Pros and Cons of External Power?

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Kipper
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Joined: May 15th, 2019, 10:44 pm

Pros and Cons of External Power?

Post by Kipper »

I’m in the brainstorming/early planning stage of designing a freshwater ROV. I’ve noticed several people power their system with a car or golf cart battery. I like this idea because I’d like my ROV to stay underwater for long periods, and from what I’ve read on-board batteries tend to deplete faster.

I’ve also read that its complicated to send power down the tether. For those of you who have experience with these systems, how would you rate the difficulty of using an external power source? Would someone with basic electrical knowledge still have difficulty building this setup?
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Bennachie
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Re: Pros and Cons of External Power?

Post by Bennachie »

The main difficulty with sending power down a tether is to do with the desire for a long, thin tether. You want a tether long enough to be useful, so usually many tens of meters, plus you want it thin enough to be flexible.

First, let's look at length. The longer the tether, the bigger the voltage drop from one end to the other. This basically means you usually want to start with a high voltage than 12v DC.

Now thickness. A given gauge of wire will only be able to handle a certain level of current (amps) before it overheats and burns out. If you want to keep your tether sensibly thin and still have enough power to run a typical ROV, you need to use a high voltage, since P=I*V (power = current multiplied by voltage).

So both length and thickness of the tether drive us towards using a high voltage to run our ROV. There are two big issues with this. The first is safety. High voltages are DANGEROUS. Work class ROVs generally operate at around 3,000 volts AC. You only make a mistake ONCE when working with that. Now a hobby ROV isn't going to need 3,000 volts but it is going to need voltages which are potentially dangerous.

Secondly, it's not entirely straightforward to generate a high DC (or AC) voltage from a low DC voltage. If you were running from a battery on the surface, probably the simplest way would be to make a motor-generator, also known as a motor-alternator, or MA set. Basically, you use a DC voltage to run a motor which is mechanically connected to an alternator which is designed to generate a high voltage AC output. You then send this high voltage down the tether and convert it to a low voltage AC current using a transformer in the vehicle, then you rectify that to a DC voltage to run your ROV. AC is generally easier to use than DC here as transformers require AC to work.

Another even simpler variant would be to use an off-the-shelf petrol generator to generate your AC voltage. US generators produce 110v AC whilst European ones generate 220v AC. Both good enough to send power down a decent length of tether.

So basically you have three choices-

1. Use a battery in the ROV. Downside is your operational time is limited.

2. Use a surface battery with no voltage conversion. Downside is you are restricted to a short tether and a low power ROV.

3. Use a surface battery and convert to a higher voltage, or use a generator. Downsides are danger, and the technical difficulties of voltage conversion.
Kipper
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Joined: May 15th, 2019, 10:44 pm

Re: Pros and Cons of External Power?

Post by Kipper »

Bennachie wrote:The main difficulty with sending power down a tether is to do with the desire for a long, thin tether. You want a tether long enough to be useful, so usually many tens of meters, plus you want it thin enough to be flexible.

First, let's look at length. The longer the tether, the bigger the voltage drop from one end to the other. This basically means you usually want to start with a high voltage than 12v DC.

Now thickness. A given gauge of wire will only be able to handle a certain level of current (amps) before it overheats and burns out. If you want to keep your tether sensibly thin and still have enough power to run a typical ROV, you need to use a high voltage, since P=I*V (power = current multiplied by voltage).

So both length and thickness of the tether drive us towards using a high voltage to run our ROV. There are two big issues with this. The first is safety. High voltages are DANGEROUS. Work class ROVs generally operate at around 3,000 volts AC. You only make a mistake ONCE when working with that. Now a hobby ROV isn't going to need 3,000 volts but it is going to need voltages which are potentially dangerous.

Secondly, it's not entirely straightforward to generate a high DC (or AC) voltage from a low DC voltage. If you were running from a battery on the surface, probably the simplest way would be to make a motor-generator, also known as a motor-alternator, or MA set. Basically, you use a DC voltage to run a motor which is mechanically connected to an alternator which is designed to generate a high voltage AC output. You then send this high voltage down the tether and convert it to a low voltage AC current using a transformer in the vehicle, then you rectify that to a DC voltage to run your ROV. AC is generally easier to use than DC here as transformers require AC to work.

Another even simpler variant would be to use an off-the-shelf petrol generator to generate your AC voltage. US generators produce 110v AC whilst European ones generate 220v AC. Both good enough to send power down a decent length of tether.

So basically you have three choices-

1. Use a battery in the ROV. Downside is your operational time is limited.

2. Use a surface battery with no voltage conversion. Downside is you are restricted to a short tether and a low power ROV.

