Walking on Water: Start Building a StriderBot





Introduction: Walking on Water: Start Building a StriderBot

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This experiment was inspired by the "robots walking on water" from Carnegie Mellon University (CMU).
CMU's robots are called STRIDE, for Surface Tension based Robotic Insect Dynamic Explorer. They mimic the way water strider insects "walk" on water.

I wanted to see how difficult it is to make a "robot" that moves on top of the water surface, purely based on surface tension. I found out that recreating the principle is not hard at all.

When carefully putting the construction on the water, the surface tension keeps it on top. When disturbed, the construction sinks, proving it is really the surface tension that does the trick.

The vibration of pager motor gives it some rudimentary propulsion (both on land and on water actually).

I hope this experiment inspires you and I look forward to hear what you can do with it. And of course, your vote is welcome.

Step 1: Materials

You need a material that is light and stiff, somewhat hydrophobic and available in shapes giving a lot of "contact edge".
A test showed that the small diameter carbon fiber reinforced rods work fine. You can find these "carbon" rods at modeling shops stocking indoor flying materials. I used two 0.8mm diameter rods, about 1 m in length. The more commonly available 1 mm diameter rods are probably also worth a try.

The other materials used are:
A small a pager motor
A small 1.5V button cell battery
Some scotch tape
A tiny piece of double sided tape

Step 2: Basic Construction

The rods are bent and kept in shape by attaching them to each other with ordinary scotch tape. At the center "node" I made support with some more scotch tape, keeping motor and battery out of the water. The motor was fixed with a tiny piece double side tape, making sure the excenter weight turns freely. The motor leads had their electric insulation removed over a couple of mm and each one is taped to one pole of the battery. I learned this method from the "Evil Mad Scientist BristleBot".

Step 3: Further Devellopment

In a first attempt to improve the propulsion somewhat I added som some small scotch tape "fins" to the back "legs". This gives indeed a slightly more noticeable forward movement when testing in the pool. This can probably be improved by repositioning the motor on a "leg" rather than on an "node". But to keep it on top, this needs a more elaborate fixing of the motor than what I had available at the poolside. This also needs some testing in order not to upset the weight distribution to much. Something for a next attempt.

With the nice ripple pattern generated, the construction makes a simple kinetic art bot. In a next stage, the principle could form the basis for an aquatic BEAM, by adding some sensory input controlling two differently oriented pager motors. You can compensate for the extra weight (we are talking of grams of course) by adding extra carbon fiber rods.



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    Rather than using cellotape to secure the joints, can I use solder instead?

    4 replies

    Not unless you use metal rods instead of carbon rods. Carbon (carbon fibre reinforced epoxy that is) can not be soldered. You might use glue, but as there is not much surface to glue, a cellotape joint will be more resilient.

    As for metal rods, these will be rather heavy, and solder will be heavier than tape. You can try, but I really recommend carbon rods of maximum 1mm diameter, available at hobby shops selling RC airplane kits and probably at kite shops to. if you can get 0.8 or 0.6 mm, this will be even better.

    Couldn't I just use JB weld (epoxy) and clamp it all together for a stronger finish?

    There is simply no need. The tape give gives an immediate hold, is strong enough and lasts a long time.

    Also it is very light. There is not a lot of surface contact, so using something like epoxy would mean adding some glue draped around the rods, adding weight.

    If you really want to ad strength, I would recommend wrapping around some sowing thread and soak that wit superglue. You could use carbon fibre thread and/or thin epoxy instead, but that would probably be overkill.

    Thanks for the fast reply, helps a lot. Haven't actually started this project though, but hoping to soon :)

    I LOVE IT!!!!!!!! must make it sometime. Keep it up!!!!!1

    Sweet ive gotta try this!

    amazing!!! however, does the motor short out after it sinks?

    2 replies

    ahhh... thank you!

    I wonder if having the motor hooked up to a small air propeller might be more propulsive without disturbing the water and sinking it?

    1 reply

    It might be tricky, but it could work.
    The motor would need to be mounted on some pillar and a problem might be the pillar acting as a lever and the propulsion force pushing the front of the construction down.

    you can do the exact same thing with a paperclip, much harder to get it to stay on top, but once it does its really cool

    1 reply

    Indeed you can with thin paperclips, and the effect is cool indeed. However as it is hard to get it to stat on top, it is clear there is no room for any propulsion. The flat metal/plastic strips used to close garbage bags or to keep electronics cables together when you by them are easier to get to stay on top. But you can not upscale it to be able to cary any propulsion. Even if you have longer strips, they are not stiff enough.

    I doubt it. Silver does not really have an interesting stiffness/weight ratio, but you can easily test it by trying if it stays on the surface of the water. First just by itself, then with a tiny extra weight.

    well its not really silver par say its silver coloured steel wire, or ... what if i were to heat it up till it glows then plunge it into cold water to make it stiff?

    Steel has a better stiffness/weight ratio than silver, but still worse than carbon fibre composite. Heating and quenching can increase hardness and to some extent strength, but it will not change the stiffness (elastic modulus). The alloy determines the stiffness.

    dang...how bout fibre glass strips impregnated with resin then formed to the shape

    That would probably work. But if you go into that effort it would be better to use carbon fibre.