Water Glider Prototype

[Byrel Mitchell] wrote in to share some details on this water glider which he has been working on with his classmates at the Nonlinear Autonomous Systems lab of Michigan Technological University. As its name implies, it glides through the water rather than using propulsion systems typically found on underwater ROVs. The wings on either side of the body are fixed in place, converting changes in ballast to forward momentum.

The front of the glider is at the bottom right of the image above. Look closely and you’ll see a trio of syringes pointed toward the nose. These act as the ballast tanks. A gear motor moves a pinion connected to the syringe plungers, allowing the Arduino which drives the device to fill and empty the tanks with water. When full the nose sinks and the glider moves forward, when empty it rises to the surface which also results in forward movement.

After the break you can find two videos The first shows off the functionality and demonstrates the device in a swimming pool. The second covers the details of the control systems.

48 thoughts on “Water Glider Prototype

  1. is it really that cool? sure it uses a fairly novel propulsion system… which is cool but it sucks… the motos could propel it much further and faster with less energy spent

    plus – it’s a freaking ROV – it can have tons of batteries – damn you might as well build one that has a gas engine and beats any electric only ROV

    1. You can propel a plane much further and faster by slapping on motors and bigger engines, but gliding is neither a novel propulsion system nor a sucky one. The Michigan guys are putting a new spin on it by taking it underwater. I’m sure they’re already working on improvements.

      Gas engines won’t work underwater without an oxygen tank, so no.

      1. It reminds me of the increased efficiency of “swimming” robots that move like fish. I like the variable ballast system… makes me wonder if you could build a robotic manta ray that combines the two technologies.

      2. lol I know they don’t work underwater :-D you could have guessed I suggested using a submarine snorkel or a buoy that has air intakes and sends air through a tube to the ROV

        Now as far as efficiency of this propulsion method goes – I am fairly certain that it takes more energy to operate those syringes than it does to power a propeller… look at the vid – it moves very little with each cycle… I am not even going to mention that with a few motors & propellers you can orient the vehicle

      3. apart from the fact I don’t think this is a good propullsion method I think that its reliability & efficiency could be improved by using a diaphragm instead of syringes

        have the motor move the diaphragm back and forth and that would change the craft volume instead of adding weight – essentially it would work in the same way but there would be less to fail and overal efficiency would improve (less friction)

    2. The design is known as a \”Slocum glider\”, invented by someone at Woods Hole. They are very low power, much lower than any other thing. They aren\’t fast, but they can stay out forever (as in years, not days). On the commercially available version, the rudders for directional control consume more power than the propulsion system.

      For oceanographic research, these have other advantages besides duration – they are basically invisible in operation, they don\’t disturb what they are trying to observe. (this would also be of value to those who are in clandestine observation and importation trades).

      Personally I would love to see large scale versions of this design as bulk transport. Unfortunately, they would be too slow for most shippers, currently fuel is cheaper than interest on cost of goods in transit.

    3. “that its reliability & efficiency could be improved by using a diaphragm instead of syringes”

      Not only that, it would use far less power if they ditched the whole arduino and half-bridge controllers and simply drove the back and forth mechanism with the mechanical switches they already have – a’la the most useless machine.

      It’s a classic case of “a little bit of thinking eliminates 95% of the circuit”.

      1. More precisely, how to wire the switch: http://www.eleinmec.com/article.asp?12

        A single switch can be used to drive the entire mechanism, leaving the designer to contemplate other more useful roles for the microcontroller. The state of the motor can be detected simply by measuring the polarity of one wire going to the motor, to know whether you’re going up or down.

      2. Or maybe a classic case of “not understanding the problem, and proposing an elegant solution to a different problem”. ;)

        These gliders are used for oceanographic research, where the point is to sample a range of depths along a path. So after taking in ballast water, you have to wait for it to sink to the desired depth (taking temperature, salinity, chlorophyll, etc. measurements all the while) before you expel the ballast, and similarly wait for it to climb before you take on ballast again.

        If it had a continuously reciprocating rig as you suggest, it would only cover a very shallow depth range. There’s ways to deal with that, too (555, anyone?), but you still need a microcontroller to take the measurements, so why not let it drive, too? It only takes the microcontroller’s attention for a few seconds at the top and bottom of each cycle, and it lets you program different depths (maybe keep it from bottoming out in coastal areas).

