TP link ethernet - fiber optic converter for tether


#1

Hi folks!

This is my first post on this forum. ROV is something i was thinking about since years and now i decided to go into it. I’m after reading tons of topics on all ROV related forums on the web and have built some knowledge base.

I divided my concerns about ROV into the following blocks:

  1. frame type, number of thrusters/DOFs
  2. materials for the frame: HDPE, alu, PVC pipes, acrylic
  3. thrusters type
  4. control system
  5. communication and tether type and this point i’d like to discuss here guys.
  6. power choise - i will use onboard battery packs.

Note my main assumptions are: cost effective, diy made parts, 100m - 150m water depths operation, tether lenghts up to 250m, video stream 1080p.

As for onboard ROV control system i’m thinking about PixHawk autopilot platform + Raspberry Pi3 + 1080p usb H.264 camera or Raspberry Pi ribbon cable camera. You may know this type of control system from ArduSub.com or bluerobotics. Yes, the idea came from those sites.
The only part of the system i do not like is the Fathom-X ethernet-twisted pair modules. It is too pricey for me at USD 159 plus shipping to Europe, VAT and customs:/

I see lot of you guys use ethernet Cat5 cables. Is it really necessary to use some kind of amplifier between onboard ROV Raspberry Pi3 ethernet connector and surface? I’m aiming at 250 meters of cable. Would it be good for 1080p video stream, telemetry and control?

Second idea is fiber optics - single fiber cable. I saw many ROV builders was talking about some sort of video-to-fiber converters + data channel (RS-485). But Why can’t we use standars fiber optics iethernet routers like this one:

http://www.tp-link.com/us/products/details/cat-43_MC111CS.html#overview

It use one fibre for two way transmission: 1550 nm wavelenght on TX and 1310 nm on RX. On the other side on the tether MC112CS should be used. According to specs 20km long hi speed transmission can be established this way.

So it can looks like this:

PixHawk+Raspberry Pi3 <-> TP-Link MC112CS <-> fiber optics tether <-> TP-Link MC111CS <-> PC/Tablet/RaspberryPi3 (i wonder if QGroundControl platform can be installed on Raspberry Pi3 ???).
Note MC112CS max power consumption is 2,5 W (according to scpes) wchich is nice for onboard powered ROVs.
What you think about such system?


#2

I am working on an ROV project with a group that builds underwater video and lighting systems. We have gone over this idea also. You have to also look at one key element, that’s your Tether and its cost. For the shorthaul fiber your looking at a tether that would contain 2 strands of MM(Multimode) fiber. Expensive! We went a cheaper route, and used a very common tether that includes a standard Coax core plus a few 14g and 18g pairs for power and features.
I found that I can run a standard non IP based camera, and inject the IP based info on a MOCA IP Extender system by D-Link or Trendnet. Cost for a this on each side is running around $50 and I can push around 80Mg of data across the link and run up to 1000ft with no issue. That gives me the ability to run a few 4Mg IP cameras, and companion computers with out any issues by placing a nano switch in the ROV, and a standard WIFI router topside behind the MOCA devices.


#3

Thank you Mike!

If i understand you right 2 strands MM fiber looks like two separate cables sticked together, right? Ifso the TP-link devices i was talking about use one strand fiber optics cable as transmission go throug one fiber string. TX and RX light signals have different wavelenghts thus making it possible to go in opposite directions through the same fibre withuot mixing/interference. Correct me if i am wrong.

Would you be so kind and explain your system in more detail? What non-IP cameras are you talking about? Coaxial tether instead of fiber sounds very interesting. I guess it is not only cheaper but more resistant to mechanical wear/bends/strain etc. Is this right?

More than one camera is also nice feature. So from you’ve said i can hook up PixHawk+RPi3 with one or more cameras directly to MOCA IP Extender? What nano swich are you using?

Thank you Mike!


#4

I found this:

So any two wires cable will be good. I think this is the best solution so far. Additionally i can try to send some pwer down the tether.


#5

Hi Mike

What is the name/brand of the tether you are using?
And where do you buy it and at what cost?

Sounds like a much better choise than the single twisted pair used on the trident if not to big/heavy.

Regards Tore


#6

Thats my sons side of things, I would have to check with him, buts its
his comercial diving cable about 3/4" thick, green, with messenger cable
inside also.


#7

Another concern would be the video lag time. Trying to drive a vehicle with a lag as small as 100ms is annoying. A lag of 250ms makes driving very difficult. Every time you add a digitizer, converter, or Ethernet switch you add more lag. Note Ethernet hubs and better than switches for this purpose as they don’t wait for the whole packet to arrive before sending it on.


#8

Well I have it all working now using TPA-311. It was a simple of plugging power to units and link. So on topside, running a cheap wifi router setup as static 192.168.2.254, The PC, and one TPA-311 all in a nice customized Pelican case. Then we have the Tether running to ROV (not Finished) ROV electronics will be oil filled(all ready tested) and has a TPA-311, then 5 port gig switch, Raspberry Pi for Mavlink with a 1.2mg camera, and Pixhawk with 2 light relays, pan tilt gimble. plugged in 2 4mg IP cameras to put some stress on the network. I was getting 3-4ms solid ping times to all devices, no video lag, all running H264.

Hubs are thing of the past, you need a SWITCH, especially with video streams, a HUB distributes same info to all devices, a switch send it to the device looking for it. Turn on your QOS for video streams, you are golden.