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Signals/tether cable recommendation


(Richard) #1

I need suggestions for what to use for signal transmission wire between the ROV and the surface. My design needs 6 signal wires (no power wires). Ideally it should be strong enough to act as a tether also. What do you use?


(Rusty) #2

Hi Richard,

Meeting all of these requirements really requires a specially designed tether cable. As I mentioned in my other post, we are working hard on tether solutions here and we should have something available in about 3 months.

In the meantime, my best suggestions is Cat5 networking cables. Cat5 cables are cheap and have 4 twisted pairs, enough for your application. You can find cables rated for extreme environments and submersion. Here’s an example: http://www.discount-low-voltage.com/Cable/Cat5E-UTP-Ethernet-Low-Voltage-Cable/QU-5710

Best,

Rusty


(Richard) #3

Thanks, Rusty. I look forward to your tether solutions. Are you familiar with an “anchor ball” setup for anchor retrieval? My ideas is to run the tether cable from the boat to the ring on the anchor ball and then down to the ROV. The anchor ball should move to stay roughly above the ROV and reduce the chance of entaglement below. It should also serve to give a rough idea of the ROV’s location.


(Rusty) #4

Hi Richard,

That’s a great idea. I assume you plan to operate mostly right under the anchor ball? If you drag it around much it could be a lot of drag to deal with.

-Rusty


(Richard) #5

Initial project is 2-axis (turn left/right) only, 2 lbs negatively buoyant, tethered. We will be working within 20 yards of the mother ship in 50 to 100 feet of water over artificial reefs off Jacksonville, Florida. Depth and minor location changes will be by pulling or releasing the tether, larger location changes by moving the boat using a trolling motor.


(Richard) #6

This looks good:

 


(Rusty) #7

Richard,

That one looks pretty good. Just keep in mind that it has solid conductors instead of stranded so it won’t be very flexible. That might not be a big deal for your application.

-Rusty


(Paul) #8

“Direct burial” Cat5e cable is intended to be buried under ground. It is not only stiff, but if flexed the solid strands may break. What you want is “Tactical” Cat5e like Rusty linked to. It is stranded, shielded and designed to be flexible / tough.


(Richard) #9

Wow, thanks for the heads up.


(Harold Scadden) #10

@Richard There are companies that sell tether cables for ROV use that has Kevlar fibers in the cable jacket. You use the Kevlar in a special termination to secure the cable to your unit. The cable itself can then be the tether / tow cable while allowing you various options for conductors. Some of the other cables use counter rotating steel armor wire either exposed on covered with thermal plastic urethane.

Here are some cable companies that I deal with and please be ready for sticker shock, but I know their stuff will do the job.

http://www.te.com/usa-en/products/families/rochester-cable.html

http://www.falmat.com/Products These guys have a catalog online … click on ROV tether cable.

http://www.southbaycable.com/ I buy stuff from these guys all the time … we have had their tow cables terminated and pulled to 22,500 lbs before break. Not bad considering it only had to hold to 18,000 lbs.

 

 


(Harold Scadden) #11

@Richard - time delay and my brain turned on. Question: What depth do you plan on going do to and how much weight must the tether be able to handle? Remember, if your unit is out in the water and there is a strong current pulling on it then that is going to act as a multiplier on the weight that your “tether” is going to feel.

Also, how do you wish to terminate the tether? Do you want a cable that is sort of coiled down a Kevlar rope style setup or are you looking for a cable that can be the towing member in addition to conducting your signals?

 


(Richard) #12

Thanks for the input Harold. I have to keep this a low-dollar venture, so some ingenuity is coming into play. I have several things working for me. I have some experience dropping GoPro cameras on a thether down to 100 ft on our artificial reefs. The buoyancy on my camera setup (3 cameras covering 360 degrees) is negative 2 lbs. That seems to be enough to handle most currents we encounter around these reefs. I use 65# PowerPro braided fishing line as a tether. Backing me up in case I lose a rig are diver friends who frequently dive these reefs and will gladly recover any lost equipment. I have already had to have them recover my camera once this year. Our reefs are ten to 30 miles offshore jacksonville. If you go to my YouTube channel (Richard Fast) you can see some of my work. I too am a nuclear submariner and I have to really watch getting too involved in thinking I want to build a miniture nuc. Blue Robotics thrusters are a major breakthrough in bringing ocean environment ROVs into the hands of amateurs with limited funds. I hope to build my 2-dimension (2 thruster) ROV for under $700. My controlling processor will be an inexpensive Parallax Basic Stamp 2. My CCTV setup will be a automobile backup camera system purchased off eBay. This is not a commercial grade project, just something to have fun with and keep me out of trouble.


(Paul) #13

I do not recommend using the waterproof connectors to take up the load from your tether. What is typically done is to rig some sort of strain relief using a braided wire or plastic cover. Lots of clever ways of accomplishing this, here’s one idea:



(Harold Scadden) #14

@Richard - Yeah another Bubble Head! I wish I had the money … I would go buy the NR-1 and use it as the Uber ROV.


(Richard) #15

My Cat5E stranded cable arrived and I tested it for breaking strength. I tested to 60 pounds tensile force around a 1- 1/2" bend radius with no signs of failure. I also bounced the weight up and down fairly rigorously to increase the loading and it held find. It’s a go for me to use as a tether as well as a signal carrier. My cost was $50 for 200 feet from cables4sure.com.


(Rusty) #16

Richard,

That’s pretty impressive. I’m really curious to know if there was any internal damage to the cable. Once you’ve got video and such through the tether it will be interested to see if quality is degraded when you pull on it.

-Rusty


(Harold Scadden) #17

@Richard - How did you terminate the cable to do the tensile strength test? Did you just secure the cable by the jacket or did you have the conductors anchored for the weight test? Depending on how you did it will make a big difference in how the cable will perform.

Also, did you do any resistance checks on the conductors before and after so you could compare the data to see if you had any strands breaking?

 


(Richard) #18

As soon as my CCTV system arrives (couple of days) I will run tests on all strands, under stress, using the video signal through the full length of cable. If I see no obvious degradation in quality I should be good to go. Other than the TV signal I only use the other strands for transmitting resistance changes from the piloting box to the microcontroller board. I measure control lever position using an RC time constant measurement.


(Harold Scadden) #19

@Richard - How big a resistance change are you looking at to have a valid RC time to measure control lever position? Normally you use a precision fixed resistor to calculate wire “line” capacitance by measuring the charge time etc. are you doing this in reverse? So you have a precision fixed capacitor and you are calculating the resistance change by the T1 charge time etc.? Inquiring minds want to know :slight_smile:

 


(Richard) #20

Here’s how it works: a variable resistor, in my case 10K ohm, is wired to ground and one lead of the tether in the topside control box. On the ROV end this lead and ground are in parallel with a .1 microfarad capacitor. I just measure the time constant and scale it to give the range of pulsewidth I need for full control of the thruster. It’s a no-brainer system and works perfectly. No precision is required. This is a proven system I used for a freshwater ROV several years back. FYI, 200 feet of 24 AWG wire has a resistance of about 4 ohms.

Keep the questions coming.