Automatic Potato Cannon Project 2010


Break Mirror of first movie -Automatic Potato Cannon

 

History

We started out by building a manual potato cannon, fuelled by ordinary hair spray, but using the same kind of parts for the chamber and the barrel. It was fired by unscrewing the lock at the top spraying in some hair spray and putting the lock back on. At the very first we used an electric lighter we put in a hole we drilled at the back of the explosion chamber. This method of ignition was kind of dangerous and bad, but we figured out we could use a spark from an ordinary battery by attaching two nails at the back of the chamber and having the tips close to each other inside the chamber, by having one cable from each, one into the battery and tapping the other cable to the other part of the battery creating sparks that ignite the hair spray. We then moved to a more powerful gas, starter gas. We then improved the ignition step by buying an ignition coil and using a button to fire our cannon.

 

 Quick stats
Barrel length: 160cm
Number of servos: 7
Controller card: Parallax servo controller
Number of pneumatics: 7 Lego and one home made.
Batteries: One 12V, two 6V.
Propellant: Starter gas
Velocity: 95-100 m/s 312-328 fps
   

 

Construction begins

At this point we decided to go automated! One of the things we wanted to fix first was to not have to stabilise the cannon with rocks behind it holding it in place from the recoil. So we built a stand.

We had one other thing in mind when building it and it was that it should be modular, so we could transport it in a car so we split up the project in several parts. The stand and recoil stabiliser, the spraying and fan out the exhaust, the ignition, the reload mechanics and a way to link all together and control it. We decided to use a parallax controller card to control servos we took from some old remote controlled cars we had lying collecting dust. We started with a bracket around the explosion chamber so we could mount the cannon on our stand later. This was mounted with a big plate at the back so we could make an opening where we could spray our fuel and also fan out the exhaust. At first we thought that it could work with some small leaks at the back with our first door. But we got aware that not even the tiniest leak was acceptable, we had to tighten up some small gaps between the plastic pipe (explosion chamber) and the plate at the back. Also we had to find a way to tighten up the door and also some way to automatically open and close it, what we came up with was pneumatics and Lego has some pneumatic parts that we made use of. For the door we used two pneumatics to give it good range. And one for each lever arm to lock the door tight. Next we welded an arm to hold the spray can and the servo to press it.
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Sparks and Pneumatics

Next thing to fix was the spark, we built a box that we fitted the ignition coil button and the servo to control it. Just in a small side note, we use some old tank to store the pressurised air, we have about 1.5 to 2 bar. At the top of the spray can arm we mounted a fan to clear out the exhaust. There was a thing with the pneumatic that we noticed at this point, it was very slow so we opened them up and found out that the inlet holes where very small, so we widened them using a drill and that gave a significant increase in speed and power.
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Reloading

Now we faced what would be the most difficult task of this project, the golf ball reload mechanism. Our initial design was to slide a slightly wider pipe on top of the barrel that had a golf ball sized hole on it so we could drop down balls there. The sliding pipe was there to make the other balls in the magazine not drop into the barrel, and make it tight enough for the explosion to not leak too much up the hole. The glide of that pipe was quite bad so we decided to make it just half a pipe and make a locking mechanism to hold it tight when fireing. We had a small incident with a fire and these ideas were scraped. We needed a more reliable solution so we redesigned it to use a hatch and another locking mechanism and also used a separate servo to handle ball drops. The new design is using a "T" shaped pipe where the golf balls are dropped into the small vertical pipe one by one by the ball dropping servo. The ball magazine can hold about 6 balls but can easily be extended if needed. There is a small spot of glue inside the long horizontal barrel so the balls can't fall back into the chamber. When the golf ball has been dropped the lid is closed by a custom made pneumatic cylinder made from electrical piping and an o-ring as seal. There are some air leaks but since the pneumatic cylinder is activated only for a short time this isn't crucial. When the top lid has been closed it is locked tight by two hasps controlled by Lego pneumatics.
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Control

To control the whole thing we used the Parallax servo controller card and wrote our own program in Java to control the flow of the process so we could easily change timing and check different parts of the cannon for debugging. The program allows us to set the number of times to fire in a sequence.
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The complete image gallery can be found here (some images are very big).

 

Contact: TheAPCTube@gmail.com

 

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