    4. I really appreciate the feedback you folks have given. You’ve pointed out several weaknesses in the design, which we’re improving in the next model. :)

      Everyone who’s pointed out that we didn’t need a microcontroller for this is completely right; this is a trivial problem for a few discrete components. However, this was intended as a protoype (and demonstration model) for the next glider, which we’re design right now. We will have a more sophisticated sensing package onboard the next, probably including inertial navigation for most runs, but with the ability to load in different sensor load for different missions. In short, we put the Arduino more to evaluate it for the real model, than because it was actually required.

      Also, as several of you noticed, we goofed on the wing loading calculations. We should have had either bigger wings or lower displacement. Nice catch!

  2. Nice work here, especially the mechanical bits. Kudos for not using duct tape and rubber bands and making it look like a professional prototype rather than a weekend special.

      1. I read your other statements as well as this one. Your deduction is lacking. To wit:

        How would you manage speed with a dumb circuit?

        How could you add functionality without a complete teardown?

        How do you compensate for water density?

        How do you compensate for pressure?

        What is the point of adding more circuitry and cost to the project when you still need a micro-controller to handle sensors, data, and navigation?

        It disturbs me the intensity of criticism on hackaday that lacks critical thought.

  3. The first question is: Why use water ballasts? Would it not be more efficient, safer, and less dependent on clean water to simply move a weight up and down the robot? That way you can actually seal the robot for good, and it can be used in muddy water. Syringes have a very low life cycle, and any moving seal is bound to break at some point.

    Second, why did they have to use an arduino? This is just getting ridiculous. Sure it’s quick and dirty for a prototype, but what this whole motherboard does can be replicated on the small breadboard using a couple of gates.

    1. I think the rationale for the water ballast system is versatility. Simply shifting the center of gravity would only provide so much movement, and it would have to be adjusted for neutral buoyancy at an appropriate depth. This, on the other hand, is capable of more-or-less surfacing & controlling its depth somewhat.

      I agree about the arduino though… unless they’re planning on more advanced features in the future?

  4. Looks like its glide ratio is quite poor. It doesn’t get much forward distance on each descent or ascent.

    Four fins on the back would provide better directional stability and improve the glide ratio.

    Putting the syringes in the back wouldn’t work because pulling water in would make the thing heavier and tip the back down. It’d go backwards.

    What would work is routing a water line to the rear for ejection and another to the front for drawing water in. Simple check valves would ensure the water goes the right way. More complex but more efficient because the pumping action never works against the desired motion.

    Nose and tail cones would also improve efficiency.

    To give it more duration, put solar cells on top, under a clear panel. Whenever the program reaches a time to transmit collected data, have it surface and the batteries will charge. Also have it surface and drift whenever the batteries are low.

    Another feature an advanced model would have is a custom built ballast chamber able to hold more water, perhaps backed up with an emergency blow out function provided by a CO2 capsule. If the ballast system fails the control system could blow out the ballast chamber so the ROV could surface and send a message to come pick it up.

    While awaiting recovery it could act as a drift buoy measuring surface temperature, insolation and via its GPS the movements of the ocean current its in.

    1. Excellent ideas. I was armchair engineering some ideas about CO2 capsule ballast control, nice to see someone else thought of it too!
      A neat idea for a large version would be on-board chemical gas production. Use water from the source environment to generate pressurization gas for the ballast tank.

      1. Its over thinking it. Use a spring with a mechanical lock. It fires, then you can always suck in a full load of water and it relocks.

        Simpler, cheaper, and reusable.

  5. Very cool… I like the ideas above about routing the expelled water out back to reduce forward friction.

    Two tips:
    1) Get rid of the solder less breadboards. They are noisy and often loose conductivity in various holes after plugging and unpluging for a while.

    2) I would make some sort of mechanical buffer for the limit switches. They will eventually break when actuated with hard surfaces.

    Very novel idea… I like it!

