Wire Selection for the Tether


#1

With the great job that Walt did on the electronics build for the tether my thoughts are now on the wire selection we should be using. A few questions that need clarification and your input on.

1. What gauge wire should we be using?

I was thinking 22 AWG, as we want to push power thru the tether to the ROV.

2. Should it be Twisted Pair or can we get away with Parallel.

3. Should it have a jacket for added strength or without and I guess this depends if we can get away with something as simple as speaker wire.

4. Can we come up with a brand and supplier that we can order from.

Regards

Dave


#2

Hey David:

This is a subject that I believe warrants a lively discussion, but in the end it's really going to come down to what your end application is, and testing, testing, testing......

I believe they're in the process of setting up a pool at OpenROV HQ, and that will be a great tool for testing not only tethers but all aspects of the vehicle performance. But as for tethers, we need to be aware that, while the HomePlug stuff is working great on OpenROV so far, I don't think anybody has tested it in the water yet. I think it's going to do just fine, since there are lots of anecdotes of other groups using HomePlug on one-off ROV designs, but there's no substitute for actual in-water testing.

There are a lot of personal opinions and ideas about what type of wire is best, but I don't feel there's enough real data to make any kind of definitive statement. One of the best things about HomePlug is that it adapts itself (through an auto-equalization process) to whatever kind of cable you're using, so we can swap out different kinds of tethers rapidly for testing purposes. In the field this is an ever bigger advantage. I know Eric is a big fan of inexpensive tethers, so that when a tether breaks or corrodes or whatever you can just change it out. But it also opens up the possibility of having multiple tethers, and picking a tether for any given mission that best suits that mission. So, to summarize my thoughts and answer some of your questions:

- The need for finding a workable connector scheme for the tether on the outside of the ROV should be bumped up in everybody's mental priority queue. I think that once we have a way for doing fast swapout of tethers, folks are going to be amazed at the capability that it brings to the ROV.

- I don't think twisted vs. untwisted makes any difference for HomePlug.

- In my mind, the outer jacket is just one more place to trap saltwater and add tether drag. If I'm going to add mass to my tether, it's going to be either in more copper, or, for exotic applications, a Kevlar strength member.

- Eric is a big fan of using the thinnest possible tether possible, to minimize water drag. One downside is tether tensile strength, so you've got to be real careful when you're driving around objects with a thin tether. I'm also concerned that the really thin tethers are going to end up being invisible in the water, so it will be tough to get yourself unstuck if you get caught somewhere. Note that once we get real HD video working properly, the tether visibility issue will be eased somewhat.

-I'm a big fan of sending power over the tether, since I'm looking at missions that need a longer run length than ~1 hr. This is going to require a thicker tether. For the OpenROV kits I'm getting in the next batch, I'm thinking 22AWG tether without an external jacket. Again, if we find a proper connector solution, then I'm not going to have to choose during construction.

- For power over tether, the amount of power you can squeeze over the tether goes up as the square of the voltage you apply to it. There is the temptation to apply as much voltage as possible, but you end up with safety issues with people working around the water. This is not a new problem, and there's a standard kicking around somewhere that defines how to safely send power through a tether where people are nearby. I haven't had the time to look this up, and it would be nice if somebody could find this and maybe get it placed on this web site as a reference document.

- To get an idea of how much power you can get over the tether, measure the resistance of one side (one conductor) of your tether. If we apply a voltage V, and the conductor resistance is R, then the maximum power you can send through is:

(V * V) / (8 * R)

Figuring out why this is so makes for an interesting Matlab simulation or scribbling on the back of an envelope. Enjoy.

-W


#3

Thanks Mark for your input and I agree that looking at connectors to make it easy to detach is a great idea. I have been looking at SeaCon connectors all be it a bit expensive one for the tether is something to look closer at. Maybe someone will come up with an inexpensive one that will work.

As for the wire I have spent a lot of time searching the web looking for wire and came across this roll that may do the trick if we had a number of people interested.

http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=230466838627&item=230466838627&lgeo=1&vectorid=229466

This is a close to 5000' of twisted pair Teflon coated cable for $995.00 aprox $62.00/300ft

I would like to have a bright orange color cable such as this one on eBay

http://www.ebay.ca/itm/Mil-Spec-Aircraft-Wire-26-19-E-M16828-4-2-conductor-1057-Mylar-teflon-1-L-/111018863032?pt=Motors_Aviation_Parts_Gear&hash=item19d93d69b8&vxp=mtr

but this has a Teflon cover which adds more weight.

