Pneumatic Catapult Help (non-engineer mentor)

Needing help in diagnosing catapult project. Project was to launch basketball (22oz) from the 3 point line (20ft) and through hoop (10ft). Ideal angle of entry would be 45 degrees. Launch height of approximate 2.5 feet, arm length of catapult is 2.5 feet, weight of arm is 3 pounds 10oz. Students used AMB Design spreadsheet to determine release velocity to be 9.4 m/s. Release angle to be 60 degrees.

They determined that approximate amount of force required for launch would be 250 newtons. The mass of the arm and ball is 2.26796 kg. The calculated acceleration was 110.45 m/s/s. (0 to 9.4 m/s over approx .4 meters.

From our collection of leftover parts we have four 3/4 inch pistons 7 inches long. Our calculations indicate that each piston should produce 117 Newtons of force at 60 psi. Pistons are 9 inches from fulcrum and are placed to be at 90 degrees to the arm at the midpoint of arm travel.

Each piston is on its own solenoid and is placed as close as possible to piston. Pistons have the output opened up to allow for better airflow. We have 2 tanks storing air at 60 psi so it doesn’t have to go through regulator at shot. We have a latch system to hold the catapult arm momentarily from deployment of solenoids before shot.

The problem is projectile is only traveling about 12 feet and max height is about 8 feet. Does anybody see any problems with our math our other suggestions to help out?

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Part of your problem may be that pneumatic pistons do not apply their full force instantly, it takes time for the air to fill the cylinder, during which the force builds towards its maximum. That 117 newtons @ 60 psi is how much force the cylinder is exerting when it is completely filled and completely extended. As such, your system is applying a steadily building force to the catapult as the cylinder extends, but it is not the full 117 you are expecting, and you are using much of the range of motion of your arm before you have the type of force you need.

One possible solution, as many teams (mine included) used in 2014, is to have the piston fired and pushing against a hard stop, such as a latch, before you fire the catapult itself. By doing this, you allow your pistons to build pressure behind the plunger that will act instantly when the hard stop is removed. This solution is more effective the more you allow the cylinder to extend before hitting the hard stop, as that will allow for more air to build pressure in the active part of the cylinder. The trade off here is that you would not be able to use the full throw of the pneumatic cylinder to move the actual catapult arm.

I will take a picture of this system on our 2014 robot as an example, I hope to post that later tonight.

Please let me know if you have more questions or if I can help any other way!

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Sounds like pairing Gas shocks with pneumatic cylinders can do the trick too?

Generally gas shocks are very slow acting. Holding the catapult while the cylinders are extended maybe half way and fully energizing then releasing the catapult will help. The flow rate into the pistons is simply too slow.

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We have a latch installed to prevent movement to allow system to pressurize. Currently it prevents any movement of the arm at all. Approximately how much extension on the piston did you have before it hit the hard stop?

in 2014, team 3081 installed an electromagnet on the underside of its piston arm to hold the arm while the piston was building pressure.

The faster a pneumatic cylinder is traveling, the less force it can apply due to the constricted air flow through the solenoid valve and the tubing/fittings. I ran some fairly detailed air flow calcs a few years ago (STRONGHOLD) for cylinders in the 1 to 1-1/2 inch diameter range, and came up with an effective pressure of about 13psi in a quickly moving cylinder when the supply was 60psi.

For our STRONGHOLD bot, rather than use four times as many cylinders, we wound up using cylinders (actually one larger one) to extend springs, latched the arm, retracted the cylinders, then launched by releasing the latch.

I’d start by adding springs / elastics in addition to the pistons, in theory you could double the force of your pistons in once direction with springs. You would however need to use a double solenoid to actuate the pistons in both directions. I believe this is the system 118 (and many other teams) used on their 2016 robots.

If you are just looking to make the cylinders fire faster, look at the bottom of this document to see some tips to get more air into the cylinders quickly and removing restrictions from the exiting air.

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I’d recommend using longer cylinders and only using the top 1/2 - 2/3rds of the travel. That way you can latch the arm down, pressurize 1/3 to 1/2 the volume of the cylinder to 60 psi, and then release. Doing it this way does not rely on the speed of air flowing into the cylinder, and allows you to use a larger diameter for more force.

We used this technique on our 2014 robot using 2 1.5inch diameter cylinders to score in the high goal. We left the other port on the cylinders open to prevent restrictions letting the air out. Even waiting for gravity to retract it, it still went fast enough to pull off a two ball autonomous.