    1. I’d suggest spring-arm microswitches. It’s pretty much what they’re designed for…
      …but if you need to keep them compact, you could also try a soft rubber block on the limit switch. Yashica used to build camera shutter that used that very method to buffer between the shutter release and the button mechanism. It only took 30 years for the rubber to break down and need a new one ;)

    2. We used to solder a soft compression spring to limit switches to buffer the compression. Another idea would to use a piece of plastic or metal that sits between the switch body and arm. Finally you could throw out the switch all together, either use an optio setup or just watch the current on the motor when it spikes reverse the direction :D

  6. A simpler solution to the syringes expelling water towards the front would be to put their intake/exhaust port on the side of the glider. No valves, just some simple surgical tubing does the trick.

  7. With the video quality, the intro and the editing work, and oh it says it is from a laboratory! i would expected at least a custom made PCB with an AVR on it or something else, not an Arduino and a breadboard,
    and the body made of clear plastic it should at least have a solar cell inside to recharge the battery,
    the syringe is a bad idea in the log run it would not last long plus it requires a lot of force to move presenting much more load on the motor, and it may come to even get suck in its place because of repeated use with salt water inside and varying temperature,

    over all it is a nice idea with a really nice presentation and demonstration but it is lacking in execution specially with the resources they have,if someone made this in their home it would be okay but from a lab in a university with resources it is a shame really

  8. (This method is not new.)
    But what is the gain? The idea is to convert static lift to thrust with the foils, of course. So the efficiency depends on the foils, and on the losses when moving the ballast cylinders (pistons?), which takes quite some energy, depending on depth.

    One obvious advantage is the lack of high speed shaft seals.

    A while ago I read about a similar glider sub exploiting the change of density of a wax with temperature, so I think there were no moving parts (except for the wax-water boundary surface?).

  9. It is only a prototype, surely they can make changes now that they have this made. Plus many of you overlook the efficiency of the propulsion if used in deep water travel. One filling of the syringes will allow it to travel a much longer distance as is sinks for 500 feet than a propeller will get you for the same energy.

    1. “Also; if you dive too deep, the back pressure will overwhelm the servo and it won’t be able to rise again.”

      How does that work? I think you would have issues with tube collapse before you have to worry about differential pressure in the capsule. Even then you can design around that like pressurizing the capsule or putting a spring bias on the plunger.

      1. >How does that work? I think you would have issues with tube collapse
        >before you have to worry about differential pressure in the capsule

        The pressure outside the capsule will rise, but the internal pressure will not. (Internal pressure will stay at 14.7 psi on one side of plunger, but the other side of the plunger will feel the full pressure of the water depth.)

      1. >Oh. I would like to see your test data of this
        >prototype versus the linked finished product.

        My point on efficiency was related to how Wave Gliders get all their energy needs from the environment. They have lasted 400+ days autonomously roving the oceans with out maintenance (after that long they still work, but need to have barnacles removed.)

  10. Good project, especially mechanical parts. But second video made me cry… Why use an Arduino instead of 555? Why use solderless breadboard in a real working device? Oh. They really need a good electronic designer in their team. :)

  11. This is a good educational project, although I was not particularly impressed by the gliding performance. Much more horizontal surface area (larger wings) could greatly improve this.
    My BSME senior project at GATECH developed a buoyancy driven glider similar in many respects to this, however we were focusing on improving the maneuverability of the propulsion method. This project is just a demonstration of an existing technology – our research sponsors were trying to improve the technology. We used a pneumatic system to produce our buoyancy changes, which is demonstrably less efficient, but plans exist to power a full scale device with chemical gas generation means, getting around the energy losses associated with compressing gasses. I submitted our project to Hackaday, but I suppose they are pretty busy and might have missed it.

  12. http://content.imamu.edu.sa/Scholars/it/net/mini%20underwater%20glider%20(mug)%20for%20education.pdf

    Their buoyancy engine greatly resembles the one described in the above paper.

    I hope they gave credit if this isn’t a case of parallel inspiration.

    I’m very interested in their source and electronics BoM. If they have released source code for a GPS waypoint navigating AUV glider that would be significant. Particularly if it is arduino.

  13. Talk about Monday night quarterbacking!
    The MTU team is doing something exciting which requires expertise and commitment in a wide range of areas. There isn’t enough time or money to excel in all of them, so the team chose to focus on something which would have the biggest impact FOR THEIR PARTICULAR SITUATION.

    There are many helpful comments suggesting alternative approaches. However, the tone of many other comments is very negative and discouraging — the posters should consider how well their projects would fare if they posted them on Hackaday! Walk a mile in a man’s shoes before you criticize him.

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