"Compressed Air" vs. "High Pressure Blower"

[DavisDad]

We are investigating the feasibility of a ball launcher using a cannon design. I want to avoid the complexity of compressed air and use a high pressure blower. Approximate blower specs:

• 2.9 psig "dead head" pressure
• 1.5 psig working pressure
• 750 CFM (200 CFM if accumulator)
• Velocity 35 ft/sec

I'd like to use an accumulator (reservoir) but if this will be considered compressed air, this will nix the design (Cv issues).

Any thoughts on this subject?

Thanks in advance!

[IndySam]

I'm on my DROID so I can't give you exact text but r73 I think is the number that states the only source of compressed air is the KOP compressor or its equivalent.

[DonRotolo]

Originally Posted by DavisDad

We are investigating the feasibility of a ball launcher using a cannon design. I want to avoid the complexity of compressed air and use a high pressure blower.

My opinion is that such a system would not be Pneumatic. I base this opinion on the lack of separation between the "pressure area" and atmosphere. That is, if no ball was there, you;d just get some wind with a barely measurable pressure difference above ambient.

Of course, my opinion doesn't count at competition.

I would strongly recommend you submit this, carefully worded please, to the official FIRST Q&A system for a ruling. That, you can take to competition.

[DavisDad]

I looked at power requirements for a blower @ 750 CFM and 40 inWC (~1.5 PSI) --> 20 HP ballpark. Yikes!

That would require something like sketch below. I think this is getting too complicated and error prone. I'm abandoning this idea...

[tsaksa]

Originally Posted by DavisDad

I looked at power requirements for a blower @ 750 CFM and 40 inWC (~1.5 PSI) --> 20 HP ballpark. Yikes!

That would require something like sketch below. I think this is getting to complicated and error prone. I'm abandoning this idea...

That sounds prudent. The use of compressed air, blowers, or even vacuum often sounds like a good way to move game pieces around. The problem is that compressed air is expensive to make. And I mean expensive in time, energy, space, and weight. This problem is even more apparent if all your power comes from a modest 12 volt battery. Take a look at the compressors used to power pneumatic tools in even a small industrial shop sometimes. They need to be quite large and powerful to keep up with even a few small tools. Pneumatics (or blowers in this case) can be a good choice to do some things on a battery powered robot. Just be very aware of the limitations and only apply them when the "expense" works out.

Your approach of applying some simple estimation techniques to see if your concept is practical is a common and valuable technique in any engineering discipline. Too often people want to cling to an initial concept that does not hold up to this type of analysis and insist on wasting time with prototypes to prove to themselves it won't work. The short build season in FIRST emulates the real world pressures engineering teams often face. The ability to brain storm and quickly come up with a bunch of good ideas for a robot seems like an important skill to succeed here. But equally if not more important is the ability to quickly sort through those ideas to find the few that are actually practical given all the other constraints of the competition.

[DavisDad]

Originally Posted by tsaksa

... Too often people want to cling to an initial concept that does not hold up to this type of analysis and insist on wasting time with prototypes to prove to themselves it won't work. ...

I'm guilty...

I couldn't let go of the idea. I had to know how the leakage around the ball would affect the performance.

I rigged up an Extrol tank (hydronic expansion tank) to a short length of Sonotube (big toilet paper tube). The ball is fairly tight with less than 1/16 gap = (tube ID - ball OD)/2. I had my son hold down the rig and I jammed a baseball bat in from the back to get a good force on the diaphragm. The ball went about 10'; OK but not great. A SWAG of the leakage is about 1/2 the volume pushed by the diaphragm leaks by the ball.

There is also an affect I've observed from testing with a reversed shop vac:

• I had the hose stuck into a bucket that fit on the OD of the Sonotube
• Before I put stops at the bottom of the Sonotube, I tried the rig letting the ball fall into the bucket where gap was large
• When I turned on the vac, the ball was held in the bucket = the rig sucked the ball back

Maybe there is some back pressure from the leaking air creating high pressure in front of ball (drag).

Extrol sketch --> http://www.arttec.net/Solar/12-24-10...gm_diagram.jpg

[Daniel_LaFleur]

Originally Posted by DavisDad

I'm guilty...

I couldn't let go of the idea. I had to know how the leakage around the ball would affect the performance.

I rigged up an Extrol tank (hydronic expansion tank) to a short length of Sonotube (big toilet paper tube). The ball is fairly tight with less than 1/16 gap = (tube ID - ball OD)/2. I had my son hold down the rig and I jammed a baseball bat in from the back to get a good force on the diaphragm. The ball went about 10'; OK but not great. A SWAG of the leakage is about 1/2 the volume pushed by the diaphragm leaks by the ball.

There is also an affect I've observed from testing with a reversed shop vac:

• I had the hose stuck into a bucket that fit on the OD of the Sonotube
• Before I put stops at the bottom of the Sonotube, I tried the rig letting the ball fall into the bucket where gap was large
• When I turned on the vac, the ball was held in the bucket = the rig sucked the ball back

Maybe there is some back pressure from the leaking air creating high pressure in front of ball (drag).

Extrol sketch --> http://www.arttec.net/Solar/12-24-10...gm_diagram.jpg

By using a rubber diaphram, you are basically creating your own compressor (without the bellows valve, and using the ball as a poppit valve).

As such you may run into issues with <R69> and proving <R73>

[R69] All pneumatic components must be COTS pneumatic devices rated by their manufacturers for working pressure of at least 125psi (with the exception of [R71]-D).