If others are interested in the twisted pair cable mentioned above let me know and maybe we can do a group buy. But I also think more testing needs to be done before we jump on this as the correct solution.

Regards

Dave


#4

Hi David:

So one thing I forgot to put on my previous post is that tether buoyancy is very important. Some particular wire might be a great deal, but if it is too buoyant or not buoyant enough, the ROV won't drive well, and you'll be unhappy with the purchase. One of the thousands of things that need to be done once the pool is set up is to take representative samples of many different types of cable, and measure their buoyancy.

-W


#5

This brings up a bigger issue, too. How do we leverage the R&D power of all of us? Making sure we're able to share learnings or distribute testing of different strategies.

I'm thinking a google doc spreadsheet with a list of tether types/distributors and their performance/length/sea vs freshwater/voltage amounts.

Just making sure the knowledge and testing isn't too redundant.

What do you think?


#6

I think as OpenROV grows it's going to get harder to keep tabs on what everybody is doing, and not end up duplicating a lot of work. Some form of centralized spreadsheets where everybody can post what they're working on, or list ideas for changes, or what have you, would be good.

For instance, I have a list of about six or seven changes that I'd like to see made on the Cape design during the next respin. Unfortunately I don't really know how to even submit this information in an organized way. I know there's an openROV-hardware page on GitHub, but it doesn't really seem like it's used for bugreporting or improvement requests. Should I get myself a GitHub account and just start posting there? Or would it better to just have a centralized spreadsheet somewhere with change requests for the Cape?

I don't know what the right answer is, but it seems like something better (or maybe just clearer to the newcomer) is needed.

-W


#7

Dave & Walt I agree with the both of you a spreadsheet would be useful. One could have a sheet with tabs for Wire and the scope of tests performed. Another Tab for Connectors, Motors, Nozzels, Change Requests etc. keeping it neat and organized is yet another issue unless we have one focal point. Let me know if I can help.


#8

Ok, this seems like a definitive area of improvement. Perhaps we can try the google doc spreadsheet to get started.

Regarding changes you’d like to see on the cape, I think the best approach is to create issues on the /electronics repo on github. That’s where we’ve been keeping those so far…


#9

Here are some SeaCon Connector quotes I got today:

there is a 2 connector female one I wanted to use for the tether on the ROV and the mating Inline connector to plug into it, Also an 8 pin connector that I was wanting for each side of the ROV for Motors,Lights and Gripper. I know these may not work well for the OpenROV platform but gives you an understanding of the cost. but maybe the two pin connector could work for the tether.

http://seaconworldwide.com/products/electrical-wet-mate/micro-wet-con/

Two Pin Connector

1. Qt.1 MCBH2M-sst w/ 20 AWG Leads $72.00 each
2. Qt.1 MCIL2F w/20 AWG Lead $27.11 each
3. Qt.1 MCDLSM Female locking sleeve $ 8.05 each
4. Qt.1 Nut & Washer –sst $ 1.48 set

8 Pin Connector
5. Qt. 2 MCBH8F-sst 18 AWG Leads $125.85 each
6. Qt. 2 MCIL8-M w/18 AWG Leads $ 70.98 each
7. Qt. 2 MCDLSM Female locking sleeve $ 8.05 each
8. Qt. 2 Nut and washer in S/S $ 1.48 set

9. Qt. 1 MCDC2F Female Dummy Connector $ 11.40 each

10. Qt. 1 MCDC2M Male Dummy Connector $ 13.97 each

11. Qt. 2 MCDC8F Female Dummy Connector $ 11.40 each

12. Qt. 2 MCDC8M Male Dummy Connector $ 24.42 each

Notes:

  1. Prices are Firm US Funds FOB San Diego CA.
  2. We will require your preferred shipper and account number.
  3. HST is not included in prices.
  4. Delivery is 3-4 weeks due to the request for #20 AWG pigtail in Items 1 and 2.

Alternatively these are stock items subject to prior sale.


