What is pneumatics?

      Pneumatic systems employ gas (or air) that is compressed under extremely high pressure. The practical use of pneumatics comes in putting that compressed gas to use, or should I say the use of the rapid expansion of compressed gas. At its most basic level a pneumatic system holds compressed gas in a specially designed tank and then we release some of that gas into an expandable chamber. The expandable part of the chamber has a rod attached to it so that as it expands the rod moves outward. Sounds pretty simple, right? Well, in theory it is, but it is in application that things get complicated.

Now lets get familiar with the parts commonly used in pneumatics

The first part of a pneumatic system may not sound like an actual part, but the main and is called the compressed gas or AIR itself.

2.Tank:

The second part of a pneumatics system is the compressed gas storage, otherwise known as the tank. Tanks range in size, weight, and proofs (rated capacities) depending upon their use. Tanks should be DOT-approved and can be made of steel or Carbon Filament wound Aluminum (like wrapping it with carbon fiber) and will either have the working pressure in psi stamped into the tank or will have a certification sticker on it. Tanks may also be measured in "bar" (not plural). 1 Bar is equivalent to about 14.5 psi which is equivalent to 100KPA which is equivalent to 1 Atmosphere. So a 69 bar tank is equivalent to 1000 psi.

Each tank should also have a burst valve to keep the tank from excessive pressure. In general the burst valve is rated for 120% of the tanks normal operating capacity. If the pressure builds to above 120% of the rated pressure the burst valve will pop open and vent the gas slowly to prevent the tank from exploding. Tanks are generally hydro-tested to twice the rated capacity to pass DOT-approval inspection. Common sources of tanks are welding supply shops, diving shops, paintball stores, and even fire extinguisher tanks!


3.Regulators:

Regulators are interesting pieces of hardware in that they can hold back 5000 psi of air and let only a enough air
through to bring the rest of the pneumatic system up to your designed operating pressure. Regulators also generally have a purge valve to allow you to purge all of the air out of a pressurized tank.

4.Buffer Tank:

Buffer tanks are not necessarily part of every pneumatic system. But, if you have the extra weight allowance and space available they are very handy to have. A buffer tank is just an extra tank in between your regulator and your valve that stores extra gas. So, what does that do for us?

Let's say that you have a pneumatic cylinder that has a 4 inch bore and a 6 inch stroke. That gives us a total of about 75 cubic inches. Now, let's say that you are using 1/2" pneumatic tubing between your regulator and your valve and on to the cylinder and we are using 250 psi of gas. If you have 24" of total tubing you have almost 5 cubic inches of compressed air in the feed lines. That will move the piston about half an inch before the regulator has to start feeding more air into chamber. The second the pressure in the feed lines drop below the regulated pressure the regulator starts letting more air through. But what if you have a regulator and is not a high flow kind? Well then your high powered pneumatic flipper just turned into a lifter.

Now, let's put a 75 CI buffer tank in line before the valve. This time the regulator spends a little longer initially
filling the feed lines and the buffer tank. But, when you fire the valves the buffer tank dumps its 75 cubic inches of compressed gas along with the 5 that was already in the lines. This time there is a lot more pressure immediately available to the cylinder and we get the "pop" that we are looking for in a flipper.

5.Pneumatic hoses and fittings:

Well, for all of the air to move around we need to have a way to move it. That's where the hoses and fittings come into
play. To get the best air flow you need to use the largest diameter hose that you can find that is rated for the pressures
that you will be using. You will also need to find matching connectors and fittings throughout the system. It does no good to
have 1/2" hoses and fittings throughout your whole system only to have a 1/8" port on your solenoid valves (okay, so it's an
extreme example but you get my drift). The 'push to connect' low pressure fittings and hoses are the easiest to work with for
prototyping and low pressures.

