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Author Topic: Animatronic hand elastic material for fingers  (Read 1512 times)
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Inspired by the video above I'm hoping to build something similar for my first project. Currently the only part which I can't seem to figure out is how the hand will be returned to it's original position. There only appear to be three potential ways of doing it.

One
An elastic material that reverts to it's original shape - but all elastic materials I know progressively lose their elasticity and stop returning completely back to their original position.

Two
Weigh the fingers down with something heavy - but this will not make the hand work if for example it is stood straight rather than be on a table flat.

Three
Alternative servos that pull the hand back - but this requires a lot of work and will make the design less compact.

Any ideas which to go for?
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It looks like the servos are connected to the fingers by cable that is almost identical to "throttle cable" or "brake cable" that you would find on a motorcycle or bicycle.  When the servos "push" the fingers go back to the straightened position.
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Hmm interesting. The ones I've found so far appear to be too thick. Can you give me a link, perhaps on ebay of the cable you are talking about.
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all elastic materials I know progressively lose their elasticity and stop returning completely back to their original position.

What sort of timescale are you dealing with? There are lots of elastic materials that maintain their elasticity over days, weeks, months. Eventually any material must degrade, but various elastic materials commonly used around the house don't degrade quickly.
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The below has a simple setup.

http://forum.arduino.cc/index.php/topic,147373.0.html
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Hmm, a simple garden hose for fingers? That sounds pretty cheap and effective.

One other thing however that may be useful to ensure the elasticity will last for longer is to attach an elastic rubber band from the tips of each finger on the back to roughly around the point of the wrist. It may require a more powerful servo but I think the elasticity will remain for longer.
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Hmm interesting. The ones I've found so far appear to be too thick. Can you give me a link, perhaps on ebay of the cable you are talking about.

In the radio control world, you can purchase tubing and small diameter steel cable that is meant to allow you to route the motion of a servo for longer distances (and to keep the moving parts out of the elements). Basically all you need is some small diameter plastic tubing, and the cable.

Or, how about this:

Run two lengths of mono-filament fishing line down plastic or vinyl tubing to the "base" of the finger mechanism. Route the fishing line to the finger such that one line pushes, and the other pulls to move the finger. Attach the other end of each line to the ends of one of the bar-linkage control horns (it looks like a "bar" with holes, and one central hole in the middle that attaches to the servo). This way, you can keep things fairly compact, and you only need a single servo for each finger.
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Elastic on the back side of the finger would have to be attached such that it would not slip off of the joint as it flexed. Wide rubber bands or wide elastic band material might work. If thin string or monofilament is used on the backside of the finger, then small tubing like coffee stirrer sticks might need to be attached to act as guides.
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Hmm interesting. The ones I've found so far appear to be too thick. Can you give me a link, perhaps on ebay of the cable you are talking about.

In the radio control world, you can purchase tubing and small diameter steel cable that is meant to allow you to route the motion of a servo for longer distances (and to keep the moving parts out of the elements). Basically all you need is some small diameter plastic tubing, and the cable.

Or, how about this:

Run two lengths of mono-filament fishing line down plastic or vinyl tubing to the "base" of the finger mechanism. Route the fishing line to the finger such that one line pushes, and the other pulls to move the finger. Attach the other end of each line to the ends of one of the bar-linkage control horns (it looks like a "bar" with holes, and one central hole in the middle that attaches to the servo). This way, you can keep things fairly compact, and you only need a single servo for each finger.

Two questions,

Diameter steel cable - is that one of those elements which can pull as well as push objects?

Also, I'm having trouble visualizing your second idea...  would you be able to do a small sketch illustrating it perhaps?
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I've looked at push/pull setups for servos on the past. I think the easiest wire to obtain might be the thin steel fishing leader wire like below. Finding small flexible tubing to run the wire in has been a little more challenging. Aquarium air tubing might be a little too big. The model aircraft/boat vendors do sell some push/pull setups.

http://www.fleetfarm.com/detail/american-fishing-wire-surfstrand-steel-leader-wire/0000000028364?utm_source=googleps&utm_medium=shopping%2Bsearch&utm_campaign=google%2Bproduct%20search&gslfah&gclid=CLfl46Gjt7cCFStp7AodZGQAUg
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One possible issue with push-pull approaches is that the fingers have several joints, and I imagine that rods stiff enough to push would be reluctant to follow the curve of the fingers. When I've used push-pull control rods for aircraft, the mechanism has always been carefully designed to that the curvature required is minimal because the rods used were not very flexible.

The nice thing about just using a spring/elastic to straighten the fingers is that you can spring each joint separately so that the movement inherently tends to be shared equally between them. I guess the elastic probably doesn't need to be very strong - typically hands are used so that they close to grip things and don't need to apply any force in the 'opening' direction.
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The "wire through a tube" thing is referred to as a Bowden cable. Might be helpful to know when you're searching.

If you're looking for a more reliable rubber band you should try silicone tubing. This is the stuff used for slingshots or water balloon launchers (tourniquets? smiley) but is also used in smaller diameters for fuel lines in hobby planes/cars/etc. and in longer lengths to launch RC gliders.

The proper type of wire you would want would be "piano wire" (most commonly used for making springs). It shouldn't be too hard to find at hardware or craft stores, but you should be able to pick it up with the other two items above at a good hobby RC store.
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Actually I came across the 'bowden cable'. Is it sufficient to use alone without tubing of some kind?

Also I was thinking the exact same thing regrading the straight metal tubes or rods which will be too heavy to allow the fingers to display realistic movement so that's why searching for an alternative.
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