Single tether control ideas.. random thoughts

[87TurboIroc]

Hi everyone, I'm just kicking ideas around in my head for my first ROV. I tend to over-complicate things during this phase and will likely do something simpler for my first version.

I've been thinking of single tether options.. multimode fiber would be even better. It would require on-board batteries but the topside support equipment would be minimal. The downside to such a system would be controls. I'm worried about control lag involved with something like a utility on a laptop reading a joystick input, then sending that via ethernet to an Arduino. But, either way this is the system I am visualizing:

Laptop -> Fiber Media converter -> Multimode Fiber -> ROV

(there is also an option a laptop fiber card for $170 )

And on the ROV Side:

topside <- Fiber Media converter <- 4 port ethernet switch <- IP camera + Arduino ethernet

This would effectively set up a 100 Mbps network to the ROV. There is a 30 meter 50/125 startech cable available for under $30. This would be about as neutral bouyancy as you can get.

Control of the ROV would be through the laptop (joystick preferably) as well as display and recording of video. If you're willing to spend the money and could make on-board batteries work, your topside equipment would be a spool of fiber and a laptop + possibly a joystick. If precision control wasn't important, you could control through a web app on the Arduino. Use a wireless router and you could in-theory control and view with a couple of wireless devices. Something like an iPad for the video and your phone for the movement/lighting controls. The AVTech IVS 1.3 megapixel network camera is one that supports these devices.

There are alot of downsides to this though as well. While the bandwidth is good, the aditional devices take up space and power. IP cameras are expensive. You can't overlay sensor data over the video either.

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Now back to reality.. I had also looked at the idea of using USB to accomplish the same. Of course you'd have to use a Cat5 cable and have a pretty strict limit to cable length.. but you could put a USB hub downside and use a USB camera, and plug the Arduino usb port in as well. This setup would be quite a bit cheaper but the bandwidth is almost useless from my guestimation.

So then I arrive back to multiple tether. I can certainly see why it is most popular. Send video direct, with overlays easily. Run a DSC and an old FM receiver downstairs and you've got easy control over your ESC's. With all these cables, might as well send some power down as well.

Ok I've probably put way too much out there for most people to bother reading all of. For those that made it this far, thanks for letting me clear my mind for the evening!

[NCUWI]

Here are the two simplest / easiest / cheapest ways to do it with a thin tether in my opinion:

1. Coax - Send control via 2.4 Ghz RC transmitter/receiver and video via VHF. There's a thread on the yahoo groups with more detail on how to accomplish this. This is similar to how GNOM does it.

2. Cat5 - Send control via RS-485 over one twisted pair and video over one (or more) twisted pair using video baluns. Technically you'd only need 2 twisted pairs if all you want is video and control comms but Cat5 cable is cheap and readily available.

Just remember that a battery powered ROV will be heavier than a topside powered system. Be sure that your tether is strong enough to pull it back up from depth if the batteries die (and/or make sure your ROV is slightly positively buoyant).

[87TurboIroc]

Interesting information about sending 2.4 down that way. I just happen to have a spare 7 channel 2.4 rx laying around and I got a couple of transmitters for my (dust collecting) helicopters. So a few ESC's and bilge motors...... some pvc pipe... and...

RS-485 makes good sense. I'll have to look into this some more.

When you refer to the weight concerns, is that for topside handling or does the weight cause problems with operation? I've been assuming a bit of weight would make it more stable, albeit at the cost of high current requirements during motor operations. Light weight is certainly preferred once its on the boat though!

[NCUWI]

When you refer to the weight concerns, is that for topside handling or does the weight cause problems with operation? I've been assuming a bit of weight would make it more stable, albeit at the cost of high current requirements during motor operations. Light weight is certainly preferred once its on the boat though!

I was just suggesting that a 50 lbs (25 kg) ROV would be more difficult to haul up from 200 foot (70 m) depth than an 8 lbs (4 kg) ROV. Plus the load on the tether would be considerably more, so you'd want to make sure you have a sufficiently strong umbilical for the task.

[thegadgetguy]

Technically, if the ROV was neutrally buoyant, changing the weight shouldn't really affect how hard it would be to pull through the water, although the initial acceleration would be more difficult. The main disadvantage of having a heavy ROV is that it takes a long time for it to get going from a standstill, and also to stop. The weight shouldn't affect the top speed either, as long the weight is in side the ROV, so to speak (inside, as in not making the ROV less hydrodynamic). I've found that, while a smaller amount of weight can make your ROV's movements more fluid, having a heavy ROV obviously makes it harder to transport, as NCUWI mentioned, and is, overall, somewhat annoying. As for control, fiberoptics obviously give you more options, but coax/CAT-5 can't be beat for the price, and provide great functionality too. Well, I think that's all I could think of at the moment - hopefully some of it will be helpful to somebody out there

[derelicte]

I think for our purposes, cat 5 has more than enough bandwidth for our data. It is also much cheaper, easier to terminate and more durable.

However for a "coolness" factor, fiber can't be beat.