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The Manta Drive: proof-of-concept for an ROV propulsion system.

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Picture of The Manta Drive: proof-of-concept for an ROV propulsion system.
Every submersible vehicle has weaknesses. Everything that pierces the hull (door, cable) is a potential leak, and if something must both pierce the hull and move at the same time, the potential for leakage is multiplied.

This Instructable outlines a drive system that eliminates the need for drive-shafts to pierce the hull of an ROV ("Remotely Operated Vehicle" - a robot submarine controlled via wire), and also removes the very real possibility of rotating impellers getting tangled or jammed by underwater plants or hanging lines.

It could also give rise to vehicles that have a much less damaging effect on the habitats they are used to investigate, because of the lack of a "wash", and because the lack of rotating impellers will reduce the risk of injuring animals the Manta Drive encounters.
 
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Step 1: The concept.

Picture of The concept.
The whole idea of the Manta Drive was inspired by a visit to an aquarium where members of the public got a chance to pilot small ROVs round an obstacle course. I got my first look at the ROVs and realised two things:

  • There were a lot of places for the water to get to the insides of the ROVs
  • The ROVs didn't look right - they were just boxes, and didn't look designed to swim. They lacked the elegance I associate with swimming animals.

Later cogitation also came to consider power - the high-revolution impellers used by the ROVs struck me as power-hungry. I may be wrong, and I have not tested the power consumption of the Manta Drive, but this is a secondary consideration.

As I wandered the aquarium, the ROVs played on my mind, and I found myself comparing them to each animal I saw. How did they compare? Could the animal's swimming motion be replicated elegantly, in a way that maintained hull integrity*?

Looking at fish like rays, sea cucumbers and stonefish, I realised that the most elegant propulsion method was the waving fin.



I also realised something important - fish don't leak. A rotating shaft needs to pierce the hull completely, working through a hole in the hull. On the other hand, a reciprocating motion (up-and-down) could work through a flexible, waterproof membrane which did could be fixed firmly around any moving parts without ripping.

I further realised that flexible membranes could wear out, but magnets don't, and magnets can act through any non-magnetic materials without restriction. Make the hull rigid, but non-magnetic, and the risk of leaks due to the drive system are completely eliminated.

* Oh, I went all Star Trek for a second there!

Step 2: Materials and tools

Picture of Materials and tools
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All I actually bought for this project were the magnets - small, countersunk neodymium magnets from ebay.

The rest was made of material I already had stocked in my shed - scrap timber, bamboo skewers and a pair of dead ball-point pens.

Similarly, no specialist tools were required - a junior hacksaw with blades for wood and metal, a hot-glue gun, drill and my multi-tool.

Health and Safety

You'll be using hot things, sharp things and very whizzy things. Be careful.
Take particular care with neodymium magnets - they can nip painfully, and will shatter if allowed to fly together.

Step 3: The frames

Picture of The frames
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I cut two empty ball-point pens into five roughly-equal lengths each - three to take the manta's ribs, two to space them out.

The frame itself is made of three lengths cut from scrap timber - the base is about 10cm long, the end sections are about 3cm long and drilled near the top, using a twist-bit the same diameter as the bamboo skewers.

I hot-glued the timber together, then threaded bamboo through the holes and the pieces of pen.

Step 4: The Ribs

Picture of The Ribs
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The actual propulsion of the Manta Drive is carried by simple ribs. These are coupled to the drive mechanism by the magnets.

Easy.

I threaded bamboo skewers into the holes of the magnets and hot-glued them in place, then glued the bamboo to three of the pieces of pen on the frame.

Step 5: The actual drive

Picture of The actual drive
The ribs are connected, via magnetic forces, to the drive mechanism.

In a finished ROV, the internal magnets would probably be moved by motors or servos. In this model, I just used more levers, shortened versions of the ribs.

Step 6: Connection and drive

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The drive isn't intended for the magnets to be in direct contact, and it defeats the object anyway.

In the final ROV, there will be a non-magnetic hull between the ribs and the drive. Non-magnetic air does the same thing, so all I needed was a set of spacers to hold the two sets of magnets apart. More scrap timber (6cm long, if you're interested), with pieces of bamboo to stop it slipping to one side.

Step 7: Working the model

Picture of Working the model
Operation is, in principle, very simple: when the levers move inside the ROV, the spines move on the outside.

The trick is to move the ribs in a useful sequence.

In this video, I made a simple "bracket" from more bamboo, slipped it over the drive levers and used it to move the levers in a basic wave sequence.

In the final ROV, the levers would be moved simply by a cam-shaft driven by a single motor. For more control, allowing "waves" of different length and frequency, each lever could be moved individually by a micro-processor-controlled servo-motor.



Step 8: Future steps

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Obviously, the model as presented in step 7 will drive nothing.

