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Thread: Awesome exoskeleton from Japan, machining included

  1. #11
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    Damn Evan, when you get that thing figured out, you can come down and help me finish the laminate flooring job I started and couldn't finish. At least send me a copy of the plans so I can make one
    _____________________________________________
    Mel Larsen
    Remember when your cup holder sat next to you and wore a poodle skirt?

  2. #12
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    Quote Originally Posted by Evan
    I am thinking along the lines of a simple framework that straps to my legs and stores energy as I squat, probably in gas springs. When I need to get back up the gas springs provide the required assist.
    Hmmm, I can't see how gas springs would work well. Maybe if small short ones placed off to the sides...

    I would think clock springs on either side of the knee myself. Only problem is if your foot slips.. Ouch. Speed limited recoil is kinda needed.

    Maybe something like a gas shock pulling on a cable, Wraped around a pully? (And the end tied through a hole in the pully, Since you only need 1/2 turn anyway), Then the speed limited retract would'nt get in your way either. You could disconnect the cables easily enough to completely disengage the rig... Or just have a length toggle mechanism...

  3. #13
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    There are many factors to consider to make something that will work well. The knees are the easiest joint to assist because they have the most limited degrees of freedom but even so the mechanism will have to be able to accommodate some twisting and side bending forces. I don't see the assist system as being the difficult part. The geometry and attaching system will probably be the most difficult. It would be nice if the system could be somewhat locked in an almost sitting position. At least, right now that seems like a good idea. It may not be. A method of easily disengaging the assist system will probably be necessary, especially for driving.

    I have checked with my doctor and the local pharmacy and as far as they know there isn't such a device on the market. When I build one, if it is successful, I will provide free plans for the system.
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  4. #14
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    In 1970 I invented what I thought was a novel spring motor. It turned out to be impractical, but for this project I derived the formula for the amount of energy (per unit volume) which can be stored in a material by stretching it. This equals one half the product of the tensile stress σ (measured in lb/in2) and the strain ε (measured as inches per inch).

    Energy = σε/2

    This will be in units of in-lb/in3.

    Of all the common materials I found that the rubber has the best potential for storing energy by stretching. Here is how it compares with music wire:

    music wire 4000 in-lb/in3
    rubber 9000 in-lb/in3

    Music wire has a much higher tensile stress (as high as 500,000 psi), compared to only 3,000 psi for rubber). But rubber has a far higher maximum strain (6 in/in) than music wire (0.016 in/in).

    So you might consider using surgical rubber tubing for your energy storage material.
    Last edited by aostling; 08-02-2011 at 02:36 AM.
    Allan

  5. #15
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    Of all the common materials I found that the rubber has the best potential for storing energy by stretching.
    A fact well known by those that build and fly model aircraft powered by elastic bands (as I used to do).

    An elastic system is definitely a contender. Some sort of damping method will be required. Elastics have the considerable advantage of being cheap, easily replaceable and adjustable too. The material is widely available in the form of exercise bands.
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  6. #16
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    Quote Originally Posted by aostling
    Of all the common materials I found that the rubber has the best potential for storing energy by stretching.
    The redundancy provided by multiple rubber tubes is also quite beneficial. If one of them ruptures, the rest will still keep the tension intact. You'll only feel a slight increase in pressure. Otherwise, a failure in a single spring arrangement could suddenly put your entire weight on your knee joint.

  7. #17
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    Power it with Warsop hammers that John posted a couple of weeks ago
    Would only take a few minutes to get the knack of running 2 at once

    http://www.youtube.com/watch?v=XnnvH5_Y0Eo

  8. #18
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    Quote Originally Posted by Evan
    An elastic system is definitely a contender.
    An elastomer can store about 9,000 in-lb per cubic inch if used in tension (or about half that much if used in torsion). Your proposed task will require storing about 500 ft-lb of energy (250 lb for man and machine, raised vertically 2 feet), or 6,000 in-lb.

    So you will need to incorporate at least 2/3 cubic inch of (say) rubber tubing into your design.

    And probably significantly more. The estimate of 2/3 cubic inch assumes you stretch it six times it's relaxed length, as rubber is capable of doing. The design need not do this of course, but the amount of rubber required will double if the strain is 3 instead of 6.
    Last edited by aostling; 08-02-2011 at 12:24 PM.
    Allan

  9. #19
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    Isn't there supposed to be a very significant creep factor with elastic/rubber which results in huge losses of stored energy unless the energy is released immediately. I think there was ballistic work done using surgical latex tubing in catapults that showed that even a few seconds delay in releasing a shot resulted in significant fall in "muzzle" energy. This would make it a poor contender if you planned to stay on your knees for any length of time.

  10. #20
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    Quote Originally Posted by Alan Smith
    Isn't there supposed to be a very significant creep factor with elastic/rubber which results in huge losses of stored energy unless the energy is released immediately.
    I was thinking Evan's machine would be an assist for some creep-free tasks, like this: http://www.youtube.com/watch?v=gEYiioT-fjo
    Allan

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