ROV Nudibranch 11 metre deep sea water test.

[PhilA]

This was an 11m deep sea water test. The issues I encountered were that as the ROV went deeper, the camera feed to the surface got progressively worse and operating the motors interfered with the feed. The control of motors was also sketchy at 11m. Recording to SD card on the onboard dash cam was fine and uninterrupted. The ROV runs fine in shallow water and on surface.

Any ideas?

https://youtu.be/6ZwGyDn9udw

[Oddmar]

Um...weird.

Not remembering your ROV design, i went and read the thread. viewtopic.php?f=18&t=1959

You have onboard batteries so it can't be voltage slump. RS485 comms and control are better than what most people have. Even filtered power. You are sending video/control through the entire length of the tether whether it's on the surface or at depth, so it can't be that. Water blocks RF/EMF so well i can't imagine any outside electronic interference.

I can only think of two possibilities.

1. Has this happened elsewhere? Could the rocks be highly magnetic? Interfering with video/coms as you get closer to the bottom? Farfetched i know.

2. Water could be seeping into your tether, shorting the wires (a little), more and more as the tether is unspooled and drug down to higher pressures. Water drains out as you reel it in and video/comms get better. To test for this i would try reading the resistance on the unused two pairs and see if it changes as you lower the rov. Or switch which two pairs you use and test the current pair.

I would use video baluns on each end of your video wire pair to ensure clean video over distance, if you don't already.

It amazes me to see the seaweed moving back and forth at nearly 35' down, i wouldn't think surface waves would have such an affect at that depth. But i'm a landlubber. I envy you your clear waters. The rivers and lakes local to my current temporary residence are all silted up from farm field runoff.

Man, that is a perplexing issue. It's gonna be bugging me the rest of the day. Thanks alot. LOL

[PhilA]

Thanks for your ideas, I will definitely add video baluns. My thru hull connections are a bit of a dogs breakfast and I think I need to tidy them up and ensure water is not interfering with signal, I'll try measuring the resistance too.

I'm also wondering if the motors draw more power the deeper it goes and this could be the issue.

I can't wait to get this fixed, there is so much better stuff to see than the video I took yesterday.

[Oddmar]

I'm also wondering if the motors draw more power the deeper it goes and this could be the issue.

Water being a non-compressible liquid, i don't think it would be any harder to push it out of the way at depth than it is to do so 1m deep.

The weight of the water column increases the pressure pushing in on the ROV, but the mass of the water stays the same. Therefore moving X mass out of the way shouldn't draw any extra power.

[Bennachie]

I'm thinking possibly a temperature problem.
Does the video feed go back to normal once you've surfaced?

Another possibility is that at depth, the video conductors and control signal conductors are getting squeezed together in your tether, leading to a bit of crosstalk.

To test if it's tether compression, send a loop of tether to depth on a weight, keeping the ROV on the surface and see if the video is affected or not.

[PhilA]

I made a new end cap over the last week and instead of using solid copper wire set in epoxy for my through hull connections I decided to try the alternative method of using insulated wire with the insulation stripped back in a section which is set in epoxy. I tested it this evening down to 50m and worked a treat. At this depth I get around 10-15 drops getting past the o-ring seals over a 10 min time frame so I should be good for 30-40mins at this depth before the leak detector is triggered. Here are some pics of the new end cap.

I really hope this fixes the camera and control issues I had. Hopefully I can take it for another test in the next few days.

[bigbadbob]

I'm not sure what thruster seals you have, if any, but I have read on here that folk using bilge pump motors as thrusters
suffer from the seals tightening on the shaft at depth. This drives the motor current up and pulls the battery voltage down
due to the internal resistance of the battery.

[PhilA]

So my thrusters are brushless and not sealed at all. The problem I had was with my thru hull connections and bad soldering to the solid rod. Once I fixed that it all ran fine and control is good at depth, I still haven't fitted the video baluns as the live feed hasn't been much of an issue. Batteries are charged ready to go, now waiting for weather to improve.

[bigbadbob]

That's good you found it.
You and me both waiting for better weather Phil, winter storms here.