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What works Underwater ? — Parallax Forums

What works Underwater ?

CounterRotatingPropsCounterRotatingProps Posts: 1,132
edited 2010-06-22 13:31 in Robotics
OK, the subject's a bit funny... and from an underwater N00B [noparse]:)[/noparse]
so don't reply: "hmmm ...·your toaster wont' work well underwater..."

I'm doing the preliminary design research for·building a submersible ROV.

Although there's a surprising number of these posted on the net, and I am looking at them, thought it would be good to ask you all:

Does a compass work? Are there depth limits?

GPS probably won't work as the signal is so weak ... but maybe OK down to 3 feet or so?

Does some frequency of radio work underwater and if so, what kind of ranges?


thanks for your assistance!
- Howard in Florida



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Comments

  • LeonLeon Posts: 7,620
    edited 2009-06-29 15:07
    VLF is used by the US Navy to communicate with submerged submarines, the transmitting antennas are several miles long. Worldwide communication is possible (one-way).

    Leon

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  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 15:37
    > transmitting antennas are several miles long

    ahh, Leon, would that be 1/4, 1/2, or full wave?? [noparse]:)[/noparse]

    Seriously though, for small scale stuff like what I'm thinking about, a mile long antenna is impractical at best. Are there other higher freqs that might work for very short distances - say around 1000 metres?

    thanks,
    -H

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  • LeonLeon Posts: 7,620
    edited 2009-06-29 15:52
    No, RF just won't work. The UK Special Boat Squadron has antennas on their heads for underwater operations, they have to poke them out of the water to communicate.

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 16:30
    > No, RF just won't work.

    Why is that, do you think? Does the water just soak up the EMF?

    Since the Earth's mag. feild permeates the surface as far as we can measure, does a compass work ?

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  • LeonLeon Posts: 7,620
    edited 2009-06-29 17:17
    Electromagnetic waves do not travel through conductive media to any extent. What do you think happens to the E field?

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 18:02
    > What do you think happens to the E field?

    What do you Brit's call a "Pop Quiz" ? [noparse]:)[/noparse])

    It's energy it transferred to, absorbed by, the conductor ? So since the VLF's wave length is so long, it doesn't interact 'locally' as much with the individual atoms of the conductor, but passes through them? If that's right, then that would mean microwaves would be really bad for underwater communication (and why a microwave heats the water in your food, but not much else [noparse]:)[/noparse]

    RE compass I should have asked: "does a compass work a depths greater than sport diving (> 150 feet) ? " - because I know a compass works from using one while scuba diving.

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  • kwinnkwinn Posts: 8,697
    edited 2009-06-29 18:32
    Longer wavelength RF (0.1-3MHz) may penetrate short distances in fresh water.
    A compass should work in either fresh or salt water.
    Sound and ultrasound travels for long distances in lakes and oceans.

    If I were to get involved in underwater communications I would start with the 38KHz ultrasonic transducers.
  • LeonLeon Posts: 7,620
    edited 2009-06-29 18:34
    You've got the right idea.

    A compass works underwater because it uses a magnetic field. They aren't affected much by conductive media.

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 19:02
    @kwinn: this will be saltwater mainly. thanks for the tip on ultrasound ... what do you think would be a reasonable distance and comm. rate for these? (There's a US Navy submarine base fairly close by --- hope the Shore Patrol doesn't show up one day ;-0 )

    @Leon: thanks ... I should have realized that the ions in the water would absorb the E-field ... pure water (de-ionized that is) doesn't conduct, if I recall right. It's been a long time since basic physics and E.E. for me... I vaguely recall wondering back then why water doesn't affect the magnetic field, but does the E-field. Consider basic E.M. theory: the electric field oscillates at right angles to the magnetic field - so why isn't a magnetic field affected too?

    Or is that the wrongly put, as a compass works underwater because the Earth's mag. oscillates at, what, 1 cm/100 years? (VVVLF [noparse]:)[/noparse] so it's always permeating?

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  • LeonLeon Posts: 7,620
    edited 2009-06-29 19:44
    It's nothing to do with the fields being at right angles, they have different properties. A better way of dealing with this is quantum electrodynamics.

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle

    Post Edited (Leon) : 6/29/2009 7:50:20 PM GMT
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 20:04
    Egads! I had quantum I - IV in school ... looks like I've forgotten almost all of it - LOL.

    Experimenting with ultrasonics looks interesting ... not much bandwidth, but maybe enough.

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  • LeonLeon Posts: 7,620
    edited 2009-06-29 20:21
    Green laser light is quite good.

