Underwater Camera, high tensile strength small diameter power over coax.  - Page 1
Forum section: electronics / projects

author: amitchell
author_rank: Regular Contributor
timestamp: May 18, 2018, 03:57:44 am
content: I want to design an underwater camera that will be towed behind a boat.The idea is to have about 50m of high tensile strength(75kg test) small diameter(1.4mm) coax that can be spooled out to: power the camera, send video back to the boat and to secure the camera.I have found a few chips that sounds promising in terms of sending the Video/power over the coax.Microchip EQCO62T20http://ww1.microchip.com/downloads/en/DeviceDoc/60001301B.pdfandTI DS90UB913A FPD-Link III SERIALIZERhttp://www.ti.com/lit/ds/symlink/ds90ub913a-q1.pdfwithTI DS15BA101 Line driverhttp://www.ti.com/product/DS15BA101With regards to the cable it appears that I require a cable of around 1.4mm diameter to limit the drag to a reasonable amount. This would seem to be the biggest challenge.Has anyone had a custom cable designed and manufactured? Does this sounds feasible?Adam

author: ajb
author_rank: Super Contributor
timestamp: May 18, 2018, 05:33:00 am
content: 75kgf at 1.4mm diameter works out to a tensile stress of 69.3kpsi or 478MPa. That's the territory of high strength steel. Kevlar strength members might be able to give you a reasonable safety factor at that diameter, but wouldn't leave room for much copper. So no, doesn't sound like a reasonable set of specs.If your concern with cable drag is that it will limit the depth of the camera while under tow, that can be counteracted with extra ballast and/or a dive wing. Obviously a dive wing will increase drag by a lot, but probably makes up for that by allowing you to spec a more reasonable cable.How much power and what sort of bandwidth do you need? That will provide some constraints on basic geometry before you even get to mechanical factors while under tow.

author: ikrase
author_rank: Regular Contributor
timestamp: May 18, 2018, 05:59:21 am
content: Seems like you could use an Ethernet camera or framegrabber with PoE?And deal with the mechanical strength separate from the data transmission?

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 18, 2018, 06:06:17 am
content: Quote from: ikrase on May 18, 2018, 05:59:21 amHack a pair of Homeplug adapters. Then you only need 2 wires for power and signal. Maybe even just one with the water acting as a return.https://forum.openrov.com/t/teardown-of-a-homeplug-adapter/305

author: amitchell
author_rank: Regular Contributor
timestamp: May 18, 2018, 01:40:33 pm
content: Quote from: ajb on May 18, 2018, 05:33:00 amI am looking at Full HD, 30FPS, plugging that into online bandwidth calculators gives me about 8-10 Mb/s.For the power, 800mW for the camera, 1.8V and 2.8V rails. Cable voltage drop compensated for at the boat end, buck with LDO followers at the Camera end.Starting with 28AWG wire 12V in at 400mA, I get ~3.5V out at 50m.

author: dmills
author_rank: Super Contributor
timestamp: May 18, 2018, 03:07:21 pm
content: 10Mb/s? 1080p30? In your dreams....You might manage that if you were working compressed domain back up the cable, but 1920 * 1080 * 30 * 20 bits per pixel (10 bit YCrCb 4:2:2 is fairly standard) is about 1.2Gb/s, if you allow for the usual overhead of using something standard like SDI then you are going to be seeing 1.485Gb/s on the wire.For 50m, you are also going to want some semi decent coax, just in order to keep the eye reasonably open.Now you could compress at the camera, at the cost of power and volume, which would get you a lower quality but far lower data rate (And horrifically increased latency).I might consider a loose jacketed fibre for the data, with power and a strength member, but use a PUR jacket, not PVC (which is rather prone to osmosis).Usually for a custom cable you are looking at ordering at least a km to get a sane price, and 10km is better, but various companies make cables for these uses as off the shel parts, but you will hate the price.Regards, Dan.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 18, 2018, 03:39:19 pm
content: Quote from: amitchell on May 18, 2018, 03:57:44 amour ISP... they installed overhead internet line (a pair of twisted cables) and now fiber optic. they pair them with a dormant thick aluminium or alloy or steel? (not sure what is but about 3-4mm diameter excluding insulator) cable just to take the tension, it doesnt carry any signal or electrical whatsoever (maybe you can use as power cable), the signal or electrical cables just got stuck next to the thick cable. maybe you can do the same, either find a supplier as said usually in reel, price will not be nice i guess, or make your own, pair or twist together a thick cable or string (for handling tension) with any smaller power/electrical/signal line you want to carry to wherever you want. something like below?... fwiw..

