Dangers of High Speed Pneumatic Gun
 

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? :cool:

Or should we just scrap the idea of a pneumatic gun? :ahh:

_Neil

Re: Dangers of High Speed Pneumatic Gun
 

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.

Re: Dangers of High Speed Pneumatic Gun
 

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.

Re: Dangers of High Speed Pneumatic Gun
 

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.

Re: Dangers of High Speed Pneumatic Gun
 

Somewhere on my web page I have a picture of my hand with a nail in it, the result of slightly careless use of a high speed pneumatic (nail) gun.

As for kicking the ball....there are some threads on pnuematic kickers, you might want to read them

http://www.chiefdelphi.com/forums/sh...ad.php?t=80534

http://www.chiefdelphi.com/forums/sh...ad.php?t=80176

http://www.chiefdelphi.com/forums/sh...ad.php?t=80474

http://www.chiefdelphi.com/forums/sh...ad.php?t=80400

Re: Dangers of High Speed Pneumatic Gun
 

Thanks Everyone! :)

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.

Thanks once again,
Neil Patel

Re: Dangers of High Speed Pneumatic Gun
 

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).

Re: Dangers of High Speed Pneumatic Gun
 

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?

Re: Dangers of High Speed Pneumatic Gun
 

Dick,

The UL safety inspectors only hand out an award. The robot inspectors are the enforcement arm for this situation.

Most of us read CD regularly...

I'm going to refer you to this post I made.

Put yourself in our shoes. What would you do? Could you look yourself in the mirror if a student gets hurt?

What do you think?

Mike

Re: Dangers of High Speed Pneumatic Gun
 

Would you be willing to share with other teams what these physics calculations were, so others can learn from them?

Thank you.


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Re: Dangers of High Speed Pneumatic Gun
 

Edit: Mike Betts covered it.


Al or Mike Can this even be tested at a regional and how would this be done?

Re: Dangers of High Speed Pneumatic Gun
 

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.


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Re: Dangers of High Speed Pneumatic Gun
 

Corey,

I have to think about it. I'm not sure that it is possible if you use a 1/8" NPT to vent...

Regards,

Mike

Re: Dangers of High Speed Pneumatic Gun
 

how exactly are you reaching this conclusion? I find it rather hard to calculate this actual IPS on youtube videos.

Re: Dangers of High Speed Pneumatic Gun
 

I believe that only valves are limited by the 1/8" NPT port, not cylinders.

Re: Dangers of High Speed Pneumatic Gun
 

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.

Re: Dangers of High Speed Pneumatic Gun
 

That was my understanding as well.

I can't say for sure, but I seem to recall a discussion of this point in another thread in CD or USFIRST forums a few days ago.

I've seen single-acting cyclinders (not sure if they were Bimba) with a plastic one-way vent valve on one end to dump air.


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Re: Dangers of High Speed Pneumatic Gun
 

I must agree that accurately calculating actual IPS of the piston using a frame analysis of a YouTube video is not feasible.

But you can certainly get a ballpark idea by looking at the piston stroke and how high and far the ball got kicked.

http://www.scribd.com/doc/6726997/ScienceofSoccer


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Re: Dangers of High Speed Pneumatic Gun
 

Daniel,

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>...

Mike

Re: Dangers of High Speed Pneumatic Gun
 

Hi Mike,

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.


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Re: Dangers of High Speed Pneumatic Gun
 

This is exactly how my team is addressing this concern.

Re: Dangers of High Speed Pneumatic Gun
 

Ether,

If I listed all my concerns, too many electrons would have to die and I'd be getting hate mail from PETE, the ASPCE and a lot of angry scientists at CERN would have nothing to annihilate their positrons with...

You, as a mentor, must assume the first line of defense when it comes to safety. If you feel out of place doing this, I very, very strongly suggest that you seek out a veteran team in your area and ask for help.

Mike

Re: Dangers of High Speed Pneumatic Gun
 

Hi Mike,

I appreciate the humor :-), and I apologize for not being clear.