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 20:49
    Probably would have to have serious error correction routines - how many schools of fish, pods of dolphins, and packs (?) of jelly fish could it tolerate before heading down to Davy Jones Locker ?

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  • xanatosxanatos Posts: 1,120
    edited 2009-06-29 23:40
    Whales work under water! Their vocalizations carry for many miles - so there's probably some possibility of certain frequency ranges in the audio frequency spectrum.

    If you have a copy of "Reference Data for Radio Engineers" - I *believe* there's a chart in there of the absorption spectrum of sea water at various RF as well as light frequencies from far IR to short wave UV. If I can dig up my copy I'll see about scanning the pages. If I recall correctly there were some significant peaks and valleys that would suggest some given wavelengths that had low scattering and absorption. If you could find some lasers that operated on those wavelengths you might have some success.

    Interesting project, keep us posted!

    Dave X
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-29 23:51
    Thanks Dave - I'll see if I can scare up a copy on the net ...

    > Interesting project, keep us posted!

    Will do ... it's a long-term deal [noparse]:)[/noparse]

    Here's one of the links that inspired this:
    ("BS2 Submersible ROV")
    http://forums.parallax.com/forums/default.aspx?f=21&m=252605

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  • Rob_WRob_W Posts: 32
    edited 2009-06-30 00:40
    I would suggest you try hacking an old fish-finder unit. It would have a sonar transducer you could mount externally on you ROV. Then you could write software to ping the unit and calculate time, speed and distance to objects in it's path just as you would do with the ultra-sonic ping sensor, like the PING DAR program written for the Basic Stamp. You may even come up with two fish-finders and use one for transmitting pulses to the ROV which would receive the sonar signals from a surface based unit, like a radio controlled aircraft.
    You would have to deal with effects the speed of sound in water, absorption, scattering, frequency loss as the distance increases, temperature, pressure and depth of the transducer, and of course the effects of biologic creatures in the sea...... But just imagine the fun you can have taking on such a project!!!!
  • kwinnkwinn Posts: 8,697
    edited 2009-06-30 05:40
    CounterRotatingProps, no idea what distance or comm rate would be possible. That would be a function of power output and receiver sensitivity. My experience has been limited to medical ultrasound, which is in the megahertz range and about two foot range. Fish finder transducers are probably your best bet. For data rate I would suggest starting slow (at the maximum desired range) and doubling the rate until it fails.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-30 14:35
    Hi Rob_W,

    thanks for making your *first* post to the forum in reply to my questions ... I appreciate that ... Welcome to the forums ! The fish-finder idea is great. There are a ton of these things around here. I don't know diddly about sonar, so this will be interesting. Any idea what frequencies these things use?

    @kwinn - neat that you work with med. ultrasound - I've had opportunities to play around with these devices - pretty amazing.

    - H

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  • kwinnkwinn Posts: 8,697
    edited 2009-06-30 17:21
    CounterRotatingProps, I googled "fish finders" and got this link ( http://www.gpscentral.ca/fishfinders.html ). Some good basic information there. I also found this link to a transducer supplier ( http://airmartechnology.com/ ). I am sure there are more suppliers and options. From what I know of ultrasound I suspect that the higher the frequency you use the shorter the range or penetration will be, but the better the resolution or higher the data rate you can achieve.

    Good luck with your project.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-30 17:34
    kwinn - thanks for the links ... found some cheapo's on ebay too. Think I'll go hang out at some boat shops and find out where they send their broken stuff (I'm not adverse to Electronics Dumpster Diving, if the "junk" is a treasure [noparse]:)[/noparse])

    > Good luck with your project

    thanks, I appreciate that.

    - Howard

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  • kwinnkwinn Posts: 8,697
    edited 2009-06-30 22:31
    Glad to be of help. I'm not adverse to electronics dumpster diving myself. Luckily we have this local recycling center where people drop off their unwanted stuff and others can buy it for pennies on the dollar. They have all kinds of stuff, not just electronics and I love to browse there.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-06-30 22:39
    Yeah, recycling places are cool --- if you don't mind the way-beyond-fermented beer smell that permeates these places ;-P ... used to do some metal sculpting, mostly with copper and brass. They are quite the resource because we have tons of air conditioners die here in Florida. The contractors use copper tubing and all kinds of weird fittings that end up in there... think I'll drop by soon and see what electronics has been dumped [noparse]:)[/noparse])

