This is a cheap and easy to build ROV (Remotely Operated Vehicle). I came up with it because I wanted to create a minimalist aquatic robot that avoided the traditional annoyance of needing to get / make waterproof motors for ROVs. This ROV uses a new form of aquatic propulsion that works via the creation of multiple standing vortices around the ROV's asymmetrical hull (if this isn't new please let me know). The propulsion system doesn't require contact of any actuator with the surrounding water, and is very easy to build and is very robust.
Previous bio-inspired ROVs have used flapping fish-like fins to move through the water. Although these fins work by creating vortices along their surfaces, the ROVVor creates constantly maintained, standing vortices around its hull. These standing vortices are maintained by a high frequency vibration of the vehicle, induced by an internal pager motor. The vortices seem propel the vehicle through the water due to the vehicle's asymmetrical hull shape which must create some imbalance in the vortice's forces applied to different sides of the vehicle.
Applications:
Due to its ease of implementation and robustness, this propulsion mechanism could potentially be useful for creating nano-scale ROVs (would need some way to vibrate the 'actuator molecule'), although at the molecular scale vortices might not work in the same way. Due to the ROVVor vortex-drive's minimal actuator displacement (compared to spinning propellers of flapping fins) it could also be useful for military applications to avoid detection. On a lighter note, the cheapness and ease with which the ROVVor can be created, and the interesting-ness of the way in which it works would make it very conducive to introducing kids to robotics in general and specifically to ROVs.
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Signing UpStep 1: Vortex-Drive Design Evolution
So I put a pager motor into a centrifuge tube (see figure I), and put it into a pool of water. It vibrated and I could see standing waves created all around it, but it didn't translocate (it didn't move through the water).
So then I tried adding little bristles to its sides (see figure II) and still nothing happened.
Finally, I starting thinking about adding a 'wave-sail' to the robot (see figure III) to take advantage of the waves created by the vibrations. I based this idea on the fact that one can use a fan on a sailboat to push the sailboat (see this great video: http://www.youtube.com/watch?v=0CrXvOKPymk). When I added this 'wave-sail' the robot started moving through the water.
However, when I trimmed the 'wave-sail' down a bunch (see figure IV) the robot still moved through the water, even faster than it had before. That's when I realized that the trimmed down 'wave-sail' was not acting as a wave sail at all. So I added sawdust to the water and saw a bunch of standing vortices all around the sides of the vehicle. I drew the vortices as I saw them as can be seen in figure IV). I then added some weights into the tube (little pebbles) and saw that the ROVVor could move even when completely submerged.
Then I thought that maybe if I put the same bendy straw on both sides of the vehicle that it would move even more powerfully through the water (see figure V) ,but when I did that it didn't trans-locate anymore! So it seems like the ability to move through the water is dependent on the vehicle's asymmetry and on the standing vortices created around it.
In the future I'm planning on trying a design in which multiple ROVVors are attached to each other (see figure VI) via a semi-flexible connection. In this way, by selecting which ROVVors to activate or deactivate one could selectively steer the whole structure in a desired direction. Because the connections are semi-flexible they would not propagate the vibrations from one vortex-drive subunit to another.
Note:
I haven't yet created a fully self contained version of the ROVVor because I couldn't find batteries small enough to fit into my centrifuge tube, and it's hard finding a water-tight tube that would fit a battery and the motor. I'll update this instructable if I find a right-sized tube and am able to implement a fully self contained version.
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Trying to bring it to bigger scale won't be energy efficient, besides other drawbacks (awful noise, payload limitation and quick wear due to vibration...).
If you want to make a bigger vehicle, more energy efficient actuators can be used, either fins as naturally evolved by fishes (also interesting are the octopus, squid and jellyfish for alternatives), or helix propellers as optimized by human engineering.
It still remains a fun experiment and a good exercise explaining the physics behind it! ;p
- A vibrating toy.
- Really?!?
- Yeah, to make a boat."
