On LEGO Compressors & Pump Testing
by Charles
Steadman |
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| Dear fellow Lego fanatic, |
| Initiation |
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First, many thanks for your superb site, the basis for all my exploration of
pneumatics. I've recently re-discovered Lego Technic with the Robotics
kits, after getting one of the first sets in 1978. Your site encouraged me
to order some pneumatic elements over the internet, but while I was waiting
for their delivery, I bought some second-hand sets containing some of these
elements. The one thing I was missing was any of the small pumps, and so I
had to use larger pumps to create compressors. When the small pumps arrived
(at last), I built up some of the compressors as shown in your pages, and
was disappointed with the results. |
| Limitations
of Pump Testing
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It seems to me that the "high-tech" testing series measures static pressure
of the system, which is not what is critical when operating pneumatic
systems. All this gives is the speed of the initial movement. The same
applies with the RCX testing, as you are only requiring one operation stroke
every three seconds. When using several pistons to recreate control loops
(and when these loops are running), there is never the high pressure in the
system that is being measured using these tests.
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| Enter
the Large Pump |
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What we actually need is air being moved down the tubes -- and lots of it!
It seems natural that the large hand pump will be able to shift several
times more air than the small pump, as it has so much larger volume. But,
there are difficulties to overcome with this pump, as follows: |
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a. A large amount of force is required to operate it -- really too much
even
for the new motor when under significant load.
Just gearing down the motor slows it down too much to have a fast
"recovery" time of pressure to the system. |
| b.
A strong structure is required to avoid the compressor breaking apart. |
| c.
There is the usual spring/no spring issue to decide. |
| Solutions |
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The solutions to these points seem to be as follows: |
| a. Two motors can easily drive a pump -- in fact, two motors can easily
drive
two pumps.
Having two motors allows the gearing to be quite quick, and balances
out
some of the stresses on the gearing. |
| b.
Provided the compressors are kept on the same horizontal level as the
drive
axle, the model can be strong without being clumsy -- this requires the
use
of the double-hole two-bricks to create the correct half-unit spacing. |
| c.
Two pumps can be mounted at a 180 degree offset, so that the
compression of
one spring is assisted by the expansion of the other -- this (almost)
removes the problem of the spring overloading the motor. |
| Another Pump Tester
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I developed a much simpler testing system than that described on your web
pages. It tests something quite different -- not the absolute pressure in
a static system nor the (related) speed of one stroke of a piston. This
system tests the continuous working speed of the compressor under load (i.e.
with air being used up at all times).
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It consists of a simple control loop
of three cylinders, the last with reverse feedback to the first. The additional switch on the test rig is just to allow the
air supply from the compressor to be switched on and off.
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When air
is applied, the three pistons extend in turn, then retract in turn. My test
consists simply of timing ten cycles of this loop, which equates to sixty
strokes of a large piston, each against the load of a pneumatic switch.
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Of course, any compressor will (given time) be able to create pressure in
the system, and it can be released when required. However, the continuous
speed of operation shows the volume of air that is being created by the
compressor, and is much more useful when translated to an operating model.
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| Results
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The results of my testing of the existing compressors using the above system
are as follows (average of several measurements):
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Tiny compressor (one small pump with pulley):
Improved dual-acting compressor (two pulleys):
Hand pump pumped like crazy:
Two hand pumps together (manually):
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60.0 seconds
55.3 seconds
30.1 seconds
21.0 seconds
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As you can see, there is a measurable but unimpressive difference between
the tiny compressor and the dual-acting compressor. However, the hand pump
thrashes both of these options comfortably (30 seconds equates to half a
second for each stroke). Linking two pumps together side-by-side and
operating them manually is even more impressive, but very tiring! My
objective was to get closer to these theoretical figures.
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Dual Large Pumps Compressor
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After many different attempts and tons of rebuilds, I created a fairly
compact two-motor two-pump compressor, which has achieved figures of 24.3
seconds for the above test -- faster than a manually-operated hand pump at
full speed. All tests were performed with brand-new Duracell
batteries.
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In the DAT file for this compressor (and on the pictures), the compressor has its own in-built
pressure limiter switch. Most of the rebuilds were because I wanted the
compressor to be as compact and tidy-looking as I could manage, and to make
the limiter switch work. It relies on a home-brew sensor, made by
de-soldering a mouse button (an Omron module from a Logitech mouse), and
gluing it into a 2x2 brick with the edge cut away. The tiny white switch
protrudes from the edge of the brick. Two electric wires come out of the
brick, and I've wired it such that depressing the switch makes a break in
the wire -- it could just as easily be wired in the opposite manner. A 2x2
plate is glued underneath to enable it to stick to other bricks properly. A
picture of the switch block and one end of the wire is shown below.
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The compressor could just as easily be made without the pressure limiter,
but it works very well with it -- the trick is to ensure that the two shock
absorbers give enough resistance to stop the piston moving until just before
the motors run out of puff. This allows the maximum pressure to be
delivered by the system, but ensures that the battery is disconnected before
the motors become overloaded. Note that to give this exact point, there is
a 2x2 tile "dropped" into the slot between the pressure switch piston and
the home-brew switch. This makes the switch activate at precisely the
correct point in the shock absorbers' travel. Different battery
arrangements and possibly different pumps will force you to remove or add to
the thickness of this adjustment device.
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Also, as shown in the pictures, it is possible to make this compressor with
most of the tubes and internal wires hidden -- one of my pet desires. It's
quite fiddly though!
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Discoveries
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Interesting points to note that I discovered during these tests:
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Old pistons can make a significant difference. Some pistons that are around
five years old (and have been left in an extended position) get results as
much as 50% worse than brand new pistons.
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In addition, bad connections or
leaky switches dramatically affect the results.
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It is important to test all
compressors and developments using a fixed environment. If you rebuild the
test system as shown, you will probably get significantly difference figures
from mine, but you should find that the relative performances are similar.
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For example, using cheap no-name alkaline batteries instead of brand new
Duracell batteries extended the time for my compressor from 24 to 43
seconds, but also extended the time of the other compressors
proportionately.
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The test rig is a great way to test piston and pneumatic switch performance.
When running, it's immediately obvious if one piston is failing to operate
as quickly as the others -- swapping it will almost always increase the
speed dramatically. Note that a sticky switch causes the same problem, so
be careful when tracing the fault.
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If you want higher ultimate pressure at the expense of recovery speed,
replace the two large double-bevel gears with the next size down, and raise
the motors in the model by one plate thickness (an easy modification). This
creates a lower speed, giving results of amount 27.5 seconds on the test
described above, but increasing the static pressure (if that's what you
want).
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I am uncertain about the effect of springs on this compressor. As
described, one spring assists the other because of the offset. But it seems
that it should be better without springs (as the energy contained in one
compressed spring can't be fully delivered to the other one). However,
swapping pumps for ones without springs makes less difference to the figures
than swapping from one pump to another one -- the difference is therefore
effectively unmeasurable for me compared with the variation from one pump to
another.
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Finally
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I do hope that this email appears constructive! I don't have a web page,
and even if I did, I would prefer any developments to be put on your page,
to retain its position as the leading authority on Lego pneumatics. So
please feel free to include these photos, DAT file, and any parts of this
email on your web page if you think they are interesting.
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CS: Thanks, Charles, for your most constructive contribution!
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LDraw
DAT file for Dual Large Pumps Compressor by Charles Steadman |
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