 + 8 = 10

steve_b

Not to resurrect this in debate....

We just hired a new guy recently who has a sonar background and he had mentioned that the transducers used in larger boats and subs has an excitation voltage upwards of 90KVA but with small current in comparison (100A's is still small compared to 90K but he didn't say what the current was!).

I guess those transducers really need quite the kick to 'sing'!

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."