Can I give you a hint: Don’t be afraid to put a battery in the water. For experimenting during a relatively short time (larger part of an hour or even hours) this is really no problem. The low conductivity of clean water makes the drain on the battery minimal. You can use a button cell and attach it with tape. This way you can see a lot better how it moves, free of long wires. Also, if you add weigth on the outside, you can keep the tube straight under the ROV and have it move straight forward more easily.
In preparing for my Striderbot I did some experiments on aquatic propulsion based on small vibrating motors. I went for a rather classical approach of putting a long flexible tail on a little Styrofoam boat, just large enough for a pager motor and a button cell battery. The tail was a 5 by 50 mm strip made out of plastic sheet similar to old fashioned overhead projector slides. I do not have it any more, nor do I have a better picture ore video. But it worked quite well (clearly faster than on the large striderbot). It was also easy to make it go reasonably straight or turn left or right by bending the tail fin. It would be interesting to compare your tube design with the tail fin and determine what are the best dimensions for each.
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Wow... now make a tube full of these as a "pumpless" pump! So lines with motors around the edge and the "straws" facing inwards.. so the water flows without the need for fan or pump blades.
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<--- water flowing through pipe.
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maybe even have the motors attached outside the pipe for easier replacement!
wow... exciting, I must try this.
Large enough to fit AA or 18650 batteries inside, and even some microcontroller and low frequency RF-module for communications (2.4Ghz such as bluetooth will not work well under water)
Due to size maybe need to upgrade the pager motor to the vibrating motor from some gamepad or similar, or DIY..
Just some experiment Ideas if you aren't sick of this game yet :)
It's hard to tell from the sketch of your vortices, but my intuition would have me believe that there is a hobbled corkscrew effect contributing to the motion... I've seen sea worms (Glycera?) swim like this (although at a dramatically slower rate of oscillation). There would be a bit of whip involved in the movement of the tube and a resulting lag in any lateral motion between the "bow" and the "stern". Since the motor displacing water in a circular motion any lag from front to back creates a corkscrew drive.
One might be able to evidence this by simply un-balancing the centrifuge tube more from front to back. If this interpretation of what is happening is correct, I guess you would get more efficiency by:
1) getting a more flexible hull,
2) mounting the motor at the tip of the tube, rather than the centre
3) fore-aft balancing the boat with an internal counterbalance from the bow suspended into the stern, and
4) keeping it smooth along the exterior direction of travel. Possibly adding flexible keels along the outside would give better acceleration and prevent the hull from accumulating spin.
If the experiments I list above worked, then one should be able to steer the boat by flexing the internal counter-balance to one side... although it may require some tricky timing with the vibrator.
Final food for thought - getting efficiency out of this it might be a question of tuning the frequency of the pager motor to the natural frequency of the tube in water. If there is a hull corkscrew frequency, I would think it would be most "motive" if it was in harmonic phase with the frequency of the vorticies produced.
When you get on to making units with more than one thruster, I think you'll have to mount it in a way that allows the thruster to vibrate (a firm fix to a frame may cut down the all-important vibrations). Maybe suspend it in a network of elastic bands?
For the multithruster version, as I mentioned in the instructable (Step 1, figure VI), and as you mentioned, yes they would have to be attached by some semi-flexible connection, I was thinking of something more like a spring, but a matrix of rubber bands could work too I guess. In an autonomous version of the multithruster design there could be radio transceivers within each sealed tube so that a centralized IC could command each of them appropriately
There are watch-cells that are the right diameter to fit into a centrifuge tube - look to cheap pen-style laser-pointers for the best way to connect them - but how are you going to switch it on and off?
(No matter how safe and well-behaved the kids are, some paranoid parent will inevitably complain about you putting something toxic in the same state as dear, fragile little Billy...)
I did some other tests where I positioned the wire leash closer to on of the walls of the bowl and the ROVVor still moved towards the wall even when it got pretty close, so it doesn't seem like any wall-reflected waves are effecting it's trajectory much, it seems like it's mostly the wire leash that's causing the curved path.