[R73] Compressed air on the Robot must be provided by one and only one compressor. Compressor specifications may not exceed nominal 12V, 1.05 cfm flow rate, 120 psi maximum working pressure. Off-board compressors must be controlled and powered by the Robot.

I'd suggest you Q&A this before you bring it to competition.

[cmrnpizzo14]

I would recommend a high pressure blower if you are really set on the idea and if it is legal, however I think that this really is not a sound idea. The consistency of it would be very iffy since the ball would be shot out with air, not exactly the best source of power. In addition, if the barrel was constructed wrong the ball would have no rotation as it flew through the air creating a knuckleball effect in which the ball would float at random through the air and would not be able to stay on target. Furthermore, you would need a LOT of air to shoot the ball any further than about 1 foot. The balls are relatively heavy and not light like most air powered projectiles would be. Don't let me stop you from trying this idea, but I really do not think that this would be the best option for any team.

[DavisDad]

Originally Posted by cmrnpizzo14

...this really is not a sound idea...the ball would have no rotation as it flew through the air creating a knuckleball effect ...any further than about 1 foot...

Hi cmrnpizzo,

All excellent points. You identified serious drawbacks of this concept and I appreciate the feedback. My son's team is going w/ a spinning disk method. I have my doubts about the accuracy achievable as I helped them build the first prototype. I'm doing this separately as my own investigation. I'm working on an unrelated project with many of the same elements (low pressure, large volume, fast acting, momentary air push).

My counter to your concerns:

1. I'm not sure how significant the knukle ball effect will be. Has anyone worked with this? I found a patent for a paintball gun barrel that has small holes in a line on the top side of the barrel to force the ball to roll at the top of the cylinder
2. I have shot the ball 10 feet in testing with only part of the force I intend to apply. Note: I'm using a diaphragm system and gave up on the blower idea.
3. I hope that the air bypassing the ball (leaking by) will create an "air slide" with consitent drag and compensate for dimensional variations in the balls. This would improve trajectory and velocity precision.
4. The components as currently concieved, would be light relative to other "pusher" systems.
5. Most imprtantly, I think I can build it.

[DavisDad]

I made a rig to test the cannon concept. The results were NOT very encouraging. The friction and air leakage losses were significant. The complexity to minimize the lost power and mechanisms to load and release the spring energy would require more work than I'm willing to bet on for this concept.

Anyway... time to focus on the working design.

[nitneylion452]

[R73]
Compressed air on the Robot must be provided by one and only one compressor. Compressor specifications may not exceed nominal 12V, 1.05 cfm flow rate, 120 psi maximum working pressure. Off-board compressors must be controlled and powered by the Robot.

Blue Box

If an alternative compressor is used, the team may be required to provide documentation to show compliance with the performance specifications.

The only difference between an on- and off-board compressor is that the off-board compressor is physically removed from the Robot. The intent of this rule is to permit teams to take advantage of the weight savings associated with keeping the compressor off-board. However, using the compressor off-board of the Robot does NOT permit non-compliance with any other applicable rules.

The compressor may be mounted on the Robot, or it may be left off the Robot and used to pre-charge compressed air in the storage tanks prior to bringing the Robot onto the Court.

[Jeff Pahl]

Standard "This is not an official reply" disclaimer applies here:

If I was being asked to inspect such a system, my first question would be "Does the pressure of the air in question at any time exceed atmospheric pressure?" If so, then it is considered "compressed" and all pneumatics / compressor rules apply.

[Daniel_LaFleur]

Originally Posted by Jeff Pahl

Standard "This is not an official reply" disclaimer applies here:

If I was being asked to inspect such a system, my first question would be "Does the pressure of the air in question at any time exceed atmospheric pressure?" If so, then it is considered "compressed" and all pneumatics / compressor rules apply.

Technically when a robot drives forward it compresses the air in front of it, thus (according to your definition) all drivetrains are illegal.

Fans and blowers, in the past, have been deemed legal and not part of the 'pneumatics system' as long as they were powered by a legal motor.

I would Q&A this question, and I would expect a blower system to be legal. Whether it's effective or not is another question

[Aur0r4]

During Lunacy, we explored the idea of using blowers to add down force and/or thrust vectoring. We abandoned the idea for feasibility issues.

Sure enough, someone at GSR added some big fans, and didn't seem to have any problems with inspection. Can't remember their number (Peterborough, I think), but it seemed to add to their maneuverability.

Compressed air has a far, far higher energy density than the blast of air from a blower, which is why FIRST hasn't seemed to come out against it, so long as the motors are legal (as stated before) and the blades are properly guarded.

[Jon Stratis]

Originally Posted by Daniel_LaFleur

Technically when a robot drives forward it compresses the air in front of it, thus (according to your definition) all drivetrains are illegal.

Fans and blowers, in the past, have been deemed legal and not part of the 'pneumatics system' as long as they were powered by a legal motor.

I would Q&A this question, and I would expect a blower system to be legal. Whether it's effective or not is another question

The difference is the accumulation of air. If you are constraining air in a high pressure environment, it's a pneumatic system. The concern here is mostly one of safety - any constrained high-pressure (in this case, greater than normal air pressure) system carries with it the risk of sudden rupture... aka something similar to an explosion. Fans and blowers, on the other hand, simply push air from one point to another - they can create a current of air that might be higher pressure than the surrounding air in the venue, but it's not constrained. The dissipation of that higher pressure stream can occur naturally, and there's no worry of rupture or danger to others since its not constrained. Of course, the design of such a fan or blower would be scrutinized for safety in and of itself, the same as any moving mechanism on a robot.