You can get pneumatic hoses and fittings that are rated for very high pressures. You can also use hydraulic lines but
they not really good for moving high volume of air in a hurry but some will work. Hydraulic lines and fittings are designed
for extremely high pressure and are sometimes sheathed with a steel mesh to help keep the hose from deforming and developing
bubbles.
It is a good idea to use PTFE (Teflon) tape on all threaded connections as it helps seal any gaps that may occur between the threads.

6.Valves:

The valve will probably be the most critical (and consequently the most expensive) part of a high power pneumatics system. It has to restrain the pressure built up on one side and be able 'pop' completely open and not restrict the air as it rushes through on its way to the cylinder. There are many types of vales that can be used; Remotely Operated, Manually Operated, and Solenoid Valves.

For the context of this help section we will just keep it to solenoid valves.

The reason that it is called a 'solenoid valve' is because there are really two parts; the valve (and valve body) and then solenoid that activates the valve. The solenoid opens a smaller valve that controls a small stream of air that then pops open the large high flow valve.

There are several different types of solenoid valves but we are just going to talk about the three most common ones used in robots. There is a 3-port, a 4-port, and a 5-port solenoid valve.

The 3-port solenoid valve is so named because it has three ports; one from the tank, one going to the cylinder, and one exhaust. Because there is only one going to the cylinder we will be using a single acting cylinder (It is possible to use a 3-port valve with a double acting cylinder but that gets into advanced design and is therefore beyond the scope of this help section). The valve opens, and pressurizes the cylinder therefore extended the ram. Then the valve closes which opens the exhaust port and the gas in the cylinder is allowed to vent which equalizes it with the outside air and the ram retracts.

A 4-port valve is designed to be used with a double acting cylinder. It has four ports; one from the tank, one to the back of the cylinder, one to the front of the cylinder, and one shared exhaust. In its normally closed position it allows pressure to build up on either the front or the back of the piston depending upon your design. When the valve activates it redirects the compressed air to the opposite side of the piston while simultaneously opening the exhaust port so that the air that is currently in the cylinder can escape. If the air in the cylinder were not allowed to escape then it would just build up pressure when the ram piston tries to move and not allow the piston to go anywhere.

A 5-port valve is also designed to be used with a double acting cylinder but has an added exhaust port. This increases the efficiency of air flow leaving the cylinder which allows it to extend or retract faster. The diagram to the right in this section shows how a 5-port solenoid valve works.

7.The Actuator/Cylinder:

The actuator is the business end of a pneumatics system. All of the parts listed above are to make the actuator (cylinder) move, and move with authority. There are three main types of actuators, each with their own advantages and disadvantages; Single Acting, Single Acting Spring Return, and Double Acting. Inside the cylinder is a disc that is sealed against the walls of the cylinder. Then there is a rod attached to the disc which extends out one end of the cylinder. The rod is where we will attach things to to make things move, usually via a clevis. There are end caps on each end of the cylinder to keep the piston from shooting out of the cylinder when the piston slams into it at high speed. Actuators are typically made out of high grade aluminum or steel (usually the stainless variety). There are also a variety of mounting styles.

A single acting cylinder has only one inlet port and therefore only one power stroke. This is usually at the back of the cylinder so that the power stroke is the 'push' stroke. These require some other means of retracting the piston to its starting position, like gravity. Because of this standard single acting cylinders have a slow reload time. On the plus side it only has one inlet and therefore you get more shots per tank full.

Single acting spring return cylinders are just like the standard single acting cylinders with the exception that they have a spring inside of them. At the completion of the power stroke the spring helps to push the piston back to its starting position. Like the standard single acting cylinder this one allows you get more shots per tank full than a double acting cylinder and it has the added bonus of a spring return to help speed up reload times. But, alas, it's not all roses. Because there is a spring inside the cylinder it will take pressure to compress it which takes away from power that you could potentially be putting into flipping the opponent. This is usually a minor issue but the bigger issue is the fact that single acting spring return cylinders tend to be longer to accommodate the spring.

The last one is the double acting cylinder. It is called double acting because it has a power stroke on the push AND pull. The picture to the left is of a double acting cylinder and you will notice an inlet port on each end cap.