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Unread 09-02-2017, 07:39 PM
pKlopp pKlopp is online now
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Pneumatapult Questions

I have heard of teams using high flow rate solenoids and gate latches in order to create a harder shot.
Which method is better?

How do you recommend I create variable shots? Would an adjustable hard stop work? What kind of hard stops have been used for pneumatic catapults?

Thanks in advance!
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Unread 09-02-2017, 07:47 PM
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Re: Pneumatapult Questions

You don't really need either to make a functional pneumatic catapult. You need to remove the fitting from the far end of a two-stroke cylinder, and either have geometry that starts at a mechanical disadvantage (which slows down the initial actuation and allows pressure to build in the cylinder - for an example, see our 2014 catapult design, which was inspired by some team I cannot remember here in CD) or else have tanks downstream of the regulator (the regulator is a huge bottleneck in terms of airflow).

High-flow solenoids do make a difference and can help eek out additional performance - but they're not what make the catapult work in the first place.

I do not like gate latches in mechanisms like these. They're an additional hazard I'd rather avoid.

There are many ways to get variable shots - a variable hardstop will certainly work. Variable actuation time can work well, too, but only if you're using the "tanks downstream of the regulator" approach (the "weird shooter geometry" approach seen in our 2014 bot above is pretty much designed for one shot and only one shot).
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Last edited by Oblarg : 09-02-2017 at 07:50 PM.
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Unread 09-02-2017, 08:01 PM
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Is it better to have a bank of small cylinders or one or two large bore cylinders?


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Unread 09-02-2017, 08:06 PM
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Re: Pneumatapult Questions

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Originally Posted by pKlopp View Post
Is it better to have a bank of small cylinders or one or two large bore cylinders?


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What do you mean by "better?"

You can, ideally, get more net airflow into more cylinders, however more cylinders take up more space.

For throwing objects like the 2014 ball, we've had good luck with 1.5''-bore cylinders. YMMV. There's no substitute for experimentation.
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FRC Drivetrain Characterization
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Unread 09-02-2017, 08:42 PM
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Re: Pneumatapult Questions

The two obvious ways to get multiple shot angles with a pneumatic catapult is to use multiple cylinders or stop early in code.

See examples here:

Catapult A has 2 cylinders (plus extras in line for more force etc) in line. One has a 3" stroke, the other has a four inch stroke. If both cylinders fire, you have 7" of effective stroke. If one does, you have 3" or 4" depending on which one fires. If neither fires, you have 0" of effective stroke. If you're good at geometry, this type of setup can get you 3 shot angles. Using the 2016 example, this could work to get an outer works, just off the batter, and batter shot all in one mechanism.

Catapult B has just one effective piston, but limit switches or beam break/laser break/whatever is called sensors placed at a few spots to be triggered by the swing of the catapult arm. Obviously takes some intelligent code and prototyping to test reaction time, but in theory one could just have the pneumatic actuator go back to the "in" position before its all the way "out".

Other ways include an adjustable hardstops, including rollers on your catapult (not quite adjusting angle but adjusting shot), and likely many others I am forgetting.
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Unread 09-03-2017, 09:09 AM
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Re: Pneumatapult Questions

In 2016 Stronghold, we used a gate latch, but the pneumatic cylinder was used to stretch springs, and the latch held the arm in the stretched position. We did the math for a "charge the cylinder and release the latch" style catapult and found that you had to latch with the cylinder about 1/3 full (don't recall the exact fraction) so that the pressure didn't fall off too quickly after release.

If you do the straight pneumatic launcher, even if you do not get the "high flow" valves and fittings, DO be sure to use valves with the full 1/8" NPT ports. Our prototype of this was inadvertently using valves with M5 ports and a Cv of about 40% of the 1/8" port. By the time we realized the error (at disassembly over a week later), we were committed to springs of some variety.

Finally, if you do opt for multiple cylinders, use a separate solenoid valve for each one. There's very little advantage in using multiple cylinders if you run all the air through the same valve bottleneck. If you use a relay, you can control the whole group together, but if you control them with the PCM, you need to use a separate channel (or channels) for each one.

(Usual caveat about the possibility of the rules changing in future years!)
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Last edited by GeeTwo : 09-03-2017 at 09:11 AM. Reason: Added relay and caveat
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