Launching the ball

[falconmaster]

Thought some of you might like this, dug out of archives, team 39 's robot was great at this and we have partnered up with them again this year. This is there 2008 robot launching this years ball.
https://www.youtube.com/watch?v=fsGUgaVG2ho

[JesseK]

Wasn't there something special about their pneumatic setup that year? They found a way around the air flow issue somehow, but I don't remember.

[Christopher149]

We found video of 39 (https://www.youtube.com/watch?v=Nn5iHFaDVgU) early on on Kickoff day, with the 2008 ball. I too would like to know if they did anything special.

[JesseK]

Ahh, now I remember. Unfortunately, <R90> prohibits that specific setup. However it may not be necessary this year, given that this ball is ~1/3 the weight.

[LH Machinist]

The pneumatic trick is called blocking the cylinder. You essentially use two pistons, one large and one small whereas the smaller cylinder acts as a release mechanism. The larger cylinder is activated, but it cannot move because the smaller cylinder is blocking its movement. When the smaller cylinder is moved it unblocks the large cylinder. The resultant force of the large cylinder is tremendous.

[MrForbes]

I don't think they used that trick. It's kind of dangerous.

[yash101]

Yeah. What if some metal snaps? There will be so much force built into the mechanism, the bot would be too dangerous for students to be around while active! I think it may be a good idea to work a little more on the design? Why not just use many small-diameter pistons, to retract quickly, and act as a team to generate a ginormous force?

[MrForbes]

We're not talking about a punching design. We're talking about an arm design. If you watch the video, you'll see....

[JCharlton]

Yeah. What if some metal snaps? There will be so much force built into the mechanism, the bot would be too dangerous for students to be around while active!

Actually the force involved is no more than the cylinder has normally when it has reached its final position. Anything not built properly can be dangerous, cylinders no more than spinning motors.

The key advantage to this arrangement is that the air has already evacuated the other half of the cylinder leading to a faster extension. Instead of having to push 60psi air out of the chamber, that volume of air has already gone. As the cylinder extends, the remaining normal pressure air will have to leave too, slowing the cylinder perhaps as it extends.

In previous years our team has used quick exhaust valves, but I believe that those are not longer permitted. Instead the return air has to pass out the solenoid valve. Haven't read the rules in that detail yet. Pity, we used it on our 2008 catapult.

[Chris is me]

I don't think they used that trick. It's kind of dangerous.

It's definitely not for the faint of heart, and getting that small cylinder to smoothly catch and release a pre-charged large cylinder is mechanically difficult... But it's a tempting way to make a linear punch, I must say.

The actual pushing force of the piston may be the same, but it's dishonest to say it is "just as dangerous as a normal piston". The piston is moving several times as fast and releasing the force all at once. (Yup, contradicting myself)

[s_forbes]

Team 39's robot (as shown in the video, modified for prototyping purposes) does not violate any of this year's pneumatic rules that I can tell. The original design used several solenoids to feed one cylinder, but this is prohibited in the 2014 rule set.

The partially filled catapult cylinders + latch approach from 2008 was very dangerous, I cringed a lot when seeing these robots used at competitions.

Here is a shot of the NERDS (team 1726) catapult from that year. Due to the geometry, the weight of the ball allowed pressure to build up in the cylinders before they had enough force to get the ball moving. This is similar to the latch approach, but if it is dry fired there is not a massive amount of stored energy being released instantaneously.

Catapults are dangerous! Make your system as safe as possible if you take this approach.

[RRLedford]

Not sure what "blocking the cylinder" actually is, but to have most of the air ahead of the retracted piston be gone when you want to fire the piston with max. peak velocity, you either need to evacuate it with a vacuum pump, or seal the exhaust port when the piston is full extended and then pull the piston back into the cylinder, creating a vacuum inside the cylinder as it retracts.

This demands some kind of retraction mechanism combined with a trigger hold/release mechanism strong enough to retain the piston with 120 psi behind it. Venting pressure during retraction and reapplying it after retraction allows for the retraction mechanism to be weaker. The key factor is to have a cycle that maintains as little air in the cylinder as possible when piston is retracted and ready to fire.

Having to push out almost no air from the cylinder, ahead of the piston disk as it accelerates gives a serious boost to peak velocity, and lengthens time in contact with the ball.

-Dick Ledford

[MrForbes]

This demands some kind of retraction mechanism combined with a trigger hold/release mechanism strong enough to retain the piston with 120 psi behind it.

you mean 60 psi?

It works fine without evacuating the other end of the cylinder.

[JesseK]

Ah, I see. The Poofs had a similar setup for the actual launch in '08.

[Jibri Wright]

Maybe the piston doesn't have to apply force directly. Maybe it can be used to apply centripetal velocity to the ball or pull the ball during launch. Food for thought.:rolleyes:

[Christopher149]

In previous years our team has used quick exhaust valves, but I believe that those are not longer permitted. Instead the return air has to pass out the solenoid valve. Haven't read the rules in that detail yet. Pity, we used it on our 2008 catapult.

