I’m looking for advice from veteran teams and mentors on pneumatics since we are a 2nd year team.
Our team was planning on using a high speed pneumatic gun to hit the soccer balls with enough velocity so they would go over the ramp.
However recently we discovered by calling Bimba that after speeds of 20 in./s on a pneumatic cylinder, the cylinder will start to break down. :eek:
We’ve done the physics calculations and we need the speed of the pneumatic piston to be atleast 5 m/s to use it as a gun. Obviously we need more speed for the gun design, but we don’t want the gun to break down in the middle of a game either.
Is there any way that we can prevent damage on these pistons, and slow the pistons at certain points during the shot to prevent damage? Is there any way to prevent damage completely?
Or should we just scrap the idea of a pneumatic gun? :ahh:
Don’t go fully pneumatic. Use the cylinder to swing a kicker bar.
Let r be a radius, L be a linear speed, and w be a rotational speed (technically it should be lower-case omega, but w is close).
L = r*w. Let’s say that you can get a linear speed of 5 units/second out of the cylinder. You attach it close to a pivot of a kicker (let’s say 5 units away from the pivot).
w is now L/r, in this case 1 radian/second.
The other end is supposed to be traveling at 20 linear units/second, let’s say.
20 = r*1. You need a 20-unit long kicking device.
Do you see what I’m getting at here? Use a swinging arm and design the placement carefully, and you can get the same effect as a pneumatic cylinder going straight in at much higher speeds.
Think along different lines. You already got your answer on how fast the cylinders can go (and here’s a hint - you don’t have the volume or the flow rate to get anywhere near that…).
So now you need to look at how much flow rate you DO have, how much pressure and volume you DO have, and you can turn a shorter stroke into a longer one.
Hint 1: Mechanical Advantage.
Hint 2: Think lever arm.
I’m not a mechanical engineer, but here’s what I’ve been pondering for the last few days. Perhaps someone who really knows can confirm or refute my hypothesis.
For a given pressure, the force you get from an actuator is proportional to the area of that actuator. The speed is inversely proportional to the volume of the actuator. You can trade force for speed with a lever arm of some sort, but that is exactly what a smaller bore size does. You can quadruple your force by using a 2x larger bore size, but that reduces your speed by 4x. In short, the mechanical power of a pneumatic actuator is limited by the size of the tube and the air pressure. Both of these are dictated by the rules, as far as I understand.
If pneumatics alone can’t do it, then a pure lever arm won’t help. A carefully crafted kicking device might be able to do a better job than a straight pneumatic puncher, but only because it makes better use of the available power.
Seems like there is no way to fix this design from the replies I have gotten so I guess the idea of a high speed pneumatic gun should be thrown in the trash.
I’ll keep on checking for replies but I think our team will move on to another concept; maybe, the kicker (pneumatic piston attached to lever arm) as most people here recommended.
A team used a pneumatic ‘gun’ in competition in 2008. That competition required moving something similar to a medicine ball. I don’t remember the team number, but they rigged a parallel arrangement of small pneumatic cylinders (and mechanically connected in series) and used valve timing to accelerate the medicine ball. The cylinders were small to obtain higher speeds from the available flow rate. Pretty interesting design.
I would expect that something like this would work for a kicker without any mechanical advantage (read lever arm).
So who thinks the FIRST safety inspectors will evaluate the Bimba guideline of 20 in/second and potentially disqualify robots because of it?
I’ve see a lot of videos and pictures of pistons tied to kicking legs during kicking motion and I have yet to see one that seemed like the piston was traveling less than 50" per second for a long distance kick. Getting 20:1 or 30:1 foot travel versus piston travel ratio is tricky especially in robots that are only 17" tall. (I think kicking hard over long distance is on the order of 40 ft/sec.)
Disconnecting the piston stroke from a kicking motion is feasible to keep the piston under the speed limit and it could be a valuable undertaking depending on whether this “20 in/second” is going to be perceived as a safety issue.
Personally, I think I am much more concerned about designs that are storing hundreds of pound-feet of energy in unrated/unqualified elastics as that would seem to be much more dangerous than speed violations on a pneumatic piston. But then again, I’m not an actuary.
I’m only a second year guy- any wise sages out there have any suggestions?
IMHO it is actually rather hard to travel 20in per second we use a 4’’ cylinder preloaded we would have to actuate this in quicker then 1/5 of a second…
A preloaded cylinder that is vented to atmosphere on the opposite side and that is restrained and released with a mechanical latch can easily exceed 20 ips by a large margin.
Many youtube videos clearly show speeds exceeding 20ips.
A cylinder firing with a speed of 1 in/s can be dangerous if used improperly. There should not be a minimum unsafe speed, or a maximum safe speed.
Also, Mike, while the UL safety inspectors can’t pull a robot out for lack of safety, anyone, including said safety inspectors, can ask the robot inspectors to reinspect a robot if they feel it is unsafe or violates a rule. Let’s say that team X passes inspection with a kicker that is deemed safe during inspection–say that it’s not fired then. On the practice field later, a member of team Y comes within inches of being hit when it accidentally goes off. Team Y can request a safety check be done to verify that team X is still safe.
You are correct… We have always used cylinders with 1/8" NPT ports before… And many valves usually don’t have NPT fittings. I wonder if this isn’t a typo…
Mike
Eric,
You are correct about requesting re-inspection.
About maximum safe speed, I’m thinking that <R94> applies here…
Any metal will fatigue and break if you hit it hard enough. There is no use in me questioning the 20"/s figure as a team can use any manufacturer.
If the team employs a precharged piston, venting to atmosphere without going through a valve with a very large cylinder port, the impact of the piston against the end of the cylinder housing could be very great indeed.
Since I can’t make an engineering decision a priori, I’d put the burden of proof for a safe design on the team via <R94>…
Is your concern that the end of the cylinder may break loose upon impact and become a projectile or otherwise create an unsafe condition?
If the kicker design includes a mechanical stop that prevents the piston from slamming into the end of the cylinder would that address your concern? In other words, in general would that meet the “the burden of proof for a safe design”?
I ask this because many many teams have built, or are busy building, kicker designs with pre-loaded cylinders.
