Subject:
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Re: SSClagorpion - Compressor
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Newsgroups:
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lugnet.technic
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Date:
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Sat, 22 May 2004 22:36:36 GMT
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Viewed:
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19451 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, Steve Hassenplug wrote:
> >
> > Here's an important question. What will be the limiting factor in how fast this
> > beast can move? Is it:
> > A) Travel time for the pneumatic cylinders
> > or
> > B) Volume of air available?
> >
> > I'm under the impression it will be limited by the volume of air available.
> >
> > An important thing to note is that the number of pistons moving is not always
> > the same, so the load on the compressor is not constant.
> >
> > A quick look at Kevin's diagram for quad-242 shows (if I read it correctly) ten
> > states. The transitions between those states include two times where four
> > pistons move, four times where two pistons move, and four times where one piston
> > moves. I think. I'm pretty sure that doesn't include timing pistons.
> >
> > That means the system expands (or contracts) twenty pistons, in ten steps, or an
> > average of two pistons, all the time.
> >
> > So, I THINK using air tanks, the compressor only has to provide enough air for
> > two pistons for the quad 242 to move at full speed. But, without air tanks, the
> > compressor has to suppily four pistons, at peek times.
> >
> > Of course SSClagorpion has twice the number of legs, but all the above logic
> > should be correct.
> >
> > Is that right?
> >
> > I'd guess the test that was posted was using a system that had a constant draw
> > (number of pistons moving is always the same), in which case, the air tanks
> > would not help it move faster, because there is no time when the compressor is
> > suppilying more air than the system will use.
> >
> > Steve
>
> Steve,
> A very interesting set of questions and logic. I don't know the answer, but
> can add insights that are related.
>
> As the available volume/pressure increases, the pistons expand and contract
> faster. I don't know if there is a linear relationship. If we double the
> pressure, does the thing expand/contract in half the time? I don't know. I can
> easily set up a test though. If I use one pump, then two pumps, then three
> pumps, I can measure the pressure (using my virgin pressure sensor), and the
> rate of transitions of a dual piston engine. I doubt that it is linear across
> the entire pressure/volume range. I also doubt though that there is some kind
> of knee in the curve, where no matter how much you increase the pressure, the
> piston rate never increases.
>
> After I get my last brickink order with three more check valve bricks, I can
> get us from one to 6 pumps.
>
> Kevin
The limiting factor with the speed of the model is composed of two things:
1. The total volume of air between the valve that is switched and the cylinders
that were previously supplied before the valve was switched.
2. The rate of supply of air from the compressors.
You can minimise 1. by using flex tubing for the straight tubes and pneumatic
tubing only for the curved tubes and in places where flexibility is necessary.
This will reduce the balloon effect and increase efficiency. Test this with a
5m long pneumatic tube between the cylinder and compressor - see how much longer
it takes to get up to pressure than with a 5cm tube!
2 is a factor of compressor power. You need at least two large pump cylinder
cycles per cylinder to be moved - three under greater load. If you have 6 pump
cylinders and three cylinders on each leg, that's a 2:1 ratio.
Also, if you use air tanks, design against the tanks leaking. Tubes between
logic stages should be short and/or use as much flex tubing as possible. Every
valve movement will use air that has to be replaced, by pressurising
intermediate tubes in the logic.
This is why I use a car tyre air compressor with pressure setting. It works
hard to maintain the pressure you set (I use 25-30psi) and leaks any excess to
the atmosphere. That way it keeps the pressure as high as it should be and
protects the Lego parts against overpressure. I see no shame in using a
non-Lego method to supply more air power (or electric power for trains) if the
official Lego method is inadequate for bigger models.
I think you need an overpressure release valve. Use two small cylinders pushing
a valve switch against elastic band tension. When the pressure in the cylinders
is greter than the elastic tension the valve will release some air. As the air
is released, the cylinders will leak, so they will be pulled back by the
elastic, closing the valve. this system allows the compressor to run constantly
or be more powerful, such that it can restore pressure quicker. Have a powerful
compressor with an overpressure release valve, rather than a wimpy compressor
that will struggle with the load.
I said a while back that if you had sets of 4 large pump cylinders driven from
pairs of RC car motors, you might need 2 or 3 sets to compress enough air. I
think that because my robot uses 6 pump cylinders (3 in each hand) and goes at
an adequate speed. The robot uses only 8 cylinders and 9 switches, where your
scorpion is on course for 50 cylinders and 100 switches. 6 large pump cylinders
is simply not enough!
Mark
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Message has 2 Replies:
 | | Re: SSClagorpion - Compressor
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| (...) Lessee, 5m tube vs. 5cm (or .05m) tube...I'd guess something approaching 100x as long? :) I'm guessing you meant to type something a little different there... Anyways, by my eye, the flex-system tube OD is roughly the same as the OD of the (...) (20 years ago, 22-May-04, to lugnet.technic)
| | | Re: SSClagorpion - Compressor
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| (...) This type of valve will be used to turn the power on/off (via polarity switch) for the compressor. Instead of needing an overpressure valve, the compressor will be turned off, saving batteries. I think we all agree, sometimes you really need (...) (20 years ago, 23-May-04, to lugnet.technic)
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Message is in Reply To:
| | Re: SSClagorpion - Compressor
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| (...) Steve, A very interesting set of questions and logic. I don't know the answer, but can add insights that are related. As the available volume/pressure increases, the pistons expand and contract faster. I don't know if there is a linear (...) (20 years ago, 21-May-04, to lugnet.technic)
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