It's been a while since our team has used pneumatics, but there seems to be very little in the rules dictating how to setup the system past the last regulator.

[cmrnpizzo14]

The pneumatic trick is called blocking the cylinder. You essentially use two pistons, one large and one small whereas the smaller cylinder acts as a release mechanism. The larger cylinder is activated, but it cannot move because the smaller cylinder is blocking its movement. When the smaller cylinder is moved it unblocks the large cylinder. The resultant force of the large cylinder is tremendous.

I think that our team used this in 2010. We didn't use the smaller cylinder to directly block the larger one but we had a latch that would hold our kicker back until that smaller cylinder released it. We would activate the solenoid to let air flow into the larger piston but it wouldn't extend due to the latch. Then releasing it we got a fairly good kick since the air was already through the solenoid.

Forgive me if I have the system wrong, it was my (and the team's) rookie year. I definitely didn't understand what was going on yet.

[carol95]

Hello, my name is Caroline Santos. I am the President for team 5023 (rookie team) from Capuchino High School. I was wondering if you could give us some tips as to how to design the robot for the FRC 2014 season. Please reply back. Thank you.

P.S: This is only our first season. Therefore, we need some help with designs.

Caroline Santos

[RRLedford]

Those here to gain insights as to how to make the best shooter should be aware of some factors influencing your choice for type of shooter and how you design/build it..
Most teams will use either a swing arm (catapult), a ram punching the ball, or spinning pinch wheels (or belts) to launch their balls.

1) Since at 2.75 LBS, the balls are fairly light, this means that a catapult arm style launcher does not require the arm to have very much structural mass, making it easier to accelerate the COMBINED mass of both catapult arm and the ball to a higher velocity with less force applied.

2) With the 60" height and 20" protrusion beyond the 112" circumference frame limits, the length of a swing arm design is rather constrained.

3) A set of spinning wheels (or belts) launcher is fairly compact, but needs to grip ball without damaging it from high slippage/friction/heat/melting.

4) Both ram and swing arm can be powered pneumatically or via winch stretched surgical tubing (or bungee cords).

5) All methods of launching need energy stored and ready to be delivered by a triggering method.

6) Best game performance requires launcher that fires at a wide range of speeds and angles.

7) High goal accurate shooting favors launcher designs that reach higher ball velocities, but it also favors launcher designs where their accuracy does not require a consistent level of ball inflation pressure, which the ram and pinch wheels designs do require for accuracy.

8) Missing high goal shots in auto is to be seriously avoided, since no assist points can accrue until all missed shot auto balls are retrieved and scored, which could easily take the whole rest of the match if defenders are good.

9) Catapult swing arms launchers are potentially highest accuracy and more consistent for handling ball pressure variation, of the three types, but they are less capable for reaching higher ball velocities.

10) Variable shooting speed & angle combined with shot triggering and trigger resetting is not a trivial design task.

11) Winch designs that retract an energy storing element (metal spring, air spring, bungee, latex rubber tubing, etc.) to store a variable level of launch energy are tricky to design, especially the triggering and trigger resetting mechanisms. However, they do favor better precision control of amount of energy delivered per shot.

12) Slow turning winch designs where the launch trigger system releases drum to free spin as ball launches have issues. Care must be taken to avoid too much slack of an unwinding cord/cable, which must be prevented from snagging, and cord/cable must also then be fully rewound before next shot is ready for launch. In addition, much of the stored energy can be wasted on accelerating the free spinning winch drum instead of delivering launch energy to the ball. Consider nylon winch drums.

13) Ram designs require that the ram velocity be absorbed by the robot frame after launched ball separates from the ram. A well optimized ball-to-ram mass ratio and a well tuned peak velocity ram acceleration allows ram rebound off the ball to help limit the ram speed at the limit of travel stopping point.

14) Swing arms also have end of stroke arm travel concerns, but if air cylinder power is used, pneumatic pressure can be used for an end of travel cushion.

15) Ram launchers only impart energy to ball while ram maintains contact with the ball. Inadequate ram speed and travel can give too little ball penetration/compression by the ram end. compressed ball acts like a spring and extends time/distance that ram stays in contact with ball.

I am sure others can add more thoughts to this list.

-Dick Ledford

[MrForbes]

Good stuff, thanks!



6) Best game performance requires launcher that fires at a wide range of speeds and angles.


That's debatable....if the variable speed/angle require complex control that can't be accomplished by your team, then a well designed single speed/angle shooter that can be easily aimed and scores consistently might be the way to go. We did that last year, and it worked great.

[RRLedford]

Good stuff, thanks!




That's debatable....if the variable speed/angle require complex control that can't be accomplished by your team, then a well designed single speed/angle shooter that can be easily aimed and scores consistently might be the way to go. We did that last year, and it worked great.

It is always best to design & build within the capabilities of your team to accomplish the goals it sets.

However, this does not ensure that meeting those goals will produce the same level of best performance that the more elite teams can & will accomplish.

I was only suggesting that variable launch speed and elevation angle will be what most elite teams will end up having.
Good results can still be accomplished with well executed simpler designs as well, as you point out.

-Dick Ledford