Subject:
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Re: SSClagorpion
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Newsgroups:
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lugnet.technic
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Date:
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Thu, 15 Apr 2004 15:46:59 GMT
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Viewed:
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8772 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, Kevin L. Clague wrote:
>
> > Pressure is in pounds per square inch. So if we know the area of the face of a
> > piston, and we know the pressure we can determine how many pounds a single
> > piston can lift.
> >
> > Force = Pressure * Area
> >
> > A pneumatic piston face is about a half an inch in diameter. The area then is
> > pi times diameter, or about 1.6. Four pistons then have the combined area of
> > 6.4.
>
>
> Mark Riley was nice enought to point out the errors in my math. The area of the
> piston care is pi * r^2 = 0.2
>
> Four times that is 0.8.
>
> So the minimal pressure needed is 10 pounds / 0.8 square inches or, 12.5 pounds
> per square inch. Still well below the 32 PSI upper limit for LEGO pneumatics.
>
> The next question is, how easy is it to get to 13 PSI with LEGO based
> compressors.
>
> If we double the number of pistons, we'd only need to get to 6 PSI, but will
> this really make much difference on piston transition time.
>
> The pistons that do the most work (force through a distance) are the horizontal
> pistons. They actually make the distance term non-zero.
>
> Back to compressors for a second...... There has always been this debate about
> which pumps are better, small pumps or large pumps. Small pumps provide less
> piston face are than larger pumps, and can drive the pressure in a closed system
> to max pressure quicker than the large pumps (even with spring removed)... but a
> pneumatic sequencer (motor for example) is not a closed system. It is always
> releasing pressure when the pistons are switching direction. A second test that
> measures the RPM of a pneumatic engine (motor?) using a small pump vs. a large
> pump showed that the large pump did better.
>
> One of the issued with large pumps is that they are large... large enough to
> affect the minimum scale of a compressor, and therefore the creation being
> made......
>
> I'd like us to make Scorpion completely out of LEGO parts. I'm not a full time
> purist, but typically this is my goal so that others can make copies. I guess
> also that I'd like to make it out of currently available parts. This rules out
> the use of the original single ported pistons. Unfortunatly this also rules out
> the use of:
>
> http://www.bricklink.com/catalogItemPic.asp?P=4692
>
> Using two of these and a double ported piston would give us a double acting
> pump, instead of the single action pumps lego makes. Presumably this would
> reduce the inherent amount of friction in the compressor.
>
> Kevin
A few things spring to mind about this:
1. I usually run robots from a car tyre air compressor at 25PSI. This gives
good performance with 1-2 large cylinders moving at a time. Using 6 large pump
cylinders as the air source, performance is half as good, probably around 13psi,
which is what you're aiming for. I suggest that 6 large pump cylinders with
springs removed might provide enough air.
2. However, the model will have to support its own weight without a gravity well
(due to losing pressure in supporting leg cylinders when other cylinders use the
air in the pipes). You'll have to either increase the source pressure so that
the minimum pressure stays above about 10psi (gaining a bit of leeway for the
friction helping to prevent movement) or put more isolation logic in place on
each leg supporting cylinder.
3. The model is going to walk, so the force on each leg is potentially a dynamic
one, if it walks fast enough. When you jump repeatedly, the force on your legs
is not equal to your weight, but significantly more - at least double, depending
on how fast you jump. Therefore the load on the legs will be at least twice the
weight of the robot divided by the number of legs that stay on the ground when a
step is taken.
Since you're going for a completely current Lego parts solution, that rules out
my 2-mini-compressors-driven-by-a-12-volt-train-motor solution. The most
powerful motors you've got are the 8475 RC buggy ones with the red plugs on.
They'll use 7.2W of electrical power but put out 2.38W each.
See http://www.philohome.com/motors/motorcomp.htm
I suggest you make a trial compressor engine using a pair of 8475 RC buggy
motors geared down and four pump cylinders with springs removed, with cranks
offset at each 90 degree angle. Offsetting the pumps by 90 degrees should make
the load on the motors smoother. The engine will need to be very strong due to
the torque required. I suggest two shafts geared together, supported by double
width beams throughout. Use 40 tooth cogs to make the cranks as they're more
sturdy than other crank parts and will allow you to gear the two shafts together
easily.
Provided the non-return valves in the pump cylinders are good, you only need to
use T-junctions to connect them together. I suggest an initial shaft speed of
1rev/second. That said, using 6 pump cylinders in my hands I pump them at a
rate of 1 pump/second, which is 6 cylinders of air/second. The engine will do
just 4, but see how well it works with the amount of motor power. You might
need several engines!
Just a thought - what's the mechanical load required to compress a large pump
cylinder at 25psi? That's the load the motors have to cope with. You could
measure the volume of a pump cylinder by using a graded syringe to collect the
air.
Overall, the load on the compressor engine will be greater than the load on the
legs, since the system can't be 100% efficient. My car tyre air compressor
needs 6 Amps at 12 Volts, so 72 Watts - that's ten RC buggy motors or 5 engines!
This is getting expensive! Surely a £20 compressor and 12V 6Ah lead acid
battery are fair game? I abandoned Lego controllers for my railway in favour of
a large PSU as soon as I got beyond 2 train motors!
Mark
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Message has 1 Reply:
 | | Re: SSClagorpion
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| (...) That makes sense, but you may want to take a look at the video of Kevin's walkers: (URL) you see, at the end of each stride, all four legs are at the bottom of their travel. So while it is true that half the legs will have to support all the (...) (21 years ago, 16-Apr-04, to lugnet.technic)
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Message is in Reply To:
| | Re: SSClagorpion
|
| (...) Mark Riley was nice enought to point out the errors in my math. The area of the piston care is pi * r^2 = 0.2 Four times that is 0.8. So the minimal pressure needed is 10 pounds / 0.8 square inches or, 12.5 pounds per square inch. Still well (...) (21 years ago, 15-Apr-04, to lugnet.technic)
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