off-season launcher
 

so now that it's the off-season, our team is building a t-shirt launcher. we plan on having three barrels that do not rotate or anything. i have some questions for the cd community so that our team can make the optimum robot.

1. what is a good diameter for the barrels? I've seen 2, 2 3/8", 3", and even 4"(and also how do you fold the shirt for the size designated)

2. if we wanted a distance of say 100 feet how large would the secondary holding tanks need to be? (we plan on using a sprinkler valve to release the air)

3. what is a good size for the hosing and specifically what is a good way to connect it to the tanks?

4. we plan on pre-loading the main tank to 120 psi with an actual big compressor and recharging between each set of shots with the 2009 compressor. (there is plenty of time for that since the barrels have to be reloaded manually.) would a 5 gallon tank be to big for what we're doing? what would a good size be?


also would the 2010 kop wheels(6wd) be good enough for a high school football game halftime(as far as the softer maybe slightly muddy ground)? what if we made them double wide?

Re: off-season launcher
 

Here's what I know from 2052's launcher-

1) It has been awhile since I have seen the launcher, but I would guess it is a 3" pipe. We roll the shirts and secure the roll with a rubber band. You can roll a tshirt to just about any size. Play around with it and once you get a method that works, stick to it.

2) This will depend a lot on the individual components. The distance will also depend far more on the PSI than the amount of air you store. 2052's is a 5 gallon tank though.

3) As large as possible. 2052's are I believe 3/4", though bigger is always better in this case. You want to maximize the amount of air leaving the tank at once and you want it to exit as quickly as possible.

4) There are trade offs here. With our 5 gallon tank at about 100 PSI we could get a long shot (over the bleachers from the track on a typical HS football field), a medium shot (middle of the stands), and a short shot (just over a 3ft fence) before recharging. The biggest issue here is the compressor. The KOP compressors are NOT designed for this use. They run VERY hot, and once you get up to around 90PSI, they start cutting out and blowing the circuit breakers. We ran two KOP compressors and still had some troubles. If you can, get a bigger compressor.

As for your last question- we run ours on the track, though I would be wary on the field. Maybe someone else can chime in on that bit.

Re: off-season launcher
 

The KOP compressor is ONLY rated for a 1 gallon tank. We used a 1 gal tank on the robot and the compressor was pushed very hard, though had no problems.
I would definitely get something more powerful.
I have drive the kOP 6 wheel robot over a grass field and it worked fine, though muddy might make it more difficult.

Re: off-season launcher
 

Tanis, by the way you answered the questions it seems you have a different set up than we plan.

this is what we are planning to do:
Main tank(120 psi)-->pressure regulators-->A secondary tank for each of the three barrels(different psi's)-->Sprinkler valve-->actual barrels.

and we haven't bought anything yet so i am actually asking what would be a good size for the main and secondary tanks for three barrels (there will be plenty of time to recharge after each set of three shots). what has worked for other teams in the past?

and one more question. if the whole system is pre-loaded with an actual compressor to 120 psi and the kop one is just used to top it off is that still too much for the kop compressor?

Re: off-season launcher
 

Instead of a sprinkler valve, you might be better off with something pressure rated for compressed gases. Sprinkler valves are designed for liquid and if a failure arises, it would likely be catastrophic. For a robot that's going to promote your team in front of people, it's probably better to be safe than sorry.

We're running 6 6" KOP "sticky" wheels on our t-shirt cannon with the center wheel dropped. They work fine on a variety of surfaces.

Re: off-season launcher
 

While it is certainly better to use components that are rated for air pressure, I found in my own (not too extensive) research that large enough solenoid valves (1" diameter) would be prohibitively expensive for some teams. However, if you know of some source that has such valves for cheap, I would definitely look into it.

And before someone says that an catastrophic failure would also be "prohibitively expensive," I guess that's just something teams will have to consider. I suppose the other option is to just wrap the sprinkler valve in layers (and layers (and layers)) of duct tape.