    Was thinking about starting a new thread call "Sonar" as it really branches off this --- or do all think it would be better just to discuss it here?
    (In just a few hours of researching that topic, I've become enthralled with it.)

    thanks
    = Howard

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  • Rob_WRob_W Posts: 32
    edited 2009-07-01 00:22
    Hi CounterRotatingProps,

    Thank You for the welcome to the forums. I have been reading the various forums and your project sparked my interest as I am a retired Navy veteran. I checked into kwinn's links and found a Garmin 90 advertised for $119.95. It is a dual frequency unit that looks promising for a "cheap" sensor. I have an old unit on an old boat I can strip down, it is an older Garmin but would be good for experimentation before you invest in a new sonar unit. I'll remove it from my boat this weekend an see what I can make work, then let you know what suitability it may provide. One major issue would be a 12 volt power supply in your ROV, the Garmin 90 utilizes 3.5 watts max, output power is 100w or 800w depending on which frequency is pinged. My question is what size is your ROV going to be approximately?
    I have a Basic Stamp board and a Propellor Control Board to test with as I am also interested in constructing a similar project as my budget will allow! Military salvage auctions are also a good source for used parts.
    Good luck with your project.... I'll be in touch. [noparse]:)[/noparse]
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-07-01 00:42
    HI Rob_W,

    you're just the person I wanted to discuss this with. There's a Navy base near and it seems like some folks there might be interested in this little project too. But I'm clueless who to call on base - any suggestions?

    RE sensors - yes, the transducers seem to be the crucial element. I've found lot's of ebay'ers selling units for dirt cheap, but the majority of the devices are screens only, missing the transducers ... as a replacement part, they run from $30 to >$500 ... I'm in the < $30 range at this point [noparse]:)[/noparse]

    So far, I've determined that the operating freqs. are between 12kHz and 400kHz, with the majority as 'dual band' units running 50kHz and 200kHz. The power ranges from the minimal wattage you listed to over 1kW. However, the spec. sheets I"ve studied so far list the duty cycles as only 2%. --- am I correct in assuming that this reflects that the outbound 'ping' is only a spike? If that's true, then comm. speed becomes more complex as %2 of 200kHz say is not much... or maybe the data could be sent in a burst mode? (Again, without toasting a few of these, I'm assuming that >10% duty cycle would merely fry the transducer.

    Thanks for taking your boat apart for the cause ! [noparse]:)[/noparse])

    Tentative size is not much more than 4 foot long, with the main tube diameter of 4 to 6 inches. (See that link above - I'm using that as an intial design.)

    Any links for the military salvage?

    My biggest issue now is how to keep the bulkheads sealed... I'd like the tethered verson to go 100 feet - so I can dive to it if needed. The final, autonomous version, I'd like to get > 1000 feet.


    thanks again!
    cheers
    Howard

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  • Rob_WRob_W Posts: 32
    edited 2009-07-03 05:12
    Hi Howard,

    1) I'm sure you will find individuals interested in helping you with your project at your local Navy base. Simply go to the front gate of the base and ask for the OOD (Officer of the Deck/Day) office. They can provide answers as to who may be able to help you. Many bases have open facilities which you can visit, ask questions and make helpful contacts in the areas of your interest.
    2) Sensors in the sea are crucial as Davey Jone's Locker is an unforgiving environment for exploration. Your budget will limit your sensors, so I would suggest the KISS rule...... Which will allow affordability in what you can afford to lose in an unforeseen natural accident, so buy sensors with frugality in mind and test them individually.
    3) Operating frequencies listed in these units are actually a tone, which is triggered in a given duty cycle; transmit a pulse and then receive/listen. When you turn on a fish finder, it triggers a "pulse out" of say the 12kHz tone, and then you see on the display a trace moving left to right "normally", representing the ping frequency. As the tone travels farther away from the transmitting sensor it is returning reflections from objects in its path which are displayed to the screen. The sonar unit is calculating two-way travel and displays a graphic representation to you. This may include both distance and bearing information which you would use to determine where to go or not to go in your program. Data logging would be a useful tool in the decision making process, depending on what you are trying to accomplish, i.e. collision avoidance or object to follow, or basic navigation.
    4) Your Welcome for the boat lobotomy....lol
    5) A 4 foot length is reasonable, diameter or hull shape would vary, depending on installed equipment.... I acquired a a 3600psi hydraulic pump that could be useful for some sort of robotic arm or propulsion assist; it can be electrically driven. Endurance of the mini sub and its ability to handle currents and many other environmental factors is key to your design success.
    6) I don't have specific links to salvage auctions.... check the OOD at your local base.... they could find the closest unit in your area.