A finished ROV will have a row of ribs down each side of the hull, significantly more ribs than three. Between the ribs, the ROV will have either a single membrane, so that ripples in the membrane will provide the propulsive force.

Reversing the direction of the wave reverses thrust.

I intend this Instructable to be freely available for others to use to build their own ROVs far more cheaply than the professional devices currently available. Using the magnetic-coupled drive, the hull could be easy to source, and easy to make water-tight.

I imagine that it would work nicely with a length of large-diameter plastic sewer pipe as the hull. Matching compression-fittings can easily close off the ends of the pipe. Modifications to allow a camera to see out, or a control cable to pass in can be made watertight very easily, because they won't need to allow motion.

For actual use, ROVs powered by the Manta Drive will, I expect, be mainly hobby vehicles, used to explore the mysteries of the local swimming pool or canal. However, I would hope that the drive could be taken up by "serious" researchers, as it could be used to make ROVs more stealthy - with a hull suitably shaped and coloured, a Manta Drive ROV could be disguised as a large stonefish, or even an actual Manta ray. This would allow them to interact with living fish more naturally, in a similar way to the BBC's Roboshark or Draper Laboratory's Robot Tuna, but with fewer technological hurdles to leap (and much more cheaply!)

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Can you put in a different YouTube video? This one is no longer available.

Great Instructable though :)
Kiteman (author)  Michael_Bell3 years ago
The video should be fine - it's working as of this message's time-stamp, and there was no reason for it to be down.
I mean the first embedded video on step1, according to YouTube, the user closed their account.
Kiteman (author)  Michael_Bell3 years ago
Woa, I didn't notice that, thanks.
Hard to see it is
Kiteman (author)  Kiteman3 years ago
Fixed.
KKHausman1 year ago

Anechoic rubber-clad foam forms a decent "skin" for this type of underwater propulsion. With a long enough transit between each of what you termed "ribs" you need only limited flexibility in line with the ribs, rather than a fully elastic membrane as a whole. You can also drive this with electromagnets above/below the permanent magnet at the end of each rib to eliminate moving parts in the body of the vessel and allow more asymmetric variations in movement between each side and in different operational modes. I tried a similar design and then used Nitinol SMA wire on either side to draw each spar upwards and downwards in sequence, reducing the external EM signature of the ROV.

wayne18811 year ago
One thing though,If the magnets are too strong, they could interfere with the motors.
AMAZING design though.
jhall302 years ago
I love this idea! I think, however, that only the front rib needs to be driven, and the others can be free-moving. Perhaps the rear rib (and the front) can be engaged or released, allowing reversal of thrust. Also, the magnetic force will decrease as the ribs reach e tops and bottoms of their paths. Perhaps a straight sided vessel with sliding magnets would reduce this problem (a similar mechanism on the outside would reduce it further).

I might try building something like this. A vertical fin in the rear (and perhaps front) could be driven the same way and make steering easy.
Kiteman (author)  jhall302 years ago
Cool - post an Instructable if you do.
Did you think about the resistance of water? Magnet system could be unefficent under water. But if you use 2 crank shafts -one for both sides- you can easily control your ROV's movements ,for example turning, just by reversing one of the motors or making it move faster than the other one. Great instructable though!
Kiteman (author)  electrophobia2 years ago
Water is non-magnetic, and so does not affect the efficiency of the system in that way.

If you look at step 8, at the bottom-right part of my sketches, you'll see I already thought of the two-crankshaft idea.

Thanks, by the way.
I was talking about the resistance force of the water. I mean, you are testing the system in air and there is much less resistance in air than it is in water. Are your magnets powerful enough to beat that force?
Kiteman (author)  electrophobia2 years ago
Oh...

As far as I know, it should be OK as long as things go steadily - too quick, and the magnets could get jolted out of synch.
This is such a good idea, if I understand it correctly you'd have 3 different part propelling the submersible forward and because of the design they'd be each helping to move the other parts within. And because it's using magnets and not gears in would be easier to stop one side physically to turn and such if it where being used as a manned submersible.
macrumpton3 years ago
I love this project, although to simplify the drive mechanism I think having the crankshaft outside the hull would be easier to build.
Kiteman (author)  macrumpton3 years ago
Putting it outside would need something to pierce the hull, which defeats the object.
what if you put a circular plate with magnets attached on the end of the exterior cam shaft .. then instead of having to run a camshaft inside the rov taking up lots of space you could have a much smaller motor with a matching magnetic wheel inside the rov to propel the exterior camshaft

if that doesnt make sense to you .. let me know .. i will try to rephrase it
Asa J4 years ago
Biomimicry, awesome. This design is closer to a cuttlefish propulsion. i read about a fish that has similar fins on top and bottom and can move in any direction without changing orientation. I.E. vertically while remaining horizontal. I think that movement was achieved by ripples originating at the extremes of the fins moving towards the center causing a vortex above the fish that sucked it upwards. Don't quote me on that but it was something like that. I wish I could remember what kind of fish it was, it would be really useful info for you I imagine.
Kiteman (author)  Asa J4 years ago
Boxfish?

