Pneumatic Actuation
Pneumatics is the term that refers to a system that uses pressurized air to achieve motion. Many types of actuation mechanisms are available, including a wide array of cylinders (linear actuation), grippers, and motors. Pneumatics add a great deal of speed and power to any actuation mechanism. However, there are a few drawbacks to consider before making the decision to use pneumatics. First, all the components are quite expensive. This is where money from corporate sponsors can help out immensely. Second, a properly designed system is more complex than an equivalent electromechanical system (electric motors, power screws, linear actuators, etc.). The third drawback is that all these components take up quite a bit of valuable space within a robot. Due to these drawbacks, a pneumatic system may not be the best choice for a beginner robotics team. It is still worth looking into, however, because the speed and power of pneumatics (in my opinion) overcome any drawbacks that may be associated with them.
To get an idea of what a pneumatic system entails, see the figure below:
In order to build a system like this, you must first understand each component’s functionality. Below, I have listed a little information about each component, and possible suppliers to buy these components from.
Nitrogen or CO2 Tank – This is what powers your system. CO2 Tanks are significantly cheaper than Nitrogen (or High Pressure Air (HPA)) tanks. However, CO2 tanks typically hold less compressed gas, and have the side effect of expelling liquid CO2 along with the gaseous CO2. This behavior creates the need for an additional flow conditioner (see Expansion Chamber below). Nitrogen and CO2 tanks are available at paintball stores and online at many locations. A few good websites to check out are:
Expansion Chamber – The expansion chamber allows the liquid CO2 another chance to warm up and decompress into gas. Expansion chambers are relatively cheap, and should keep most of the harmful liquid out of the valve and actuator. Expansion chambers can also be found in paintball stores and online.
Regulator – The output of either the Nitrogen or CO2 tanks is about 800psi. Solenoid valves and actuators can only safely operate at about 100psi. The regulator decreases pressure in the airlines to levels that you specify (by turning a screw). Regulators are standard paintball parts, just like the expansion chamber and the tank. You have to be careful about which regulator you order. Not all paintball regulators can decrease the pressure below 100psi. Look for regulators that say they have an adjustable output pressure of 0-500psi.
Pressure Gage – Now we’re getting into the more industrial parts. Paintball stores sell pressure gages, but typically they have a range between 0 and several thousand psi. Now that the pressure is below 100psi, you need a gage that has a range between 0 and ~100psi. These gages can be obtained from pneumatics suppliers. Two good places to look are:
Solenoid Valve – This is the switch for your system. The solenoid valve takes electrical signals and uses them to open and close the air lines to the actuator. There are many types of valves, so it will take some research on your part to find valves that are suited to you application. One thing to look for is a solenoid coil that is within your battery’s operating range (i.e. if you have a 12V battery, don’t buy a valve with a 120VAC solenoid coil). Suppliers of solenoid valves are basically the same as for pressure gages.
R/C / Electronic Switch Interface – This is a nifty device that interfaces perfectly between your RC receiver and the solenoid valve. It interprets the pulse-width modulated signal from the receiver to a high (positive voltage) and low (no voltage) signal for the solenoid valve. I have seen similar items several places, but the ones we used were from Team Delta (http://www.teamdelta.com)
Actuator – Finally, its showtime! If you have successfully gotten all the other components to work, this is where the reward comes in. Pneumatic actuators have very good power and speed. For example, a cylinder with a bore of 2.5 inches produces about 500 lbf when operated at 100psi! The speed of the cylinder can be controlled using flow control fittings. Speeds can range from very slow opening to very fast, taking only about a tenth of a second to open (depending on the cylinder stroke)
Final Notes:
One design consideration is how much air you will consume while operating your actuators. One guideline I have used is the ideal gas law (PV=nRT). Find out how much air, n, you can hold in your tank (I usually hold T constant, and R is a constant anyway). Then figure out how much air each actuator will use per stroke (V = cross-sectional area * stroke). This will at least give you some idea of how much actuation your tank will hold out for.
Like all robotic components, pneumatic components have the potential to be extremely dangerous. Therefore, heed all safety precautions and design limits of the components you are using.