3. Use a surface battery and convert to a higher voltage, or use a generator. Downsides are danger, and the technical difficulties of voltage conversion.
Thanks for such a detailed explanation. This was really helpful.

Based on your post I might consider batteries more closely. I’m a little inexperienced with self-made electrical systems. I mainly want to make a simple starter ROV anyway, and I don’t want to overcomplicate things while I’m still learning. I’ll keep researching. Thanks for the help.
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Lucas
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Re: Pros and Cons of External Power?

Post by Lucas »

Bennachie hit the nail on the head in terms of the issues surrounding sending power over the tether.

I think you will find that the limits of current battery technology are not actually that limiting in practice for hobby purposes. I'm running a 4S 10Ah LiPo and getting around 2 hour runtimes (although I'm in pretty calm waters).
Oddmar
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Re: Pros and Cons of External Power?

Post by Oddmar »

As he said,
1. Use a battery in the ROV. Downside is your operational time is limited.


Limited, but not necessarily Short. More battery = longer deployment time. Spare battery pack topside = x2.

The problem i see with ROV's like the Trident, for example, is it's really just an expensive toy. It can't pick anything up...the only thing you can do is go down and look at things. And not very well at that...it is SO Light the tether is constantly pulling it left/right, or as you try to move the tether drag pulls it off heading so you have to constantly correct/fight in order to simply look at (whatever), especially if there is any current. People also seem to be having problems with it not working correctly, not charging, or firmware/update issues.

If you make an ROV larger/heavier the tether and environment have less effect on it. Bigger and more difficult to transport but steadier/smoother underwater. An easy way to make an ROV heavier is to add batteries, which has the added benefit of extending the runtime. And with an ROV that you have to build yourself, as long as you have enough 'drive' to see the project through to the end, at least you will have an intimate understanding of every part and can easily troubleshoot any problems that may arise.
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Bennachie
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Re: Pros and Cons of External Power?

Post by Bennachie »

Limited, but not necessarily Short. More battery = longer deployment time. Spare battery pack topside = x2.
Indeed. It seems that it doesn't seem too hard to get a couple of hours run time from an on-board battery powered ROV, which is plenty. Obviously heavy use of the thrusters will shorten the deployment time, so expect longer duration in lakes and ponds than in rivers or the open sea.
The problem i see with ROV's like the Trident, for example, is it's really just an expensive toy. It can't pick anything up...the only thing you can do is go down and look at things. And not very well at that...it is SO Light the tether is constantly pulling it left/right, or as you try to move the tether drag pulls it off heading so you have to constantly correct/fight in order to simply look at (whatever), especially if there is any current. People also seem to be having problems with it not working correctly, not charging, or firmware/update issues.
This is true. Off the shelf 'toy' ROVs are basically just that- expensive toys. You can't do much with them, you don't learn anything from them, and you'll probably get bored with it pretty quickly.
If you make an ROV larger/heavier the tether and environment have less effect on it. Bigger and more difficult to transport but steadier/smoother underwater. An easy way to make an ROV heavier is to add batteries, which has the added benefit of extending the runtime. And with an ROV that you have to build yourself, as long as you have enough 'drive' to see the project through to the end, at least you will have an intimate understanding of every part and can easily troubleshoot any problems that may arise.
Also true, however I'd say it's good to start small & simple. Don't bite off more than you can chew. If you're new to ROVs it is probably best to start with a small observation ROV with minimal bells & whistles. You'll learn a lot from that and then be able to apply it to bigger, more complex vehicles without risking investing a lot of money into a project that you may not complete.
pwoosam
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Joined: Jul 16th, 2019, 3:25 pm

Re: Pros and Cons of External Power?

Post by pwoosam »

Have there been any efforts with a hybrid type system? Like a small/medium sized battery on-board. And a generator or large battery topside. Then just charge the on-board battery through the tether.
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Bennachie
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Re: Pros and Cons of External Power?

Post by Bennachie »

pwoosam wrote:Have there been any efforts with a hybrid type system? Like a small/medium sized battery on-board. And a generator or large battery topside. Then just charge the on-board battery through the tether.
I don't really see the point in that. It would only make sense if the ROV were spending long periods of time idle in the water. Then it might make sense to trickle charge the battery down the tether.

Normally, if the ROV is idle you would simply recover it to the surface and swap/charge the batteries.
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Lucas
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Re: Pros and Cons of External Power?

Post by Lucas »

Agreed with Bennachie - barring a unique use-case, it sounds like an unnecessary complication to an already complicated vehicle.
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