Alternative wire robot-surface proposal


#1

Last week i meet @Roy_Petter_Dyrdahl_T in a maker space in Madrid, Spain. That was the first time i know about OpenROV project, a great one!

After examine the robot structure, the board and all the staff i think in a alternative way to connect the robot with the control laptop in the surface, i find so fragile the actual cable and needlessly complex the PLC conversion system.

My proposal is use a FTP CAT5e outdoor cable with a messenger wire. The steel messenger wire can be locked to the robot with a carabiner and the FTP wire can have a RJ45 IP68 waterproof connector. With this solution, the battery’s can be removed from the robot using POE standard for supply trough the FTP cable. I just thinking about this idea for the last days and i have this pros and cons.

Pros:

  • Better wire quality, in fault case, we can tow away the robot with out problems
  • Batteries can be removed from the robot, and we can use other kind of battery’s or a external supply
  • If use the RJ45 connector, the robot and the cable are different thins an that is better for maintenance and transportation.

Cons:

  • Alternative thing to replace the weight of the batteries is needed in the robot if this is necessary
  • The RJ45 connector can give some problems, also the cable can be connected directly to the control unit
  • The actual control unit need some changes, the new cable must go in first step to a supply stage and then to the BeagleBoneBoard, the fast and cheap solution is use a POE splitter

Some URL’s:
A sample of FTP CAT5e outdoor cable with messenger wire
https://www.shireeninc.com/osc/outdoor-cat5e-ftp-shielded-1ft-734.html

A sample of RJ45 IP68 connector

More information about POE

POE splitter sample
http://nodalis.net/open-mesh/accesorios-nodos-wifi-mesh/17-inyector-poe-para-nodo-mr3201a.html

Let me know about you opinion of this idea
Thanks!


#2

If you haven’t already done so, I recommend picking up a copy of The ROV Manual. I have been using it quite extensively for my larger observation class OpenROV.

With that, here are some other points to consider for your proposal:

  1. Tether drag and weight. Although stronger, that CAT5e cable you mentioned has a much greater diameter and will have considerable drag in any sort of current. You want the tether to be as thin as possible, while still having enough strength to be reeled in. I have reeled in my OpenROV with no problems with the stock wire. Also, you will have to make your thicker cable neutrally buoyant, otherwise it is going to make your ROV negatively buoyant with the more tether you let out.

  2. Power over the tether. You are going to lose A LOT of power over the tether itself. @Walt_Holm is a better expert on power loss for tethers, but this is why batteries work well for observation class ROVs. Even larger OC ROVs such as the Hydroacoustics ROV still use batteries. Frankly batteries are safer and less complex than a topside power source.

  3. The RJ45 IP68 connector. From looking at it and the specs, I would not recommend this connector if you intend to go deeper than 1m for any prolonged length of time. There are no pressure ratings on the spec sheet, so I would be wary of using it at any sort of depth.


#3

Sorry Kevin but this simply isn’t true. First off, if current exists everything in the water will be equally effected. Your ROV will be effected just as much as the tether. Heck, even air bubbles and brine shrimp will be effected. Where tether diameter will have an effect is when you try to tow that tether with the ROV. The more drag your tether has the more power the thrusters will need to generate to over come that drag. As for a negatively buoyant tether having an effect on the buoyancy of the ROV, that would depend upon the size of the rov and just how heavy that tether is. If you find that its an issue, you can easily fix it by adding a small float a on the tether a short distance from the ROV. That should counter what little effect the tether may have.

As for tether strength vs tether diameter, there’s a reason why every commercial ROV in the world has a tether with a breaking strength of 1000+ pounds. Do I need to say more? :wink:


#4

You’re both right??

Yes if you’re drifting if you are trying to maintain station or drive into the current you are battling the current with your thrusters and the amount of drag comes into play

OK simplified and idealised in the depths we are basically talking about (less than 100m some weird stuff can happen in significantly deeper water) essentiality there is a current velocity gradient due to water movement (current) with mathematically a zero condition just above the bottom (think 0.00001mm above the bottom) to the max strength at the surface

Consider the existing OpenROV with its 2 pair twisted wire 1.3mm in dia (say for the twisted pair the effective cross section exposed to the current or dragged through the water of say 3mm) and at 60m depth (say 100m tether out) that gives a “face” of 0.3m2 whist the ROV “face” is (0.25x0.18) 0.045m2 hence the tether has 6.7 times the apparent “face” compared with the ROV with a total face of 0.345 m2

Whist near the bottom, the face of the ROV (and 13% of the total area expose to current) is subject to less force (due to the boundary layer) than the tether with the largest force that the unit has to overcome being the drag on the tether

Given the wire indicated in the link has a cross sectional diameter of 6.5mm in the same conditions this has a 0.65 m2 giving more than twice the exposed face and “likely” to be greater than twice the drag


#5

My thoughts on this subject is that the basic OpenROV will continue to use battery power for a long time, for the reasons mentioned above:

  • Reduced technical complexity
  • Reduced cost
  • Reduced physical footprint (transportation, etc.)

However, there are many applications where power over the tether makes sense, i.e. the commercial applications that we use on our vessel. I would love to learn more about power over tether, specially how we can drive price and complexity down. I suggest we keep this thread for any information we can discover on the subject.