Re: off-season launcher
 

If you're looking to build a t-shirt launcher, then why do what everybody else is doing and make a pneumatic canon? Our team decided to try a different strategy, that was both cost effective, crowd-pleasing, and a far shooter! Say hello to the t-shirt SLINGSHOT! Just get some surgical tubing, a long robot, and....BAM! A self-firing, enjoyable, and can shoot more than t-shirts robot! We made ours out of things we found in our robotics room.

Below is the original version (we've been working on it)!

Re: off-season launcher
 

i agree that a catastrophic explosion would be bad but we plan to put the three barrels inside of a large diameter piece of PVC so even if something does explode it will be contained.

again if someone could chime in on tank sizes(main and secondary) that would be very helpful.

and after looking around i found most of the valves are 24 volt ones. our team isn't the richest(we hope more people will sponsor us one they see this robot) is there a place to get cheap 12 to 24 volt converters. i've looked around and it seems most are around $50 that i've seen.

Re: off-season launcher
 

This is an off season project - why not just run 2 batteries in series? The minibot batteries are 12v each.

Re: off-season launcher
 

Excellent idea. I would caution that care must be taken here to not then wire the subsequent 24 VDC potential into the PD board. I'm sure that the transformer(s) there would not like the new voltage, nor would the downstream components.

To be safe, I would search places like allied electronics and newark for a 2-turn transformer. This would give you your voltage conversion from 12 VDC to 24 VDC. Just make sure that the transformer is rated for the right power (watts) or amperage, depending on how the spec is written.

Re: off-season launcher
 

Ah, you are correct there. We have one barrel hooked up through a sprinkler valve to our tank. On the other end of the tank we have a compressor with regulators.

Though I am by no means an expert in pneumatics and the underlying physics, I am fairly certain that a pre-charged 5 gallon tank would prove just as problematic. Keep in mind that 120 PSI means 120 pounds of pressure per square inch. As you have 5 gallons worth of volume, you still have far more total stress on the compressor.

I could be wrong there though, so don't quote me!

Re: off-season launcher
 

i've searched around and found out that an AC sprinkler valve on a DC circuit can be run on lower voltage since DC isn't as affected by either a resistance or an inductor and thus has more current. 12 volts should be fine.

Re: off-season launcher
 

Now, I know you said not to quote you, but I just wanted to clarify something(s). I don't know if this is what you were trying to say, but if it is, then it is just an extension on that.

1) The piston in the compressor has a set (frontal) surface area [which for the sake of simplicity we will say is one square inch]. If the compressor is filling a 1-inch cube to 120 psi, it would need to push with 120 lbs of force to move the piston (1x120=120). Now, if we increase that to 5 gallons, 1x120 still comes out to be 120 lbs. The volume of the tank doesn't change the amount of force needed to push the piston in the compressor. The (frontal) surface area of the piston and the pressure are the only factors involved in this.

2) Volume and surface area are not related in any particular manner, without specifying a specific shape such as a sphere. Thus, the total force applied to a vessel at any given pressure not directly related to the vessels volume, while it is directly related to its surface area.

3) The compressor will still have to do more work to compress a larger volume of air simply due to the fact that more air must be moved, however, at a given pressure, work per compressor cycle (piston stroke) is constant.

Re: off-season launcher
 

A catastrophic failure would be more than expensive. It has the potential to seriously injure team members or spectators regardless of how much duct tape you wrap around it. Is that really a risk you, your team and your school are willing to take to save a few bucks?

Re: off-season launcher
 

Hence why I put "prohibitively expensive" in quotes the second time.
I would still like to know what valve 254 uses - might make my entire point moot.

Re: off-season launcher
 

From our team website:
1" NPT MAC Solenoid Valve at ~17.0 cV Flow Capacity with a 23 ms Response Time.

For the increased reliability, safety, performance and peace of mind, buying a quality valve was well worth the investment.

