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Hi,
Single LEGO pneumatic switches are not capable of controlling a pneumatic
system that uses both pressure and vacuum to drive a given cylinder at the same
time. A single switch does not let you capture the exhaust inherint when a
piston contracts or expands.
By combining four switches, four T's, hoses and a tricky linkage, I've created
a very reliable composite switch that can contain the exhaust.
Here is a top view of the switch:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190018.jpg
Port 1 allows air pressure in. Port 2 and four are connected to a double acting
piston's ports. Ports 3 and 5 are combined together and provide access to the
piston's exhaust. The piston on the left was used for testing.
In the above configuration port 1, and port 3/5 are interchangable.
I combined two of these composite switch/pistons to make a simple timing circuit
for testing.
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190001.jpg
The above timer circuit goes through a repeated pattern:
top - contracted, bottom - contracted
top - expanded, bottom - contracted
top - expanded, bottom - expanded
top - contracted bottom - expanded
top - contracted bottom - contracted (notice the repeat starting here)
I created a compressor that creates both pressure and vacuum:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190002.jpg
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190003.jpg
I combined the pressure/vacuum pump and timer circuit like this:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190004.jpg
When the pressure/vaccum pump is connected to batteries and turned on, the
timing circuit runs as desired.
How does it work?
By only using half of a pneumatic switches capabilities we can avoid using the
switches feature to exhaust out the handle hole. By two switches we can route
pressure to either port of the piston without exhaust. By pairing each of the
original switches with another switch (also half used) we can run the exhaust
back through the second switches.
The switch pair are in one of three states:
first second
on off
off off
off on
By combining the pairs together using a linkage with hysteresis, and rubber
bands to automatically return switches to off position, we never have leaks.
Here is the three states of the composite switch:
Pressure goes to left piston port:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190020.jpg
Pressure is off:
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=1497306
Pressure goes to right piston port:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190021.jpg
Here is the back side of the composite switch:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190017.jpg
Here is a birds eye view of the composite switch:
http://www.brickshelf.com/gallery/kclague/vacuumpressure/pb190016.jpg
Hypothetically, these pistons and switches could be combined to create a an
engine that uses both pressure and vacuum at the same time. I'll leave that as
an excercise for the student.
This implementation is a proof of concept. It is pretty large, but it
functions. I may work up other implementations I've thought of.
Please let me know what you think.
Kevin
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