for a 1/4” pipe size, and
was rated to 10,000 PSI,
operated on 24V just like
Protobot, and only weighed
three pounds. As a solenoid
valve, we wouldn’t need
any other sort of trigger
mechanism, and we could
add control with a simple
relay.
Since the solenoid
valve is such a great
solution for the purge
valve, why didn’t we use
one for the main cannon
valve? Why muck around
with torsion springs and
trigger mechanisms like we
did in our last project?
That’s because for the
purge valve, a small pipe
size and small valve flow is fine. It doesn’t matter if the
nitrogen takes a little bit of a leisurely time exiting the
tanks; as long as it purges the tanks, that’s all we care
about. For the main cannon valve, we need to minimize
pressure drop with a large diameter valve. Large diameter
high pressure solenoid valves exist — but they are
prohibitively heavy ( 50 pounds plus — way heavier than
even two pugs) and prohibitively expensive.
One of the last specialized components we needed was
a pressure transducer. The transducer was essential to allow
us to take readings of the pressure in the tank, and to
ensure that we didn’t overfill it. We tracked down a
transducer from the appropriately named Transducers
Direct. The TDH30 series we acquired could handle up to
5,000 PSI, and gave a 0-5V output. Paired with an
appropriate panel meter, we would be able to take pressure
readings as accurate as the Naughty and Nice List after it’s
been checked twice.
The last of the major components was the pressure
regulator. We planned to fill the cannon through a
nitrogen tank, and a regulator would need to go
between the nitrogen tank and the cannon. We
selected a 6,000 PSI regulator from Harris, which was
a beefy unit that would allow us to use a 6,000 PSI
nitrogen tank for filling. We were debating between
using a 6,000 PSI regulator or a cheaper 3,000 PSI
regulator that would probably require multiple 3,000
PSI tanks to fill the cannon. With all of our 5,000 PSI
and even 10,000 PSI parts, chickening out and going
for the bare minimum pressure rating on the
regulator seemed like ending up on the Naughty List
on December 24th. So, we maintained our
commitment to high pressure and went with the
higher rated component.
Silver Bells, Steel Fittings
After numerous visits from Santa Bot at Robot Central,
we had all the components for the cannon. Putting it all
together would be somewhat similar to our experience with
the PVC cannons, in the sense that it would involve putting
primer and sealant on the parts, twisting them together,
and ensuring that the cannon tanks and barrel line up in the
same plane. Since we were dealing with steel and not PVC,
there was a little more prep work that needed to be done.
The steel pipe from Totten Tubes came with beautiful
threads, but the inside of the pipe was a bit messy (likely
from the lubricant and chips and whatnot from the thread
cutting). We ideally wanted a nice smooth barrel for our
projectiles, and we wanted to clean out the tank pipes so
that we didn’t have any griminess knocking around. So, how
do you smooth out the inside of a three inch pipe without a
large reamer or a lathe or some other sophisticated
SERVO 12.2016 57
PREPPING THE END CAPS.
PROPER TOOLS FOR
PROPER TAPPING.
BANANA FOR SCALE.