Re: off-season launcher
 

An ASCO 8210G004 12 VDC solenoid valve is $218 LIST. You can probably get this cheaper if you go to a distributor and explain that you are a school related team. This is a 1" NPT valve, with a full bore orifice. If you step up to the 1 1/2" NPT 8210G022 12VDC solenoid valve, you get a much higher Cv (22.5 vs 13) but your price goes up to $405.

Both of these valves are only rated for 125 PSIG, so be careful not to try climbing above that. The biggest thing with building the air cannon is making sure that you can get an appropriate amount of air into the barrel in as short amount of time as possible. That's why it seems that higher pressures are better. You'll only need a limited amount of pressure if you can get the flow high enough. This of course also depends on your secondary accumulator (tank) volume. Make it too small and you won't have enough air to accelerate the projectile to a proper velocity, make it too large and you are just wasting energy (assuming equal barrel lengths).

A note on compressor usage: Do not expect kit compressors to last very long if you are using them constantly. They are surely not rated for anything higher than a 10-15% duty cycle. This means that they are only rated to run at the pressure rating for 10-15% of time. In other words, 6-9 minutes out of every hour. Running them more than that risks overheating the compressor and burning something out.

Re: off-season launcher
 

Out of curiosity, have you ever heard of an incident involving a sprinkler valve failure?

There are plenty of stories about and warnings against using PVC to store air, but I have never before heard someone claim that using a sprinkler valve would be dangerous.

Sprinkler valves are typically very solid. I wish I had a data sheet on hand for one of them. If someone does, could you read off the ratings?

Re: off-season launcher
 

Typically solid when used for gardening? I don't understand the argument here. Sprinkler valves are rated for water pressure. A plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would. Use a valve rated for air pressure if you are even remotely concerned with safety. Water is incompressible and behaves completely different than air when pressurized. Please refer to the countless other threads here where this has been discussed before. Bottom-line; please don't cut corners when people’s safety is at risk.

Re: off-season launcher
 

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with compressed air.

As for "a plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would:" Not true. PVC used as an air storage tank almost always fails at the end caps or connection points, typically due to the a failure in the cement used to join two parts. If it isn't an endpoint, then it's because there was a crack somewhere along the body. If you are operating at reasonable pressures, a PVC tank will not spontaneously explode.

I have very carefully watched every thread on CD relating to air cannons and PVC air storage. I would never use PVC to store air. However, you may notice that every single team in those threads intends to use (and many have been using) sprinkler valves. None of them have reported any problems, and no one before has ever called them out on the sprinkler valve.

Re: off-season launcher
 

I still encourage you to do further research before using a sprinkler valve. While there may not be any documented failures that you have found thus far, the associated risks should not be taken lightly. Explaining in near absolutes how something will not fail is not helping your case here, either. I do not believe you know how all PVC storage tanks fail, and also do not believe you know all of the failure modes of a sprinkler valve. Objectivity is key, and building something safe which your school, students, and team can be certain won't harm anyone is my only concern. I posted initially because it seems like many people on these forums are not taking the safety of these cannons seriously. Please do not take my posts as personal attacks, because they are far from that. Using internet forums, youtube videos, and tech blogs to determine the safety of processes, mechanisms, materials, and hardware scares me and I hate seeing corners being cut with regard to safety in an effort to save a few bucks. Please just be as cautious as you can when proceeding.

Re: off-season launcher
 

Why go through haphazard testing on a component's limits while another product is already rated by the company's engineers through rigorous testing and with a high safety factor to perform the exact same task in a safe manner? It would be like testing the limits of 150 pound strength steel cable and because under my own tests I see it is fit to lift 170 pounds of material instead of purchasing something rated at 175 pounds or 200.

The same conclusions drawn about the unintelligent PVC storage tanks should be applied to other points of potential failure. Although I'm not an engineer like Travis, common sense would tell me to just purchase the valve that can handle the pressure well beyond what you will be using. Its not something sensible to cut corners with.

+0.02