cases, routing the exhaust air through
a muffler is the simplest way to avoid
the hiss associated with actuator air
being vented.
Pneumatic Actuator: Typically, air
cylinders are used to get motion from
compressed air. With air cylinders,
you usually see two types: single acting and double acting cylinders. A
single acting cylinder will have a single port for air to enter/exit (
Figure
8). These cylinders usually rely on a
spring to return them to a start position and will move to the end of their
travel when air is applied. They will
return to their start position when air
pressure is exhausted.
Double acting cylinders have two
ports, typically designated A and B.
When pressure is applied to the A
port, the cylinder rod will move to
the end of its travel. You then apply
pressure to the second port (typically
designated B) in order for the cylinder rod to return to its start position.
Also, you must remember that if pressure is applied to the A port, the B
port must be routed to exhaust (and
vice versa) in order to allow the cylinder to move.
In most cases, single and double
acting cylinders will move a rod in
and out in order to perform work.
The rod may extend from only one
end of the cylinder or, in some cases,
may extend from both ends (
Figure
9). Some cylinders are mounted in
rigid enclosures with guide rods that
will allow the device to move precisely without rotating or deviating from
its path.
There are also rotary actuators
that will rotate from one position to
another (i.e, 0 to 180 degrees) when
air is applied (Figure 10). These are
sometimes used to rotate a turntable
to position parts or can be used to
open/close a door on a mechanism.
Air Reservoir: (Figure 11) Though
this is usually optional, some designs
may call for a higher volume of air
than may be provided by the compressor tank itself. If you want to
store up "extra" or if your device is at
the end of a long hose run, you can
place an air reservoir near your solenoid valve and cylinder in order to
insure a sufficient supply of air for a
given pneumatic assembly.
SCARY AIR!
My first foray into using pneumatics was when I was building props for
my haunted house — the Spiders
Preyground (see Resources). I wanted
to build a prop affectionately called a
"Trash Can Trauma." This is a specially
modified trashcan that can pop its
top on command to give folks a good
startle. I researched designs online
and they all used pneumatics to provide the motive force to get a fast
acting and reliable pop-up action.
After looking over a few plan sets I
found on the Internet, I started compiling a list of the things I would need
to make my own TCT.
I found everything I needed for
my first pneumatic prop in my junk
bin, on eBay, and through a couple of
surplus component websites. In short
order, I had a 17 inch Bimba air cylinder, a Surplus Center five port, two
way solenoid valve, and a pressure
regulator. I mounted all the parts on a
piece of scrap wood (Figure 12) and
then added some simple electronics
(solid state relay, colored light bulb,
■ Figure 8. A 1.75" single-acting,
spring return pneumatic cylinder.
■ Figure 9. A dual-acting guided
cylinder with 1" of travel.
PERSONAL ROBOTICS
and a strobe light). I then placed the
device into a galvanized trashcan.
When connected to power and given
only 45 psi of air, this made for a
very fun and scary Halloween prop.
Simply press a button and the trashcan lid pops up with a scary skull
■ Figure 10. A Festo brand adjustable
rotary actuator with 180 degrees of
rotation.
■ Figure 11. A typical, inexpensive
five gallon air reservoir.
■ Figure 12. The trashcan pop-up
mechanism before mounting.
February 2009 83
