Subject:
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Re: New technique: three position piston control
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Newsgroups:
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lugnet.technic, lugnet.robotics
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Date:
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Wed, 28 Sep 2005 22:25:54 GMT
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Viewed:
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4607 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, Kevin L. Clague wrote:
> > In lugnet.technic, Mark Bellis wrote:
> >
> > <snip>
> > >
> > > Interesting stuff! Should make walkers move more smoothly, with the possibility
> > > of more groups of legs. The trick will then be reversing all the groups, since
> > > it was easier with just two, using two polarity reversers.
> > >
> > > I have a centre-stop mechanism for pneumatic steering for a JCB. It's not part
> > > of an automatic system though, since I don't yet have an application for that.
> >
> > Was this single or dual piston?
>
> Mark,
> Did I miss your answer on this?
> Kev
No, I was so busy at work that I missed a couple of days online!
I had a look at both, but a dual piston scheme works better for steering because
of equalising the piston area and pushing at least 3 switches. In fact I have
two cylinders in parallel at the moment. One problem I can foresee with that is
that there is a net force towards the middle due to the auto-expansion effect,
so the mechanism in the middle has to have good compressive strength.
Another advantage of the dual piston scheme is that it's possible to eliminate
the leaks from the tops of the cylinders by not using those nozzles. This is
something that also hinders precision in positioning. My pneumatic point
actuation scheme works very well using just the bottom nozzles, and I've got it
down to a minimum of parts in the mechanism in order to fit it within the
scenery. It has a very low profile above ground too. I hope to fit it to a
scenic module soon.
I think the single piston scheme could be equalised with an elastic band to
counter the auto-expansion effect. This would fit better in a small space in
the vehicle chassis, probably longitudinally and operating the steering via a
bell crank. The band would be optimised to give enough tension at the desired
mid-point.
Perhaps stopping in exactly the same place from both directions is not so
critical in a walker. In a vehicle with steering it is important to be exactly
straight at the mid-point, and to have the mid point the same in both directions
without overshoot.
Not using an automatic system has the advantage of fewer switches to load the
cylinders. Creeping up to the mid-point is slower under more load. In the dual
piston scheme, the auto-expansion effect is perhaps not enough force to move the
load.
I optimised the switches down from 4 to 3. I think it works better with a rigid
structure than with studless beams. There are still 3 switches offset by
different amounts.
I used some axle joints to optimise cylinder position limiting in normal logic
systems (without offset switches) in my Octopus arm. The base of one axle hole
is on the end switch and the other end hits the top of the cylinder, limiting
its contraction to the right amount. This reduces air usage on the logic board,
speeding things up a bit.
Mark
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