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Re: [SeattleRobotics] electroadhesion

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  • Kevin Ross
    Hmmmm... Interesting calculation. Since current is not really an issue here (other than that which would leak), I suppose it is possible to crank the voltage
    Message 1 of 3 , Apr 9, 2010
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      Hmmmm... Interesting calculation. Since current is not really an issue here
      (other than that which would leak), I suppose it is possible to crank the
      voltage up to something obscene to get the adhesion. That would help with
      the separation to some degree.

      If you think about it, 'static cling' has a ridiculously high voltage with
      little current capacity.

      --------------------------------------------------
      From: "Peter Balch" <peterbalch@...>
      Sent: Thursday, April 08, 2010 3:17 AM
      To: <SeattleRobotics@yahoogroups.com>
      Subject: [SeattleRobotics] electroadhesion

      > http://www.sri.com/rd/electroadhesion.html
      >
      > When first read about the SRI electroadhesion robot, I wondered (like most
      > people I suppose) what sort of voltage it was using. They were claiming
      > 0.5
      > to 1.5 N/sq.cm which seemed huge.
      >
      > So lets do some back-of-the-envelope calculations. I'm sure someone will
      > correct me if I get it wrong.
      >
      > The force between the plates of a capacitor is
      >
      > F = permittivity * A * V^2 / (2 * d^2)
      >
      > A is area; V is voltage; d is separation of the plates.
      >
      > The permittivity of air is around 8.85e-12
      >
      > Assume that the robot's adhesion plate is a square of side L; then
      > rearranging:
      >
      > V = d/L *sqrt(2*F/permittivity)
      >
      > If we need a force of 1kg weight (10N) then
      >
      > V = d/L * sqrt(20/permittivity)
      > V = d/L * 1,500,000
      >
      > If L 20cm and d = 0.1mm then
      >
      > V = 0.0001/0.2 *1,500,000
      > V=750
      >
      > 750 Volts is achievable relatively easily but getting the separation of
      > the
      > adhesion plate from the wall to be 0.1mm is quite hard.
      >
      > Presumably, the adhesion plate is a conductive sheet covered in an
      > insulator. How thick must the insulator be? It can't be too thin or its
      > insulation will break down. It can't be thick or d gets too big (assume a
      > relative permittivity of the insulator of 3 to 4).
      >
      > SRI claim 1N/sq.cm, so V = 4.7kV if d=0.1mm. Either they're using a big
      > voltage or they're managing a much smaller separation.
      >
      > Peter
      >
      >
      >
      > ------------------------------------
      >
      > Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
      >
      >
      >
      >
    • Peter Balch
      Kevin ... Well, I think it s right. If the answer had come out with a different number of zeros then I d have been suspicious. ... Yes but the voltage required
      Message 2 of 3 , Apr 10, 2010
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        Kevin

        > Hmmmm... Interesting calculation.

        Well, I think it's right. If the answer had come out with a different number
        of zeros then I'd have been suspicious.

        > Since current is not really an issue here
        > (other than that which would leak), I suppose it is possible to crank the
        > voltage up to something obscene to get the adhesion. That would help with
        > the separation to some degree.

        Yes but the voltage required is proportional to the separation. I think a
        separation of 1mm is much more easily achieved but 7.5kV is harder to
        generate. SRI say they use compliant electrodes.

        Sure, you can _eventually_ generate a high voltage but switching it on and
        off is much harder. (I reckoned 4.7kV was needed to give the 1N/sq.cm that
        SRI claim.)

        The highest voltage I've made in a battery-powered circuit is around 500V.
        As you'd expect, I used an oscillator, a small mains transformer connected
        backwards then a few stages of diode-pump. It took an appreciable time - a
        couple of seconds - to reach its operating voltage. Would it take 10 times
        longer to reach 4.7kV? From my understanding of diode-pumps, yes.

        The SRI website talks about turning the adhesion on/off in 50mS.
        http://www.sri.com/rd/electroadhesion.html

        > If you think about it, 'static cling' has a ridiculously high voltage with
        > little current capacity.

        Sure. SRI say 20 microwatts/Newton. But you have to be able to turn it on
        and off. I've no experience of handling kV on a robot-sized pcb. Maybe it's
        easier than I imagine.

        Or maybe you don't have to be able to turn it on and off. This wall-climber
        just uses Scotch Tape:
        http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.122.5937&rep=rep1&type=pdf

        And why 20 microwatts/Newton? Why microwatts? 4.7kV at 20uW is 4nA. Where is
        that current going? Is that leakage or is that the current needed to
        charge/discharge the cap for every centimetre travelled? If we assume a
        200sq.cm plate for the two tracks (It looks like a tracked vehicle) and
        0.1mm separation then I think that's around 2nF. So at 4.7kV, Q = 9.4
        microCouloumbs which is 20uW for 0.5 seconds.

        In other words, their 20uW claim could mean they're charging their whole cap
        over a period of 0.5sec. That seems reasonable. But then how do they turn it
        off in 50mS? Of course, they may not be using a 20cm x 20cm plate or 0.1mm
        separation or 4.7kV. It would be nice to get some real figures.

        If it's a tracked vehicle, why aren't they just using a permanent electret?

        > the structural design of female evening gowns (see "A Stress Analysis of a
        > Strapless Evening Gown: Essays for a Scientific Age") and am struck by the
        > amazing things that could be done with electroadhesive clothing. The mind
        > boggles.

        And then there's a power failure ...

        Surprisingly, electroadhesion has been suggested for use in clothing
        technology (see: Principles of Electroadhesion in Clothing Technology - P.M.
        Taylor & G.J. Farnworth).

        Peter


        Here's a company that sells battery-powered electroadhesive display boards.
        Nowhere do they say what force the boards generate.
        http://www.justick.com/Products.aspx
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