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HPA is NOT pure oxygen. Please don't get them confused. It is non-flammable breathable air under high pressure, which is normally used in SCUBA diving.
CO2 has many advantages over HPA. First, it's easy to get a hold of. Common sources of CO2 are paintball stores and welding supply shops. Since it has been the de facto standard for use in paintball, the parts are cheap and easy to find. Also, it is considered less dangerous than HPA since the pressures involved are much lower.
A major design goal of Deadblow was continuous rapid fire. When you demand a lot of air from a CO2 system in a continuous way, the liquid CO2 has a hard time converting to gas fast enough and the whole system starts to freeze up. As the temperature goes down in a CO2 system, the pressure goes down, which is a major drag. In contrast, the air pressure of HPA remains constant regardless of temperature and volume of air demanded.
As with all pneumatic systems, you need a few basic things. First, you need a pneumatic tank on your robot for storing the air. If you buy a paintball system (which is what Deadblow is equipped with), it will also come equipped with a regulator. Unless you have tons of weight to burn, do not attempt to use a welding regulator. The regulators that come with the tanks work great and you don't have to do any special machining, adapting, or plumbing to get the regulator to work with the tank. Look for a carbon-wrapped aluminum tank, which is common in paintball HPA systems. Depending on the amount of air you need, you may have to purchase more than one to get the required onboard capacity. Deadblow has two tanks.
Second, you need a valve. Your choices are single-acting and double-acting. The difference is that single-acting valves can only act in one direction, while double-acting valves are powered in both directions. For example, let's say that you have a hammer weapon that swings up and down. With a single-acting valve, you can only fire the hammer in one direction. In this case, we'll say that direction is down. In order to reset the weapon and bring the hammer head back up, you'll need a spring or winch or some other means of pulling the hammer back up, since the valve is passive in that direction and provides no force. In contrast, a double-acting cylinder, which is powered in both directions, will be able to retract itself with no extra components. Unfortunately, a double-acting cylinder consumes twice as much air as a single-acting cylinder. Also, be sure to find a 12 or 24 volt DC valve. Unless you're running a 120 VDC drive system (which a few robots do), stay away from the 120 volt valves.
You will need a servo switch to trigger the valve. This takes a signal from the receiver and activates the valve by connecting 12 or 24 volts to the input leads of the valve. This can be a store-bought ON/OFF servo switch (such as High Sky or other brands), a Dan Danknick Special, or even homemade with a regular servo and a momentary-ON leaf switch.
Oh yeah, and you'll need a cylinder and air fittings. Usually, the working pressure of the cylinder and air fittings is 150 psi. The manufacturer will rate the power of a cylinder based on the diameter (simple, really), and two gauges: one that goes up to 4000 psi to check how far you're filling the tank, and one that goes up to 200 psi to set your working output pressure. Don't think that you don't need an output gauge. Sure you can set the pressure and remove the gauge to save weight, but if your regulator psi knob gets bumped or loosened at the event, you will have no idea what your output pressure is, and if it's too high, you could blow your fittings and lose all your air.
Get the high pressure air fittings- braided steel air lines from the paintball store. For the low pressure fittings, the place where you get the air cylinder should also sell plastic tubing rated for 150 psi. By the way, all paintball pneumatics use NPT (national pipe thread) fittings, which can be purchased at your local hardware store in the plumbing section. |
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The advantage of the fittings in the paintball store is that they are compact and pretty. (And in this sport, looks count for a lot...) If you purchase surplus tanks, regulators, and/or fittings, you find that they are threaded with MS or military-style threads. Good luck trying to find adapters.
That takes care of what you need onboard the robot. Next, you'll need a SCUBA tank (or several SCUBA tanks) to hold the air for refilling the robot. This can be bought at a local SCUBA store. Optionally, you may want to purchase a gauge to check the capacity of your SCUBA tanks. This should also come from the SCUBA store. The adapter from the SCUBA tank to fill the robot should come from the paintball store.
Unless you are a certified diver, some shops will not sell you equipment or refill your tanks. This is what scares off many hobbyists, and rightly so, since HPA is dangerous and every SCUBA or paintball tank is potentially a deadly missile.
Also, compared to a CO2 system, HPA is very expensive. Tanks with regulators can cost between $200 and $500 each. The valve should be in the $25-75 region depending on the configuration and air flow capacity. The servo switch will run you $20-40, and you should budget $50-75 in fittings, air lines, air gauges, and other miscellaneous connecting devices.
A new SCUBA tank will cost around $250 and you will probably need more than one. The filling rig (from the paintball store) is about $125 and the air pressure gauge for checking the SCUBA tank ran about $200.
Before attempting to put together an HPA system, you should do your homework. Buy as many paintball magazines as you can and read all the articles comparing HPA and CO2. I'm sure there are many more arguments for and against HPA than I've listed here. Go to a paintball store and talk to the guys behind the counter. These guys deal with this stuff every day. Go to a dive shop and explain exactly what you want to do. Ask questions and research until you're absolutely comfortable with the idea of HPA and the dangers involved.
Listen, this is not a technology for a beginning robot builder. If you are really set on an air-powered system, then start with CO2. It's easier and cheaper to get into. Nothing is worse than wasting hundreds of dollars on a technology that may not even work for you in the end.
And finally, SAFETY, SAFETY, SAFETY. If you've finally decided that HPA is the way to go for you, then put your credit card down and ask your paintball supplier and dive shop supplier EXACTLY the right and safe way to handle the items that you buy from them. This will help keep you out of trouble and from getting hurt, maimed, or worse. As with all of robot building, common sense one of the most important skills you need to have.
pneumatic: air-powered
CO2: Carbon Dioxide. See other articles on its use and handling.
cylinder: pneumatic cylinder, basically a piston with a hole on each side of the diaphragm. Put air into one side, the diaphragm moves away from that side and either extends or retracts.
regulator: used to bring (regulate) the HPA air pressure (usually about 3000 psi) down to the working pressure of 150-200 psi.
valve: also called a pneumatic solenoid valve. It's basically an air valve that takes an input air pressure (from your tank) and routes it to the cylinder. Usually, valves have two outputs: A and B. A single acting valve is basically an airflow on/off switch, and routes air into A, exhausting through B. A double acting valve routes air into A, exhausting through B in when not energized and then switches the direction when activated, so that the cylinder is powered in both directions.
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