Sunfish?
senchele4 years ago
I thought of doing this same thing several years ago! Way to go!! I didn't think about using magnets. I figured using a long cam in the middle would reduce weight and give the same effect. I would dream of using it for submersibles, as well as air craft. Carbon filaments epoxied to thinfilm. I also thought it would work the other way around, and place the membrane inside a tube and jet the water/air through it. I never got to try it, but it is nice to see someone else working on it!
Kiteman (author)  senchele4 years ago
Thanks.
was thinking of using this kind of propulsion for a rc air ship someone has technically already done this but mine will be steampunkish here is link; http://www.youtube.com/watch?v=UxPzodKQays
Wish I had seen this instructable a long time ago.  A couple years ago I made a proof-of-concept for a magnetically coupled fish tail with no mechanical coupling between the tail and the drive mechanism.  The fish tail shown is on a metal swivel, but the next thing would be to eliminate the swivel and use something flexible, probably just the same material as the tail.  The idea was to make a propulsion system that would be very quiet, i.e., stealthy.  I used a servo motor rigged to spin constantly with neomagnets mounted on top, and with neomagnets also mounted on the tail.  Next I wanted to try electromagnetic coils so that there would be no noise from a motor. 

http://www.youtube.com/watch?v=NohFbnIj7GE 

Kiteman (author)  el_roboto_loco5 years ago
That's nice - does it actually move?
Yeah, it actually moves.  Works just like a fish.  Of course it needs a lot of refinement.
k0ldburn5 years ago
I just had a thought about this. What if the wings weren't made watertight and adjusted to changes in pressure naturally, but then a center hull had a master ballast system like a normal ROV that could push the craft close enough to the surface to be retrieved? The cavities of the wing should be able to equalize to the pressure outside the craft quickly enough to adapt and not be crushed, right?

Also, I remember hearing about Speedo making a new nearly frictionless bodysuit that sparked some controversy for helping as many people break records as it did. It's just a thought, I'm in no position to spend $257+ on a pair of shorts to destroy for an ROV concept. If it isn't usable, what about normal wetsuit material?

I like this idea and I'll definitely try to push it further as soon as I have time and materials. 
ultimachris5 years ago
wow, very impressive... i will definitely be taking this concept further.


How do you think turning will be handled?- you can't turn with both wings doing the same thing.


I'll keep you posted
Kiteman (author)  ultimachris5 years ago
I thought of running each wing separately - look at the bottom-right of the sketch in step 8.

I was just proving an idea, though - others (you) will be able to take it much further.

(Don't forget to post an Instructable.)
eyerobot5 years ago
What about using some form of latex as the shell of this vehicle?
wow talk about the next davinci
sh_joe5 years ago
hardlec6 years ago
I hope this is not too late: It would be interesting to see how well this will work. The idea of a "swimming" ROV or MPV is quite interesting. Magnetohydrodynamics has been disappointing. Propellers have hit their development limit. Have you considered making your "joints" linear induction motors?
Jaycub6 years ago
This could be simplified even more by just using one rib, and having the rest of the wing trail it and pivot on it freely within the range allowed by a "pizza with slice missing" shaped blocker.
setok706 years ago
can you use gear system like in the ornithopter to produce the up and down motion for the propulsion. I been working on my own idea in the same general direction I stuck with the up and down motion with the idea that I have . I like the design and good ideal. WELL DONE
crapflinger7 years ago
you could do this without the possible drawbacks(and possible fin loss) of the magnet concept by just sheathing the entire ROV in vinyl or latex or something...so build mechanically attached ribs that could be independantly controlled or cam controlled or whatever controlled then just cover the whole thing with a flexible skin...it would probably be easier to accomplish turning without having to muck with the magnet parts...
Kiteman (author)  crapflinger7 years ago
The trouble with flexible skins is that pressure deforms them, and the deeper the ROV goes the greater the pressure, and the greater the amount of deformation on the skin. This would eventually split the skin or jam it in the moving parts.
What if you had everything inside working in liquid (maybe that 'dry water' used in offices to extinguish flames without water damage) as well? Fish don't deform because they have (uncompressable) liquid inside - maybe that's the key?
mineral oil
Kiteman (author)  mwwdesign7 years ago
If you fill it with something besides air, you're going to run into buoyancy issues. Plus, part of the reason for producing this concept was to keep cost and complexity down for amateurs.
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