pneumatic catapults

[traxxasracr1]

A few team members and I (who just graduated and are kinda bored) are making a robot for Stronghold out of leftover parts. I was curious as to what size pneumatic cylinders you guys used for your pneumatic catapults and how long your arm was/where the pistons mounted ect... Just so we can get an idea of where to start. Thanks for your input

[Wren Hensgen]

The last time I did this, we used some fairly large (2-7/8ths) bore cylinders to throw the 2014 Aerial Assist ball.

A couple things to keep in mind:

Assuming you're using a decently sized cylinder, it should be a priority for you to keep your hoses to a minimum, and use a solenoid with enough flow rate to handle the shot.

You can latch a cylinder in place (we used a gate latch for this), or brake it, so that the pressure has a moment to build behind the piston. We found that even a quarter second gave us a significantly harder shot. This also gives you an adjustment, varying the charge time by fractions of a second results in different powers and arcs.

It's better to have a solid mechanical stop for the arm of the catapult. Don't slam the cylinder against the end of the travel if you can avoid it.

You'll get a better exhaust rate, and thus, a better throw, if you simply don't retract the cylinder. Leave the top port empty in order to do this legally.


Good luck!

[Billfred]

Some other notes we've found:

- Multiple small cylinders will fill faster than one big one. (You can also plumb just one of these for retraction, if you don't want to do a spring return.)
- A 1" webbing strap makes a great hard stop for catapults.
- High cV is the order of the day. Automation Direct sells one that's aimed that way, but I don't know the part number offhand.

[GeeTwo]

In addition to the items above:

The volume of each cylinder (or more precisely, the volume filled by each solenoid valve in completing the stroke) is more important than the bore or stroke separately, though a short, wide cylinder will be connected nearer the axis of launcher rotation, and a long, thin one near (or past) the boulder.

IIRC when I did our calculations, somewhere around 8 or 10 cubic inches was optimal. This was using airflow through a standard 1/8" NPT port and a 12" ball movement during the launch; a high flow port or a longer throw would allow a larger cylinder, while a shorter ball movement or smaller port would require smaller volumes, and thus more solenoid valves.

For best results, include an air tank on the low pressure side of the regulator. If the air has to work its way through the regulator during the stroke, you won't get the advantage of high-flow or multiple solenoid valves.

[Tom Ore]

https://grabcad.com/library/2016-525-swartdogs-2

You can download our CAD from FRCDesigns to see what we did.

We had two tanks on the high pressure side and two on the low pressure side. We used two cylinders on the catapult. We varied the shot distance by changing the length of time the solenoids were on and by putting a delay start on one of the solenoids. This gave us a decent shot from a wide range of distances. It took a bit of calibrating to find the best settings for different distances. The competition bot didn't work quite as well as the practice bot because we never had time to really dial it in. We used the sames settings from the practice bot but even small changes in valves and the robot make a difference.

[Joey1939]

In 2014, my team used two large pneumatic pistons on a catapult to launch the ball. We quickly discovered that every little advantage you give the piston in extending quickly makes a large difference. A piston by itself couldn't launch the ball with enough force. First, pistons can extend faster than they can pull, so make sure the catapult is setup to shoot when the piston extends. Second, holding the piston back for a second lets it build up pressure and extend more quickly when released. We tried using a gate latch but had so much trouble getting the gate latch to release when we wanted (In retrospect we should have used a little piston). Then in the offseason we used an electromagnet, which worked amazingly.

[troy_dietz]

Make sure to remove the fittings from the cylinder on the venting side to allow for maximum airflow out of the cylinder.

We used surgical tubing (cut in half to further reduce its stregnth) to retract the catapult after each shot instead of plumbing a retracting cylinder.

[Joey1939]

Originally Posted by troy_dietz

Make sure to remove the fittings from the cylinder on the venting side to allow for maximum airflow out of the cylinder.

We used surgical tubing (cut in half to further reduce its stregnth) to retract the catapult after each shot instead of plumbing a retracting cylinder.

You don't want to leave the retracting side empty because then you will get stuff inside the piston. We found a really nice filter that screwed into the piston.

