Servo - December 2016

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