#10

IMCA Code of PRactice for The Safe Use of Electricity Under Water


#11

the MCBH connectors seem to be stainless steel version(Note the -ss on the end of the number), you could probably save some money on the brass versions.


#12

FYI Only: Here is another Quote I got on Teledyne Impulse Connectors these are in Canadian Currancy:

http://teledyneimpulse.com/products/wetplug.aspx

LPMBH-2-MP $69.00 Low Profile, Mini Bulkhead, 2 Pin, Male
LPMBH-2-MP $3.00 Nut & Waser
LPMDC-2-FS $25.00 Low Profile, Dummy Connector, 2 Pin, Female
LPMIL-2-FS $70.00 Low profile mini inline, 2 pin, female

LPBH-9-FS $180.00 Low Profile, Bulkhead, 9 Pin, Female, Brass
LPBH-9-FS $3.00 Nut and Washer
LPIL-9-MP $110.00 Low Profile, Inline, 9 Pin, male

MCBH(WB)-2-MP SS $117.00 Micro Mini Bulkhead, Waterblocked, 2 Pin, Male,
MCBH(WB)-2-MP-SS $3.00 Nut and Washer Stainless Steel
MCIL-2-FS $68.00 Micro Mini Inline, 2 Pin, Female, w/ 2 ft pigtail
MCDLS/F $20.00 Micro Delring Locking Sleeve, Female



#13

FYI I just found this ad on eBay for 22AWG twisted pair 19 strands teflon coated MIL Spec wire no shield or cover.

http://www.ebay.com/itm/230870691431?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

$59.00 with free shipping great price!

It is yellow, so it's nice and brite under the water and there is 500' per roll. They show as having 10 rolls they say in part of the ad that they do not sell to anyone outside the US even if they have a US delivery Address. I ordered one from Canada and asked to have it shipped to a US Destination so I hope they except the order, I should know shortly. If they do not except my order maybe I can arrange with one of you to buy it for me and have it shipped to my US Address and I will do an electronic transfer of funds to you. please let me know if you can help out if needed.


#14

http://www.ebay.com/itm/121068970687?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

This is what I'm using on mine (x3 rolls). They are also available through Jameco's website. I'm sure they would ship outside of the US if the above doesn't work out. Basically the same price per foot, as the Jameco wire is $10 shipping.


#15

Thanks Philip for your feedback, Looks like the item I ordered will ship as it probably is not classified as a Military.


#16

We've ordered a number of samples from Teledyne and are going to be testing them all. I think Colin put together a spreadsheet with a list of tests and options, that way we're not doubling up on experiments and testing (I'll ask him today).

Will be publishing test results as soon as they happen.

We'll also be buying the best option in bulk for the kits. We should be able to buy more for individual sale, too.



#17

Also- for the time being, you can buy Ethernet cable and extract the twisted pairs (usually 26 or 24AWG) from that!


#18

Hi I was just reading your comment and thought I could suggest creating Dropbox account and sharing a folder to document R&D? You could upload a spreadsheet with a revision box and share it that way. And Dropbox is free.

https://www.dropbox.com/


#19

looking at this "safe use of underwater electricity" manual Brian found, thanks Brian, I am just looking at the tables and not reading it in depth or anything. It lists 24V as a safe voltage for a 40mA current reaching the human body. The currents going to the motors of the rov are obviously higher, but I think we can assume that will drop drastically at short distances from the rov? And if a diver was between the cathode and the anode then thats where problems could develop but thats not the case at all for us either in the sub or in sending power down a tether. In one of the appendices (A2.13) they cover a fairly drastic 2000A DC situation where a diver just needs to be 1.5m away from the cathode for a 40mA charge. I am not going to calculate this now but I am reading we should be pretty safe with this. Right?


#20

Yeah- I'm still trying to understand this fully myself. For the motors, we'll be running at a lower voltage (like between 8 and 12v nominally) which I think is pretty darn safe. The power going down the tether (if we end up doing that) is where we may have issues. I've read elsewhere that 40v is the limit of what is considered to be a "safe" DC voltage, but I'm not sure what the associated current with that is. It seems that if the ROV consumed 40W nominally, then at 40v, the tether would be transferring something like 1A of current. ... then again, there's a voltage drop across the tether so I'm not sure how that effects things either.

E