    I am in Jacksonville, Florida; so we may be able to get in touch for further development ideas...
    7) Water-tight integrity is key..... I can weld a steel hull, which could carry heavy duty equipment/propulsion etc......
  • kwinnkwinn Posts: 8,697
    edited 2009-07-03 05:36
    Howard, the outgoing pulse is fairly short duration on sonar units. The time between pulses is relatively long in order to listen for echoes. Obviously the greater the range you want the greater the time between pulses. For transmitting data you could pulse width modulate the ping and decrease the time between them.

    Like most things in life it's a trade off.

    PS - the recycling center I go to is pretty clean. No fermented beer smell or any other odor. They also keep it fairly neat and well organized (for a recycling center). The place is mainly run by volunteers from the local school who work there to earn their community service credits.

    Post Edited (kwinn) : 7/3/2009 5:42:48 AM GMT
  • Rob_WRob_W Posts: 32
    edited 2009-07-05 14:04
    Hi Howard,

    I dissected my boat this weekend and after a struggle with watertight plastic, I was able to disassemble my fish finder and discovered water damage on the circuit board. Grrrrr...
    I did find an owners manual which states the ping frequency to be 192Khz with an output power of 275 watts peak to peak. I am hoping to be able to restore the circuit board to an operating condition, however if that doesn't pan out, plan "B" is to acquire another unit of which I already have a prospect if my friend can find his old one in his chaotic garage...lol
    I am planning on using 4 additional receiver modules in construction of an ROV. These will pick up the audio from whichever ping frequency turns out to be. I plan on using the receivers to calculate a relative Doppler effect, in order to determine approximate direction and distance to detected objects; i.e. fish or any other object that returns an echo. That is my main objective for an ROV at this point, which should prove to be a great learning experience for me in my quest to integrate both my Basic Stamp and my Propellor Control Board into my project. The receiver modules are home-constructed from off the shelf RadioShack parts. I am a not a trained electronics tech, but I do have some background from self-studying and home hobby experimentation. Parallax is a great site for old dogs to learn new tricks [noparse]:)[/noparse] ..... I love this place!! The people here are superb in their quest to help people learn..... Life is good!!
  • ceruleanplainsceruleanplains Posts: 11
    edited 2009-07-07 04:34
    I also would like to get something that dives deep with the final project (~750 ft). I'm not really interested in something that's fully autonomous, but that's at least semi autonomous. I'm in the Alexandria, VA area looking to put something in a few rivers in the area then take final variants to places like the Inside Passage in Alaska.

    I'm not sure about the validity of the use of a depth gauge to communicate just because of the noise in the water, but I could be wrong. You'd have to have a nice filter but it might work. I was going along with a relay attennae concept on the surface as a buoy that you could rectify into a different wavelength under the water as something more reasonable myself for wireless. However, my application is for transmission of video & i'm not an expert at trying to pull that off. so it was automatic tether for me.


    Few observations:

    Something that's going to depths > 1000 ft is likely to cost > $10k.
    The pressure, salt water intrusion on seals, etc. is a nightmare

    i'm just testing my concept at shallow depths with PVC pipe per the other Parallax posting. My first ROV was 4' x 3' and was a pain to make neutrally buoyant (~100 lbs of ballast). My latest attempt is much much smaller and uses a wet hull structure with most of the electronics out of the water with control & power through a tether. It also cost << less. Bigger isn't always better. In fact it's a pain in the butt underwater.

    I definitely recommend you doing the buoyancy calcs ahead of time to assist in sizing your concept before you go too far. If you have access to milling machines, etc. then you're way ahead of me in that area. But as an amateur, days matter for construction and fixing leaks was another bain of the other design. Fiberglass, epoxy and all that fun stuff to seal leaks on edges... then hours of sanding and testing on top of that.

    My learned opinion is that whatever technology concepts you can demonstrate independently such as sensors, comm, etc. do so outside your vehicle in order to make them modular. Then try and incorporate the modules in the final app. You can even do this with the hull.
  • kwinnkwinn Posts: 8,697
    edited 2009-07-07 14:17
    I am far from an expert on underwater ROV's or autonomous vehicles but I have often wondered if it would be possible to solve the pressure and water leakage problems by filling the interior of the vehicle with an inert non conductive liquid. Has this been done or even considered?
  • ceruleanplainsceruleanplains Posts: 11
    edited 2009-07-09 19:15
    yeah non-conductive oil has been used.
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