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 18, 2018, 03:43:37 pm
content: fibre for the data, kevlar for strength, and for power, DC at as high a voltage as the cable will stand.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: May 18, 2018, 04:15:05 pm
content: Quote from: amitchell on May 18, 2018, 03:57:44 amI am a custom harness design engineer and what you are asking is very difficult. However, you may be able to partner up with some of the custom cable manufacturers and come up with a solution. there are several out there that do video.Here are links to a couple that may get you started.https://www.gore.com/products/categories/cables-cable-assemblieshttps://www.carlisleit.com/products/wire-and-cableThese companies will work with you towards a solution, however, be aware that you will have to have your requirements well defined and understand the given solution. Also be aware that the end resultWILLbe expensive.Hope this helps...

author: C
author_rank: Super Contributor
timestamp: May 18, 2018, 04:46:26 pm
content: There are many places that use cable that might work.You have the well logging cable that is used to put electronics thousands of feet in well.You have the many underwater sensor systems a UAV.google "well logging cable"note: that 50 meters is scrap for deep well logging places.C

author: BrianHG
author_rank: Super Contributor
timestamp: May 18, 2018, 06:14:40 pm
content: Use the power over SDI standard, example.http://www.sct.com.tw/SiteShare6/AP/DownloadProductFile2.php?ProductFn=SDIVPD&ComDIR=org/ee/smartcabling&lang=SC&ispreview=SDI can handle 6gb/sec and is uncompressed raw 30 bit 4:2:2 video with multiple uncompressed audio channels + and ancillary data channel.The above .pdf might uses custom channel for the reverse direction RS232 com port. You may then use the SDI's ancillary channel to send control code with the video in the video's transmitting direction.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: May 18, 2018, 06:46:32 pm
content: You folks are all forgetting his 1.4mm diameter requirement. This is the very difficult part.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 18, 2018, 07:27:06 pm
content: Quote from: tpowell1830 on May 18, 2018, 06:46:32 pmwe find his reason "to limit the drag" is unreasonable... any larger than human power, say 2-10HP outboard (among the smallest a fisherman can have) can drag 1cm thick 50m long rope no problem, the problem will be the payload (camera). its like he's up to some sort of racing competition with minimum outboard engine available. since he left this info open, we may made some assumptions. anyway if he has to, he can ask somebody to wrap some fishing lines to whatever signal/power cable of his choosing together, hmmv.

author: amitchell
author_rank: Regular Contributor
timestamp: May 18, 2018, 09:59:31 pm
content: Wow lots of neat stuff to consider, that is what makes this community great. Helping out the little guy with the big ideas and limited knowledge.So I did a bit of research and found a patent for a similar idea, essentially exactly what I am looking for. I am now looking at my project as a modernization of the following system.https://patents.google.com/patent/US6091443A/en?oq=US6091443Aand a link to the product:http://www.walkerdownriggers.com/why.choose.strike.vision.htmThey have done this with a 3 conductor cable that has an outside diameter of 0.055in from the sales page, 0.070in from the patent file. That is right at the diameter I was looking for.Quote from: dmills on May 18, 2018, 03:07:21 pmI was dreaming about it last night, and yes that figure was for H.264 medium quality compressed. Uncompressed is going to be easier in terms of hardware, considering I know jack all about video signal.Quote from: tpowell1830 on May 18, 2018, 04:15:05 pmThat is quite helpful actually, another one I found.https://falmat.com/products/marine-subsea/Quote from: BrianHG on May 18, 2018, 06:14:40 pmI am going to read up on this now, thank you.Quote from: mikeselectricstuff on May 18, 2018, 03:43:37 pmHmm I know nothing about fibre, what kind of hardware would do you think I should be looking at? Thanks for the suggestion.Fun Fun.

author: amitchell
author_rank: Regular Contributor
timestamp: May 18, 2018, 10:20:43 pm
content: I found this cablehttps://www.carlisleit.com/products/wire-and-cable/high-performance-coaxial-cable/m17-169-00001Likely good enough in terms of diameter and specs.I wonder if some dielectric thickness could be sacrificed to add the strength member (thicker shield element?) I assume a balanced trade off with cable performance must be calculated for.

author: dmills
author_rank: Super Contributor
timestamp: May 18, 2018, 11:35:52 pm
content: Those are 50 ohm cables, if you were trying for something using standard video gear you want 75 ohm, and at 52dB per hundred feet @ 1GHz I don't see it working.Now have you actually tried underwater video? More often then not issues of available light and silt levels make a HD picture largely pointless, especially if under tow at the sorts of speeds that make a 1.4mm cable in any way relevant to the drag.Prove the concept with a PAL or NTSC camera and a low band video modulator to put the signal on the wire as RF (Makes sending the power the other way simple), I think you will discover wonders when it comes to trying to keep water out of the doings (Including the doings on deck), and that PUR overmolds, aluminium bronze and very carefully made O ring grooves will be very much a thing in your future.Regards, Dan.