Instructables

Vortex-Drive Micro ROV (ROVVor)

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Introduction:
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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Step 1: Vortex-Drive Design Evolution

Picture of Vortex-Drive Design Evolution
 
 
I came up with the ROVVor after thinking about how to create a minimalist, cheap, and easy to build ROV. I figured the first thing I would try would be an aquatic version of the popular land-based 'brushbot'  which can be built with just a toothbrush head and a vibrating pager motor.

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.
 
wolfgang641 year ago
Hi, this (scaled up) might have uses as subarine sonar counter measure.Small moble and sonicly loud with a little tuning you could probably make it mimic the sigiture of a boomer or attack boat.
mooseo1 year ago
What about using a 50 ml Falcon tube? It seems like you could fit a lot of stuff in one of those... not sure if it's too big to work with the little pager motor, though. Maybe I'll order one up and give it a try.
cuvette mooseo1 year ago
Vibration is mainly interesting for micro robotics.
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
shojidoug1 year ago
If you head on down to your local adult store you will find any number of small self contained water proof vibrating ah.. erm... toys :)-
"What would you bring along if you were to be left alone on an island?
- A vibrating toy.
- Really?!?
- Yeah, to make a boat."
leevonk (author)  shojidoug1 year ago
good point, might have to tape some Styrofoam to them to get them to neutral buoyancy or positive buoyancy, but that might be a good way to make a scaled up model
Fascinating!

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.
DSC06329.jpg
leevonk (author)  masynmachien1 year ago
thanks for the the battery info, that's good to know and will help out a lot at least until suitably sized tube/motor combos can be found. That's cool that the fin worked, that method would make the system fully enclosed too, and since it uses a vibrating motor it would be really easy to make. Thanks for the ideas!
sitearm1 year ago
@leevonk; Hi! I really like your "design evolution" diagram and explanation. This is an elegant presentation and idea and I've tweeted it. Cheers! : ) Site
So you do two like a catamaran :

\-----
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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.

--------------------------------------
====== ====== ======
/----- /----- /-----

<--- water flowing through pipe.

\----- \----- \-----
====== ====== ======
--------------------------------------

maybe even have the motors attached outside the pipe for easier replacement!

wow... exciting, I must try this.
I'm pretty sure that this is how a solid-state automotive fuel pump works!
dausmus1 year ago
Did you consider hearing-aid batteries? And with a tiny controller and a small microphone, notch filter to eliminate the motor sounds, and bandpass to catch only control tones, you may be able to remotely control these using sonic tones in the water, eliminating wires entirely. No need to add weights, let the controls be that weight.
leevonk (author)  dausmus1 year ago
I like the audio information transfer idea, that's a really neat concept. A similar solution which would use less power could be using light (LEDs) to communicate between modules but that would work less consistently as it would vary more dependent on the turbidity of the water. However, compaired to either of those options, really small, low power, RF transmitters and RF receivers still might be the best option since I think they would need less power and take up less space than a tiny speaker / microphone (I think).
As you say, LED is OK except water absorbs so much light energy it becomes difficult to get significant distance. Radio is good, but very low frequencies have to be used (see how submarine communication is done while underwater), again due to absorption and propagation difficulties with water. Sonic energy, however, is very nicely propagated, depending on frequencies chosen (same principle as hydrophones).
leevonk (author)  dausmus1 year ago
hm, yeah, I looked up a little info on submarine RF and it seems that freshwater is not much of a problem for RF but salt water really prohibits RF a lot, so maybe sound would be better.
leven1 year ago
What about the tube you get effervescent tablets in?
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..
Fun Prototype, and great idea!

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.
Kiteman1 year ago
I don't think you need to find batteries to fit the tube, since you're going to need to control it from outside the tube anyway.

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?

leevonk (author)  Kiteman1 year ago
If the battery was inside it would make it safer for kids to make and play with, and it would operate a lot like a land-based 'brushbot' does (i.e. with no real control, but still buzzing around nonetheless). Also, with larger sizes there's no reason that the control circuitry (autonomous IC or radio receiver) couldn't be put inside of the enclosure too.
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
Ah, I get you. You're going to have to think about buoyancy as well - as near to neutral as you can.

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?
leevonk (author)  Kiteman1 year ago
watch batteries are just barely too big to fit into the tubes I used. If I get a bigger tube though the laser pointer battery holder is a great idea, connecting to batteries securely is always a pain. I was thinking of putting one of the leads to the batteries in the cap of the tube so that it would turn off whenever the cap is removed and turn on when the cap was in place. I was able to adjust the bouyancy pretty accurately by just adding small pebbles until it floated below the surface, but lead shot might work better to save space in the tube.
Try plastic test tubes or cigar tubes. (The disposable plastic tubes that higher end cigars come in.) Check tobacco shops or google plastic cigar tubes OR plastic test tubes for places to buy on line.
If this is for kiddies, don't use lead shot!

(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...)
LOL, why not try some mercury for the ballast. Then the parents will really have something to complain about.
bronchitis1 year ago
This is a really awesome propulsion idea. Just need to add an arduino and a few more motors and you have yourself and underwater quadra-ROVer.
sgsax1 year ago
Would hearing aid batteries fit any better than watch batteries? Having them on-board would also provide built-in ballast.
rimar20001 year ago
Very interesting!
Indeed!
leevonk (author)  rimar20001 year ago
Thanks!
Tomdf1 year ago
That little guy moves pretty fast. I really like the quadmotor idea, reminds me of one of those water strider bugs.
water_2.jpg
shabaki1 year ago
This is brilliant, i would love to see this technology expanded, i wonder if this could be used in something like pontoons attached to a small boat ( thinking of how it would work scaled up).
leevonk (author)  shabaki1 year ago
Right now it seems like it's a pretty inefficient form of propulsion, so I think its applications, if any, could be in niche markets where it's imperative for the actuator to not touch its surroundings. For instance in some environment with very caustic liquid, Also there might be a niche advantage of ifs low actuator movement profile in terms of minimizing detectability.
fozzy131 year ago
This is a cool idea. I watched the video at the end and it occurred to me that you may want to test your ROV in differently shaped containers.  Considering the propulsion method, it seems like the size and shape of the container and the walls could affect performance.
leevonk (author)  fozzy131 year ago
yeah, I just remembered that I had not addressed that so I added some text to the video page about that. The ROVVor is moving in a curved trajectory due to the wire 'leash' suspended from above. Even though the wires don't look taught they are holding the vehicle with enough force to pull on it as it drives (this propulsion method isn't very strong, at least at this scale).

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.
I noticed that the wires seemed to be moving the craft slightly too. Still, try it in a rectangular container if you get a chance! I'd like to see if it : )
leevonk (author)  fozzy131 year ago
I'll give it a try. I'm pretty sick today but maybe tomorrow. Maybe I'll be able to get a photo of the vortices too