Our piston setup was weighted weird because the pistons were backwards (so that extend launched the catapult) and gravity was our return.

[jspatz1]

Our Stronghold catapult used 1.06" diameter cylinders. The moment arm to the cylinder clevis was 4", and the arm length to the center of the ball was 27".

https://youtu.be/kKJDhcgQxkY?t=34

[troy_dietz]

Originally Posted by Joey1939

You don't want to leave the retracting side empty because then you will get stuff inside the piston.

We didn't have any issues throughout the season, but I could see that becoming an issue depending on how you decide mount everything.
We're usually pretty careful about metal shavings.

[GeeTwo]

Originally Posted by Joey1939

You don't want to leave the retracting side empty because then you will get stuff inside the piston. We found a really nice filter that screwed into the piston.

Our piston setup was weighted weird because the pistons were backwards (so that extend launched the catapult) and gravity was our return.

That seems reasonable. As a simple solution, a bit of cheesecloth and a couple of tie-wraps might also serve to keep swarf, dust, and such out.

Gravity return is perfectly usual for this sort of launcher. Even so, you will want to reduce the weight of the ball carriage to a minimum so that most of the energy goes into the projectile rather than in beating the carriage against its stop. This means mounting the cylinder body to the robot and mounting the piston shaft to the ball carriage. This sort of mount will also require less "loose" tubing.

Edit: And hoping that the use of piston to refer specifically to the portion of a linear pneumatic actuator which moves relative to the cylinder and fittings doesn't draw IndySam's wrath...

[adciv]

In 2014 we used a pneumatic catapult. After some experimentation throughout the season here's what we found:

1) The Automation Direct solenoids are the best we've seen. The higher CV rating the better. Andymark lists them on their site but I recommend going direct.
2) Use a large number of working pressure tanks, the regulator is a choke point.
3) Use Pressure Sensors to see what's REALLY going on in your system. One thing you'll find is the piston pressure is substantially below your working pressure when firing. You then use these sensors to see how any tweaks you perform affect your performance over the entire stroke.
4) Either use single solenoids or depower both sides of a double solenoid when not firing. We found the electrical transition does have a noticeable effect.

[Brandon Holley]

You can see the inner workings of our catapult in the below image. We used 4x 7/8" bore cylinders, 4" stroke each.



We did not use the super high Cv solenoids automation direct sells (mainly because they are GIGANTIC). We used some smaller sized SMCs that we have used many times along our history.

One thing we did do though was limit the runs of tubing to as small as possible. To do this, we mounted the manifold & solenoids right next to the catapult cylinders.

Additionally, we created a 'pre-charge' tank @60psi to remove the pressure regulator from the flow path. This meant we had a black plastic Clippard tank @ 60psi right next to our manifold, which was right next to our cyilnders. Each shot from our catapult used about 1/3 of the black Clippard tank, which again helped as we maintained fairly consistent pressure in the entire system throughout each shot.

Good luck! We shot approximately ~12,000 test shots on 10 different prototypes to reach the final configuration we liked .

-Brando

[jvdiv]

We built a pneumatic catapult for RI3D on 'Snow Problem. We used a pair of 0.75" bore 7" stroke cylinders with high cv valves (Norgren v60). The black tanks are at 60 psi. The catapult is gravity return, top of the cylinders are left open. The distance from the pivot point to the piston mounting point is 6.5". The length from the pivot to the center of the cradle is 20.5". I would be happy to answer any questions.
Imgur Album

[Cothron Theiss]

This is a bit of a tangent, but what do people typically use as the fulcrum in the catapult? Do you just drill holes and run a strong bolt through, or do you incorporate bearings and go with something a bit more engineered? In 2014, we had an excessively overpowered kicker (A poor design led to us putting a dangerous amount of power into that kick.) that ended up bending several rods we were using as fulcrums. The hardstop was actually between the fulcrum rod and where our surgical tubing was mounted, so when the kicker arm hit the hardstop, the kicker arm applied a pretty large amount of side loading on the intended fulcrum rod because the corner of the hardstop began acting as the fulcrum whenever it hit the hardstop.

So long story short, what methods do you all use to mount the catapult arm and/or use as a fulcrum?