"Compressed Air" vs. "High Pressure Blower"

[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.

[Bob Steele]

Originally Posted by Tristan Lall

As long as FIRST doesn't pretend that a ducted fan is a traction device,¹ I'm happy. (In all seriousness, the distinction between accumulating pressure and blowing air seems to be a good, practical one.)

¹ I'm not making that up. In 2009, a fan thrusting the robot downward was allegedly a traction device. That was ridiculous.

Tristan this is an interesting comment... why did you use the fan?

My guess is that it was used to increase the "apparent weight" of the robot which would increase the normal force on the wheels which would increase the friction of the wheels with the surface. Therefore this would increase traction..
hence a traction device.

That being said.. .the battery and every other part on the robot are also traction devices by this definition ..

interesting

[Daniel_LaFleur]

Originally Posted by Bob Steele

Tristan this is an interesting comment... why did you use the fan?

My guess is that it was used to increase the "apparent weight" of the robot which would increase the normal force on the wheels which would increase the friction of the wheels with the surface. Therefore this would increase traction..
hence a traction device.

That being said.. .the battery and every other part on the robot are also traction devices by this definition ..

interesting

Increasing the nominal downward force was, indeed, the reasoning of the rule. Fans / props were allowed that year as long as they did not increase the nominal downward force of the robot (I know, we used propellers horizontally for additional thrust).

As far as 'every other part' on the robot ... all those needed to be within the 120LB max weight (plus battery and bumpers) and thus were already accounted for in the maximum traction attainable.

[Tristan Lall]

Originally Posted by Bob Steele

Tristan this is an interesting comment... why did you use the fan?

My guess is that it was used to increase the "apparent weight" of the robot which would increase the normal force on the wheels which would increase the friction of the wheels with the surface. Therefore this would increase traction..
hence a traction device.

That being said.. .the battery and every other part on the robot are also traction devices by this definition ..

interesting

The fan wasn't mine. Other teams wanted to increase their traction on the slick fibreglass surface. The Q&A ruled against it.

But like you said, if that was a traction device, why wasn't everything with weight or downward momentum also a traction device? (They contribute to the normal force, which determines the traction.) And if the weight limit was considered "accounted for", why couldn't underweight robots use a fan to bring themselves to an equivalent normal force? Note that there was no theoretical limit on momentum (though admittedly there were practical ones).

And from an enforcement point of view, how is a referee supposed to know when a ducted, vectoring fan is exerting a downward force, and when it's just thrusting horizontally or off? Failing that, under what authority would FIRST have asked inspectors to disapprove of that mechanism? It was completely unworkable.

Let's hope it's never an issue again....

[Jeff Pahl]

Originally Posted by Tristan Lall

And from an enforcement point of view, how is a referee supposed to know when a ducted, vectoring fan is exerting a downward force, and when it's just thrusting horizontally or off? Failing that, under what authority would FIRST have asked inspectors to disapprove of that mechanism? It was completely unworkable.

Let's hope it's never an issue again....

Amen to that!!

[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.