I wasn't asking you to list all your concerns. Just to clarify your comment about "burden of proof for a safe design", since what I mentioned IS what many "veteran" teams are doing.


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Re: Dangers of High Speed Pneumatic Gun
 

Thanks for replying, Mike.

I guess the reason I posted is that the Bimba 20 in/sec recommendation has been floating around CD for over a week and I'm still seeing very experienced CD contributors pointing new teams toward designs that that seem to violate it. And as far as I know, it took my posting to get robot inspectors to weigh in with opinion.

If a ball is going 20 ft in the air then the likely speed is on the order of over 20 feet per second- (I have a Excel projectile spreadsheet that I found on the internet and modified. If I can get the permission from the original author, I will post it here.) The foot is traveling faster than the ball in the videos I've seen. I can approximate the travel distance ratio I've seen as maybe 6:1 at best.

I do want to do things safely and not get in arms race. The pneumatic piston for our kicker that my team was using was only traveling 16in/sec (measured with a 240 fps camera), but other team members saw video of low-profile robots that showing very long distance pneumatic kickers with elastic assist and wanted greater distance for our robot.

I would imagine that some teams will not have the initiative to contemplate computation of their piston speed based on their ball speed and their leverage, or to actually measure their piston speed. Both of those approaches to understanding piston speed are challenging to accomplish.
I'm not sure robot inspectors will be consistently equipped to make such an evaluation.

FIRST encourages novice teams to use pneumatics and did not choose to highlight this 20 in/second rule and its safety impact on kicker designs.

FIRST seemingly encourages use of surgical tubing for elastic energy storage.

As to your questions of could I look myself in the mirror if a child gets hurt...Yes, I could. If I let the concept of me having no relation to a child ever with any potential of getting hurt, I certainly wouldn't ever drive a child anywhere. In fact, I would have never had children of my own or associate with children.

Question for you: have you ever exceeded the posted speed limit (by even one mile per hour) with a child in the car?

Life is not about risk avoidance, it's supposed to be about competent risk management. We are teaching these kids that training wheels are important, but that sometimes removing the training wheels has a time and a place. It's important that children are taught not only to follow rules but to know the relative danger of things. It's important that they understand the difference between misusing a mechanism that will cause it to wear out too soon and misusing a mechanism in a manner that would substantially increase danger: personally, I do not know which category that exceeding the 20in/second falls under.

My intent is not that the pneumatic rules get bent...the intent of my posting was to encourage that they be enforced consistently so the arms race does not get out of hand. And I really do not understand how a leaky piston gasket could cause a significant safety concern. I would be much more concerned about kicker designs that are improperly using the end of the cylinder as the stop. (Even though there is not an explicit "rule" against such a thing).

I guess I would also suggest that elastics and springs and gravity need their own section of the robot rules. Something like: if it stores more than 50? lb-ft of energy, the design needs qualified parts that are rated for the stress.

I like robot inspectors- I think they are REALLY wonderful people that are put under totally unfair stress at the competitions and are not appreciated anywhere near enough. FIRST needs good inspectors- I thank you for your service.

Re: Dangers of High Speed Pneumatic Gun
 

For most teams, following the rules will insure a safe design. <R94> is a catch all rule when teams do anything that is questionable. It is their responsibility to prove that they are, indeed, safe. It should be self explanatory...

Think of it as a "design review" that any engineering design must pass before a product goes to manufacturing.

Please use common sense.

If you use a pneumatic cylinder part number 123 made by XYZ, you should have the design or test documentation to prove that it meets the rules.

If the inspector watches it operate and he is concerned that something might fail and, in so doing, cause drivers, referees, spectators, et al to be in jeopardy, he will ask you to prove your design is safe.

If you can not, he or she may require modification or removal of that device before you will be allowed to compete.

Mike

Re: Dangers of High Speed Pneumatic Gun
 

Amen to that.


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